Ionic Liquids as Solvents: From Green Chemistry Foundations to Advanced Biomedical Applications

Abstract

This article traces the transformative journey of ionic liquids (ILs) from their discovery as molten salts to their current status as versatile 'designer solvents.' Aimed at researchers, scientists, and drug development professionals, it explores the foundational principles of ILs, including their unique, tunable physicochemical properties. The scope extends to their methodological applications in diverse fields such as biomass processing, drug delivery, and CO2 capture, while also addressing critical challenges like toxicity, biocompatibility, and solvent recovery. Finally, the article provides a comparative analysis of different IL generations and their validation for sustainable and safe use in pharmaceutical and biomedical innovations, offering a comprehensive resource for leveraging ILs in advanced scientific applications.

From Molten Salts to Designer Solvents: The Evolutionary Journey of Ionic Liquids

The field of ionic liquids, now a major subject of study in modern chemistry, traces its origins to foundational work conducted over a century ago. While contemporary research produces thousands of papers annually, the initial discoveries that established this domain were isolated breakthroughs that went largely unnoticed for decades. This technical examination details the seminal contributions of Paul Walden and other early pioneers who first documented salts that remained liquid at low temperatures, establishing the fundamental principles that would later enable the diverse applications of ionic liquids in green chemistry, electrochemistry, and industrial processes. The historical development of these materials demonstrates how disparate research threads eventually converged to create a unified field characterized by intentional design of ionic systems with specific physicochemical properties [1] [2].

The Walden Breakthrough: Ethylammonium Nitrate

Experimental Context and Motivation

In 1914, Paul Walden documented the first ionic liquid, ethylammonium nitrate ([EtNH3][NO3]), while investigating the relationship between molecular size and conductivity in molten salts. His specific research aim was to identify molten salts that would remain liquid at equipment-compatible temperatures, thereby avoiding the specialized adaptations required for high-temperature experimentation. This practical consideration led him to explore organic ammonium salts with melting points below approximately 100°C, enabling conventional laboratory techniques rather than those needed for traditional inorganic molten salts studied at 300-600°C [1] [2].

Walden's key insight recognized that these low-melting-point salts provided experimental conditions approximating those of conventional aqueous and non-aqueous solvents while maintaining ionic characteristics similar to high-temperature molten salts. This allowed him to apply the established theoretical frameworks of van't Hoff's osmotic theory and Arrhenius's electrolytic dissociation theory to these novel systems, despite their complex association/dissociation behavior [2].

Synthesis Methodology and Characterization

The synthesis of ethylammonium nitrate followed a straightforward neutralization reaction:

Reagents:

• Ethylamine (C₂H₅NH₂)
• Concentrated nitric acid (HNO₃)

Experimental Protocol:

• Combine equimolar amounts of ethylamine and concentrated nitric acid under ambient conditions
• The neutralization reaction proceeds spontaneously: C₂H₅NH₂ + HNO₃ → [C₂H₅NH₃][NO₃]
• Purify the resulting salt to obtain pure ethylammonium nitrate
• Characterize the melting point using standard laboratory equipment

Key Physical Properties:

• Melting point: 12°C [1] [2] [3]
• Appearance: Colorless liquid at room temperature
• Conductivity: Exhibited significant ionic conductivity

Walden's measurements focused primarily on electric conductivity and molecular size (determined via capillarity constant). His analysis revealed that these organic salts at low temperatures exhibited behavior corresponding to experiences with inorganic molten salts at much higher temperatures, with association phenomena complicating the complete dissociation of simple ions [2].

Walden's Experimental Workflow

Post-Walden Developments: The Formative Decades

Following Walden's discovery, the potential of low-melting-point salts remained largely unexploited for nearly four decades, with only isolated developments appearing in the literature. The first significant industrial application emerged in a 1934 patent describing the use of "liquefied quaternary ammonium salts" such as 1-benzylpyridinium chloride and 1-ethylpyridinium chloride for dissolving cellulose at temperatures above 100°C. The resulting solutions enabled chemical modifications of cellulose to produce threads, films, and artificial masses—a herald of modern cellulose processing using ionic liquids [2].

The period immediately following World War II witnessed renewed interest in low-temperature molten salts, particularly for electrochemical applications. In 1948, researchers applied mixtures of aluminium(III) chloride and 1-ethylpyridinium bromide for the electrodeposition of aluminum. The phase diagram for this [C₂py]Br-AlCl₃ system revealed a narrow composition window (63-68 mole percent AlCl₃) where the mixture was liquid at or below room temperature. This system featured eutectics at 1:2 (45°C) and 2:1 (-40°C) molar ratios with a maximum at the 1:1 molar ratio (88°C), attributed to bromochloroaluminate species formation in the melt [1] [2].

Key Historical Developments (1914-1980s)

Table 1: Major Early Developments in Ionic Liquids Research

Year	Researcher(s)	System/Discovery	Key Properties/Applications	Significance
1914	Paul Walden	Ethylammonium nitrate [EtNH₃][NO₃]	Mp: 12°C; Protic ionic liquid	First documented room-temperature ionic liquid [1] [2]
1934	-	Liquefied quaternary ammonium salts	Cellulose dissolution >100°C	First industrial patent applying ionic liquid principles [2]
1948/1951	Hurley & Weir	[C₂py]Br-AlCl₃ mixtures	Electroplating; Liquid at RT with eutectic at -40°C	Recognized benefits of low MP salts for electrodeposition [1] [2]
1963	John Yoke	Alkylammonium chlorocuprates	Room-temperature "oils"	Expanded range of accessible ionic liquid systems [1]
1970s	Warren Ford	Tetraalkylammonium tetraalkylborides	Low viscosity; Effects on organic reaction rates	Studied toxicity and antimicrobial activity [1]
1972	George Parshall	[Et₄N][GeCl₃] and [Et₄N][SnCl₃]	Mp: 68°C and 78°C; Platinum-catalyzed hydrogenation	Early catalytic applications in ionic liquids [1]
1975	Osteryoung Group	[C₄py]Cl-AlCl₃	Room-temperature liquid range: 60-67% AlCl₃	Electrochemistry of organometallic complexes [1] [2]
1982	Wilkes et al.	1-Alkyl-3-methylimidazolium chloroaluminates	Wide liquid composition range	Introduced imidazolium cations, now most popular for ILs [1] [2]

Methodological Advances in Early Ionic Liquid Research

Electrochemical Applications: Chloroaluminate Systems

The replication and extension of early ionic liquid research requires specific methodologies, particularly for handling moisture-sensitive systems like chloroaluminates. The experimental protocol for aluminum electrodeposition developed by Hurley and Weir exemplifies the technical requirements:

Reagents and Equipment:

• 1-Ethylpyridinium bromide ([C₂py]Br)
• Anhydrous aluminum chloride (AlCl₃)
• Inert atmosphere glove box (essential for water-sensitive systems)
• Standard electrochemical cell components

Experimental Protocol:

• Prepare 1-ethylpyridinium bromide through quaternization of pyridine with bromoethane
• Dry all reagents and equipment thoroughly to exclude moisture
• In an inert atmosphere glove box, mix [C₂py]Br and AlCl₃ in a 2:1 molar ratio
• Heat gently (if necessary) to form a homogeneous liquid phase
• Characterize the phase behavior across different composition ratios
• Employ as an electrolyte for aluminum electrodeposition at room temperature

Critical Considerations:

• The [C₂py]Br-AlCl₃ system is only liquid at room temperature at specific compositions
• Water sensitivity necessitates strict exclusion of moisture to prevent decomposition
• Phase diagram analysis reveals optimal composition windows for practical applications [1] [2]

Structural Investigations: Debating Ionic Liquid Organization

Early research on imidazolium-based ionic liquids sparked significant controversy regarding their internal structure, with competing theories requiring specialized investigative approaches:

Conflicting Structural Models:

• Hydrogen-Bonded Network: Proposed interionic interactions primarily through hydrogen bonding [1]
• Stacked Structure: Suggested cations arranged with anions positioned above and below the imidazolium ring plane [1]

Experimental Resolution Methods:

• Raman Spectroscopy: Identify specific ion interactions and coordination environments [1]
• X-ray Crystallography: Determine solid-state structures where possible
• Physical Property Measurements: Correlate structural hypotheses with observed properties

This debate was ultimately resolved by recognizing that imidazolium ring protons can act as hydrogen bond donors only with sufficiently strong hydrogen bond acceptors, while stacked structures dominate with larger anions that are poor hydrogen bond acceptors [1].

The Research Toolkit: Essential Materials for Early Ionic Liquid Research

Table 2: Key Research Reagents and Experimental Components in Early Ionic Liquid Studies

Reagent/Category	Specific Examples	Function/Role	Key Characteristics
Organic Cations	Ethylammonium, 1-Ethylpyridinium, 1-Butylpyridinium, 1-Ethyl-3-methylimidazolium	Positively-charged component	Bulky, asymmetric structure preventing efficient crystal packing
Anions	Nitrate, Halides (Cl⁻, Br⁻), Tetrahalogenoaluminates (AlCl₄⁻, Al₂Cl₇⁻)	Negatively-charged component	Ranging from simple inorganic to complex metal-containing species
Metal Salts	Aluminum chloride (AlCl₃), Copper(I) chloride (CuCl), Germanium chloride (GeCl₃)	Anion precursor, Lewis acid component	Forms complex anions; Adjusts Lewis acidity of final ionic liquid
Specialized Equipment	Inert atmosphere glove box, Sealed electrochemical cells, Moisture-free glassware	Handling and containment	Essential for moisture-sensitive compositions (e.g., chloroaluminates)
Characterization Tools	Conductivity apparatus, Melting point apparatus, Phase diagram analysis	Physical property determination	Key for establishing fundamental ionic liquid behavior

Early Ionic Liquid Research Components

The pioneering work on ionic liquids from Walden's 1914 discovery through the subsequent decades established fundamental principles that continue to guide research today. Walden's recognition of the relationship between ion size, symmetry, and melting point created the conceptual foundation for the field, while later investigators expanded the chemical diversity and practical applications of these unique materials. The experimental challenges encountered by early researchers—particularly regarding moisture sensitivity and structural characterization—established methodological approaches that would enable the rapid expansion of ionic liquid science in later years. These initial forays into molten salts at ambient temperatures demonstrated the profound implications of intentionally designing ionic systems with specific physicochemical properties, paving the way for the extensive development and application of ionic liquids as designer solvents in modern chemical research and industrial processes.

Ionic liquids (ILs), a class of materials often defined as salts with melting points below 100 °C, have evolved from academic curiosities to cornerstone solvents in green chemistry and pharmaceutical research [4] [5] [6]. Their journey began in 1914 with Paul Walden's report on ethylammonium nitrate, but significant interest emerged with the discovery of air- and water-stable imidazolium-based ILs in 1992 [4] [6]. The defining feature of ILs is their inherent tunability; their physicochemical properties can be precisely tailored by selecting different cation-anion combinations, making them "designer solvents" for specific applications [4] [7] [8]. This guide details the key properties that define ILs as solvents, providing researchers with the foundational knowledge to select and utilize them effectively, particularly in drug development.

Historical Development and Generations of Ionic Liquids

The evolution of ionic liquids is categorized into distinct generations, each expanding their capabilities and aligning with advancing sustainability goals [7] [9].

Table 1: Generations of Ionic Liquids

Generation	Key Characteristics	Typical Applications	Limitations
First Generation	Low melting point, high thermal stability; sensitive to water/air [8] [9].	Electrochemistry, electroplating [8].	Often toxic, poorly biodegradable [8] [9].
Second Generation	Air- and water-stable; tunable physical/chemical properties [7] [8] [9].	Catalysis, synthetic chemistry, electrochemical systems [7].	High toxicity, poor biodegradability [9].
Third Generation	Bio-derived ions (e.g., cholinium, amino acids); low toxicity, good biodegradability [7] [8] [9].	Biopharmaceutical applications, drug delivery, green chemistry [7] [8].	-
Fourth Generation	Focus on sustainability, biodegradability, and multifunctionality [7].	Next-generation green technologies, precision medicine [7].	-

This evolution has enabled the development of specialized subclasses like Active Pharmaceutical Ingredient Ionic Liquids (API-ILs), where the IL itself is formed from a pharmacologically active ion, and Surface-Active ILs (SAILs), which exhibit amphiphilic properties and can self-assemble [4] [9].

Diagram 1: The evolution of ionic liquids from academic discovery to advanced commercial applications shows a clear trend towards sustainability and specialized functionality.

Core Physicochemical Properties

The utility of ILs as solvents stems from a unique combination of physicochemical properties, which are modular and can be optimized for specific applications.

Tunable Property Spectrum

Table 2: Key Physicochemical Properties of Ionic Liquids

Property	Description & Impact	Influencing Factors	Typical Range/Value
Melting Point	Defines the liquidus range; crucial for application temperature [4] [10].	Ion size, symmetry, charge delocalization, intermolecular forces [10] [6].	< 100 °C (definition); many are liquid at room temperature [4] [8].
Viscosity	Affects mass transfer, reaction rates, and pumping efficiency; generally higher than molecular solvents [11] [10].	Alkyl chain length, anion type, strength of Coulombic & hydrogen-bonding interactions [11] [9].	0.3 to over 189 Pa·s [11].
Thermal Stability	Determines the upper temperature limit for applications [4] [10].	Nature of cation-anion combination [4].	Up to 672 K (~399 °C) for glycerol-derived ILs [11].
Vapor Pressure	Negligible volatility reduces solvent loss, inhalation risk, and environmental emissions [4] [12] [10].	Ionic nature and strong Coulombic forces [4].	Extremely low / non-volatile [4] [12].
Solvation Power	High capacity to dissolve diverse substances, from polar compounds to metals [4] [11].	Selection of cation and anion [4].	Tunable from highly polar to non-polar [9].
Polarity	Governs miscibility and solvation behavior; can be finely adjusted [9].	Choice of ions and alkyl substituents [9].	Broadly tunable [9].
Density	Important for product separation and flow dynamics [11].	Molecular weight and packing of ions [11].	1.03 – 1.40 g/cm³ [11].
Electrochemical Stability	Defines the voltage window for electrochemical applications [6].	Redox stability of the constituent ions [6].	Wide electrochemical window [6].

Structure-Property Relationships

The properties of an IL are dictated by the structures of its constituent ions. Cations are typically bulky and organic (e.g., imidazolium, pyridinium, ammonium, phosphonium), while anions can be inorganic or organic (e.g., chloride, [BF₄]⁻, [PF₆]⁻, [Tf₂N]⁻) [8] [10]. The large size and asymmetric nature of the ions prevent efficient crystal packing, leading to low melting points [6]. Properties can be fine-tuned by modifying ion structures; for example, increasing alkyl chain length on a cation can enhance lipophilicity and reduce viscosity to a point, but may also increase toxicity [9].

Experimental Protocols for Key Characterization

For researchers, accurately determining the properties of synthesized or commercial ILs is critical. Below are standard protocols for key measurements.

Melting Point and Thermal Stability

Objective: To determine the melting point (T_m) and thermal decomposition temperature (T_d) of an ionic liquid. Principle: Melting point is the temperature at which a solid transitions to a liquid. Thermal decomposition temperature indicates the onset of thermal degradation. Methodology:

• Sample Preparation: Ensure the IL is pure and thoroughly dried to remove water, which can significantly affect results [10].
• Melting Point (T_m): Use a melting point apparatus or Differential Scanning Calorimetry (DSC). In DSC, seal a small sample (2-5 mg) in an aluminum pan and run a heating cycle (e.g., from -50°C to 100°C at 5°C/min under N_2 flow). The onset of the endothermic peak corresponds to T_m [10].
• Thermal Decomposition (T_d): Use Thermogravimetric Analysis (TGA). Load 5-10 mg of sample into a platinum pan and heat (e.g., from 25°C to 600°C at 10°C/min under N_2). T_d is typically reported as the onset temperature of mass loss or the temperature at which a certain percentage (e.g., 5%) of mass is lost [11] [10].

Viscosity Measurement

Objective: To measure the dynamic viscosity of an ionic liquid as a function of temperature. Principle: Viscosity is the resistance of a fluid to flow. Methodology:

• Sample Preparation: Dry the IL sample completely and store under an inert atmosphere. Viscosity is highly sensitive to water content and air bubbles.
• Instrumentation: Use a rotational rheometer with a cone-and-plate or concentric cylinder geometry. A capillary viscometer can also be used for simpler measurements.
• Measurement: Equilibrate the sample at a set temperature. Apply a controlled shear rate and measure the resulting shear stress. The viscosity is calculated from this ratio. Repeat measurements across a temperature range (e.g., 20°C to 80°C) to model the temperature dependence, which often follows an Arrhenius-like behavior [11] [10].

Solubility and Solvation Power

Objective: To assess the capacity of an ionic liquid to dissolve a target compound (e.g., a poorly soluble Active Pharmaceutical Ingredient (API)). Principle: The solubility of a solute in a solvent is determined by the balance of intermolecular forces. Methodology:

• Shake-Flask Method: Place an excess amount of the solid API into a vial containing the IL.
• Equilibration: Seal the vial and agitate it in a temperature-controlled shaker bath for a sufficient time (e.g., 24-48 hours) to reach equilibrium.
• Separation: Centrifuge the mixture to separate the undissolved solid from the saturated solution.
• Quantification: Dilute an aliquot of the supernatant with a suitable solvent (e.g., methanol) and analyze the concentration of the dissolved API using a calibrated analytical technique such as High-Performance Liquid Chromatography (HPLC) or UV-Vis spectrophotometry [4] [11].

Diagram 2: A standard workflow for characterizing the key physicochemical properties of an ionic liquid, highlighting the critical steps from sample preparation to data integration.

The Scientist's Toolkit: Key Research Reagent Solutions

Selecting the right ions is the first step in designing an IL for a specific application. The table below catalogs common ions and their functional roles in research.

Table 3: Research Reagent Solutions: Common Ionic Liquid Components

Reagent (Ion)	Type	Key Function & Properties
1-Butyl-3-methylimidazolium ([C₄C₁im]⁺)	Cation	A versatile, widely studied cation; contributes to low melting points and good chemical stability [4] [10].
Cholinium	Cation	A bio-derived, low-toxicity cation from the third generation of ILs; essential for biocompatible applications [8] [9].
Bis(trifluoromethylsulfonyl)imide ([Tf₂N]⁻)	Anion	Hydrophobic anion that imparts high thermal and electrochemical stability, and low viscosity [13] [6].
Hexafluorophosphate ([PF₆]⁻)	Anion	Imparts hydrophobicity and is commonly used in extractions and electrochemical applications [4] [10].
Tetrafluoroborate ([BF₄]⁻)	Anion	Offers moderate hydrophilicity and is used in synthesis and catalysis [4] [10].
Docusate	Anion	A pharmaceutically accepted anion used to form API-ILs, enhancing drug solubility and absorption [9].
Amino Acid-based Anions	Anion	Bio-derived, chiral anions used to create low-toxicity, biodegradable ILs (Bio-ILs) [9].
Glycidyl Ethers / Epichlorohydrin	Precursor	Renewable platform molecules for synthesizing tailored, bio-based IL families [11].

Ionic liquids represent a paradigm shift in solvent technology, moving from static, single-purpose solvents to dynamic, tunable media defined by their customizable physicochemical properties. Their journey from simple chloroaluminates to sophisticated, bio-inspired fourth-generation ILs underscores their growing alignment with the principles of green and sustainable chemistry. For researchers in drug development and beyond, mastering the relationship between an IL's ionic structure and its emergent properties—such as negligible volatility, thermal stability, and unparalleled solvation power—is the key to unlocking new possibilities in synthesis, analysis, and formulation. As this field progresses, the continued development of biodegradable, non-toxic, and highly functional ILs promises to further solidify their role as indispensable solvents for 21st-century scientific innovation.

Ionic liquids (ILs), organic salts with melting points below 100 °C, have undergone a remarkable evolution since their discovery. Their development is characterized by a distinct generational shift, moving from highly specialized, air-sensitive systems to modern, biocompatible materials designed for integration with biological systems. This journey reflects a broader paradigm in materials science, where the emphasis has moved from fundamental property exploration to targeted functional design, particularly for biomedical and pharmaceutical applications. The history of ILs began over a century ago with the synthesis of ethylammonium nitrate by Walden in 1914, but it was not until the 1980s and 1990s that significant interest grew, leading to the classification of ILs into four key generations [1] [7] [14].

The classification of ILs into generations provides a powerful framework for understanding their historical development and future trajectory. First-generation ILs, primarily explored as green solvents, were dominated by chloroaluminate systems and were often sensitive to air and water [1] [14]. Second-generation ILs introduced enhanced stability and tunable physicochemical properties, expanding their use into catalysis and electrochemistry [7] [14]. The pivotal turn toward biological applications came with the third-generation, which incorporated bio-derived ions to create biodegradable and biocompatible ILs [15] [7]. Today, fourth-generation ILs combine these attributes, focusing on multifunctionality, sustainability, and intelligent design for applications in precision medicine and green technology [7]. This review will trace this generational shift, highlighting the key properties, applications, and experimental methodologies that define each stage, with a particular focus on the breakthrough from air-sensitive salts to biocompatible pharmaceutical tools.

The Historical Pathway: Four Generations of Ionic Liquids

Table 1: Key Characteristics of the Four Generations of Ionic Liquids

Generation	Time Period	Primary Focus	Example Cations	Example Anions	Key Applications
First	1914 - ~1990	Air/Moisture-sensitive solvents	Ethylammonium, Alkylpyridinium	[NO₃]⁻, Chloroaluminates	Electrochemistry, Green Solvents
Second	~1992 - 2000s	Air/Water-stable & tunable properties	1-Ethyl-3-methylimidazolium [EMIM]⁺	[BF₄]⁻, [PF₆]⁻	Catalysis, Lubricants, Electrochemical Systems
Third	~2000 - Present	Biocompatibility & Biodegradability	Choline, Amino Acids	Amino Acids, Fatty Acids, Carboxylates	Drug Formulation, Biomedicine, Pharmaceuticals
Fourth	Emerging	Smart & Multifunctional Materials	Functionalized Bio-Ions	Functionalized Bio-Ions	Precision Medicine, Targeted Drug Delivery, Sustainable Tech

The evolution of ionic liquids is a story of continuous refinement and purposeful design. The first-generation began with Paul Walden's 1914 report on ethylammonium nitrate, but these early melts were largely ignored for decades [1]. A significant rediscovery occurred in the 1950s with Hurley and Weir's work on room-temperature chloroaluminate melts for electroplating [1]. These systems, however, were notoriously difficult to handle, requiring inert-atmosphere glove boxes due to their extreme sensitivity to moisture [1]. This high barrier to entry limited their widespread adoption.

The development of the second-generation was catalyzed by the synthesis of air- and water-stable ILs based on the 1-ethyl-3-methylimidazolium cation with anions like tetrafluoroborate ([BF₄]⁻) and hexafluorophosphate ([PF₆]⁻) in 1992 [14]. This breakthrough unlocked the potential of ILs as truly tunable "designer solvents" [16]. Their remarkable stability and customizable properties (e.g., polarity, hydrophobicity, viscosity) spurred research across diverse fields, including organic synthesis, catalysis, and lubricants [7] [14]. Despite their versatility, concerns regarding toxicity and poor biodegradability persisted, limiting their use in biomedical fields [15].

The need for safer materials led to the third-generation of ILs. This generation prioritized biocompatibility and sustainability by utilizing ions derived from natural, renewable sources [15] [7]. Choline, an essential B-group vitamin, and amino acids became the cornerstone cations and anions for these bio-ILs (Bio-ILs) [15]. These components are generally recognized as safe (GRAS) by regulatory authorities like the FDA, making them ideal candidates for pharmaceutical applications [15]. The third-generation represents the critical shift from simply exploiting the physical properties of ILs to engineering their chemical structures for specific biological interactions and low environmental impact.

Currently, the emerging fourth-generation of ILs focuses on multifunctionality and smart materials [7]. These ILs are designed to be biodegradable, recyclable, and capable of performing multiple tasks, such as simultaneous drug delivery and biological sensing [7]. They are engineered with tailored functionalities for next-generation applications in precision medicine, advanced energy storage, and sustainable industrial processes, marking the frontier of IL research and development [7].

The Rise of Biocompatible ILs in Biomedicine and Drug Delivery

The advent of third-generation ILs has opened up transformative applications in biomedicine and pharmaceuticals. Their tunable nature allows them to address some of the most persistent challenges in drug development, particularly the poor solubility, low bioavailability, and instability of many therapeutic compounds [17]. By acting as solvents, stabilizers, and permeation enhancers, biocompatible ILs have revolutionized drug delivery strategies.

A primary application is in overcoming solubility barriers. A significant proportion of new drug candidates exhibit poor aqueous solubility, which limits their absorption and efficacy [17]. ILs can dramatically enhance the solubility of these hydrophobic drugs. Furthermore, a powerful strategy is the creation of Active Pharmaceutical Ingredient-Ionic Liquids (API-ILs), where the drug molecule itself is incorporated as either the cation or anion of the IL [17]. This approach can convert a crystalline solid drug into a liquid salt, improving its bioavailability and enabling new delivery routes [17].

Another major application is in the stabilization of biopharmaceuticals. Therapeutic proteins, peptides, and nucleic acids are often fragile and prone to denaturation or aggregation. Choline-based ILs, such as choline dihydrogen phosphate ([Chol][DHP]), have demonstrated a remarkable ability to stabilize proteins, preventing unfolding and preserving biological activity [16]. For instance, studies have shown that certain choline ILs can increase the melting point of insulin and monoclonal antibodies, significantly delaying their aggregation [18]. This stabilizing effect is crucial for the long-term storage and transport of biologic drugs.

Transdermal drug delivery has particularly benefited from IL technology. Biocompatible ILs like choline and geranic acid (CAGE) have been developed as effective permeation enhancers [19] [18]. CAGE can fluidize the lipids in the skin's stratum corneum, the main barrier to drug absorption, facilitating the delivery of not only small molecules but also large biopharmaceuticals like insulin and nucleic acids (siRNA, mRNA) [18]. This enables non-invasive, needle-free administration of drugs that would otherwise require injection. The following workflow generalizes the process of developing and evaluating a biocompatible IL-based transdermal system, as exemplified by CAGE.

Finally, ILs have shown intrinsic biological activity. By carefully selecting cation-anion pairs, ILs can be designed to possess inherent antimicrobial or anticancer properties [14]. Their mechanism of action often involves disrupting pathogen cell membranes or interacting with intracellular organelles and biomolecules [14]. This dual functionality—serving as both a drug delivery vehicle and an active therapeutic agent—exemplifies the multifunctional potential of fourth-generation ILs.

Experimental Protocols: Working with Biocompatible ILs

Synthesis of a Choline-Based Bio-IL (e.g., Choline Geranate [CAGE])

The synthesis of biocompatible ILs is typically straightforward. The following protocol for preparing Choline Geranate (CAGE), a well-studied IL for transdermal delivery, is representative [19].

• Objective: To synthesize a biocompatible ionic liquid from choline and geranic acid.
• Principle: A neutralization reaction between a chonium base (choline hydroxide) and a stoichiometric amount of geranic acid.
• Materials:
  • Choline hydroxide aqueous solution (e.g., 45% w/w in water)
  • Geranic acid (>95% purity)
  • Deionized water
  • Magnetic stirrer and stir bar
  • Round-bottom flask
  • Rotary evaporator or vacuum oven
  • Analytical balance
  • NMR spectrometer for characterization
• Procedure:
  • Stoichiometric Calculation: Calculate the required masses of choline hydroxide and geranic acid to achieve the desired molar ratio (a common effective ratio for CAGE is 1:2, choline to geranic acid).
  • Mixing: In a round-bottom flask, add the calculated amount of choline hydroxide solution. Place the flask on a magnetic stirrer and begin stirring.
  • Acid Addition: Slowly add the calculated mass of geranic acid to the stirring choline hydroxide solution.
  • Reaction: Continue stirring the mixture for 12-24 hours at room temperature. The reaction is a simple acid-base neutralization and will proceed to completion.
  • Water Removal: Remove the water from the resulting ionic liquid mixture using a rotary evaporator under reduced pressure. Alternatively, dry the sample in a vacuum oven at elevated temperature (e.g., 40-50°C) for several hours.
  • Characterization: Confirm the structure and purity of the resulting IL using (^1)H NMR spectroscopy. Determine the water content via Karl-Fischer titration and analyze thermal properties using Differential Scanning Calorimetry (DSC) [19].

Protocol for Evaluating Protein Stability in Aqueous IL Solutions

Assessing the stabilizing effect of ILs on proteins is crucial for their application in biopharmaceuticals [16].

• Objective: To determine the stabilizing effect of a choline-based IL on a model protein (e.g., lysozyme) against thermal aggregation.
• Principle: The stability of a protein in solution with and without IL additives is monitored under thermal stress by measuring turbidity (light scattering) as an indicator of aggregation.
• Materials:
  • Model protein (e.g., Lysozyme)
  • Biocompatible IL (e.g., Choline Dihydrogen Phosphate, [Ch][DHP])
  • Buffer solution (e.g., Phosphate Buffered Saline, PBS, pH 7.4)
  • UV-Vis Spectrophotometer with temperature-controlled cuvette holder
  • Centrifuge and microcentrifuge tubes
• Procedure:
  • Sample Preparation: Prepare a series of protein solutions (e.g., 1 mg/mL lysozyme in PBS) containing varying concentrations of the IL (e.g., 0.1 M, 0.5 M, 1.0 M). Prepare a control sample with no IL.
  • Thermal Stress: Incubate all samples at a denaturing temperature (e.g., 65°C) in a heating block.
  • Turbidity Measurement: At regular time intervals (e.g., 0, 10, 20, 30, 60 minutes), remove aliquots from the heated samples. Centrifuge briefly to pellet any large aggregates.
  • Absorbance Reading: Transfer the supernatant to a cuvette and measure the absorbance at 360 nm (or 600 nm) using a UV-Vis spectrophotometer. A increase in absorbance at this wavelength is indicative of light scattering from protein aggregates.
  • Data Analysis: Plot absorbance versus time for each IL concentration. A lower rate of increase in turbidity in the IL-containing samples compared to the control indicates a stabilizing effect against thermal aggregation.

Table 2: The Scientist's Toolkit: Key Reagents for Biocompatible IL Research

Reagent / Material	Function / Role	Example in Use
Choline Hydroxide / Chloride	Cation precursor for synthesis of biocompatible ILs. Considered non-toxic and "generally regarded as safe" (GRAS).	Base cation in choline-geranate (CAGE) for transdermal delivery [15] [19].
Amino Acids (e.g., Glycine, Alanine)	Serve as either anions or cations for bio-ILs. Provide chiral centers, biodegradability, and low toxicity.	Choline-glycine IL for drug solubilization and antimicrobial activity [15].
Fatty Acids / Carboxylic Acids (e.g., Geranic Acid)	Anion precursors that impart hydrophobicity and specific biological interactions (e.g., permeation enhancement).	Geranic acid in CAGE enhances skin penetration of biologics [19] [18].
Deep Eutectic Solvents (DES)	Eutectic mixtures of Lewis/Brønsted acids and bases, often with lower cost and simpler preparation than traditional ILs.	Choline Chloride-Urea DES for solvating poorly soluble drugs or as a reaction medium [15].
Differential Scanning Calorimeter (DSC)	Instrument for characterizing thermal properties of ILs (melting point, glass transition) and protein stability (melting temperature).	Determining the melting point of a synthesized IL or the increased thermal stability of a protein in [Ch][DHP] [19] [16].
Karl-Fischer Titrator	Essential instrument for accurately determining the water content in hygroscopic ILs, a critical quality attribute.	Measuring and controlling residual water in CAGE after synthesis [19].

The journey of ionic liquids from air-sensitive curiosities to sophisticated biocompatible materials underscores a profound shift in materials science. The generational classification provides a clear narrative of this evolution: from first-generation ILs focused on fundamental properties as solvents, to the stable and tunable second-generation, and finally to the transformative third- and fourth-generations designed for integration with biological systems. This progression has been driven by an increasing emphasis on sustainability, functionality, and human health.

The impact of biocompatible ILs in drug delivery and biomedicine is already significant, offering innovative solutions to longstanding challenges such as poor drug solubility, low bioavailability, and the instability of biopharmaceuticals [17]. However, the journey is not complete. Future research will focus on the clinical translation of IL-based formulations, with several choline-derived ILs already advancing into clinical trials [17]. Key challenges that remain include comprehensive long-term biosafety studies, scalable and cost-effective manufacturing processes, and regulatory harmonization for these novel chemical entities [17].

The convergence of IL technology with artificial intelligence, nanomedicine, and additive manufacturing (e.g., 3D-printed drug formulations) presents unprecedented opportunities [17] [7]. AI can accelerate the design of optimal cation-anion pairs for specific therapeutic tasks, while advanced fabrication techniques can enable the creation of personalized IL-based drug delivery systems. As these trends continue, ionic liquids are poised to move from being merely "green solvents" to becoming indispensable components of next-generation precision medicine, fully realizing the potential of the generational shift toward biocompatibility and intelligent design.

The field of ionic liquids (ILs) has undergone a remarkable evolution, transitioning from simple, curiosity-driven molten salts to sophisticated, task-specific materials engineered for advanced applications. This journey is categorized into four distinct generations that reflect the growing understanding of cation-anion diversity beyond traditional imidazolium-based structures [7]. First-generation ILs, primarily studied as novel green solvents, gave way to second-generation ILs designed with specific physicochemical properties for applications in catalysis and electrochemistry. The paradigm truly shifted with third-generation ILs, which incorporated bio-derived and task-specific functionalities for biomedical and environmental applications, culminating in the current fourth-generation ILs that emphasize sustainability, biodegradability, and multifunctionality [7]. This historical progression underscores a fundamental principle: the limitless tunability of IL properties through strategic selection and functionalization of cationic and anionic components. As research expands, moving beyond conventional imidazolium systems has unlocked unprecedented opportunities for designing ILs with precision-engineered functions for applications ranging from pharmaceutical sciences to energy storage and environmental remediation.

Fundamental Chemistry: Cation and Anion Structural Diversity

The physicochemical properties and application potential of ionic liquids are fundamentally governed by the structural diversity of their cationic and anionic components. Understanding this diversity is essential for the rational design of task-specific ILs.

Cationic Architectures Beyond Imidazolium

While imidazolium remains a prevalent cation core, numerous alternative structures offer distinct advantages for specific applications.

• Pyrrolidinium and Piperidinium: These saturated, cyclic ammonium cations exhibit enhanced electrochemical stability and wider electrochemical windows, making them particularly valuable for energy storage applications such as lithium-ion and lithium-metal batteries [20]. Their structural rigidity contributes to higher thermal stability compared to their aromatic counterparts.

• Phosphonium: Quaternary phosphonium cations demonstrate superior thermal stability (often exceeding 300°C) and chemical inertness toward reactive metals [20]. These properties have enabled their use in demanding applications including aerospace lubricants, hypergolic fuels, and high-temperature industrial processes. Phosphonium-based ILs have shown 30% longer maintenance intervals in industrial bearings within steel mills compared to conventional fluids [20].

• Cholinium (Vitamin B4 Derivative): As a bio-derived cation, cholinium offers significantly reduced toxicity and enhanced biodegradability compared to conventional ILs [20]. This biosourced platform aligns with green chemistry principles and has found applications in pharmaceutical formulations and biomass processing.

• Pyrrolinium: Recent research has demonstrated the utility of task-specific pyrrolinium cations in analytical chemistry. For instance, 1-(2-hydroxy-3-(isopropylamino)propyl)methylpyrrolinium chloride has been successfully employed for the selective microextraction of pharmaceutical compounds like sertraline from complex matrices including water and urine samples [21].

• Ammonium: Including simple structures like ethylammonium nitrate (one of the earliest known ILs with a melting point of 12°C) [22], these cations continue to offer valuable platforms for fundamental studies and applications requiring minimal molecular complexity.

Anionic Diversity and Its Functional Implications

The selection of anions equally dictates IL properties, enabling fine-tuning for specialized applications.

• Fluorinated Anions (BF₄⁻, PF₆⁻): These anions provide high conductivity and electrochemical stability, enabling operation at higher voltages (3-5V) in electrochemical devices [20]. However, concerns regarding HF release upon hydrolysis and potential toxicity have prompted research into fluorine-free alternatives.

• Bulky, Charge-Diffuse Anions (TFSI, NTf₂⁻): Bis(trifluoromethylsulfonyl)imide and similar anions create ILs with low lattice energies, reduced melting points, and enhanced hydrophobicity. Their charge delocalization contributes to wider electrochemical windows and improved stability in demanding electrochemical environments [22].

• Amino Acid-Based Anions: These bio-derived anions enhance biocompatibility and sustainability. Alanine-based anions, for example, have been investigated in modular IL libraries for biomedical applications, showing reduced cytotoxicity compared to traditional anions [23].

• Solvate Ionic Liquids (SILs): An emerging subclass where cations are chelated by neutral ligands (typically oligoethers like triglyme or tetraglyme) paired with charge-diffuse anions [13]. SILs maintain characteristic IL properties while offering simplified synthesis and cost effectiveness, positioning them as promising candidates for next-generation energy storage applications.

Table 1: Cation and Anion Diversity in Ionic Liquids

Component	Type/Example	Key Properties	Representative Applications
Cations	Imidazolium (e.g., [CₙMIM]⁺)	Moderate stability, tunable polarity	General solvents, catalysis
	Pyrrolidinium/Piperidinium	High electrochemical stability	Battery electrolytes, supercapacitors
	Phosphonium	Exceptional thermal stability (>300°C)	High-temperature lubricants, aerospace
	Cholinium	Low toxicity, biodegradable	Pharmaceutical formulations, green chemistry
	Pyrrolinium	Task-specific functionality	Analytical microextraction
Anions	Fluorinated (BF₄⁻, PF₆⁻)	High conductivity, wide EWindow	Electrolytes for energy storage
	TFSI/NTf₂⁻	Low lattice energy, hydrophobic	Advanced electrochemical devices
	Amino Acid-based (e.g., Ala⁻)	Biocompatible, sustainable	Biomedical applications
	Chloride (Cl⁻)	Hydrogen bond accepting	Fundamental studies, synthesis

Structural-Property Relationships and Toxicity Considerations

The relationship between IL structure and its biological effects represents a critical consideration for pharmaceutical and biomedical applications. Systematic studies using modular IL libraries have revealed that cytotoxicity is predominantly influenced by the cationic alkyl chain length rather than the specific cation head group or anion identity [23]. Research across multiple cell lines (bEnd.3, 4T1, HepG2), 3D cell spheroids, and patient-derived organoids consistently demonstrates that ILs with short cationic alkyl chains (scILs, C1-C4) exhibit minimal cytotoxicity, while those with long chains (lcILs, ≥C8) show dramatically increased toxicity [23].

The mechanism underlying this structure-toxicity relationship involves the formation of IL nanoaggregates in aqueous environments. Cryo-TEM and molecular dynamics simulations reveal that both scILs and lcILs form nanoaggregates (~5 nm for scILs vs. ~12.5 nm for lcILs), but their intracellular trafficking and biological fates differ significantly [23]. scILs are typically restricted to intracellular vesicles, whereas lcILs accumulate in mitochondria, inducing mitophagy and apoptosis. This understanding enables rational design of safer ILs for biomedical applications, with scILs demonstrating 30-80 times greater tolerance than lcILs in murine and canine models across various administration routes (oral, intramuscular, intravenous) [23].

Machine learning approaches are advancing toxicity prediction for ILs. Models using random forest (RF), multi-layer perceptron (MLP), and convolutional neural network (CNN) algorithms have been developed to predict toxicity toward biological systems including Vibrio fischeri, acetylcholinesterase (AChE), and leukemia rat cell lines (ICP-81) [24]. Interpretation tools like SHAP (Shapley Additive exPlanations) analysis quantify the contribution of molecular features to toxicity predictions, facilitating the design of greener ILs with reduced ecological impact.

Task-Specific Applications and Experimental Protocols

Pharmaceutical Analysis and Drug Delivery

Application: Determination of Sertraline in Real Water and Urine Samples

Task-specific pyrrolinium-based ILs enable highly efficient and environmentally friendly microextraction of pharmaceutical compounds [21]. The homogeneous in situ solvent formation microextraction (ISFME) protocol achieves exceptional sensitivity and selectivity through the designed interactions between the IL and target analyte.

Table 2: Research Reagent Toolkit for Pharmaceutical Microextraction

Reagent/Material	Specification/Function
Task-Specific IL	1-(2-hydroxy-3-(isopropylamino)propyl)methylpyrrolinium chloride; selective complexation with sertraline
Hydrophobizing Agent	Sodium hexafluorophosphate (NaPF₆); induces phase separation by increasing IL-analyte complex hydrophobicity
Sertraline Standard	Analytical standard for calibration and quantification
Real Water Samples	Environmental water matrices for method validation
Urine Samples	Biological matrices for clinical application
Centrifuge	Phase separation post-hydrophobization (5000 rpm, 5 min)
FTIR, NMR, MS	Characterization of synthesized IL and complex

Experimental Protocol: TSIL-ISFME for Sertraline Determination

• IL Synthesis and Characterization: Synthesize 1-(2-hydroxy-3-(isopropylamino)propyl)methylpyrrolinium chloride through nucleophilic substitution reaction. Characterize the structure using FTIR, NMR, and mass spectroscopy techniques to confirm successful synthesis [21].

• Sample Preparation: Acidify water or urine samples to pH 3.0 using hydrochloric acid to ensure sertraline exists predominantly in its ionic form, enhancing interaction with the TSIL.

• Homogeneous Extraction: Add the hydrophilic TSIL (500 μL) to the aqueous sample (10 mL) containing sertraline. The system remains homogeneous, eliminating phase boundaries and maximizing extraction efficiency. Stir vigorously for 3 minutes to facilitate complex formation between the IL and sertraline.

• Phase Separation Induction: Introduce NaPF₆ (100 μL, 10% w/v) as a hydrophobizing agent. The PF₆⁻ counterion exchanges with Cl⁻, increasing the hydrophobicity of the IL-sertraline complex and inducing phase separation.

• Collection and Analysis: Centrifuge at 5000 rpm for 5 minutes to complete phase separation. Collect the sedimented IL phase and analyze using HPLC-UV with external standard calibration at 275 nm.

• Method Validation: Under optimal conditions, this method achieves a limit of detection (LOD) of 2.4 μg L⁻¹, limit of quantification (LOQ) of 8.0 μg L⁻¹, linear dynamic range of 5.0-200 μg L⁻¹, relative standard deviation of 2.6%, preconcentration factor of 192, and recovery rates of 99.0-103.4% in real samples [21].

Biomedical Applications and Drug Delivery Systems

Application: scIL Nanoaggregates as Insoluble Drug Carriers

The systematic understanding of IL nanoaggregate behavior has enabled their application as carriers for poorly soluble drugs. Short-chain ILs (scILs) with C1-C4 alkyl chains demonstrate excellent biocompatibility and ability to enhance drug bioavailability [23].

Experimental Protocol: Formulation and Evaluation of scIL-Drug Systems

• IL Selection and Characterization: Select scILs based on cationic alkyl chain length (C3-C4 recommended for optimal balance of solubilization and biocompatibility). Characterize nanoaggregate formation using cryogenic transmission electron microscopy (Cryo-TEM) and dynamic light scattering (DLS) to confirm size distribution (~5 nm for C3MIMCl) [23].

• Drug Loading: Incorporate insoluble drugs (e.g., megestrol acetate, a semi-synthetic progestin) into scIL nanoaggregates using solvent evaporation or direct dispersion methods. Maintain drug:IL ratio of 1:10 (w/w) for optimal loading and stability.

• In Vitro Biocompatibility Assessment:

  • Evaluate cytotoxicity across multiple cell lines (bEnd.3, 4T1, HepG2) using CCK-8 assay after 24h exposure.
  • Validate in 3D models: Treat HepG2 cell spheroids and patient-derived liver cancer organoids with scIL formulations (400 μM) and assess viability using live/dead staining.
  • Confirm minimal cytotoxicity for scILs compared to lcILs (C12MIMCl reduces viability to <5%) [23].

• In Vivo Tolerance and Bioavailability:

  • Administer scIL-drug formulations via oral, intramuscular, and intravenous routes in murine and canine models.
  • Monitor tissue distribution and mitophagy/apoptotic markers to confirm superior safety profile of scILs (30-80 times greater tolerance than lcILs).
  • Compare bioavailability against commercial formulations; scIL-megestrol acetate demonstrates enhanced bioavailability over commercial tablets in canine models [23].

Energy Storage and Electronics

Application: ILs as High-Voltage Electrolytes

The surging demand for advanced battery technologies, particularly from the electric vehicle sector, has driven the adoption of IL-based electrolytes. ILs offer wider electrochemical windows (3-5V), better thermal stability, and reduced flammability compared to traditional organic electrolytes [20].

Experimental Protocol: Formulating IL Electrolytes for Lithium-Ion Batteries

• Component Selection: Prioritize pyrrolidinium cations (e.g., N-methyl-N-propylpyrrolidinium) paired with fluorinated anions (PF₆⁻) or TFSI for their electrochemical stability and conductivity. For solvate ionic liquids (SILs), combine glymes (G3/G4) with lithium salts (LiTFSI) [13].

• Electrolyte Formulation: Prepare 1.0 M solutions of lithium salt in the selected IL. Dry under vacuum at 80°C for 24 hours to achieve water content <10 ppm, critical for battery performance and longevity.

• Electrochemical Window Determination:

  • Use linear sweep voltammetry (LSV) with a three-electrode cell (working: glassy carbon; reference: Ag/Ag⁺; counter: platinum).
  • Scan from open circuit potential to ±5V at 1 mV/s.
  • Confirm electrochemical window exceeds 4.5V for high-voltage cathode compatibility [20].

• Cell Assembly and Testing:

  • Fabricate coin cells (CR2032) with high-voltage cathodes (e.g., nickel-rich layered oxides LiNi₀.₈Mn₀.₁Co₀.₁O₂) and lithium metal anodes.
  • Include polypropylene separators soaked with IL electrolyte.
  • Perform galvanostatic cycling at C/10 rate between 2.5-4.5V for 100 cycles.
  • Evaluate capacity retention (>80% after 100 cycles) and Coulombic efficiency (>99.5%) compared to conventional electrolytes.

Future Perspectives and Computational Design

The future of ionic liquid development lies in leveraging advanced computational methods to navigate their vast chemical space efficiently. With theoretical combinations numbering in the billions, only a minute fraction of potential IL structures have been synthesized and characterized [25]. Machine learning approaches are now enabling predictive design of novel ILs with tailored properties.

Conditional variational autoencoders (CVAEs) represent a promising generative approach for expanding IL chemical space. These models can propose novel cation and anion structures with a high likelihood of forming low-melting-point ILs (<373 K) [25]. When coupled with molecular dynamics simulations, this approach has validated that 13 out of 15 generated ILs possess desirable melting characteristics, demonstrating the power of computational prediction in accelerating IL discovery.

The integration of interpretable machine learning models with quantum chemical calculations further enhances our understanding of structure-property relationships. SHAP analysis quantifies the contribution of molecular features to toxicity predictions, while electrostatic potential calculations reveal the structure-activity relationships between IL components and biological effects like acetylcholinesterase inhibition [24]. This multidimensional computational approach provides a robust foundation for the rational design of next-generation ILs with optimized performance and minimal environmental impact.

As research progresses, the expansion beyond traditional imidazolium systems will continue to yield ILs with enhanced functionality, sustainability, and application specificity. The convergence of synthetic chemistry, computational design, and multidisciplinary application knowledge positions ionic liquids as key enablers of technological advancement across pharmaceuticals, energy storage, and environmental technologies.

Harnessing Tunable Properties: Cutting-Edge Applications in Science and Industry

The global shift toward a sustainable, bio-based economy necessitates the development of efficient processes for converting lignocellulosic biomass into valuable products. Among the most promising technological advances is the IonoSolv process, which utilizes protic ionic liquids (PILs) for selective biomass fractionation. The history of ionic liquids (ILs) dates back to 1914 when Paul Walden first reported the synthesis of ethylammonium nitrate [1], a low-melting-point salt that would later be recognized as the first protic ionic liquid. However, ILs remained a scientific curiosity for decades until their rediscovery in the late 20th century, when researchers began exploring their unique properties as green solvents for various applications [1] [26]. The term "ionic liquid" now encompasses a diverse class of salts with melting points below 100°C, characterized by negligible vapor pressure, high thermal stability, and tunable physicochemical properties based on cation-anion combinations [27] [26]. The evolution of IL applications has progressed from early electrochemical studies to their current role in biorefining, positioning the IonoSolv process as a transformative approach for lignocellulose deconstruction within the broader context of sustainable solvent development.

The IonoSolv Process: Fundamental Principles and Mechanisms

The IonoSolv process represents a significant advancement in ionic liquid-based biomass processing, specifically utilizing low-cost protic ionic liquids (PILs) derived from amines and inorganic acids [28] [27]. Unlike conventional pretreatment methods that primarily target cellulose digestibility, IonoSolv selectively dissolves lignin and hemicellulose fractions while leaving cellulose as an intact solid pulp [28]. This selective fractionation enables separate valorization pathways for each biomass component, moving beyond traditional biorefinery models that often treat lignin as a waste product.

The development of IonoSolv emerged from earlier discoveries with aprotic ionic liquids (AILs). In 2002, Swatloski et al. first demonstrated the remarkable ability of ILs to dissolve cellulose [29], opening new pathways for biomass processing. Subsequent research by Pu et al. revealed that certain ILs could also effectively dissolve lignin [29]. These foundational discoveries paved the way for the IonoSolv process, which leverages the unique properties of PILs—simple synthesis via acid-base neutralization, lower cost, and tolerance to moisture—making them particularly suitable for industrial-scale biomass processing [27].

Molecular Mechanisms of Biomass Fractionation

The effectiveness of IonoSolv pretreatment stems from the coordinated action of IL cations and anions in disrupting the complex lignocellulosic matrix:

• Anion Functionality: Hydrogen sulfate ([HSO₄]⁻) anions in commonly used PILs like triethylammonium hydrogen sulfate ([TEA][HSO₄]) act as Brønsted acids, catalyzing the cleavage of ether bonds (particularly β-O-4 linkages) in lignin and hydrolyzing hemicellulose [28] [27]. This selective bond cleavage enables the dissolution of lignin and hemicellulose while preserving the cellulose structure.

• Cation Interactions: The ammonium-based cations (e.g., [TEA]⁺ or [DMBA]⁺) facilitate penetration into the biomass structure through hydrophobic interactions with lignin aromatics, disrupting π-π stacking and lignin-carbohydrate complexes [29].

• Synergistic Effects: The combination of cation and anion actions results in effective delignification and hemicellulose removal, significantly increasing cellulose accessibility for subsequent enzymatic hydrolysis [28] [27].

The following diagram illustrates the mechanism of IonoSolv fractionation and the resulting biomass components:

Diagram 1: IonoSolv Biomass Fractionation Mechanism

Experimental Implementation and Process Optimization

Standard IonoSolv Pretreatment Protocol

Based on established methodologies for processing grassy biomass like Miscanthus × giganteus [28] [30], the following protocol details a representative IonoSolv pretreatment:

• Ionic Liquid Synthesis: N,N-dimethyl-N-butylammonium hydrogen sulfate ([DMBA][HSO₄]) is synthesized by dropwise addition of 5M H₂SO₄ (1 mol, 200 mL) to N,N-dimethyl-N-butylamine (1 mol, 101.19 g) in an ice bath with continuous stirring [30]. The reaction proceeds for 5 hours, after which excess water is removed by heating at 40°C under reduced pressure. The water content is adjusted to 20 wt% as measured by Karl Fisher titration [30].

• Biomass Preparation: Feedstock is size-reduced using a hammer mill and sieved to a particle size of 1-3 mm to balance pretreatment effectiveness and grinding energy requirements [28].

• Pretreatment Reaction: Biomass is mixed with IL at 10-50 wt% loading in a stirred reactor (scale: 10 mL to 1 L) and heated to 120-150°C for 45-90 minutes [28]. Efficient slurry mixing is critical for heat and mass transfer, especially at higher solid loadings.

• Fraction Recovery: After pretreatment, the cellulose-rich pulp is separated by filtration and washed with water-acetone mixtures (1:1 v/v) to prevent lignin redeposition [28] [29]. Lignin is recovered from the filtrate by adding water, adjusting pH to 2-3 to protonate phenolic hydroxyl groups, and filtering the precipitate [29]. IL is recycled from the aqueous phase by evaporation or membrane processes [26].

Process Optimization Strategies

Recent research has identified key parameters for optimizing IonoSolv pretreatment:

Table 1: Key Optimization Parameters for IonoSolv Pretreatment

Parameter	Optimal Range	Impact on Process Efficiency	Scale-up Considerations
Biomass Loading	20-50 wt%	Higher loading (20 wt%) reduces lignin reprecipitation and improves glucose yields due to better heat/mass transfer with efficient mixing [28]	Loadings >20 wt% require powerful stirring systems; impacts reactor CAPEX [28]
Particle Size	1-3 mm	Larger particles (1-3 mm) provide higher glucose yields than fine powders due to reduced surface area for lignin re-precipitation [28]	Reduces energy consumption for biomass grinding by up to 60% [28]
Temperature	120-150°C	Higher temperatures improve delignification but may lead to cellulose degradation and IL decomposition [28] [27]	Requires balance between efficiency and solvent stability; affects energy input
IL Water Content	15-20 wt%	Maintains pretreatment efficiency while reducing IL viscosity and corrosion potential [28] [30]	Enhances process safety and reduces equipment requirements

Analytical Methods for Characterizing Output Streams

Comprehensive analysis of IonoSolv fractions ensures optimal process control and valorization potential:

• Pulp Composition: Gravimetric determination of pulp yield followed by compositional analysis using NREL standard methods to quantify glucan, xylan, and lignin content [28].

• Enzymatic Saccharification: Assessment of cellulose digestibility by incubating pulp with commercial cellulase cocktails (e.g., CTec2) in buffer (pH 4.8-5.0) at 50°C for 72 hours, followed by glucose quantification via HPLC [28].

• Lignin Characterization: Structural analysis using HSQC NMR to identify interunit linkages (β-O-4, β-β, β-5) and gel permeation chromatography (GPC) for molecular weight distribution [28].

• IL Purity and Recovery: Karl Fisher titration for water content, HPLC to monitor sugar and degradation product accumulation, and NMR to assess structural integrity after recycling [26].

Research Reagent Solutions for IonoSolv Processing

Table 2: Essential Research Reagents for IonoSolv Experiments

Reagent/Chemical	Function in Process	Technical Specifications	Application Notes
Triethylammonium hydrogen sulfate ([TEA][HSO₄])	Primary pretreatment solvent	Protic IL synthesized from triethylamine + H₂SO₄; cost ~$0.78/kg [28] [27]	Tolerant to biomass moisture; 99% recovery demonstrated [28] [27]
N,N-Dimethyl-N-butylammonium hydrogen sulfate ([DMBA][HSO₄])	Alternative PIL for pretreatment	Protic IL with hydrogen sulfate anion; water content typically adjusted to 20 wt% [30]	Effective for nanocellulose production from pulps [30]
Acetone-Water Mixture (1:1 v/v)	Anti-solvent for cellulose precipitation and washing	HPLC grade solvents; mixture optimal for lignin solubility control [29]	Prevents lignin redeposition on pulp during washing; reduces saccharification inhibition [28]
Commercial Cellulase Cocktails (CTec2, etc.)	Enzymatic saccharification of cellulose-rich pulp	Standardized enzyme activity; dosage typically 15-20 mg protein/g glucan [28]	Susceptible to IL inhibition; thorough washing of pulp essential [28] [31]
Alkaline Hydrogen Peroxide (H₂O₂/NaOH)	Oxidation system for nanocellulose production	1-3% H₂O₂ with 0.1-0.5M NaOH; solid loading 1:10 g/g [30]	Converts IonoSolv pulps to carboxylated CNCs; 1h reaction sufficient [30]

Output Valorization and Integrated Biorefinery Applications

Cellulose Valorization Pathways

The cellulose-rich pulp produced by IonoSolv pretreatment serves as a platform material for various value-added products:

• Bioethanol Production: High-glucan pulp (≥85% cellulose) enables glucose yields up to 98% after enzymatic hydrolysis, providing optimal feedstock for fermentation [28]. Recent paradigm shifts have transformed lignin from an inhibitor to a potential enhancer in enzymatic hydrolysis systems through structural tailoring [31].

• Nanocellulose Production: IonoSolv pulps can be converted to carboxylated cellulose nanocrystals (CNCs) via alkaline H₂O₂ oxidation (1h, 1:10 solid loading), producing electrostatically stable, needle-like CNCs with 58-63% crystallinity [30]. This route eliminates the need for toxic chemicals and complex purification steps associated with conventional CNC production.

Lignin Valorization Opportunities

IonoSolv lignin retains a relatively uncondensed structure with abundant β-O-4 linkages, making it suitable for various applications:

• Polymer Precursors: Potential for epoxy resins, dispersants, and surfactants [29].

• Carbon Materials: Production of carbon fibers, activated carbons, and composite materials [29] [27].

• Aromatic Chemicals: Depolymerization to benzene, vanillin, guaiacol, and other platform chemicals [29].

The following workflow illustrates the integrated valorization pathways for IonoSolv fractions:

Diagram 2: IonoSolv Integrated Biorefinery Workflow

Scale-up Challenges and Sustainability Considerations

Technical Hurdles in Commercial Implementation

Despite promising laboratory results, scaling IonoSolv technology presents several challenges:

• IL Recycling and Economics: Efficient IL recovery is critical for economic viability, with targets of ≥97% recovery for ILs costing $2.5/kg [26]. Current recovery methods include antisolvent precipitation, membrane separation, and distillation, each with specific energy and efficiency trade-offs [27] [26].

• Materials Compatibility: The acidic nature of PILs like [TEA][HSO₄] necessitates corrosion-resistant reactors, increasing capital costs [27]. Limited data exists on long-term material compatibility at industrial scale.

• Water Consumption: Pulp washing requires significant water volumes, creating energy-intensive wastewater treatment demands. Optimization of washing protocols can reduce water use by 30-50% while maintaining saccharification efficiency [28] [26].

Environmental Impact Assessment

The green credentials of IonoSolv processing require careful lifecycle assessment:

• Toxicity Concerns: While initially hailed as green solvents, certain ILs have demonstrated environmental persistence and potential ecotoxicity [32]. Recent studies have detected IL residues in environmental matrices, highlighting the need for comprehensive risk assessment [32].

• Energy Balance: The low vapor pressure of ILs reduces atmospheric emissions but shifts environmental impacts to energy-intensive recycling processes [26]. Integration with renewable energy sources improves overall sustainability.

• Circular Economy Potential: IonoSolv technology aligns with circular economy principles by converting waste biomass into multiple value streams while minimizing waste generation [27] [30].

The IonoSolv process represents a significant maturation in the application of ionic liquids for biomass valorization, building upon a century of IL development since Walden's initial discovery. By enabling selective fractionation of lignocellulosic biomass into high-purity cellulose, lignin, and hemicellulose streams, IonoSolv technology addresses critical bottlenecks in biorefining efficiency and economics. Recent advances in process intensification—including higher biomass loadings, optimized particle sizes, and improved washing protocols—have enhanced the commercial viability of this approach.

Future development should focus on four key areas: (1) designing next-generation ILs with improved recyclability, reduced toxicity, and lower cost; (2) integrating advanced IL recovery technologies such as membrane separation and aqueous biphasic systems; (3) developing standardized analytical protocols for IL purity assessment after recycling; and (4) demonstrating pilot-scale operations to validate technoeconomic models. As research continues to transform lignin from a "barrier component" to a "functional carrier" [31], the IonoSolv process is poised to play a pivotal role in the transition toward circular, carbon-neutral biorefining systems that fully utilize the complex molecular architecture of lignocellulosic biomass.

The development of efficient and safe drug delivery systems represents a paramount objective in modern pharmaceutical research and therapeutic innovation. Conventional delivery platforms face persistent challenges that substantially limit their clinical utility, including poor aqueous solubility of many drug candidates, structural instability under physiological conditions, and nonspecific biodistribution that results in insufficient drug accumulation at target sites while inducing off-target toxicity [17]. These limitations have underscored the urgent need for advanced delivery technologies capable of overcoming multiple pharmacological barriers simultaneously.

The convergence of materials science and biomedical engineering has propelled ionic liquids (ILs) to the forefront of next-generation drug delivery solutions. As organic salts that remain liquid below 100°C, ILs exhibit unparalleled molecular design flexibility owing to their modular cation-anion combinations [17]. This structural tunability enables precise tuning of critical pharmaceutical parameters including solubility, stability, and biocompatibility. The term "designer solvents" aptly describes ILs because their physicochemical properties can be custom-designed through strategic selection of cation-anion pairs, allowing formulators to tailor polarity, hydrophobicity, hydrogen-bonding capacity, and thermal stability for specific pharmaceutical applications [18] [33] [9].

Historical Development and Classification

Evolution of Ionic Liquids

The historical development of ionic liquids spans more than a century, marked by key discoveries that have progressively expanded their pharmaceutical applicability:

• 1914: Paul Walden first reported the physical properties of ethyl ammonium nitrate ([EtNH3][NO3]) with a melting point of 13-14°C, pioneering the concept of ionic liquids [33] [9].
• 1950s: Discovery of halide salt ionic liquids based on mixtures of aluminum chloride and ethyl pyridinium bromide, primarily used for metal deposition and as Lewis acid catalysts [33].
• 1992: Wilkes and Zaworotko developed air- and water-stable imidazolium-based ILs with anions like [BF4] and [PF6], overcoming handling issues associated with earlier generations [33] [9].
• 2007: Introduction of Active Pharmaceutical Ingredient-Ionic Liquids (API-ILs) with the synthesis of ranitidine docusate, representing a paradigm shift in pharmaceutical formulation [9].
• Present: Third-generation ILs featuring biologically active ions with low toxicity, reduced manufacturing costs, and good biodegradability suitable for biopharmaceutical applications [9].

Classification of Pharmaceutical ILs

The evolution of ionic liquids has led to the development of several specialized categories for pharmaceutical applications:

Table 1: Classification of Ionic Liquids for Pharmaceutical Applications

IL Category	Composition Features	Key Characteristics	Pharmaceutical Applications
First-Generation	Chloroaluminate chemistry	Low melting point, high thermal stability, but sensitive to water and air	Limited due to instability and toxicity
Second-Generation	Imidazolium/pyridinium with [BF4], [PF6], [NTf2]	Air and water stable, adjustable properties, but high toxicity	Industrial applications with limited biological use
Third-Generation (Bio-ILs)	Cholinium, betainium, amino acid-derived ions	Low toxicity, good biodegradability, biocompatible	Topical, transdermal, and oral drug delivery
API-ILs	API as either cation or anion paired with counterion	Enhanced solubility, eliminates polymorphism, improved bioavailability	Direct formulation of active pharmaceuticals
SAILs	Long alkyl chains in cation/anion	Surface-active, self-assembling, micelle formation	Solubilization enhancement, nanocarrier systems

The following diagram illustrates the historical evolution and classification of ionic liquids in pharmaceutical applications:

Mechanisms of Action: Enhancing Solubility and Permeability

Solubilization Enhancement Mechanisms

Ionic liquids employ multiple synergistic mechanisms to enhance the solubility of poorly water-soluble drugs, which represent approximately 80% of new drug candidates and 40% of marketed oral drugs [9]:

• Ionic Interaction and Hydrogen Bonding: ILs can form multiple ionic bonds and hydrogen bonds with drug molecules, disrupting the crystal lattice energy and enhancing dissolution [17].

• Hydrophobicity Tuning: By adjusting the alkyl chain length on cations or selecting appropriate anions, the hydrophobicity of ILs can be precisely tuned to match the physicochemical properties of specific drug molecules [17] [9].

• Surface Activity: Surface Active Ionic Liquids (SAILs) incorporate long alkyl chains that enable self-assembly into micellar structures, providing a hydrophobic core for solubilizing non-polar compounds [9].

• Polymorphism Prevention: API-ILs eliminate polymorphism issues associated with solid dosage forms by preventing nucleation and crystal growth through ionic interactions between drug and counterion [9].

Permeability Enhancement Strategies

Ionic liquids enhance drug permeability across biological barriers through several documented mechanisms:

• Stratum Corneum Modification: For transdermal delivery, ILs transiently fluidize the lipid bilayers of the stratum corneum, creating temporary pathways for drug permeation without causing permanent damage [18] [34].

• Tight Junction Modulation: Certain ILs can reversibly open tight junctions in epithelial barriers, enhancing paracellular transport of macromolecules [17].

• Membrane Fluidity Enhancement: ILs interact with phospholipid bilayers to increase membrane fluidity, facilitating transcellular transport of encapsulated drugs [18].

Table 2: Quantitative Enhancement of Drug Properties Using Ionic Liquids

Drug/Drug Category	IL Formulation	Solubility Enhancement	Permeability Increase	Bioavailability Improvement
Hydrophobic Small Molecules	Imidazolium-based ILs	5-200 fold increase	2-8 fold transdermal flux	3-10 fold increase
Proteins/Peptides	Choline-geranic acid (CAGE)	Maintains native structure	2-4 fold skin penetration	Enables transdermal delivery
Nucleic Acids	Lipid-derived ILs	Stable encapsulation >95%	Effective cellular uptake	Demonstrated gene silencing
Biologics	Bio-IL nanocarriers	Prevents aggregation >20°C melting point elevation	Crosses biological barriers	Enables oral delivery

Experimental Protocols and Methodologies

Protocol: Formulation of IL-Based Transdermal Delivery Systems

This protocol details the preparation of ionic liquid-loaded ethosomes for enhanced transdermal delivery of biopharmaceuticals, based on recently published methodologies [18]:

Materials Required:

• Dimyristoyl-phosphatidylcholine (phospholipid component)
• Choline-based ionic liquid (e.g., choline geranate)
• Drug candidate (e.g., insulin, siRNA, small molecule)
• Ethanol (pharmaceutical grade)
• Phosphate buffer saline (PBS, pH 7.4)
• Rotary evaporator with vacuum source
• High-pressure homogenizer or probe sonicator
• Dynamic light scattering apparatus for size measurement

Procedure:

• IL-Ethosome Preparation:

  • Dissolve 2% w/v dimyristoyl-phosphatidylcholine in a 30:20:50 ratio of ethanol:ionic liquid:PB7.4
  • Hydrate the lipid film with the ethanol-IL-PBS mixture at 35°C for 1 hour with gentle agitation
  • Incorporate the drug candidate at this stage for active loading
  • Subject the mixture to high-pressure homogenization at 15,000 psi for 3 cycles or probe sonication at 40% amplitude for 5 minutes (30-second pulses)

• Characterization:

  • Measure particle size and zeta potential using dynamic light scattering (target size: 100-200 nm; PDI <0.25)
  • Determine encapsulation efficiency via ultracentrifugation at 100,000×g for 30 minutes and HPLC analysis of supernatant
  • Assess in vitro drug release using Franz diffusion cells with regenerated cellulose membranes

• Ex Vivo Permeation Studies:

  • Use excised human or porcine skin mounted in Franz diffusion cells
  • Apply 1 mL of IL-ethosome formulation to the donor compartment
  • Maintain receptor phase at 37°C with continuous stirring
  • Sample receptor fluid at predetermined intervals (0.5, 1, 2, 4, 8, 12, 24 h) for drug quantification

The following workflow diagram illustrates the key steps in developing and evaluating IL-based transdermal drug delivery systems:

Protocol: Synthesis of API-Ionic Liquids

The synthesis of Active Pharmaceutical Ingredient-Ionic Liquids represents a sophisticated approach to drug formulation where the active molecule itself becomes part of the ionic liquid structure [17] [9]:

Materials:

• Pharmaceutical compound with ionizable group (acidic or basic)
• Appropriate counterion (e.g., choline, docusate, amino acid derivatives)
• Solvents: methanol, ethanol, dichloromethane (anhydrous)
• Rotary evaporator
• Freeze dryer
• Analytical HPLC for purity assessment

Procedure for Anionic API-ILs:

• Acid-Base Neutralization:

  • Dissolve the acidic drug (1.0 equivalent) in minimal anhydrous ethanol
  • Slowly add a solution containing the cationic counterion (1.05 equivalents) in ethanol with stirring at room temperature
  • Continue stirring for 12-24 hours under nitrogen atmosphere

• Purification and Isolation:

  • Remove solvents under reduced pressure using a rotary evaporator (40°C water bath)
  • Wash the resulting ionic liquid with cold diethyl ether (3×20 mL) to remove non-ionic impurities
  • Dry under high vacuum (0.1 mmHg) for 48 hours until constant weight is achieved
  • Characterize by NMR, MS, and DSC to confirm formation and purity

• Quality Control Parameters:

  • Purity >98% by HPLC
  • Water content <1% by Karl Fischer titration
  • Glass transition temperature (Tg) or melting point below 100°C
  • Solubility profile in aqueous and biological media

Therapeutic Applications and Clinical Translation

Transdermal Drug Delivery Systems

Ionic liquids have demonstrated remarkable success in transdermal drug delivery, particularly for biopharmaceuticals that traditionally required injection [18] [34]:

Diabetes Management: IL-based transdermal formulations have achieved prolonged glycemic control in diabetic models through sustained insulin release. Choline-geranic acid ILs (CAGE) enabled non-invasive insulin delivery with bioavailability comparable to subcutaneous injection [17] [18].

Dermatological Therapeutics: Clinical trials have validated the efficacy of IL-based topical treatments. CGB-500, a topical treatment for atopic dermatitis, demonstrated a 98% improvement in disease severity index compared to 28% for placebo in a phase 2a clinical trial [35].

Oncology Applications: IL-based delivery systems have shown potent anti-tumor responses in nucleic acid immunotherapy. siRNA delivery against specific oncogenes achieved significant tumor growth inhibition in preclinical models with enhanced tumor targeting and reduced off-target effects [17] [18].

Oral Drug Delivery Advancements

For oral delivery, ILs address the critical challenge of poor bioavailability of BCS Class II and IV drugs [9]:

Solubility Enhancement: API-ILs have demonstrated 5-200 fold increases in aqueous solubility for poorly soluble drugs, directly translating to enhanced oral bioavailability [9].

Metabolic Stability: IL formulations protect drugs from enzymatic degradation in the gastrointestinal tract, particularly for peptide-based therapeutics [17] [9].

Mucosal Permeation: Surface-active ILs enhance drug permeability across the intestinal mucosa through transient opening of tight junctions and membrane fluidization effects [9].

Table 3: Clinical Development Status of Selected IL-Based Therapeutics

Therapeutic Area	IL Platform	Drug Candidate	Development Status	Key Outcomes
Dermatology	CAGE IL Technology	CGB-500 (Atopic Dermatitis)	Phase 2b Trial Ongoing	98% improvement in severity (Phase 2a)
Dermatology	CAGE IL Technology	CGP-501 (Alopecia Areata)	Phase 1 Initiating 2025	Preclinical efficacy demonstrated
Onychomycosis	Choline-geranic acid IL	Topical Antifungal	Phase 2 Completed (NCT05202366)	Significant improvement vs placebo
Diabetes	IL-transethosomes	Transdermal Insulin	Preclinical	Sustained glucose control >24 hours
Oncology	IL-nanoemulsions	siRNA/mRNA Therapeutics	Preclinical	Tumor growth inhibition >70% in models

The Scientist's Toolkit: Key Research Reagents and Materials

Successful implementation of ionic liquid technology in pharmaceutical formulation requires specific reagents and materials optimized for this emerging field:

Table 4: Essential Research Reagents for IL-Based Drug Formulation

Reagent/Material	Specifications	Functional Role	Application Notes
Choline-Based Salts	≥98% purity, choline bicarbonate or chloride	Cation source for biocompatible ILs	Preferred for third-generation Bio-ILs with low toxicity profiles
Amino Acid Derivatives	Natural and unnatural amino acids with protection groups	Anion source for API-ILs	Enhances biocompatibility and active targeting
Fatty Acids (C8-C18)	Pharmaceutical grade, saturated/unsaturated	Hydrophobic component for SAILs	Chain length determines CMC and self-assembly properties
Phospholipids	Hydrogenated or natural, >95% purity	Lipid component for nanocarriers	Compatible with IL integration in ethosomal systems
Imidazolium Salts	Various alkyl chain lengths (C2-C16)	Versatile IL cations for non-biological applications	Limited to external use due to toxicity concerns
Pharmaceutical Buffers	PBS, HEPES, acetate buffers	Aqueous phase for formulations	Must maintain stability and activity of biologics
Permeation Enhancers	Terpenes, surfactants, solvents	Synergistic permeability enhancement	Use at minimal effective concentrations to minimize irritation

Future Perspectives and Concluding Remarks

The integration of ionic liquids into pharmaceutical development represents a paradigm shift in drug formulation strategy. The unparalleled tunability of ILs enables formulators to simultaneously address multiple challenges in drug delivery, including poor solubility, limited permeability, and low stability [17]. The emergence of API-ILs has further blurred the distinction between active ingredient and delivery system, creating opportunities for fundamentally new approaches to pharmaceutical design [9].

Future development in this field will likely focus on several key areas:

Intelligent Delivery Systems: Next-generation ILs incorporating stimuli-responsive mechanisms will enable spatiotemporally controlled drug release in response to specific physiological triggers [17].

Personalized Therapeutics: The modular nature of IL design facilitates the creation of patient-specific formulations tailored to individual metabolic characteristics and disease states [17].

Biologics Delivery: IL-based platforms show exceptional promise for stabilizing and delivering increasingly important biologic drugs, including proteins, peptides, and nucleic acids [18].

Regulatory Harmonization: As IL-based pharmaceuticals advance through clinical development, establishing standardized regulatory frameworks will be essential for streamlined clinical translation [17].

The remarkable progress in ionic liquid technology for pharmaceutical applications underscores their potential to redefine contemporary drug delivery paradigms. With continued research addressing challenges in long-term biosafety, scalable manufacturing, and regulatory approval, IL-based formulations are poised to make significant contributions to next-generation precision medicine.

Ionic Liquids (ILs) are a class of compounds defined entirely of ions with a melting point below 100 °C [3] [36]. Their unique properties, such as extremely low vapor pressure, high thermal stability, wide liquidus range, and tunable physicochemical characteristics, have positioned them as green alternatives to volatile organic solvents [37] [3]. The capacity to tailor IL properties by selecting different cation-anion combinations has unlocked applications across diverse engineering fields, including carbon capture, electrochemistry, and lubrication [37] [3]. This whitepaper provides an in-depth technical examination of the roles and methodologies of ILs within these domains, framed within their historical development as sophisticated solvents.

Historical Development and Properties of Ionic Liquids

A Brief Historical Context

The history of ILs dates back to 1914 with Paul Walden's report on ethylammonium nitrate, a salt liquid at room temperature (m.p. 12 °C) [1] [37] [3]. However, this discovery remained a chemical curiosity for decades. The modern era of ILs began in the mid-20th century with the work of Hurley and Weir, who used mixtures of alkylpyridinium halides and aluminium chloride to electroplate aluminium at room temperature [1] [36]. A significant breakthrough came in the 1980s and early 1990s with the introduction of 1,3-dialkylimidazolium cations and, later, air- and water-stable anions like hexafluorophosphate (PF₆⁻) and tetrafluoroborate (BF₄⁻) [1] [3]. This evolution expanded the application of ILs from electrochemical studies to broader uses as solvents and catalysts in organic synthesis [37] [36].

Fundamental Properties and Tunability

The immense versatility of ILs stems from their tunable nature. By varying the structures of the cationic and anionic constituents, properties such as hydrophobicity, viscosity, solubility, acidity, basicity, and electrochemical stability can be precisely engineered for specific applications [37] [3]. Table 1 summarizes key properties of some common ionic liquids.

Table 1: Characteristic Properties of Common Ionic Liquid Families

Cation Type	Example Anion	Melting Point (°C)	Thermal Stability (°C)	Key Characteristics	Typical Applications
1-Ethyl-3-methylimidazolium (EMIM)	Cl⁻, NO₃⁻, BF₄⁻, NTf₂⁻	-21 to 12 [1] [3]	~150-400 [37]	Low viscosity, good conductivity	Electrochemistry, catalysis, biopolymer processing [3]
1-Butyl-3-methylimidazolium (BMIM)	PF₆⁻, BF₄⁻, NTf₂⁻	~0 to -20 [3]	~150-400 [37]	Hydrophobic (with PF₆⁻), widely studied	Solvent for organic reactions, gas separation [37]
Tetraalkylammonium	Amino acids, NO₃⁻	Variable [1]	~200-300 [37]	Often biodegradable	Bio-ILs, pharmaceuticals [37]
Tetraalkylphosphonium	Halides, amino acids	Variable [3]	High (>300)	High thermal stability, low toxicity	Lubricants, extraction processes [3] [36]

Ionic Liquids in Carbon Dioxide (CO₂) Capture

Carbon capture and storage (CCS) is a critical technology for mitigating climate change, projected to account for up to 25% of emission reductions in aggressive mitigation scenarios [38]. ILs have emerged as promising candidates for CO₂ capture due to their high solubility for CO₂ and low energy requirements for regeneration compared to conventional amine-based processes [39] [40].

Mechanisms and Experimental Protocols

The primary mechanisms for CO₂ capture using ILs involve physical absorption, chemical reaction, and electrochemically-mediated processes.

• Absorption with Task-Specific ILs: So-called "task-specific" ILs can be designed with functional groups (e.g., amines) that chemically bind to CO₂. The general methodology involves:

  • Synthesis: A task-specific IL is synthesized, for example, by tethering an amine group to the cation of a conventional imidazolium-based IL [37].
  • Absorption Experiment: A known volume of the IL is placed in a high-pressure reactor or a gas absorption cell. A stream of CO₂ or a CO₂/N₂ mixture is bubbled through the liquid at a controlled temperature and pressure.
  • Analysis: The amount of CO₂ absorbed is measured gravimetrically (by weight increase) or by monitoring the gas volume decrease using a mass flow meter or pressure sensors. The captured CO₂ can be regenerated by heating the IL or by applying an electrochemical potential swing [38].

• Electrochemical pH-Swing Capture: This method leverages the electrochemistry of ILs or aqueous solutions containing ILs to modulate pH and drive CO₂ capture and release [38].

  • Cell Setup: An electrochemical cell is constructed with appropriate working, counter, and reference electrodes. The electrolyte can be an IL or an aqueous solution with a dissolved redox-active capture agent.
  • Capture Cycle: At the cathode, the reduction of a molecule (e.g., H₂O or a quinone) generates a base (OH⁻), which reacts with dissolved CO₂ to form (bi)carbonate ions, effectively capturing CO₂.
  • Release Cycle: At the anode, the reverse oxidation reaction occurs, generating an acid (H⁺) that protonates the (bi)carbonate, releasing pure, high-pressure CO₂ gas [38].

The following workflow diagram illustrates the two primary CO₂ capture pathways using ionic liquids.

The Scientist's Toolkit: Key Reagents for CO₂ Capture

Table 2: Essential Research Reagents for CO2 Capture using Ionic Liquids

Reagent / Material	Function / Role	Example & Notes
Task-Specific ILs	Chemically bind CO₂ via functional groups (e.g., amines).	Amino-functionalized imidazolium acetate. High capture capacity but can have slower kinetics [37].
Physical Absorption ILs	Dissolve CO₂ via physical intermolecular interactions.	[BMIM][PF₆], [EMIM][NTf₂]. Tunable solubility, faster regeneration [3].
Electrochemical Cell	Provides the platform for pH-swing or mediated capture processes.	H-cell or flow cell with electrodes (e.g., Pt, carbon). Enables energy-efficient regeneration [38].
Redox-Active Capture Agents	Molecules that change oxidation state to mediate CO₂ capture/release.	Quinones (e.g., AQDS). Reduced form binds CO₂, oxidized form releases it [38].

Electrochemical Applications and Conversion of CO₂

Electrochemical methods are gaining traction not only for capturing CO₂ but also for converting it into valuable products, a process known as Carbon Capture and Utilization (CCU) [38] [40]. ILs play a crucial role in enhancing the efficiency of these processes.

Electrochemical CO₂ Conversion Protocol

A key application involves the electrochemical reduction of captured CO₂ into chemicals and fuels, such as carbon monoxide, formic acid, or ethylene glycol [40] [41]. The detailed methodology is as follows:

• Electrolyte Preparation: The electrolyte is prepared, often consisting of an IL (e.g., [BMIM][BF₄] or [EMIM][NTf₂]) or a mixture of an IL with a supporting salt in a solvent. The IL stabilizes reaction intermediates and lowers the overpotential required for CO₂ reduction [37] [41].
• Electrode Preparation: The working electrode, typically a metal catalyst (e.g., Cu, Ag, or a proprietary catalyst), is fabricated. In advanced systems, the catalyst may be a gas diffusion electrode to improve mass transfer of gaseous CO₂ [41].
• Electrochemical Cell Assembly: A multi-compartment electrochemical cell (e.g., an H-cell or a flow cell) is assembled to separate the products formed at the anode and cathode.
• Electrolysis and Product Analysis: CO₂ gas is purged through the catholyte compartment. A controlled potential or current is applied using a potentiostat/galvanostat. The products are quantified using techniques like gas chromatography (for gaseous products such as CO and CH₄) and high-performance liquid chromatography (for liquid products such as formate and ethylene glycol) [41]. Electrochemical Impedance Spectroscopy (EIS) may be used to characterize the system's resistance [41].

The logical flow of an integrated capture and conversion system is shown below.

Ionic Liquids in Lubrication and Advanced Materials

Beyond carbon management, the unique physicochemical properties of ILs, including their high thermal stability, low volatility, and excellent tribological properties, make them high-performance lubricants and lubricant additives [3] [36].

Lubrication Mechanisms and Material Protection

ILs function as lubricants by forming a robust, adsorbed film on metal surfaces, preventing direct metal-to-metal contact. Their design flexibility allows for the creation of ILs with specific anti-wear and extreme-pressure additives. Phosphonium-based ILs, for instance, are noted for their excellent stability and lubrication performance [3]. A key advanced application is in Surface Engineering, where ILs are used in concepts like SCILL (Solid Catalyst with Ionic Liquid Layer), where a thin layer of IL coating a solid catalyst can also provide lubricating and anti-corrosion effects in reactive environments [36].

Ionic liquids have evolved from academic curiosities into versatile solvents and functional materials at the heart of green engineering. Their tunable nature allows for tailored applications in critical areas such as carbon capture, where they offer energy-efficient pathways; electrochemistry, where they enable the conversion of CO₂ into valuable products; and lubrication, where they provide high-performance, non-volatile alternatives. As the historical development of ILs continues, their integration into industrial processes is poised to play a significant role in advancing sustainable technologies and mitigating environmental impact. Future research will likely focus on reducing synthesis costs, thoroughly assessing environmental impact and biodegradability, and scaling up the most promising laboratory innovations for industrial deployment.

Ionic liquids (ILs), a class of materials defined as organic salts with melting points below 100°C, have undergone significant generational evolution since their discovery by Paul Walden in 1914 [9] [14]. This progression has transformed them from chemical curiosities into sophisticated pharmaceutical tools. First-generation ILs, primarily used as green solvents in electrochemical applications, demonstrated attractive physical properties including low volatility and thermal stability but suffered from sensitivity to air and water, poor biodegradability, and significant toxicity [15] [42]. Second-generation ILs brought improved stability and tunable physicochemical properties, yet retained considerable toxicity profiles that limited their biomedical applications [7] [43].

The emergence of third-generation ILs marked a paradigm shift toward biocompatibility and sustainability. These ILs incorporate biologically relevant ions such as choline, amino acids, and fatty acids, offering reduced toxicity, enhanced biodegradability, and improved safety profiles [15] [43] [14]. Within this category, Bio-ILs—specifically those derived from choline and amino acids—have emerged as particularly promising for pharmaceutical applications, addressing critical challenges in drug formulation and delivery while aligning with green chemistry principles [15] [44].

Classification and Structural Characteristics of Bio-ILs

Choline-Based Bio-ILs

Choline serves as an ideal cation for pharmaceutical Bio-ILs due to its status as an essential nutrient recognized as "Generally Regarded as Safe" (GRAS) by the U.S. Food and Drug Administration [15] [43]. As a precursor to the neurotransmitter acetylcholine and phospholipid components of cell membranes, choline offers inherent biocompatibility [43]. Choline-based Bio-ILs are typically synthesized through neutralization reactions between choline hydroxide or choline bicarbonate and various acids, including amino acids, fatty acids, and carboxylic acids [43]. Common synthetic procedures involve mixing these components in organic solvents for 12-24 hours at room temperature or elevated temperatures (approximately 40°C), followed by filtration to remove excess acids and drying under high vacuum [43].

Research has demonstrated numerous choline-based Bio-IL systems with pharmaceutical relevance. Foulet et al. developed a series of choline-amino acid ILs (e.g., choline-glycine, -serine, -proline, -alanine, -histidine, and -valine) and evaluated their toxicity and antimicrobial activities [43]. Raihan et al. prepared similar choline-containing amino acid ILs (glycine, alanine, proline, serine, leucine, isoleucine, and phenylalanine) to investigate cytotoxicity and drug solubilization efficiency [43]. Beyond amino acids, choline has been combined with various organic acids including germanic acid, citronellic acid, octanoic acid, decanoic acid, hexenoic acid, salicylic acid, and glutaric acid to enhance transdermal delivery of therapeutic molecules [43].

Amino Acid-Based Bio-ILs (AAILs)

Amino acids serve as versatile building blocks for Bio-ILs, capable of functioning as either cations or anions due to their amphoteric nature [44] [45]. This dual functionality, combined with their natural abundance, low cost, and chiral centers, makes them ideal for designing sustainable ILs with low toxicity and high biodegradability [15] [44] [45].

AAILs with amino acid anions are typically synthesized in two steps: initial conversion of an organic halide to organic hydroxide via ion exchange, followed by neutralization of the amino acid by the organic hydroxide [45]. In contrast, AAILs with amino acid cations are obtained in a single step by acidifying the neutral amino acid with a strong acid [45]. To minimize hydrogen bonding and reduce melting points, amino acids are sometimes first converted to amino acid ester chlorides before metathesis reactions with metal salts [45].

The structural versatility of AAILs enables precise tuning of their physicochemical properties for specific pharmaceutical applications, positioning them as sustainable alternatives to conventional ILs and organic solvents [44].

Physicochemical Properties and Their Pharmaceutical Relevance

Viscosity and Solubility

Viscosity significantly influences the application of Bio-ILs in pharmaceutical processes, particularly as solvents in catalysis, extraction, and drug delivery systems [45]. AAILs containing the same anion but different cations exhibit varying viscosities. For instance, AAILs with imidazolium or amino acid ester cations demonstrate comparable viscosities, both being less viscous than those based on quaternary ammonium cations [45].

Strong hydrogen bonding capabilities, particularly through carboxyl groups, generally result in higher viscosity. For 1-ethyl-3-methylimidazolium (Emim)-based AAILs, reported viscosities start at 486 mPa·s at 25°C [45]. Tetrabutylphosphonium (P4444)-based AAILs exhibit lower viscosity due to weakened intermolecular interactions, though values remain relatively high (>344 mPa·s at 25°C) [45]. Choline-based AAILs typically show high viscosities ranging from 102 to 107 mPa·s, with the specific amino acid anion significantly influencing this property [45].

Regarding solubility, the miscibility of room-temperature AAILs with organic solvents depends on the side chain structure of the amino acid anion. For example, [Emim][Glu] and [Emim][Asp], containing amino acid anions with two carboxyl groups, are insoluble in chloroform [45]. AA ester saccharinates, nitrates, and halogenates are typically miscible with water, ethanol, and acetone [45].

Thermal Properties

Thermal stability varies significantly among AAIL classes based on cation and anion selection. For the alanine anion, decomposition temperatures (Td) decrease in the following order based on cation: phosphonium > imidazolium > tetraalkylammonium > choline [45]. Specific Td values for Ala-based ILs range from 286°C for phosphonium cations to 150°C for choline cations [45].

For triethanolammonium [TEA] salts, thermal stability varies with the amino acid anion, decreasing in the order: His > Arg > Glu2 > Pro > Trp > Ser > Gly > Leu > Ile > Thr > Met > Ala > Asp2 > Gln > Phe > Glu > Val > Lys > Asp > Asn [45]. Phosphonium-based AAILs generally demonstrate lower melting points and viscosities than ammonium-based AAILs with similar structures [45].

Table 1: Key Physicochemical Properties of Selected Bio-ILs

Bio-IL System	Viscosity (mPa·s at 25°C)	Thermal Stability (Decomposition Temp.)	Aqueous Solubility	Key Pharmaceutical Applications
[Emim][Gly]	486	Varies by cation	Hydrophilic	Drug solubilization, extraction processes
[P4444][Lys]	>344	High (>250°C)	Hydrophilic	Biocatalysis, drug synthesis
[Ch][Gly]	121	Moderate (~150°C)	Highly soluble	Transdermal delivery, oral formulations
[Ch][Trp]	5640	Moderate	Variable	Sustained release formulations
[N2222][L-Ala]	81	Moderate (~162°C)	Hydrophilic	Green synthesis, pharmaceutical processing

Pharmaceutical Applications and Mechanisms of Action

Enhanced Drug Solubility and Bioavailability

A significant challenge in pharmaceutical development is that approximately 80% of new drug candidates and 40% of marketed drugs exhibit poor aqueous solubility, limiting their therapeutic efficacy [9]. Bio-ILs address this challenge through multiple mechanisms that enhance drug solubility and bioavailability.

The unique ionic environment of Bio-ILs can significantly improve the solubility of poorly water-soluble drugs (BCS Class II and IV) through various intermolecular interactions including hydrogen bonding, ionic interactions, and π-π stacking [9] [17]. For oral drug delivery, Bio-ILs enhance bioavailability by improving drug solubilization in gastrointestinal fluids and increasing permeability across intestinal mucosa [9]. Surface-active ILs (SAILs), which incorporate long alkyl chains into cations or anions, can self-assemble in aqueous solutions to form micelles, liposomes, and other colloidal systems that further enhance drug solubilization [9].

Active Pharmaceutical Ingredient Ionic Liquids (API-ILs)

A revolutionary application of Bio-IL technology is the development of Active Pharmaceutical Ingredient Ionic Liquids (API-ILs), where the drug molecule itself forms either the cation or anion of the IL [9]. First introduced in 2007 with the synthesis of ranitidine docusate, API-ILs represent a paradigm shift in pharmaceutical formulation [9].

API-ILs provide multiple advantages over conventional crystalline drug forms, including enhanced solubility, elimination of polymorphism issues, improved thermal stability, and increased bioavailability [9] [17]. By selecting appropriate counterions for parent APIs, pharmaceutical scientists can exert precise control over physicochemical and biological properties [9]. Three primary types of API-ILs have been identified: (1) those created directly through ionic binding using APIs as anions or cations; (2) ionic prodrugs formed via covalent linkage before IL conversion; and (3) dual active API-ILs utilizing both approaches [9].

Drug Delivery Applications

Bio-ILs have demonstrated significant potential across various drug delivery routes:

Oral Delivery: Bio-ILs enhance oral bioavailability of poorly soluble drugs by improving solubilization in GI fluids and increasing permeability across intestinal mucosa [9]. Their tunable properties allow design of systems that resist degradation in the stomach and release payload in specific intestinal regions [9].

Transdermal Delivery: Bio-ILs function as effective chemical permeation enhancers in transdermal drug delivery systems [42]. They facilitate transcellular and paracellular transport by disrupting stratum corneum integrity, fluidizing lipids, creating diffusional pathways, and extracting lipid components [42]. Choline-geranic acid IL (CAGE) has demonstrated particular efficacy in transdermal applications, with multiple clinical trials targeting conditions including rosacea, onychomycosis, and atopic dermatitis [17].

Nanocarrier Systems: Bio-ILs enable the creation of advanced nanocarriers including nanoparticles, micelles, and nanoemulsions that improve drug targeting, protect therapeutic agents from degradation, and provide controlled release profiles [17] [42]. For instance, cholinium oleate ([Cho][Ole]) combined with sorbitan monolaurate (Span-20) forms stable micelles that significantly enhance transdermal delivery of paclitaxel [42].

Experimental Protocols and Methodologies

Synthesis of Choline-Based Bio-ILs

Materials:

• Choline hydroxide or choline bicarbonate solution
• Desired acid component (amino acid, fatty acid, or carboxylic acid)
• Organic solvent (e.g., methanol, ethanol)
• Filtration apparatus
• Vacuum drying system

Procedure:

• Combine choline hydroxide or choline bicarbonate with slightly more than an equimolar amount of the desired acid in an appropriate organic solvent [43].
• Mix the reaction components continuously for 12-24 hours at room temperature or elevated temperature (approximately 40°C) to ensure complete reaction [43].
• Filter the resulting mixture to precipitate and remove any excess acids [43].
• Remove the aqueous organic solution under high vacuum pressure to isolate the pure choline-based Bio-IL [43].
• Characterize the resulting Bio-IL using appropriate analytical techniques including NMR, FT-IR, and mass spectrometry to confirm structure and purity.

Synthesis of Amino Acid-Based ILs (AAILs)

AAILs with Amino Acid Anions:

• Convert organic halide to organic hydroxide using ion exchange (metathesis with silver oxide or anion exchange resin in hydroxide form) [45].
• Neutralize the amino acid with the resulting organic hydroxide in aqueous or alcoholic solvent [45].
• Remove solvent under reduced pressure to obtain the AAIL.

AAILs with Amino Acid Cations:

• Acidify the neutral amino acid with a strong acid (e.g., HCl, H₂SO₄) in aqueous solution [45].
• Concentrate the solution and precipitate the AAIL using appropriate solvents.
• Recrystallize if necessary to obtain pure product.

Alternative Ester Route:

• Convert amino acid to amino acid ester chloride using thionyl chloride or other chlorinating agents [45].
• Perform metathesis reaction with metal salts (e.g., LiNTf₂, NaBF₄) to obtain the target AAIL [45].
• Purify through appropriate techniques (extraction, recrystallization).

Diagram 1: Bio-IL Synthesis Workflow. This flowchart outlines the primary synthetic pathways for choline-based and amino acid-based ionic liquids, highlighting key steps and alternative routes.

Characterization Techniques

Comprehensive characterization of Bio-ILs is essential for pharmaceutical applications:

Structural Analysis:

• Nuclear Magnetic Resonance (NMR) spectroscopy ( [9]H, [7]C, 2D techniques) to confirm chemical structure and purity
• Fourier-Transform Infrared (FT-IR) spectroscopy to identify functional groups and intermolecular interactions
• Mass spectrometry (ESI-MS, MALDI-TOF) for molecular weight confirmation and impurity profiling

Physicochemical Properties:

• Thermogravimetric analysis (TGA) for thermal stability assessment
• Differential scanning calorimetry (DSC) for phase behavior and melting point determination
• Rheological measurements for viscosity profiling across temperature ranges
• Solubility studies in pharmaceutically relevant solvents and simulated biological fluids

Performance Evaluation:

• In vitro drug release studies using Franz diffusion cells or USP dissolution apparatus
• Permeability assessments using artificial membranes or cell monolayers (e.g., Caco-2 for intestinal permeability)
• Compatibility studies with pharmaceutical excipients and packaging materials

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 2: Essential Research Reagents for Bio-IL Pharmaceutical Development

Reagent/Material	Function/Application	Examples/Specific Types	Key Considerations
Choline Salts	Cation source for Bio-IL synthesis	Choline chloride, choline hydroxide, choline bicarbonate	GRAS status; commercial availability; purity requirements
Amino Acids	Anion or cation source for AAILs	Glycine, alanine, proline, serine; natural and unnatural varieties	Chirality; side chain functionality; cost and availability
Fatty Acids	Hydrophobic component for SAILs	Oleic acid, decanoic acid, octanoic acid	Chain length; degree of unsaturation; purity
Organic Solvents	Reaction medium for synthesis	Methanol, ethanol, acetonitrile	Anhydrous conditions; removal post-synthesis; residual limits
Purification Materials	Isolation and purification	Filtration systems; vacuum ovens; chromatography media	Solvent removal; byproduct separation; quality control
Characterization Tools	Structural and property analysis	NMR, FT-IR, MS, TGA, DSC	Method validation; equipment sensitivity; data interpretation

Structure-Property Relationships and Design Principles

The pharmaceutical performance of Bio-ILs is governed by fundamental structure-property relationships that enable rational design strategies:

Cation Effects: Cation structure significantly influences toxicity profiles, with choline-based cations demonstrating superior biocompatibility compared to imidazolium and pyridinium counterparts [42]. Cation size and symmetry affect viscosity, with asymmetric cations typically yielding lower viscosities [45].

Anion Effects: Anion selection critically determines hydrogen bonding capacity, thermal stability, and biodegradability. Amino acid anions with additional functional groups (e.g., aspartate with second carboxyl group) enable enhanced intermolecular interactions but may increase viscosity [45].

Alkyl Chain Considerations: Increasing alkyl chain length generally enhances lipophilicity and membrane permeability but may also increase toxicity through enhanced membrane disruption [9]. Chain lengths can be optimized to balance solubilization capacity with biocompatibility.

Hydrogen Bonding: The extensive hydrogen bonding network in Bio-ILs significantly impacts their physicochemical behavior, including viscosity, thermal stability, and solvation capabilities [14]. Strategic manipulation of hydrogen bonding density enables tuning of drug release profiles.

Diagram 2: Bio-IL Structure-Property-Performance Relationships. This diagram illustrates the critical connections between structural features of Bio-ILs, their resulting physicochemical properties, and ultimate pharmaceutical performance.

Future Perspectives and Research Directions

Despite significant advances in Bio-IL technology, several challenges and opportunities remain for future research and development:

Toxicity and Biocompatibility: While third-generation Bio-ILs demonstrate improved safety profiles compared to earlier generations, comprehensive long-term toxicity studies and detailed structure-toxicity relationship analyses are still needed [9] [42]. Understanding metabolic fate and elimination pathways represents another critical research direction.

Scalability and Manufacturing: Developing cost-effective, scalable synthesis and purification methods remains essential for widespread pharmaceutical adoption [45]. Continuous manufacturing approaches and quality-by-design principles need adaptation to Bio-IL production.

Regulatory Considerations: Establishing regulatory frameworks and standardization protocols specific to Bio-IL-based pharmaceuticals will be crucial for clinical translation [17]. This includes developing appropriate characterization methods, quality control standards, and safety assessment protocols.

Advanced Applications: Emerging research directions include stimuli-responsive Bio-IL systems for targeted drug delivery, Bio-IL-based biomaterials for tissue engineering, and multifunctional systems combining therapeutic and diagnostic capabilities [17] [14]. Integration with emerging technologies such as 3D printing and artificial intelligence-driven design also presents exciting opportunities.

As research progresses, Bio-ILs are poised to make increasingly significant contributions to pharmaceutical development, enabling more effective, safer, and sustainable therapeutic products that address unmet medical needs while aligning with green chemistry principles.

Navigating Challenges: Toxicity, Biocompatibility, and Process Optimization

Ionic liquids (ILs), a class of low-melting point salts, have garnered significant scientific and industrial interest since the emergence of their second generation in 1992, which introduced air and water-stable varieties [46] [47]. Their unique properties—including extremely low vapor pressure, high thermal stability, and low flammability—led to their initial branding as "green solvents" [48]. This perception was primarily rooted in their minimal atmospheric pollution compared to volatile organic compounds. However, this label created a paradox: their high chemical stability, low volatility, and considerable water solubility result in high persistence in aquatic and terrestrial environments, posing potential threats to ecosystems and human health [48]. Early research focused predominantly on exploiting their tunable physicochemical properties for applications in synthesis, catalysis, and electrochemistry, while largely overlooking their biological and environmental impacts [46] [1]. This article examines the historical context and evolving understanding of the cytotoxicity and ecotoxicity of early ILs, framing this knowledge within the modern imperative to design safer, sustainable chemicals.

Historical Context and Early Generations of Ionic Liquids

The development of ILs is marked by distinct generational shifts, which reflect a growing understanding of their functionality and biological interactions.

Table 1: Generations of Ionic Liquids

Generation	Key Characteristic	Example	Melting Point	Key Toxicity Concern
First Generation	Low-melting point, air & water sensitive [46] [1]	Ethylammonium nitrate [EtNH₃][NO₃] (Walden, 1914) [47] [1]	12 °C [1]	High reactivity with moisture [1]
Second Generation	Air and water stable [46]	1-ethyl-3-methylimidazolium tetrafluoroborate [C₂mim][BF₄] (Wilkes & Zaworotko, 1992) [46]	< 100 °C	Initial assumptions of being "green" [48]
Third Generation	Task-specific, tunable biological activity [46]	Functionalized imidazolium-based ILs for metal extraction [46]	Variable	Designed functionality, potential API use [46]

The foundational research by Wilkes and Zaworotko in 1992 set the stage for the widespread application of second-generation ILs [46]. Their statement that the 1-ethyl-3-methylimidazolium cation was "an ideal candidate for general use" due to its moderate size, shape promoting cation stacking, and ability to engage only in weak hydrogen bonding, inadvertently directed early research efforts away from systematic toxicity assessments [47]. The focus was instead on their promising physicochemical properties for industrial applications. It was only when ILs began to be considered for biological applications, such as enzyme catalysis and drug delivery, that their cytotoxicity profiles became a critical area of investigation [49].

Cytotoxicity of Ionic Liquids: Mechanisms and Experimental Evidence

Fundamental Mechanisms of Cytotoxicity

The cytotoxicity mechanisms of ILs are multifaceted, with research indicating two primary pathways:

• Cellular Membrane Damage: The amphiphilic nature of many ILs allows them to penetrate and disrupt lipid bilayers, compromising membrane integrity and function [50] [48]. The cationic head groups of ILs interact strongly with the negatively charged phospholipid heads in cell membranes.
• Induction of Oxidative Stress: ILs can trigger the overproduction of reactive oxygen species (ROS), leading to oxidative damage of cellular components such as lipids, proteins, and DNA [49] [48]. This can subsequently initiate apoptosis (programmed cell death).

Experimental Data on Model Ionic Liquids

Substantial experimental evidence has demonstrated the dose-dependent cytotoxic effects of ILs on various human cell lines. Studies on two common early ILs—1-butyl-3-methylimidazolium bromide ([Bmim]Br) and 1-butylpyridinium bromide ([Bpy]Br)—highlight these concerns.

Table 2: Experimentally Determined IC₅₀ Values for [Bmim]Br and [Bpy]Br [49]

Ionic Liquid	MCF-7 (Breast Cancer) IC₅₀ (μmol/L)	HeLa (Cervical Cancer) IC₅₀ (μmol/L)	HEK293T (Embryonic Kidney) IC₅₀ (μmol/L)
[Bmim]Br	841.86	538.38	654.78
[Bpy]Br	341.74	333.27	328.98

The data shows that [Bpy]Br is consistently more toxic than [Bmim]Br across all tested cell lines [49]. Furthermore, non-cancerous HEK293T cells were also significantly affected, indicating that the toxicity is not selective and poses a general biosafety risk.

Key Experimental Protocols for Cytotoxicity Assessment

A typical experimental workflow for assessing IL cytotoxicity involves the following key methodologies [49]:

• Cell Culture: Human cell lines (e.g., MCF-7, HeLa, HEK293T) are cultured in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum at 37°C and 5% CO₂.
• IC₅₀ Determination (Real-Time Cell Analysis):
  • Cells are trypsinized and resuspended at a density of 1×10⁵ cells/mL.
  • An aliquot (300 μL) is transferred to each well of an E-Plate L8.
  • Cells are treated with ILs in a concentration range (e.g., 10–5000 μmol/L).
  • The impedance-based cell status is monitored every 30 minutes for 24 hours to determine the half-maximal inhibitory concentration (IC₅₀).
• Apoptosis Assay (Flow Cytometry with Annexin V/PI Staining):
  • Cells are treated with ILs at varying concentrations based on their IC₅₀ values.
  • After treatment, cells are harvested, washed, and resuspended in a binding buffer.
  • Cells are stained with Annexin V-FITC and propidium iodide (PI) for 15 minutes in the dark.
  • Flow cytometry analysis distinguishes live (Annexin V⁻/PI⁻), early apoptotic (Annexin V⁺/PI⁻), late apoptotic (Annexin V⁺/PI⁺), and necrotic (Annexin V⁻/PI⁺) cell populations.
• Cell Cycle Analysis (Flow Cytometry):
  • IL-treated cells are fixed, permeabilized, and stained with a DNA-binding dye like propidium iodide.
  • The DNA content of cells is analyzed by flow cytometry to determine the percentage of cells in each phase of the cell cycle (G0/G1, S, G2/M).

Figure 1: Key Cytotoxicity Pathways Induced by Ionic Liquids.

Structural Drivers of Toxicity and Environmental Impact

Structure-Activity Relationships (SARs)

A comprehensive analysis of cytotoxicity data reveals clear structure-activity relationships [50] [48] [24]:

• Cationic Head Group: Toxicity often follows the order: pyridinium > imidazolium > pyrrolidinium > ammonium. The aromaticity and charge delocalization of the head group influence binding to cellular targets.
• Alkyl Chain Length: A critical factor known as the "side chain effect." Cytotoxicity generally increases with the length of the alkyl side chain on the cation, typically peaking at a chain length of 6-8 carbon atoms. Longer chains enhance lipophilicity, facilitating better membrane integration and disruption [48].
• Anion Effect: While the cation often dominates the toxic profile, the anion can modulate toxicity. Common anions like [Br⁻], [BF₄⁻], and [PF₆⁻] can hydrolyze and release potentially harmful species (e.g., HF from [BF₄⁻]) [48].

Environmental Impact and Ecotoxicity

Despite their low volatility, ILs pose a significant risk to aquatic and terrestrial environments due to their high water solubility, mobility, and low biodegradability [48]. They have shown toxic effects on various organisms, including bacteria, crustaceans, and fish [49] [48]. The same structural features that drive cytotoxicity, particularly alkyl chain length, also govern their ecotoxicity. Upon release into the environment, ILs can undergo translocation and retention, and their lipophilicity influences their potential for bioaccumulation in organisms [48].

The Modern Toolkit: Predictive Modeling and Safer Design

The vast chemical space of ILs makes experimental testing of all variants impractical. Modern research leverages computational methods to predict toxicity and guide the design of safer ILs.

• Machine Learning (ML) Models: Quantitative Structure-Activity Relationship (QSAR) models use theoretical molecular descriptors to predict IL toxicity [24]. For example, random forest (RF) and multi-layer perceptron (MLP) models have been successfully developed to predict the toxicity of ILs towards Vibrio fischeri and leukemia rat cell lines (ICP-81) with high accuracy [24].
• Interpretability Analysis: Tools like SHAP (Shapley Additive exPlanations) are used to interpret ML models, quantifying the contribution of specific molecular features to the predicted toxicity. This provides actionable insights for chemists, highlighting which structural elements to avoid or modify to reduce toxicity [24].

Figure 2: Workflow for Machine Learning-Based Toxicity Prediction and Interpretation.

Table 3: The Scientist's Toolkit: Key Reagents and Materials for Cytotoxicity Research

Item	Function/Brief Explanation
Specific Ionic Liquids (e.g., [Bmim]Br, [Bpy]Br)	Model compounds for investigating structure-activity relationships and mechanisms of cytotoxicity [49].
Human Cell Lines (e.g., MCF-7, HeLa, HEK293T)	In vitro models for assessing cell-type-specific toxicity and therapeutic potential against cancer cells [49].
DMEM with Fetal Bovine Serum	Standard cell culture medium providing essential nutrients and growth factors for maintaining cell lines [49].
Real-Time Cell Analysis (RTCA) System	Label-free, impedance-based system for continuous monitoring of cell proliferation, morphology, and IC₅₀ determination [49].
Annexin V-FITC / Propidium Iodide (PI)	Fluorescent dyes used in flow cytometry to distinguish between live, early apoptotic, late apoptotic, and necrotic cell populations [49].
MTT Reagent (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)	Used in colorimetric assays to measure cell metabolic activity as a proxy for cell viability and proliferation [49].

The journey of understanding ionic liquids has evolved from an initial focus on their versatile physicochemical properties to a nuanced recognition of their cytotoxicity and environmental impact. Evidence shows that early ILs like [Bmim]Br and [Bpy]Br can induce significant dose-dependent cytotoxicity, apoptosis, and cell cycle arrest in human cells, with structural features such as the cationic head group and alkyl chain length being key determinants. The historical "green solvent" paradigm has been rightfully challenged, making way for a more responsible approach.

Future progress hinges on the integration of robust experimental toxicology with advanced predictive computational tools like machine learning. This combined strategy enables the in silico design of ILs with minimal hazardous properties before they are ever synthesized. As the field moves towards a third generation of task-specific ILs, this knowledge provides a foundational framework for designing innovative, effective, and sustainable ionic liquids that are truly green by design, ensuring their safety for both human health and the environment.

Ionic liquids (ILs) have emerged as transformative materials in pharmaceutical research, representing a paradigm shift from traditional organic solvents. Defined as organic salts with melting points below 100°C, ILs possess exceptional properties including negligible vapor pressure, high thermal stability, and tunable physicochemical characteristics [15]. Their evolution through three distinct generations reflects a growing emphasis on biocompatibility and sustainability, particularly for biomedical applications. The pharmaceutical industry faces persistent challenges with drug solubility, bioavailability, and formulation stability, with many drug candidates exhibiting poor aqueous solubility and polymorphic conversion issues [15]. Traditional organic solvents like dimethyl sulfoxide and ethanol present acute toxicity concerns, limiting their pharmaceutical applicability [15]. Third-generation ILs, specifically designed with biocompatible ions from natural sources, offer a promising alternative that addresses both formulation challenges and safety imperatives.

Historical Development: From Electrochemistry to Pharma

The development of ionic liquids spans more than a century, with multiple independent discoveries gradually converging into a unified field. The earliest documented ionic liquid, ethylammonium nitrate ([EtNH3][NO3]) with a melting point of 12°C, was reported by Paul Walden in 1914 [1]. This serendipitous discovery, also representing the first protic ionic liquid (PIL), went largely unexplored for decades. The modern era of IL research began in the 1950s when Hurley and Weir developed room-temperature molten salts from ethylpyridinium bromide and aluminum chloride for electroplating applications [1]. The 1980s witnessed critical advancements with the introduction of 1,3-dialkylimidazolium cations by John Wilkes' group, which dramatically improved room-temperature stability and transport properties [1]. This period also saw early investigations into the biological implications of ILs, including studies of enzyme activity in IL-water mixtures and the first use of ILs as stationary phases in gas chromatography [1]. The following diagram illustrates the key milestones in the historical development of ionic liquids:

The taxonomy of ionic liquids has evolved significantly through three generations, each with distinct characteristics and applications:

Table 1: Generational Evolution of Ionic Liquids

Generation	Time Period	Key Characteristics	Example Components	Primary Applications
First Generation	1914-1980s	Air and water sensitivity, unique physical properties	Imidazolium with BF₄⁻ or PF₆⁻ anions	Electrochemistry, synthesis
Second Generation	1990s-2000s	Air and water stability, tunable physicochemical properties	Diverse cation-anion combinations with improved stability	Catalysis, lubricants, materials science
Third Generation	2000s-Present	Biocompatibility, low toxicity, biodegradability	Choline, amino acids, fatty acids, natural products	Pharmaceuticals, drug delivery, biomedical applications

This historical progression demonstrates a clear trajectory toward increased biocompatibility and expanded pharmaceutical relevance, with third-generation ILs specifically engineered to address toxicity concerns while maintaining the advantageous properties of their predecessors.

The Biocompatibility Imperative: Rational Design Principles

Toxicity Challenges in Conventional Ionic Liquids

First-generation ILs, particularly those containing imidazolium cations with fluorinated anions, demonstrated significant toxicity and poor biodegradability [15]. Studies revealed that these ILs could cause aquatic toxicity and persist in the environment, creating barriers to their pharmaceutical application [15]. The fundamental challenge stems from the potential of ILs to interact with cellular membranes and disrupt essential biological processes. Research has established that the cationic alkyl chain length significantly influences toxicity, with longer chains generally associated with greater membrane disruption and cytotoxicity [23]. This structure-activity relationship necessitates careful design considerations for pharmaceutical applications.

Designing Biocompatible Ionic Liquids

Third-generation ILs employ biocompatible ions primarily derived from natural sources, addressing the toxicity limitations of earlier generations. The rational design of these ILs focuses on selecting cation and anion components with established safety profiles and biological compatibility:

Table 2: Biocompatible Ionic Liquid Components and Their Properties

Component Type	Example Compounds	Source/Biological Role	Key Properties	Pharmaceutical Advantages
Cations	Choline derivatives	Essential nutrient, precursor to acetylcholine	GRAS status, biodegradability	Low toxicity, membrane compatibility
Anions	Amino acids (glycine, proline, alanine)	Protein building blocks	Natural abundance, metabolic pathways	Biodegradability, chiral centers
Anions	Fatty acids (geranic acid, octanoic acid)	Natural lipids	Surfactant properties, metabolic pathways	Enhanced permeation, self-assembly
Anions	Carboxylic acids (salicylic acid, glutaric acid)	Metabolic intermediates	Hydrogen bonding capacity	Solubilization, crystallinity control
Anions	Non-nutritive sweeteners	Food additives	GRAS status, sweet taste	Patient compliance, masking bitterness

The United States Food and Drug Administration (FDA) has recognized choline as "generally regarded as safe" (GRAS), establishing a foundation for its use in pharmaceutical applications [15]. Choline serves as a precursor to the neurotransmitter acetylcholine and is an integral component of phospholipids in cell membranes [15]. Similarly, amino acid-based ILs leverage the natural abundance, metabolic pathways, and chiral nature of amino acids to create biodegradable, low-toxicity solvents [15].

The following diagram illustrates the strategic approach to designing biocompatible ionic liquids:

Structure-Property Relationships: Quantitative Insights

Cationic Alkyl Chain Length and Cytotoxicity

Recent systematic studies have elucidated the critical relationship between IL structure and biological compatibility. A comprehensive investigation evaluating 61 structurally diverse ILs revealed that cationic alkyl chain length is the primary determinant of cytotoxicity, overshadowing the effects of cationic head groups or anions [23]. The study demonstrated that ILs with shorter cationic alkyl chains (C1-C4) exhibited minimal cytotoxicity, while those with longer chains (C8+) showed dramatically increased toxicity across multiple cell lines [23].

Table 3: Cytotoxicity of Imidazolium-Based ILs by Alkyl Chain Length

Ionic Liquid	Cationic Alkyl Chain Length	Cell Viability (bEnd.3 cells) at 400 μM	Cell Viability (HepG2 cells) at 400 μM	Cell Viability (4T1 cells) at 400 μM
C1MIMCl	Methyl (C1)	>90%	>90%	>90%
C3MIMCl	Propyl (C3)	>85%	>85%	>85%
C8MIMCl	Octyl (C8)	~40%	~35%	~30%
C12MIMCl	Dodecyl (C12)	<10%	<5%	<5%
C16MIMCl	Hexadecyl (C16)	<5%	<5%	<5%

This structure-toxicity relationship was consistent across two-dimensional cell cultures, three-dimensional cell spheroids, and patient-derived organoids, confirming the fundamental nature of this relationship [23]. The mechanistic basis for this correlation involves the formation of IL nanoaggregates in aqueous environments, with longer alkyl chains facilitating greater membrane disruption and intracellular damage [23].

Biocompatible ILs in Drug Solubilization and Delivery

Biocompatible ILs have demonstrated remarkable capabilities in enhancing the solubility and bioavailability of poorly soluble drugs. Choline-based ILs have been particularly effective, with studies showing significant improvements in drug solubility compared to conventional solvents:

Table 4: Drug Solubilization Efficacy of Biocompatible Ionic Liquids

Ionic Liquid	Drug Compound	Solubility Enhancement	Key Findings	Reference
Choline-geranate	Megestrol acetate	~5-fold increase vs commercial tablet	Enhanced oral bioavailability in canine models	[23]
Choline-amino acid ILs	Sparingly soluble drugs	2-10 fold increase vs water	Improved transdermal delivery, reduced polymorphism	[15]
Benzyl-functionalized imidazolium ILs	N/A (intrinsic activity)	IC₅₀: 3.99-5.20 μM vs MCF-7 cells	Superior to tamoxifen (IC₅₀: 15.41 μM), high selectivity	[51]
Choline-organic acid ILs	Small and large molecules	Varied by compound	Enhanced transdermal delivery, maintained stability	[15]

The application of choline and geranic acid-based ILs as tumor ablation agents has demonstrated improved killing effects when combined with doxorubicin, highlighting the potential of ILs in cancer therapy [23]. Similarly, benzyl-functionalized imidazolium ILs have shown promising anticancer activity against human breast cancer cells (MCF-7), with IC₅₀ values superior to tamoxifen and high selectivity indices [51].

Experimental Protocols: Assessing Biocompatibility and Efficacy

Cytotoxicity Assessment Protocol

Comprehensive evaluation of IL biocompatibility requires a multi-faceted approach employing both in vitro and in vivo models. The following protocol represents current best practices for assessing IL cytotoxicity:

• Cell Line Selection: Utilize a panel of cell lines representing different tissues, including:

  • Mouse brain endothelial (bEnd.3) cells for blood-brain barrier models
  • Human hepatocellular carcinoma (HepG2) for hepatic metabolism assessment
  • Cancer cell lines relevant to intended application (e.g., MCF-7 for breast cancer)
  • Primary cells or stem cell-derived models when possible [23]

• Viability Assays:

  • Cell Counting Kit-8 (CCK-8): Incubate cells with ILs at gradient concentrations (25, 100, 400, and 1600 μM) for 24 hours. Add CCK-8 solution and measure absorbance at 450 nm after 2-4 hours [23].
  • Live/Dead Staining: Use calcein-AM (for live cells) and propidium iodide (for dead cells) following manufacturer protocols. Quantify using fluorescence microscopy or flow cytometry [23].
  • 3D Spheroid Models: Culture cells in low-attachment plates to form spheroids. Treat with ILs and assess viability using ATP-based assays or live/dead staining [23].

• Mechanistic Studies:

  • Apoptosis Assessment: Analyze caspase activation using fluorogenic substrates or Western blotting. Measure phosphatidylserine externalization via annexin V staining [51].
  • Mitochondrial Function: Evaluate mitochondrial membrane potential using JC-1 or TMRM dyes. Assess mitophagy via LC3-I/II conversion and p62 degradation [23].
  • Reactive Oxygen Species (ROS): Detect intracellular ROS using DCFH-DA or CellROX reagents [51].

Synthesis of Choline-Based Biocompatible ILs

The synthesis of choline-based ILs typically follows a straightforward neutralization reaction:

• Materials:

  • Choline hydroxide or choline bicarbonate solution
  • Desired acid (amino acids, fatty acids, carboxylic acids)
  • Organic solvents (methanol, ethanol) for purification
  • Filtration apparatus and high vacuum system

• Procedure:

  • Dissolve slightly more than an equimolar amount of the desired acid in minimal water or alcohol.
  • Slowly add choline hydroxide or choline bicarbonate solution with stirring at room temperature.
  • Continue stirring for 12-24 hours at room temperature or elevated temperature (up to 40°C).
  • Remove solvents under reduced pressure using a rotary evaporator.
  • Purify the resulting IL by dissolving in an organic solvent, filtering to remove excess acid, and drying under high vacuum.
  • Characterize the final product using NMR, mass spectrometry, and elemental analysis [15].

• Quality Control:

  • Determine water content using Karl Fischer titration.
  • Assess purity through chromatographic methods (HPLC, GC).
  • Confirm chemical structure via spectroscopic methods (FTIR, NMR).

The following diagram illustrates the experimental workflow for evaluating ionic liquid biocompatibility and mechanisms:

The Scientist's Toolkit: Essential Research Reagents

Successful investigation of biocompatible ILs requires specific reagents and materials carefully selected for their relevance to pharmaceutical applications:

Table 5: Essential Research Reagents for Biocompatible Ionic Liquid Studies

Reagent Category	Specific Examples	Function/Application	Key Considerations
Biocompatible Cations	Choline derivatives, amino acid-based cations	IL core structure providing biocompatibility	Purity, moisture content, storage conditions
Biocompatible Anions	Amino acids, fatty acids, carboxylic acids	IL counterions determining physicochemical properties	pKa, hydrogen bonding capacity, metabolic fate
Cell Culture Models	bEnd.3, HepG2, MCF-7, primary cells, stem cells	Cytotoxicity assessment, mechanism studies	Passage number, culture conditions, authentication
Viability Assays	CCK-8, MTT, resazurin, ATP-based assays	Quantification of cell viability and proliferation	Linearity range, interference with test compounds
Apoptosis Detection	Annexin V, caspase substrates/assays, TUNEL	Mechanism of cell death	Timing of assessment, positive controls
Mitochondrial Probes	JC-1, TMRM, MitoTracker, ROS indicators	Mitochondrial function and oxidative stress	Concentration optimization, loading conditions
Animal Models	Murine models, canine models	In vivo tolerance, biodistribution studies	Ethical approvals, species differences, administration routes
Analytical Instruments	Cryo-TEM, NMR, HPLC, mass spectrometry	IL characterization, purity assessment, quantification	Method validation, sensitivity, resolution

Mechanisms of Action: From Molecular Structure to Biological Response

Nanoaggregate Formation and Cellular Interactions

Third-generation ILs exhibit unique mechanisms of biological interaction that distinguish them from conventional solvents. A critical discovery is that ILs form nanoaggregates in aqueous environments rather than existing as discrete molecules [23]. Cryogenic transmission electron microscopy (Cryo-TEM) studies have revealed that ILs with short cationic alkyl chains (scILs) form nanoaggregates of approximately 5 nm, while those with long cationic alkyl chains (lcILs) form larger aggregates of approximately 12.5 nm [23]. Molecular dynamics simulations confirm that amphiphilicity drives nanoaggregate formation, with cationic alkyl chains embedded inside cationic heads paired with anions [23].

These nanoaggregates follow distinct intracellular trafficking pathways:

• scILs are primarily restricted within intracellular vesicles, limiting their interaction with critical organelles [23].
• lcILs accumulate in mitochondria, inducing mitophagy and apoptosis through direct membrane interactions [23].

The differential biological effects of scILs versus lcILs have been confirmed in vivo, with scILs exhibiting 30-80 times greater tolerance than lcILs across various administration routes (oral, intramuscular, intravenous) [23].

Pharmaceutical Performance Enhancement Mechanisms

Biocompatible ILs improve drug formulation and delivery through multiple mechanisms:

• Polymorphism Control: ILs can inhibit or direct drug crystallization, preventing the formation of less soluble polymorphs and improving consistency between production batches [15].
• Solubilization Enhancement: The dual nature of ILs, containing both hydrophilic and hydrophobic regions, enables solubilization of diverse drug molecules regardless of their inherent polarity [15] [52].
• Permeation Enhancement: Choline-based ILs have demonstrated particular efficacy in enhancing transdermal and intestinal permeation of both small and large molecules [15].
• Stabilization: ILs can protect drugs from enzymatic and non-enzymatic degradation, extending shelf-life and improving in vivo performance [15].

The development of third-generation ionic liquids represents a significant advancement in pharmaceutical technology, addressing the critical challenge of biocompatibility while maintaining the exceptional solvent properties of ILs. The systematic design of ILs using GRAS components like choline and amino acids provides a foundation for creating truly biocompatible drug formulation platforms. Current research demonstrates that these materials can significantly enhance drug solubility, formulation stability, and delivery efficiency while mitigating the toxicity concerns associated with earlier IL generations and traditional organic solvents.

Future developments will likely focus on expanding the library of biocompatible ions, optimizing IL structures for specific pharmaceutical applications, and advancing our understanding of IL interactions with biological systems at the molecular level. The integration of computational modeling, high-throughput screening, and sophisticated characterization techniques will accelerate the rational design of next-generation ILs tailored to specific therapeutic needs. As regulatory frameworks adapt to these novel materials, biocompatible ILs hold exceptional promise for addressing persistent challenges in drug development and delivery, potentially enabling the formulation of previously undeliverable therapeutic compounds.

The evolution of ionic liquids (ILs) spans multiple generations, transitioning from first-generation chloraluminate salts studied primarily for their electrochemical properties to sophisticated fourth-generation ILs designed for enhanced sustainability and multifunctionality [7]. Throughout this development, a persistent challenge has hindered their transition from laboratory curiosities to mainstream industrial application: the economic and operational feasibility of solvent recovery and recyclability at scale. While their unique properties—negligible vapor pressure, high thermal stability, and structural tunability—initially positioned them as ideal "green" replacements for volatile organic compounds, their implementation in continuous industrial processes demands robust recovery systems to justify substantial initial investments [53]. This technical analysis examines the specific hurdles and emerging solutions within this critical aspect of ionic liquid technology, providing a framework for researchers and process engineers navigating scale-up challenges.

Economic Hurdles in Ionic Liquid Scale-Up

The economic viability of ionic liquid processes is predominantly governed by high initial costs and the efficiency of recycling operations. A comprehensive understanding of these factors is essential for realistic techno-economic analysis and process planning.

Production and Recycling Cost Analysis

The synthesis of high-purity ionic liquids remains a significant cost driver. Industrial-grade ILs typically command prices between $50-200 per kilogram, and can exceed $500 per kilogram for specialized formulations, compared to conventional solvents at $2-10 per kilogram [53] [20]. This differential necessitates high recovery rates to achieve economic sustainability. Table 1 summarizes the key economic factors impacting IL process viability.

Table 1: Economic Factors in Ionic Liquid Process Scale-Up

Factor	Impact Level	Description	Geographic Relevance	Impact Timeline
Manufacturing Cost Differential	High (-1.3% CAGR Impact) [20]	Unit costs of ILs eclipse $500/kg vs $5/kg for conventional organics [20].	Global, with higher impact in price-sensitive markets	Medium term (2-4 years)
Recycling Process Efficiency	Critical	Inefficient recovery directly impacts operational expenses; >95% recovery rates are often essential for viability [54] [53].	Global	Immediate
Limited Eco-toxicity Data	Medium (-0.9% CAGR Impact) [20]	Scarcity of standardized datasets slows regulatory approvals (e.g., REACH) and time-to-market [20].	Europe, with global spillover effects	Short term (≤ 2 years)
Feedstock Volatility	Medium (-0.7% CAGR Impact) [20]	Fluorinated anions (e.g., BF₄⁻, PF₆⁻) depend on HF supply chains, causing cost fluctuations [20].	Global	Medium term (2-4 years)

Lifecycle and Techno-Economic Assessments

Lifecycle assessment (LCA) and techno-economic analysis (TEA) frameworks are increasingly critical for evaluating the true cost and environmental impact of IL-based processes. While ILs are often marketed as "green" solvents, comprehensive LCAs reveal that their environmental credentials are heavily dependent on efficient recycling and regeneration. For instance, in lignocellulosic biomass pretreatment, LCA and TEA studies indicate that ILs can have a higher eco-toxicity impact than conventional solvents unless efficient recovery and recycling strategies are implemented [26]. The energy-intensive nature of IL production means that the cumulative energy demand (CED) and global warming potential (GWP) of a process are highly sensitive to the number of reuse cycles. Techno-economic models show that recycling and reuse strategies must account for the initial investment in recovery equipment, energy consumption during recycling, and the value of recovered ionic liquids to determine true commercial feasibility [53].

Operational Hurdles in Recovery and Recycling

Translating laboratory-scale recovery techniques to continuous industrial operations presents distinct challenges related to process stability, material compatibility, and purity management.

Technical Barriers in Industrial Implementation

The path to industrial-scale IL recovery is fraught with technical obstacles that impact both efficiency and cost.

• High Viscosity and Mass Transfer Limitations: The elevated viscosity of many ILs negatively impacts pumping efficiency, mixing, and mass transfer rates in large-scale reactors and separation units. This often necessitates specialized equipment design, higher energy inputs for agitation, and elevated operating temperatures to reduce viscosity, which can in turn compromise IL stability [54] [53].
• Thermal and Chemical Stability in Continuous Operation: Although ILs are celebrated for their thermal stability, prolonged exposure to industrial processing temperatures can induce decomposition. This is particularly relevant in processes like distillation or membrane filtration, where thermal and mechanical stress can lead to the formation of degradation products that contaminate both the IL and the product stream, altering solvent properties and reducing process efficiency [54].
• Material Compatibility: Many conventional industrial construction materials, such as certain grades of stainless steel, exhibit degradation when exposed to ionic liquids over extended periods, especially those with halogenated anions. This necessitates the use of expensive corrosion-resistant alloys or specialized coatings, significantly increasing capital expenditure and maintenance costs [53].
• Purity Maintenance and Impurity Accumulation: With each recovery cycle, impurities—including water, residual substrates, decomposition products, and extracted impurities from biomass or other feedstocks—can accumulate in the IL phase. For example, in biomass pretreatment, lignin residues, sugars, and proteins can foul the IL [26]. In battery metal recycling, residual metal ions can alter extraction kinetics [54] [55]. This accumulation gradually diminishes IL performance and necessitates costly purification steps or eventual disposal.

Recovery Methodologies and Experimental Protocols

Several core methodologies are employed for IL recovery, each with specific operational considerations and scalability challenges.

Antisolvent Precipitation

This is a common method, particularly in biomass processing where ILs are used to dissolve cellulose or lignin.

• Detailed Protocol: The IL-rich process stream is mixed with a counter-solvent (antisolvent) such as water, ethanol, or acetone in a controlled manner. Common volume ratios of antisolvent to IL mixture range from 2:1 to 10:1. The mixture is agitated at a controlled temperature (typically 25-70°C) for a predetermined time (e.g., 30-120 minutes) to ensure complete precipitation of the dissolved solute. The solid precipitate is then separated via filtration or centrifugation. The IL remains in the antisolvent phase, requiring subsequent separation, typically via thermal distillation or membrane processes [26].
• Scale-up Challenges: High volumes of antisolvents create significant downstream separation and recovery demands. Solvent loss and energy-intensive distillation for antisolvent recycling contribute substantially to operational costs. Ensuring consistent mixing and precipitation kinetics in large-scale vessels is also non-trivial [26] [53].

Membrane Separation

This technique leverages pressure-driven processes for IL recovery from aqueous streams.

• Detailed Protocol: The IL-containing solution is fed under pressure into a membrane system, typically employing nanofiltration (NF) or ultrafiltration (UF) membranes with molecular weight cut-offs (MWCO) tailored to the IL's size. For example, a study might use a ceramic NF membrane with a 450 Da MWCO. The process is conducted in a cross-flow filtration unit, where temperature, transmembrane pressure (e.g., 10-30 bar), and flow rate are optimized to maximize IL rejection and permeate flux. The IL-rich retentate is recycled, while the permeate, containing water and smaller molecules, is discarded or further treated [26].
• Scale-up Challenges: Membrane fouling by impurities significantly reduces flux and increases downtime for cleaning. The high viscosity of concentrated IL solutions can limit permeate flux, requiring careful management of operating parameters. The capital cost of large-scale membrane systems and membrane replacement frequency are also key economic considerations [26].

Distillation and Evaporation

Used for separating ILs from volatile solvents or for concentrating IL streams.

• Detailed Protocol: The solution is fed into a thin-film evaporator or a falling-film evaporator, which provides a large surface area for heat transfer. Due to the low volatility of ILs, the process is operated under vacuum (e.g., 0.01-0.1 bar) and elevated temperatures (often 80-150°C, depending on the IL's thermal stability). The volatile component is vaporized, condensed, and collected, while the concentrated IL is collected from the bottom of the evaporator [26] [53].
• Scale-up Challenges: The high thermal stability of ILs is advantageous, but prolonged exposure to elevated temperatures can still lead to decomposition. The energy consumption for vaporization is substantial, making this one of the most energy-intensive recovery steps. Designing heat exchange systems that can handle the high viscosity and unique thermal behavior of ILs requires specialized engineering [53].

Liquid-Liquid Extraction and Stripping

This is particularly relevant in hydrometallurgical applications, such as the recovery of metals from spent lithium-ion battery cathodes, and for reclaiming ILs from aqueous phases.

• Detailed Protocol: The loaded IL phase is brought into contact with a stripping agent. For metal recovery, this could be an acidic solution (e.g., dilute HCl or H2SO4) or pure water. The two-phase system is vigorously mixed in a stirred tank for a set period (e.g., 30-60 minutes) at a controlled phase ratio (e.g., O/A = 1:1 to 1:3) and temperature. The metals transfer into the aqueous stripping phase, which is then separated. The regenerated IL can be recycled back to the extraction stage [54] [55]. Stripping efficiency is highly dependent on the IL and the extracted metal; for instance, Co(II) stripping efficiency with water can vary dramatically based on the IL's anion [54].
• Scale-up Challenges: Formation of stable emulsions can complicate phase disengagement. The process generates secondary waste streams (e.g., spent stripping solutions) that require treatment. Inefficient stripping leads to a gradual buildup of contaminants in the IL, reducing its extraction capacity over multiple cycles [54] [55].

Table 2: Comparison of Key Ionic Liquid Recovery Methods

Method	Typical Applications	Key Operational Parameters	Energy Intensity	Key Scale-up Challenges
Antisolvent Precipitation	Biomass processing, Pharmaceutical crystallization [26]	Antisolvent ratio (2:1 to 10:1), Temperature (25-70°C) [26]	Medium-High (due to antisolvent recycling)	Solvent recovery & loss, Waste stream volume [26]
Membrane Separation	Aqueous stream concentration, Purification [26]	Transmembrane Pressure (10-30 bar), MWCO, Cross-flow velocity [26]	Low-Medium	Membrane fouling, Viscosity-limited flux [26]
Distillation/Evaporation	Solvent removal, IL concentration [26] [53]	Temperature (80-150°C), Pressure (0.01-0.1 bar) [53]	High	Thermal degradation, High energy demand [53]
Liquid-Liquid Extraction/Stripping	Hydrometallurgy (e.g., Battery recycling) [54] [55]	Strippant concentration, Phase ratio, Mixing time [54]	Low-Medium	Emulsion formation, Secondary waste [54]

The Scientist's Toolkit: Key Research Reagent Solutions

Successful research into IL recovery relies on a suite of specialized reagents and materials. Table 3 details essential components for developing and optimizing recovery protocols.

Table 3: Essential Research Reagents for Ionic Liquid Recovery Studies

Reagent/Material	Function in Recovery Research	Typical Examples & Notes
Antisolvents	Induces solute precipitation; dilutes IL for further processing.	Deionized Water, Ethanol, Acetone, Ethyl Acetate. High purity is critical to avoid IL contamination [26].
Stripping Agents	Liberates target solutes (e.g., metals) from the IL phase.	Mineral acids (HCl, H₂SO₄), Organic acids, Pure water. Selection depends on the IL-solute complex strength [54] [55].
Nanofiltration/Ultrafiltration Membranes	Separates ILs from smaller molecules/solvents based on size & charge.	Ceramic (Al₂O₃, TiO₂) or polymeric membranes. MWCO selection (200-1000 Da) is vital for high IL rejection rates [26].
Stationary Phases for Chromatography	Analyzes IL purity and identifies degradation products post-recovery.	C18 columns, Ion-exchange resins. Used for HPLC and LC-MS to monitor IL integrity and impurity profile [53].
Corrosion-Resistant Materials	For constructing reactors and piping to withstand ILs at high T.	Hastelloy, Tantalum, Teflon-lined systems. Essential for long-term stability studies under process conditions [53].

Visualization of Recovery Processes and Challenges

The following workflow diagram synthesizes the major recovery pathways and the critical decision points that influence the efficiency and economic outcome of ionic liquid recycling.

IL Recovery Process and Key Hurdles

The scale-up of ionic liquid technologies is inextricably linked to solving the dual challenges of economic viability and operational reliability in solvent recovery and recycling. While significant hurdles remain, the trajectory of research points toward increasingly sophisticated solutions. The future of IL recycling lies in the development of integrated, multi-technique approaches—such as combining membrane pre-concentration with low-energy distillation, or designing task-specific ILs with built-in triggers for easier separation (e.g., thermomorphic or pH-switchable systems). Furthermore, the application of AI and machine learning for predicting IL degradation pathways and optimizing recovery cycles holds promise for accelerating the development of closed-loop, economically sustainable IL processes [56] [53] [20]. As these innovations mature, they will solidify the role of ionic liquids as not merely green solvents, but as robust and recyclable platforms for next-generation chemical and pharmaceutical manufacturing.

Corrosion, defined as the destructive electrochemical dissolution of metals, poses a significant problem impacting numerous industry sectors, leading to economic losses, technical performance deterioration, and environmental damage [57]. The global economic impact is staggering, with estimates suggesting corrosion is responsible for businesses losing approximately $2.5 trillion annually [58]. Traditional corrosion protection methods have relied on techniques such as protective coatings, cathodic protection, alloying, and chemical inhibitors [59] [60]. However, the emergence of ionic liquids (ILs) as versatile, tunable solvents has introduced a transformative approach to corrosion control. This technical guide explores the intersection of material compatibility and corrosion mitigation, framed within the groundbreaking context of ionic liquids research.

Ionic liquids, organic salts that are liquid below 100°C, have evolved through three distinct generations. From initial air- and water-sensitive formulations to today's advanced task-specific and bioactive ILs, their development has revolutionized multiple fields, including corrosion science [9]. Their unique properties—including negligible volatility, high thermal stability, ionic conductivity, and wide electrochemical windows—make them ideal for engineering protective interphases on reactive metal surfaces [61]. This review provides an in-depth examination of corrosion mechanisms, the role of ionic liquids in corrosion control, experimental methodologies for assessment, and advanced mitigation strategies, serving as a comprehensive resource for researchers and engineers tackling material degradation challenges.

Fundamentals of Metallic Corrosion

Electrochemical Mechanisms

At its core, metallic corrosion is an electrochemical process involving oxidation and reduction reactions. Each metal structure functions as an electrochemical cell with anodic and cathodic sites [60]. At the anode, metal oxidation occurs, releasing electrons and ferrous ions (e.g., Fe → Fe²⁺ + 2e⁻). These electrons travel through the metal to the cathode, where they combine with water and oxygen to produce hydroxyl ions (e.g., O₂ + 2H₂O + 4e⁻ → 4OH⁻). The resulting ferrous and hydroxyl ions then interact to form ferrous hydroxide, which subsequently reacts with oxygen to produce hydrated ferric oxide, commonly known as rust [60].

Forms of Corrosion

• Uniform Corrosion: Generalized material degradation across a surface.
• Localized Corrosion: Includes pitting and crevice corrosion, often more dangerous due to its concentrated nature.
• Galvanic Corrosion: Occurs when two dissimilar metals interact in the presence of an electrolyte, where the less noble metal corrodes preferentially [59].
• Stress Corrosion Cracking: Results from combined tensile stress and corrosive environment.

Ionic Liquids: Historical Development and Properties

Evolution of Ionic Liquids

The history of ionic liquids dates to 1914 when Paul Walden first reported a room-temperature ionic liquid, ethylammonium nitrate [9]. However, significant interest emerged in 1992 with the report by Wilkes and Zaworotko of the first air- and water-stable imidazolium-based ILs [9]. This breakthrough ignited widespread research into their application across diverse fields, including catalysis, electrochemistry, and materials science.

The three generations of ionic liquids illustrate their evolving applications [9]:

• First-Generation ILs: Focused on low melting points and adjustable physical properties but exhibited sensitivity to air and water.
• Second-Generation ILs: Featured enhanced air and water stability with highly tunable physical and chemical properties.
• Third-Generation ILs (Bio-ILs): Designed with a emphasis on low toxicity, biodegradability, and biocompatibility, making them suitable for pharmaceutical and biomedical applications.

Classification and Key Properties

Ionic liquids are characterized by their modular structure consisting of organic cations and inorganic/organic anions. This structural versatility enables precise tuning of properties for specific applications, including corrosion control [17]. Key properties relevant to corrosion science include:

• Low volatility: Reduces evaporative losses and environmental contamination
• High thermal stability: Enables use in high-temperature industrial processes
• Ionic conductivity: Facilitates electrochemical applications
• Wide electrochemical windows: Allows operation under extreme potentials
• Solvation capability: Can dissolve diverse organic and inorganic compounds

Table 1: Common Ionic Liquid Cations and Anions in Corrosion Science

Cations	Anions	Key Characteristics	Industrial Applications
Imidazolium	Bis(trifluoromethylsulfonyl)imide (TFSI)	High thermal stability, good conductivity	Corrosion inhibitors, electrochemical devices
Pyrrolidinium	Bis(fluorosulfonyl)imide (FSI)	Enhanced electrochemical stability	Battery electrolytes, metal protection
Phosphonium	Organophosphates	Excellent surface activity, protective films	Mg and Al alloy protection
Cholinium	Carboxylates	Low toxicity, biocompatibility	Green corrosion inhibitors

Ionic Liquids in Corrosion Control: Mechanisms and Applications

Protection Mechanisms

Ionic liquids control corrosion through multiple sophisticated mechanisms, primarily by forming protective films on metal surfaces. The specific protection strategy depends on the IL composition and application method:

Solid-Electrolyte Interphase (SEI) Formation Research has demonstrated that ILs containing fluorinated anions (e.g., TFSI, FSI) facilitate the creation of passivating SEI layers on reactive metals. These layers, typically composed of metal fluorides (e.g., LiF, MgF₂) and breakdown products of the IL anions, effectively passivate the metal surface while permitting ion transport [61]. For instance, studies have shown that the SEI formed on lithium metal in C3mPyrFSI ionic liquid contains LiF, LiOH, Li₂S, LiSO₂F, and NSO⁻ species, which collectively provide excellent passivation [61].

Corrosion Inhibition Imidazolium-based ionic liquids (IBILs) have garnered significant attention as promising corrosion inhibitors for stainless steels in various corrosive environments [57]. These ILs function through adsorption onto metal surfaces, forming a protective layer that impedes corrosive species access. Their effectiveness stems from their high thermal stability, ionic conductivity, and low volatility [57]. The adsorption process is influenced by factors including the length of alkyl chains attached to the imidazolium ring, the nature of the anion, and the specific metal substrate.

Material-Specific Applications

Stainless Steels IBILs have demonstrated exceptional effectiveness as corrosion inhibitors for stainless steels across diverse corrosive media. The protection efficiency depends on the specific IL structure and environmental conditions [57].

Aluminum and Magnesium Alloys Ionic liquids containing organophosphate anions have proven particularly effective for forming nanoscale protective films on magnesium and aluminum alloys. These films, often less than 100 nm thick, provide substantial corrosion resistance in chloride-containing environments [61]. Research indicates that phosphate-based ILs can offer superior protection compared to fluoride-forming ILs for these lightweight structural metals.

Copper and Brass The corrosion behavior of copper and brass in various ILs has been systematically investigated, revealing significant variations in compatibility depending on the specific IL chemistry [62].

Table 2: Ionic Liquid Applications for Specific Metal Protection

Metal/Alloy	Effective IL Classes	Protective Mechanism	Protection Efficiency
Stainless Steels	Imidazolium-based ILs	Adsorption inhibition, film formation	High (varies with substituents)
Magnesium Alloys	Organophosphate ILs, TFSI-based ILs	Salt layer formation (Mg₃(PO₄)₂, MgF₂)	Up to 100-fold reduction in corrosion [61]
Aluminum Alloys	Carboxylate ILs, Organophosphate ILs	Passivating salt deposition	Significant corrosion reduction
Lithium Metal	Pyrrolidinium FSI/TFSI	SEI formation (LiF, Li₂S, etc.)	>99% cycling efficiency [61]
Carbon Steel	Choline-based ILs, Carboxylate ILs	Surface adsorption, barrier layer	Good to excellent

Experimental Methodologies for Corrosion Assessment

Standard Testing Protocols

Electrochemical Testing Electrochemical techniques represent the cornerstone of corrosion assessment, providing quantitative data on corrosion rates and mechanisms:

• Potentiodynamic Polarization: Measures current response to controlled potential changes, generating Tafel plots for corrosion rate calculation.
• Electrochemical Impedance Spectroscopy (EIS): Applies small-amplitude AC signals across a frequency range to characterize interface properties and protective layer effectiveness.
• Open Circuit Potential Monitoring: Tracks the corrosion potential over time to assess material stability.

Accelerated Life Testing Recent advances involve exposing energized complete drive units or components to mixed-gas environments containing common industrial pollutants (chlorine, nitrogen dioxide, sulfur dioxide, hydrogen sulfide) at elevated concentrations, temperature, and humidity [58]. This approach provides realistic assessment of real-world performance under accelerated conditions.

Environmental Classification Standards

The revised IEC 60721-3-3:2019 standard classifies environmental corrosiveness by measuring the thickness of metal corrosion on test surfaces over 30 days, categorized by G-classifications (G1 to GX) [58]:

• G1 (Mild): <300 Å/30 days
• G2 (Moderate): 300-1000 Å/30 days
• G3 (Harsh): 1000-2000 Å/30 days
• GX (Severe): >2000 Å/30 days

This performance-based classification provides more practical guidance for material selection than previous chemical-based standards.

Diagram 1: Corrosion testing workflow for material compatibility assessment.

Mitigation Strategies and Technical Protocols

Integrated Corrosion Protection Framework

Effective corrosion mitigation requires a systematic approach combining material selection, environmental control, and protective technologies:

Material Selection and Design

• Utilize corrosion-resistant alloys (e.g., nickel aluminum, 310 stainless steel) where economically feasible [63]
• Consider galvanic compatibility when joining dissimilar metals
• Implement design strategies that minimize crevices and stagnant areas

Environmental Control

• Maintain climate-controlled environments where possible
• Implement proper air filtration (MERV 8 for recirculated air; MERV 11-13 for outdoor intake) [58]
• Control relative humidity to prevent condensation
• Remove airborne contaminants through adequate ventilation

Surface Engineering and Coatings

• Apply conformal coatings on PCBs and electronic components
• Utilize thermal spray overlays with corrosion-resistant materials [63]
• Implement high-IP enclosures (e.g., IP55/UL Type 12) with proper gasketing [58]
• Employ ionic liquid-based surface treatments for enhanced protection

Ionic Liquid Application Protocol

Based on published research, the following technical protocol describes the application of ionic liquids for corrosion protection:

Surface Pretreatment

• Mechanically polish the metal surface with successively finer abrasives (e.g., 400-1200 grit SiC paper)
• Ultrasonically clean in acetone and ethanol for 10 minutes each
• Dry under nitrogen stream
• (Optional) Acid pickling in 10% HNO₃ for 2 minutes for stainless steels

IL Application and Film Formation

• Prepare ionic liquid solution (neat or 1-5% v/v in volatile solvent such as ethanol)
• Apply to pretreated surface via:
  • Spin coating (3000 rpm, 30 seconds)
  • Dip coating (withdrawal rate 2 mm/s)
  • Spray coating (10-15 psi N₂ pressure)
• For chemical film formation:
  • Immerse specimen in IL solution at 25-60°C for 2-24 hours
  • Maintain agitation if necessary
• Rinse gently with ethanol to remove excess IL
• Cure at 60-100°C for 1-2 hours

Quality Assessment

• Visual inspection for uniform coverage
• Electrochemical testing to verify protection efficiency
• Surface analysis (SEM/EDS, XPS) for film characterization

Diagram 2: Ionic liquid corrosion protection mechanism.

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Research Reagents for Ionic Liquid Corrosion Studies

Reagent/Material	Function/Application	Key Characteristics	Example References
Imidazolium-based ILs (e.g., [BMIM][TFSI])	Corrosion inhibition studies	High thermal stability, tunable hydrophobicity	[57]
Pyrrolidinium FSI/TFSI ILs	SEI formation on reactive metals	Electrochemical stability, fluoride formation	[61]
Organophosphate ILs	Mg and Al alloy protection	Nanoscale protective films	[61]
Choline-based ILs	Green corrosion inhibitors	Low toxicity, biocompatibility	[9]
Carbon steel specimens	Fundamental corrosion studies	Standardized composition, industrial relevance	[62]
Stainless steel alloys	Industrial application studies	Varying corrosion resistance grades	[57]
Electrochemical workstation (potentiostat)	Corrosion rate quantification	Three-electrode cell configuration	[62]
Surface analysis (XPS, SEM-EDS)	Protective film characterization	Elemental composition, morphology	[61]

The field of ionic liquid-assisted corrosion protection continues to evolve rapidly, with several promising research directions emerging. The convergence of IL technology with artificial intelligence, nanomedicine, and additive manufacturing presents unprecedented opportunities for developing personalized therapeutic platforms and advanced protective systems [17]. Future research priorities include:

• Development of standardized testing protocols specifically for IL-based corrosion protection
• Investigation of long-term stability and degradation pathways of IL protective layers
• Exploration of synergistic effects between ILs and traditional corrosion inhibitors
• Advancement of computational methods for predicting IL-metal interactions
• Optimization of application techniques for industrial-scale implementation

In conclusion, ionic liquids represent a paradigm shift in corrosion control strategies, offering tunable, effective protection across diverse industrial applications. Their unique properties and flexible design enable tailored solutions for specific material compatibility challenges. As research progresses, ionic liquids are poised to play an increasingly vital role in extending equipment lifespan, reducing maintenance costs, and improving operational safety across industrial sectors. The integration of fundamental corrosion science with innovative ionic liquid technology promises to deliver next-generation materials protection strategies that address the pressing challenges of industrial degradation in demanding environments.

Benchmarking Performance: A Comparative Analysis of Ionic Liquid Generations and Alternatives

Ionic liquids (ILs), a class of materials composed entirely of ions with melting points below 100°C, have undergone significant generational evolution since their early discovery. The foundational work on ionic liquids dates back to 1914 with Paul Walden's synthesis of ethyl ammonium nitrate, but the field remained relatively niche until the late 20th century when air- and water-stable forms were developed, igniting widespread scientific interest [26] [64]. This evolution has been characterized by a deliberate shift from simply exploiting their physical properties toward designing them for specific applications with increasing consideration of their environmental and toxicological profiles. The classification into first, second, and third generations provides a useful framework for understanding this development, each representing a distinct philosophy in IL design and application [65]. The following diagram illustrates this evolutionary pathway and the primary design focus of each generation.

This review provides a comprehensive technical comparison of the first three generations of ionic liquids, with a specific focus on their performance characteristics and safety profiles. Within the broader context of research on the history and development of ionic liquids as solvents, we will analyze quantitative data on their physicochemical properties, detail experimental methodologies for their evaluation, and discuss their suitability for pharmaceutical and biomedical applications, providing drug development professionals with a clear framework for selection and use.

Generational Classification and Core Characteristics

The distinct generations of ionic liquids represent a clear evolution in design philosophy, moving from fundamental property exploration to advanced, application-specific functionality with integrated safety considerations.

First-Generation Ionic Liquids

The first generation of ILs, developed primarily for electrochemical applications such as electroplating, were characterized by combinations of dialkylimidazolium or alkylpyridinium cations with metal halide anions (e.g., chloroaluminates) [65]. The primary design goal for these early ILs was the achievement of specific physical properties, such as low melting points, high thermal stability, and broad liquid ranges, rather than specific chemical functionality or environmental compatibility [7]. Consequently, a significant limitation of many first-generation ILs is their sensitivity to moisture and air, which can lead to decomposition or the release of hazardous species [65]. From a sustainability and safety perspective, these ILs often exhibit low biodegradability, high toxicity to aquatic organisms, and high preparation costs, limiting their modern applications, especially in biological fields [65].

Second-Generation Ionic Liquids

Second-generation ILs marked a pivotal shift toward "designer solvents." These ILs, typically composed of cations like dialkylimidazolium, alkylpyridinium, ammonium, or phosphonium paired with anions such as tetrafluoroborate (BF₄⁻) or hexafluorophosphate (PF₆⁻), are stable in both air and water [65] [9]. This stability unlocked a wider range of applications. Their defining feature is their highly tunable nature; by modifying the structures of the anions and cations—for instance, by altering alkyl chain lengths or incorporating specific functional groups—scientists can precisely tailor physical and chemical properties like melting point, viscosity, hydrophilicity, and solvation power for specific tasks [65]. This generation found extensive use in catalysis, as alternative reaction media in chemical synthesis, and in electrochemical systems [7]. However, despite their functional advantages, many second-generation ILs are still plagued by high toxicity and poor biodegradability, presenting environmental and health concerns [9].

Third-Generation Ionic Liquids

Third-generation ILs were developed specifically to address the toxicity and sustainability shortcomings of their predecessors. This generation incorporates bio-derived and biologically compatible ions from natural sources, such as choline (cation) and amino acids, fatty acids, or lactate (anions) [65] [9] [44]. A significant driver for their development was the need for low-toxicity materials for biomedical and pharmaceutical applications [65]. A prominent subset of third-generation ILs is Active Pharmaceutical Ingredient-Ionic Liquids (API-ILs), where the ion pair itself constitutes a pharmaceutical drug, potentially enhancing solubility, stability, and bioavailability while circumventing polymorphism issues common in solid crystalline APIs [9] [64]. The key advantages of third-generation ILs include low toxicity, good biodegradability, and often lower manufacturing costs, making them ideal candidates for applications in drug delivery, biosensing, and other biomedical fields [65] [9].

Table 1: Core Characteristics and Compositions of Ionic Liquid Generations

Generation	Exemplary Cations	Exemplary Anions	Primary Design Philosophy	Key Differentiating Properties
First	Dialkylimidazolium, Alkylpyridinium	Metal Halides (e.g., AlCl₄⁻)	Achieve desirable physical properties (low mp, thermal stability)	Air/moisture sensitive, high toxicity, low biodegradability
Second	Dialkylimidazolium, Ammonium, Phosphonium	[BF₄]⁻, [PF₆]⁻, [Tf₂N]⁻	Tailor physicochemical properties for specific applications	Air/water stable, highly tunable, but often toxic
Third	Cholinium, Amino Acid-based	Amino Acid-based, Fatty Acids, Lactate	Incorporate bio-derived ions for biocompatibility & low toxicity	Low toxicity, good biodegradability, often pharma-compatible

Comparative Performance and Safety Analysis

A critical understanding of IL generations requires a direct comparison of their quantitative performance metrics and safety parameters. The following table synthesizes data related to their stability, toxicity, and primary application domains.

Table 2: Performance and Safety Comparison of Ionic Liquid Generations

Parameter	First-Generation ILs	Second-Generation ILs	Third-Generation ILs
Thermal Stability	High [65]	High (e.g., up to 400°C for some [Tf₂N]⁻-based) [65]	Moderate to High (depends on bio-ion) [9]
Air/Water Stability	Low (sensitive to hydrolysis) [65]	High [65]	High [9]
Toxicity (General)	High (especially to aquatic environments) [65]	High (e.g., imidazolium with long alkyl chains) [65] [9]	Low (e.g., cholinium-amino acid combinations) [9] [44]
Biodegradability	Very Low [65]	Low to Very Low [65] [9]	High (e.g., esters, long alkyl chains enhance it) [9]
Exemplary Applications	Electrolytes, electroplating [65]	Catalysis, organic synthesis, separations, electrochemistry [7] [65]	Drug delivery (API-ILs), biomedicine, green synthesis [7] [65] [9]

Analysis of Key Trade-offs

The data in Table 2 highlights the fundamental trade-offs between performance and safety that have driven IL development. While first and second-generation ILs often exhibit superior thermal stability and a well-established range of physicochemical properties, this comes at the cost of significant environmental and health hazards [65]. The poor biodegradability of many early ILs raises concerns about their environmental persistence [9]. Third-generation ILs, particularly Amino Acid-Based ILs (AAILs) and other Bio-ILs, explicitly sacrifice some of the extreme, tunable properties to achieve drastically improved safety profiles [44]. For instance, the presence of an ester functional group in the side chain of an IL cation can significantly enhance its biodegradability, a key consideration for green chemistry principles [9]. This makes third-generation ILs the only viable option for most pharmaceutical and internal biomedical applications, where low toxicity and metabolic processing are paramount.

Experimental Protocols for Performance and Safety Evaluation

For researchers developing or working with ILs, standardized experimental protocols are essential for generating comparable data on performance and safety. Below are detailed methodologies for key evaluations.

Protocol: Synthesis of a Third-Generation Amino Acid-Based Ionic Liquid (AAIL)

This is a common two-step metathesis reaction for producing AAILs.

• Synthesis of the [Cation]OH Precursor: An ion-exchange resin is typically used to convert a halide salt of the desired cation (e.g., cholinium chloride) into the corresponding hydroxide ([Cation]OH) solution.
• Neutralization and IL Formation: The [Cation]OH solution is added dropwise to a stirring, equimolar aqueous solution of the chosen amino acid (e.g., glycine) in an ice bath. The reaction is a simple acid-base neutralization.
  • Reaction Condition: Maintain temperature at 0-5°C during addition to prevent racemization or decomposition of the amino acid.
• Water Removal: The water is removed from the resulting mixture via rotary evaporation.
• Purification: The residual solid is typically washed thoroughly with an organic solvent like ethyl acetate or acetonitrile to remove any unreacted starting materials.
• Final Drying: The pure AAIL is obtained by drying under high vacuum for 24-48 hours to remove trace solvents and water [44].

Protocol: Assessing IL Toxicity via Cytotoxicity Assay

The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay is a standard colorimetric method for measuring cell metabolic activity, indicating cytotoxicity.

• Cell Seeding: Seed appropriate cell lines (e.g., Caco-2 for intestinal models, HeLa for general cytotoxicity, or specific primary cells) in a 96-well plate and allow them to adhere for 24 hours.
• IL Exposure: Prepare a dilution series of the IL in the cell culture medium. Remove the growth medium from the cells and add the IL-containing medium. Include wells with only medium (blank) and cells with no IL (vehicle control).
• Incubation: Incubate the cells for a predetermined period (e.g., 24, 48, or 72 hours) at 37°C and 5% CO₂.
• MTT Addition: After incubation, add a specific volume of MTT solution to each well and incubate for 2-4 hours to allow for the formation of formazan crystals.
• Solubilization: Carefully remove the medium and dissolve the formed formazan crystals in a solvent like dimethyl sulfoxide (DMSO).
• Absorbance Measurement: Measure the absorbance of the solution in each well at a wavelength of 570 nm using a microplate reader. The measured absorbance is directly proportional to the number of viable cells.
• Data Analysis: Calculate the percentage of cell viability relative to the untreated control. The half-maximal inhibitory concentration (IC₅₀ or EC₅₀) can then be determined from the dose-response curve [9] [66].

Protocol: Evaluating Solubilization Enhancement for a Poorly Soluble Drug

This protocol tests the ability of an IL to enhance the solubility of a Biopharmaceutics Classification System (BCS) Class II/IV drug.

• Sample Preparation: Prepare the IL (e.g., a third-generation API-IL or a solution of a drug in a biocompatible IL) and the pure crystalline drug as a control.
• Dissolution Test: Use a standard USP dissolution apparatus. Place the IL-formulated drug and the control in separate vessels containing a dissolution medium (e.g., simulated gastric or intestinal fluid without enzymes, pH 1.2 or 6.8, respectively) maintained at 37°C.
• Agitation and Sampling: Operate the paddles at a specified speed (e.g., 50 rpm). Withdraw samples automatically or manually at predetermined time intervals (e.g., 5, 10, 15, 30, 45, 60, 90, 120 minutes).
• Filtration and Analysis: Immediately filter the withdrawn samples through a syringe filter (e.g., 0.45 µm) to remove any undissolved particles. Analyze the concentration of the drug in the filtrate using a validated analytical method, typically High-Performance Liquid Chromatography (HPLC) with UV detection.
• Data Comparison: Plot the drug release profile (cumulative amount dissolved vs. time) for both the IL formulation and the control. Key metrics for comparison include the dissolution efficiency at a specific time point and the time taken for 80% of the drug to dissolve (T₈₀) [9] [64].

The Scientist's Toolkit: Key Reagents and Materials

Selecting appropriate reagents is fundamental to ionic liquid research. The following table details key materials used in the synthesis and application of ILs, particularly in a pharmaceutical context.

Table 3: Essential Research Reagents for Ionic Liquid Research

Reagent/Material	Function/Role	Exemplary Use Case
1-Butyl-3-methylimidazolium chloride ([C₄C₁im]Cl)	Second-generation IL solvent/catalyst	Lignocellulosic biomass pretreatment; solvent in organic synthesis [26] [64].
Cholinium Chloride	Cation precursor for third-generation Bio-ILs	Synthesis of low-toxicity ILs with anions like amino acids or ibuprofenate for drug delivery [9] [44].
Amino Acids (e.g., Glycine, Proline)	Anion precursor for AAILs	Creating biodegradable, low-toxicity ILs for pharmaceutical formulations and green chemistry [44].
Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)	Anion source for electrochemical ILs	Forming solvate ionic liquids (SILs) with glymes for advanced battery electrolytes [13].
Docusate (Dioctyl sulfosuccinate)	Pharmaceutically-acceptable anion	Forming API-ILs to improve drug solubility and permeability (e.g., Ranitidine Docusate) [9].

Decision Framework and Future Perspectives

The choice between IL generations is not a simple matter of selecting the "latest" but involves a careful balance between performance requirements and safety constraints. The following decision diagram provides a logical pathway for selecting the appropriate IL generation based on application requirements.

The future of ionic liquids lies in the continued refinement of fourth-generation ILs, which focus on sustainability, multifunctionality, and smart materials [7]. Key research frontiers include the development of ILs with built-in recyclability to improve the economics and green credentials of industrial processes like biomass pretreatment [26]. In the pharmaceutical sphere, the exploration of dual-functional API-ILs and surface-active ILs (SAILs) for targeted drug delivery and nanocarrier formation represents a cutting-edge area set to expand [9] [64]. Furthermore, the integration of computational modeling with experimental science will accelerate the rational design of task-specific ILs, minimizing synthetic effort and empirical screening [13]. As the field matures, the trend is unequivocally moving toward materials that do not force a choice between high performance and environmental and biological safety, but rather integrate them seamlessly.

The historical development of solvent use in chemical processes is marked by a significant paradigm shift, driven by the urgent need for sustainable and environmentally benign alternatives. This shift is central to the broader thesis on the history and development of ionic liquids as solvents. Traditional organic solvents, characterized by their volatility, toxicity, and environmental persistence, have long posed substantial risks to both human health and ecosystems [67] [68]. In response, the principles of green chemistry have catalyzed the search for safer substitutes, propelling the advancement of ionic liquids (ILs)—a class of solvents composed entirely of ions with melting points below 100°C [68]. Initially discovered in 1914 but largely unexplored until the early 21st century, ILs have since witnessed exponential growth in scientific interest and application diversity [26]. Their unique properties, including negligible vapor pressure, high thermal stability, and structural tunability, have positioned them as promising candidates for a sustainable solvent platform across fields ranging from biomass processing and battery recycling to carbon capture and pharmaceutical synthesis [55] [26] [69]. This review provides a comparative assessment of the green credentials of ionic liquids against traditional organic solvents, contextualized within their developmental history and evaluated through the lenses of performance, sustainability, and future prospects.

Fundamental Properties and Historical Trajectory

The evolution of ionic liquids from a scientific curiosity to a platform for green solvent design is a key narrative in modern chemical research. The trajectory of solvent use reflects a growing adherence to green chemistry principles, which emphasize waste reduction, hazard minimization, and energy efficiency [68]. Traditional organic solvents such as benzene, chloroform, and toluene are often volatile, flammable, and toxic, contributing to environmental pollution and occupational health hazards [67] [68]. Their environmental impact is significant; for instance, chlorinated solvents like trichloroethylene are persistent, mobile in soil and groundwater, and associated with serious health risks including cancer and organ failure [67].

In contrast, ionic liquids are salts in the liquid state, typically consisting of organic cations and inorganic or organic anions. Their journey began with Paul Walden's 1914 synthesis of ethyl ammonium nitrate [26]. However, their potential remained largely untapped until the late 20th century, when their unique properties sparked widespread interest. The number of science citation index (SCI) publications on ILs grew from just a few in 1996 to over 5000 by 2018, surpassing the growth rates of many other scientific fields [26]. A key characteristic of ILs is their negligible vapor pressure, which virtually eliminates inhalation risks and atmospheric volatile organic compound (VOC) emissions—a major advantage over conventional solvents [55] [68]. Furthermore, their properties can be finely tuned by altering the cation-anion combination, enabling their design for specific applications [26] [68]. This tunability underpins their designation as "designer solvents".

Table 1: Fundamental Properties Comparison between Organic Solvents and Ionic Liquids

Property	Traditional Organic Solvents	Ionic Liquids
Vapor Pressure	High, volatile [67] [68]	Negligible [55] [68]
Flammability	Often high, significant fire risk [68]	Typically non-flammable [68]
Thermal Stability	Variable, often low [55]	High, often >300°C [55]
Toxicity Profile	Often toxic, carcinogenic (e.g., benzene) [67]	Ranges from low to high; tunable [68]
Biodegradability	Often low, persistent [67]	Variable; design for biodegradability possible [26]
Structural Tunability	Limited	Virtually unlimited "designer solvents" [26] [68]

Quantitative Comparison of Environmental and Economic Footprints

A rigorous assessment of green credentials necessitates a quantitative examination of environmental, health, and economic indicators. While the fundamental properties of ILs are promising, their overall sustainability must be evaluated across their entire life cycle, from production and use to disposal [68].

Environmental and Health Impacts

Traditional organic solvents are a major source of volatile organic compounds (VOCs), which contribute to smog formation and poor air quality [67]. Exposure, whether through inhalation or skin contact, can lead to acute health issues like headaches, dizziness, and nervous system effects, as well as chronic problems including increased cancer risk and organ damage [67]. In contrast, the low volatility of ILs drastically reduces airborne exposure risks and VOC emissions [55]. However, the green label for ILs is not unconditional. Their toxicity and biodegradability vary significantly with their chemical structure. For example, the toxicity of imidazolium-based ILs often increases with the length of the alkyl chain on the cation [68]. Some hydrophobic ILs can persist in the environment and bind strongly to sediments [68]. Therefore, a full life-cycle assessment (LCA) is critical to validate their environmental credentials [26] [68].

Economic and Market Considerations

The global market for green solvents, which includes bio-based solvents, ILs, and deep eutectic solvents, is experiencing robust growth. It is projected to reach USD 5.51 billion by 2035, growing at a compound annual growth rate (CAGR) of 8.7% [70]. This growth is driven by stringent government regulations and rising consumer demand for eco-friendly products. A major hurdle for ILs is their high production cost compared to conventional solvents [26]. This cost is primarily attributed to expensive raw materials and energy-intensive synthesis and purification steps [26]. While some protic ionic liquids (PILs) like triethylammonium hydrogen sulfate ([TEA][HSO4]) have been developed for cost-effective biomass processing [26], the economic challenge remains a significant barrier to widespread industrial adoption.

Table 2: Economic and Environmental Impact Comparison

Parameter	Traditional Organic Solvents	Ionic Liquids
Global Market Growth	Mature, regulated, and declining in some segments [70]	Growing rapidly (CAGR 8.7%) as part of green solvents market [70]
Production Cost	Generally low	High, due to synthesis and purification [26]
Recyclability & Reuse	Often difficult or hazardous; frequently disposed of	High potential for multiple recovery and reuse cycles [55] [26]
Waste Generation	High VOC emissions and hazardous waste [67]	Low emissions; potential for integrated waste valorization [69]
Regulatory Pressure	Increasingly restricted [67] [70]	Promoted as sustainable alternatives, though some require assessment [70]

Experimental Protocols and Applications

The theoretical advantages of ILs are realized in practical applications, where their performance is tested against traditional solvents. Detailed experimental protocols highlight their unique roles and operational methodologies.

Protocol 1: Ionic Liquid-Based Recycling of Lithium-Ion Batteries

Objective: To selectively recover valuable metals (e.g., Li, Co, Ni) from spent lithium-ion battery cathodes using ILs as leaching agents [55].

• Key Research Reagents:
  • Ionic Liquid Leachant: Typically a functionalized IL like 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]) acts as both solvent and acid source [55].
  • Hydrogen Peroxide (H₂O₂): Used as an oxidizing agent to enhance metal dissolution kinetics [55].
  • Spent Cathode Material: Pretreated by discharging, dismantling, and separating cathode black mass (e.g., NCM or LCO type) [55].
• Methodology:
  • Pretreatment: The spent LIBs are discharged, manually dismantled, and the cathode active materials are separated from the current collector (aluminum foil) [55].
  • Leaching Process: The cathode powder is mixed with the IL aqueous solution in a reactor. A typical solid-liquid ratio is 1:20 g/mL. The mixture is heated to 150-200°C under continuous stirring for several hours [55].
  • Separation & Purification: After leaching, the solid residue (graphite, impurities) is separated by filtration. The leachate, containing dissolved metal ions, undergoes selective precipitation or solvent extraction to isolate individual metals [55].
  • IL Recovery: The spent IL is regenerated by removing metal impurities and water, allowing it to be recycled into the leaching process [55].

Protocol 2: Chemical Recycling of Plastics with Aqueous Ionic Liquids

Objective: To depolymerize mixed plastic waste (e.g., PET, PLA) into monomeric components using an aqueous IL system [71].

• Key Research Reagents:
  • Cholinium Lysinate ([Ch][Lys]): A biocompatible IL that demonstrates high efficiency in depolymerizing PET and PLA [71].
  • Mixed Plastic Waste: Sourced from post-consumer products, washed, and shredded into small flakes (<5 mm) [71].
• Methodology:
  • Feedstock Preparation: Plastic waste is sorted, cleaned, and shredded into small flakes to increase surface area [71].
  • Depolymerization Reaction: The plastic flakes are combined with an aqueous solution of [Ch][Lys] in a pressurized reactor. A typical reaction is conducted at 160°C for 6 hours with constant agitation [71].
  • Product Recovery: After the reaction, the mixture is cooled. The monomer products (e.g., terephthalic acid from PET) precipitate out or are separated from the IL solution. The reported yield can be as high as 99% for terephthalic acid [71].
  • IL Regeneration: The aqueous IL stream is purified and concentrated for direct reuse in subsequent batches, demonstrating the process's circularity [71].

Performance and Sustainability Analysis

The application of ILs in diverse industrial sectors showcases their potential to address critical environmental challenges, from electronic waste to greenhouse gas emissions.

Resource Recovery and Circular Economy

ILs are revolutionizing waste valorization. In lithium-ion battery recycling, they achieve high extraction rates for cobalt, nickel, and lithium under milder conditions compared to traditional pyrometallurgical or hydrometallurgical processes, which are energy-intensive and generate secondary pollution [55]. Similarly, in plastic waste recycling, IL-based depolymerization converts polymers like PET into valuable monomers with yields exceeding 99%, enabling a closed-loop recycling system and reducing reliance on virgin petrochemicals [71] [69]. This aligns with the principles of a circular economy, turning waste streams into resources.

Carbon Capture and Environmental Remediation

ILs, particularly task-specific ionic liquids (TSILs), show great promise for CO₂ capture. Their high solubility for CO₂, thermal stability, and low volatility make them attractive alternatives to amine-based scrubbing technologies, which are energy-intensive and suffer from solvent degradation [69] [72]. For instance, functionalized imidazolium-based ILs have demonstrated CO₂ absorption capacities as high as 25.2 mol/kg IL [69]. This application not only mitigates greenhouse gas emissions but also opens pathways for utilizing CO₂ as a feedstock for fuels and chemicals, creating a synergistic approach to environmental remediation [72].

Table 3: Performance of Ionic Liquids in Key Green Applications

Application Area	Exemplar Ionic Liquid	Reported Performance	Comparative Advantage
LIB Metal Leaching [55]	[BMIM][HSO4]	>99% Co, Ni extraction	Milder conditions, less secondary pollution vs. strong inorganic acids
Plastic Depolymerization [71]	Cholinium Lysinate	99% monomer yield (PET)	Processes mixed waste, eliminates organic solvents
Lignocellulosic Biomass Pretreatment [26]	[EMIM][CH3COO]	High enzymatic digestibility	Efficient deconstruction under mild conditions, recyclable
CO₂ Capture [69]	Functionalized Imidazolium ILs	25.2 mol CO₂ / kg IL	High capacity, low volatility, tunable for specific gases

The Scientist's Toolkit: Key Research Reagents

The experimental and industrial application of ionic liquids relies on a core set of reagents and materials. The following table details essential components in the IL research toolkit.

Table 4: Essential Research Reagents for Ionic Liquid Applications

Reagent/Material	Function/Description	Exemplar Use Case
Imidazolium Cations (e.g., [BMIM]⁺)	Common organic cation providing a versatile platform for IL synthesis; properties are tuned via alkyl chain length [55] [68].	Synthesis of [BMIM][Cl] for biomass dissolution [26].
Acetate Anion ([CH₃COO]⁻)	Anion with strong hydrogen-bond-breaking ability, effective for dissolving cellulose and biopolymers [26].	[EMIM][OAc] for biomass pretreatment [26].
Choline Chloride	Low-cost, biodegradable quaternary ammonium salt; a common component of Deep Eutectic Solvents (DES) [68].	Forming DES with urea for green extraction [68].
Deep Eutectic Solvents (DES)	Mixtures of H-bond donors/acceptors with low m.p.; considered low-cost, simpler-to-synthesize IL analogues [68].	Extraction of metals or bioactive compounds [73] [68].
Triethylammonium Hydrogen Sulfate ([TEA][HSO4])	Protic Ionic Liquid (PIL); cost-effective, selective for lignin removal in biomass processing [26].	Low-cost biomass pretreatment under hydrous conditions [26].
Spent Battery Black Mass	Pre-processed cathode material (e.g., LiNixCoyMnzO₂) from spent LIBs, serving as the target feedstock for metal recovery [55].	Serves as the secondary resource for Co, Ni, Li leaching experiments [55].

Challenges and Future Perspectives in Ionic Liquid Development

Despite their considerable promise, the path to the widespread industrial adoption of ionic liquids is fraught with challenges that must be addressed through future research and development.

A primary obstacle is the high cost of many ILs, stemming from expensive precursors and complex, energy-intensive synthesis and purification processes [26]. While low-cost protic ionic liquids (PILs) offer a partial solution [26], scalable and economical production routes are still needed. Furthermore, comprehensive life-cycle assessments (LCA) and techno-economic analyses (TEA) are crucial to validate the environmental and economic sustainability of IL-based processes [26] [74]. Some LCAs have indicated that ILs can have a higher eco-toxicity impact than conventional solvents unless efficient recovery strategies are implemented [26].

The toxicity and biodegradability of ILs remain significant concerns. Not all ILs are inherently green; their environmental impact is highly structure-dependent [68]. Future research must focus on the rational design of biodegradable and non-toxic ILs, for instance, those derived from natural products like choline [68] [71]. Another challenge is the high viscosity of many ILs, which can impede mass transfer and process efficiency in applications like CO₂ capture [69] [72].

Looking forward, the integration of artificial intelligence (AI) and machine learning is poised to accelerate the discovery and optimization of next-generation ILs [73]. AI can predict the physical properties and toxicity of novel IL structures, guiding the design of safer and more efficient solvents. The market for green solvents, including ILs, is projected to grow significantly [70], indicating strong industrial interest. Future breakthroughs will likely come from interdisciplinary efforts that combine synthetic chemistry, process engineering, and sustainability science to develop ionic liquids that truly fulfill their promise as green and sustainable solvents.

The historical development of solvent technology is marked by a persistent effort to balance efficiency with environmental and safety considerations. For decades, industrial processes across chemical, pharmaceutical, and petrochemical sectors have been heavily reliant on conventional organic solvents, which account for 80-90% of the total mass in many formulations and contribute significantly to hazardous waste production [75]. These volatile organic compounds (VOCs) pose serious environmental and health risks, including carcinogenicity, mutagenicity, respiratory problems, and environmental contamination [75]. This concerning backdrop has fueled the search for safer, more sustainable alternatives aligned with the 12 principles of green chemistry, ultimately leading to the emergence of ionic liquids (ILs) and later, deep eutectic solvents (DES) [75].

ILs, initially celebrated as green solvents due to their negligible vapor pressure, represented a significant advancement in solvent technology [76]. However, as research progressed, concerns regarding their toxicity, poor biodegradability, and high production costs began to emerge [75] [76]. These limitations created an opportunity for DES to emerge as a potentially more sustainable and cost-effective alternative [75]. This whitepaper provides a comprehensive technical comparison between ILs and DES, examining their historical development, fundamental properties, and applications, with particular emphasis on their relevance to researchers and drug development professionals seeking environmentally benign solvent solutions.

Historical Development and Fundamental Principles

The Emergence and Evolution of Ionic Liquids

The history of ionic liquids spans over a century, with the first documented example, ethylammonium nitrate ([EtNH3][NO3]) with a melting point of 12°C, reported by Paul Walden in 1914 [1]. However, this discovery remained largely unexplored for several decades. The modern era of IL research gained momentum in the 1950s with Hurley and Weir's work on room-temperature molten salts for electrodeposition, specifically using mixtures of 1-ethylpyridinium bromide and aluminum chloride [1]. The field expanded significantly in the 1980s and 1990s with the introduction of air- and water-stable ILs, particularly those based on the 1,3-dialkylimidazolium cation by John Wilkes and colleagues [1] [5]. This breakthrough ignited widespread academic and industrial interest, leading to the classification of ILs into three generations based on their intended properties and applications [9].

Table: Generations of Ionic Liquids

Generation	Primary Focus	Key Characteristics	Example Applications
First	Desired Physical Properties	Low melting point, high thermal stability, low vapor pressure	Electrochemistry, synthesis
Second	Tunable Physicochemical Properties	Air and water stability, adjustable properties	Catalysis, specialized synthesis
Third	Biocompatibility & Sustainability	Low toxicity, biodegradability, bio-based components	Pharmaceuticals, biotechnology

The interest in ILs has grown exponentially, with publications surpassing 80,000 and patent families reaching 17,000 by 2018 [5]. Their applications have diversified dramatically, extending into energy storage, pharmaceuticals, biomass processing, and carbon capture [5] [77].

The Rise of Deep Eutectic Solvents

Deep Eutectic Solvents represent a more recent innovation in solvent technology, first systematically reported by Abbott et al. in 2003 with the description of a mixture of choline chloride and urea [75] [78]. DES are defined as mixtures of two or more components—typically a Hydrogen Bond Acceptor (HBA) such as a quaternary ammonium salt and a Hydrogen Bond Donor (HBD) such as urea, acids, or polyols—that form a eutectic mixture with a melting point significantly lower than that of either individual component [75] [78]. This pronounced depression in freezing point occurs due to strong, complex hydrogen bonding networks between the components, which disrupts the crystal lattice of the pure substances [78] [79].

The interest in DES has witnessed phenomenal growth, with publication numbers exceeding 2,000 in 2022 alone [75]. This surge is largely attributable to their specific advantages over ILs, including superior biodegradability, ease of preparation from inexpensive, often bio-based components, and reduced synthesis costs [75] [79]. DES are categorized into five main types based on their composition, with Type III (quaternary ammonium salt + HBD) being the most prevalent for extraction and separation applications [75] [78].

Figure 1: Classification and Composition of Deep Eutectic Solvents (DES). DES are primarily composed of a Hydrogen Bond Acceptor (HBA) and a Hydrogen Bond Donor (HBD), combining to form different types. Natural DES (NADES) represent a specific subclass utilizing bio-derived components.

Comparative Analysis: Properties and Green Credentials

Physicochemical Properties and Tunability

Both ILs and DES share several key properties that make them attractive as alternative solvents, including low volatility, non-flammability, and high thermal stability [75] [80]. However, they differ significantly in other aspects.

ILs exhibit a remarkable degree of tunability. By selecting different cation-anion pairings from a vast combinatorial library (potentially up to 10¹⁸ ternary ILs), properties such as viscosity, polarity, hydrophobicity, and solvent miscibility can be precisely engineered for specific applications [76] [9]. For instance, amino groups can be incorporated to enhance water solubility, while -CF₃ groups can increase hydrophobicity for targeting non-polar contaminants [80].

DES are similarly tunable, though through different mechanisms. Their properties are adjusted by varying the structure and molar ratio of the HBA and HBD components [78] [80]. For example, the viscosity and basicity of DES can be modulated by adjusting water content, temperature, or by incorporating basicity-enhancing agents like guanidine [80]. The alkyl chain length of DES components can also significantly influence solvatochromic parameters such as hydrogen bond acidity (α) and basicity (β), allowing for fine-tuning of their solvation properties [80].

Environmental and Economic Considerations

The initial designation of ILs as "green solvents" was primarily based on their negligible vapor pressure, which minimizes atmospheric emissions and inhalation risks compared to VOCs [76]. However, a more comprehensive assessment has revealed significant drawbacks. Many ILs, particularly early generations, demonstrate high toxicity toward aquatic organisms and poor biodegradability [75] [76]. Their synthesis often involves expensive starting materials and complex, energy-intensive purification steps, resulting in high production costs that can limit large-scale industrial application [75] [79]. Furthermore, certain fluorinated anions used in ILs share structural similarities with persistent perfluoroalkyl substances (PFAS), raising concerns about their potential for bioaccumulation and long-term environmental persistence [76].

DES address many of the limitations of ILs. They are generally composed of biocompatible and biodegradable components, such as choline chloride and natural carboxylic acids, leading to lower overall toxicity [75] [79]. Their preparation is remarkably straightforward, typically involving the simple mixing of two components, often with little to no need for purification, making them inexpensive and easy to manufacture on a large scale [75] [78]. The components of DES are also frequently sourced from renewable resources, further enhancing their sustainability profile [78] [81].

Table: Comprehensive Comparison of ILs and DES Properties

Property	Ionic Liquids (ILs)	Deep Eutectic Solvents (DES)
Composition	Discrete ions (organic cation + organic/inorganic anion)	Mixture of HBA and HBD
Vapor Pressure	Negligible	Negligible
Tunability	Very high (via cation/anion selection)	High (via HBA/HBD selection and ratio)
Typical Synthesis	Multi-step, often requiring purification	Simple mixing of components
Cost	Relatively high	Low
Biodegradability	Often poor	Generally good to high
Toxicity	Can be high; dependent on structure	Typically low
Green Credentials	Questioned due to toxicity/persistence	Generally considered green & sustainable

Applications in Research and Industry

Pharmaceutical and Drug Development Applications

The pharmaceutical industry has shown growing interest in both ILs and DES for overcoming challenges associated with poorly soluble active pharmaceutical ingredients (APIs), a problem affecting approximately 40% of marketed oral drugs [9].

A significant innovation in IL technology is the development of API-ILs, where an active pharmaceutical ingredient is engineered into an ionic liquid form. This approach can effectively address issues of polymorphism, enhance thermal stability, and dramatically improve solubility and bioavailability [9]. For instance, transforming neutral paracetamol into an ionic liquid paired with a docusate counterion has been shown to enhance its solubility and delivery [9]. Furthermore, third-generation ILs (Bio-ILs) derived from biological cations like cholinium are particularly suited for pharmaceutical applications due to their low toxicity and high biocompatibility [9].

DES have also proven valuable in pharmaceutical applications. They can act as efficient solubilizing agents for poorly soluble drugs and have been successfully used to enhance permeability across biological barriers, facilitating their use in oral and transdermal drug delivery systems [9]. Their role as benign reaction media for organic synthesis and biocatalysis further underscores their utility in pharmaceutical manufacturing [75] [81].

Established and Emerging Industrial Applications

Ionic Liquids have found commercial or pilot-scale applications in numerous fields. In the energy sector, they are used as electrolytes in advanced batteries (e.g., lithium-ion, lithium-sulfur) and supercapacitors due to their wide electrochemical windows and non-flammability [77]. They are also employed as stationary phases in gas chromatography and as catalysts in various industrial chemical reactions [1] [76]. Other growing application areas include carbon capture, lubricants, and metal electroplating [77].

Deep Eutectic Solvents have demonstrated exceptional utility in extraction and separation processes. They are highly effective for extracting phenolic compounds, pesticides, and other organic contaminants from water, with some studies reporting extraction efficiencies exceeding 80% [80]. In biomass processing, DES selectively solubilize lignin or hemicellulose, leaving behind a cellulose-rich material that is more susceptible to enzymatic hydrolysis, thereby facilitating biofuel production [81]. For example, cholinium arginate (ChArg), a bio-based IL, has shown remarkable efficacy in delignifying apple pomace waste, a common agri-food residue [81]. DES also show promise in metal processing applications such as electrodeposition and the recovery of precious metals from electronic waste [75] [78].

Experimental Protocols and Methodologies

Synthesis and Preparation Protocols

DES Synthesis via Heating and Stirring: This is the most common method for preparing DES [75].

• Weighing: Precisely weigh the Hydrogen Bond Acceptor (HBA), such as choline chloride, and the Hydrogen Bond Donor (HBD), such as urea, in the desired molar ratio (e.g., 1:2 for ChCl:Urea, also known as reline).
• Mixing: Combine the solid components in a round-bottom flask or similar vessel.
• Heating/Stirring: Heat the mixture with continuous magnetic stirring at approximately 60-80°C until a homogeneous, colorless liquid forms. This usually takes between 30 minutes and 2 hours, depending on the components.
• Cooling and Storage: Allow the formed DES to cool to room temperature. Store it in a sealed container, preferably in a dry environment, though most DES are not highly moisture-sensitive [79].

Alternative DES Synthesis via Freeze-Drying: This method is suitable for water-soluble components [75].

• Aqueous Solution Preparation: Dissolve the HBA and HBD in deionized water.
• Freeze-Drying: Subject the aqueous solution to freeze-drying (lyophilization) to remove the water under vacuum at low temperatures.
• Collection: A viscous liquid or low-melting-point solid DES is obtained after the complete removal of water.

IL Synthesis (General Overview): The synthesis of ILs is more varied and complex than for DES. A common method for imidazolium-based ILs involves [1] [9]:

• Quaternization (Alkylation): An amine (e.g., 1-methylimidazole) is reacted with an alkyl halide (e.g., chlorobutane) to form a halide salt of the desired cation (e.g., [BMIM]Cl).
• Anion Metathesis: The halide anion is exchanged for the target anion (e.g., [PF₆]⁻, [BF₄]⁻, [Tf₂N]⁻). This can be achieved by reacting the halide salt with a metal or acid salt containing the desired anion (e.g., KPF₆, LiTf₂N).
• Purification: The resulting IL must be rigorously purified to remove residual halides and other impurities. This often involves multiple washes with water and organic solvents, followed by prolonged stirring with activated carbon and final vacuum drying at elevated temperatures [1].

Protocol for Biomass Pretreatment Using DES/ILs

A representative experimental protocol for the pretreatment of lignocellulosic biomass (e.g., apple pomace fibers) is outlined below, based on a recent case study [81].

Table: Research Reagent Solutions for Biomass Pretreatment

Reagent/Material	Function/Description	Example from Study
Biomass Feedstock	The lignocellulosic material to be fractionated.	Apple pomace fibers (agri-food waste).
DES/IL Solvent	The solvent medium for pretreatment, selectively dissolves lignin/hemicellulose.	Cholinium Arginate (ChArg, Bio-IL) or ChCl:LA (1:10, NADES).
Heating/Mixing System	Provides controlled temperature and agitation for the reaction.	Oil bath with magnetic stirrer.
Filtration Setup	For solid-liquid separation after pretreatment.	Büchner funnel and vacuum flask.
Washing Solvent	Removes residual solvent from the treated solid fraction.	Water, ethanol, or a similar solvent.
Analytical Tools	For characterizing the pretreated material.	FTIR, TGA, enzymatic hydrolysis with DNS assay.

Objective: To fractionate lignocellulosic biomass (apple fibers) into a cellulose-rich material (CRM) and a lignin-rich stream, enhancing the enzymatic digestibility of the cellulose.

Procedure:

• Loading: Combine 1.0 gram of dried and milled apple fiber biomass with 20 grams of the selected solvent (e.g., ChArg IL or ChCl:LA DES) in a suitable reaction vessel.
• Pretreatment: Heat the mixture to 90°C with constant stirring for 1 hour.
• Separation: After the reaction time, cool the mixture and add an anti-solvent (e.g., water or an ethanol-water mixture) to precipitate dissolved components. Separate the solid residue (the cellulose-rich material, CRM) from the liquid fraction via vacuum filtration.
• Washing: Thoroughly wash the solid CRM cake with the anti-solvent to remove any residual IL or DES.
• Analysis:
  • Solid Fraction (CRM): Dry the CRM and analyze it using Fourier-Transform Infrared Spectroscopy (FTIR) to identify chemical structure changes and Thermogravimetric Analysis (TGA) to assess thermal stability and composition. Perform enzymatic hydrolysis on the CRM using cellulase enzymes, and quantify the released glucose using a DNS (3,5-dinitrosalicylic acid) assay to determine the sugar yield improvement.
  • Liquid Fraction: This contains dissolved lignin and hemicellulose. Lignin can be recovered by further dilution with water or pH adjustment, followed by centrifugation or filtration.
• Solvent Recovery: The IL or DES in the liquid fraction can potentially be recovered and reconcentrated for reuse, a key factor for process sustainability and economics [81].

Figure 2: Experimental Workflow for Biomass Pretreatment using ILs or DES. The process involves pretreatment of biomass with the solvent, followed by separation into solid and liquid fractions for subsequent analysis and valorization.

The historical trajectory of solvent development, from conventional VOCs to ionic liquids and now to deep eutectic solvents, reflects a continuous evolution toward more sustainable and tailored technological solutions. While ionic liquids broke new ground as designer solvents with unparalleled tunability and opened doors to countless applications in electrochemistry, catalysis, and materials science, concerns regarding their toxicity, cost, and environmental persistence have tempered their initial "green" label.

Deep eutectic solvents have emerged as a powerful, often more sustainable, and cost-effective alternative. Their simplicity of preparation, biocompatibility, and utilization of renewable feedstocks position them strongly within the principles of green chemistry. In the context of pharmaceutical research and drug development, both classes offer distinct advantages. ILs, particularly API-ILs and third-generation Bio-ILs, provide a sophisticated tool for manipulating the properties of active pharmaceutical ingredients. Meanwhile, DES excel as benign reaction media and effective excipients for enhancing drug solubility and permeability.

The choice between ILs and DES is not a simple substitution but a strategic decision based on the specific requirements of the application. ILs may still be preferred where extreme tunability or specific electrochemical properties are paramount. However, for most applications prioritizing sustainability, cost-effectiveness, and low toxicity—especially in biomass processing, extraction, and pharmaceuticals—DES represent a compelling and rising green alternative. Future research will undoubtedly focus on deepening the understanding of their structure-property relationships, long-term environmental impact, and scaling up their most promising applications for industrial adoption.

Ionic liquids (ILs), a class of salts that exist in liquid state at relatively low temperatures, have emerged as transformative solvents across diverse scientific and industrial domains. Their unique properties, including negligible vapor pressure, high thermal stability, and tunable physicochemical characteristics, have positioned them as environmentally friendly alternatives to volatile organic solvents. Within the context of a broader thesis on the history and development of ionic liquids as solvents, this review explores their application efficacy in two seemingly disparate fields: transdermal drug delivery and lignocellulosic biomass pretreatment. Despite their different end goals—enhanced therapeutic outcomes and sustainable biofuel production—both fields leverage the fundamental ability of ILs to overcome formidable biological and structural barriers: the stratum corneum in skin and the recalcitrant lignin-cellulose matrix in biomass.

This technical guide provides an in-depth examination of case studies validating IL efficacy through quantitative metrics, detailed experimental protocols, and visualizations of key mechanisms. By presenting structured data and methodological frameworks, this review serves as a resource for researchers and scientists seeking to implement or advance IL-based technologies in their respective fields.

Ionic Liquids in Transdermal Drug Delivery

Mechanism of Action and Barrier Permeability

The skin, particularly the stratum corneum, presents a formidable barrier to transdermal drug delivery (TDD). The stratum corneum, approximately 10-20 μm thick, consists of dead keratinocytes (corneocytes) embedded within a lipid matrix comprising ceramides, cholesterol, and free fatty acids [82]. This structure forms a highly cohesive, hydrophobic barrier that restricts passive penetration of molecules, particularly those exceeding 500 Da [82] [83].

Ionic liquids enhance transdermal permeability through multiple mechanisms:

• Lipid matrix disruption: ILs interfere with the highly organized lipid bilayers of the stratum corneum, reducing their barrier integrity [84].
• Protein conformational changes: ILs can alter the structure of keratin within corneocytes, facilitating transcellular transport [85].
• Solvation power: ILs act simultaneously as solvents and permeation enhancers, improving drug solubility and stability, particularly for labile biopharmaceuticals [85] [86].

Table 1: Key Properties of Ionic Liquids for Transdermal Drug Delivery

Property	Impact on Transdermal Delivery	Representative ILs
Tunable hydrophilicity/lipophilicity	Enables customization for specific drug properties and skin penetration pathways	Choline-based ILs, lipid-derived ILs [85]
High ionic conductivity	Suitable for integration with physical enhancement methods like iontophoresis	Imidazolium-based ILs [82]
Low volatility	Prevents solvent evaporation, ensuring consistent formulation composition and safety	Various ILs with negligible vapor pressure [85] [87]
Thermal stability	Allows for processing and storage under a wide range of temperatures	Pyrrolidinium, phosphonium-based ILs [87]

Case Study: Delivery of Biopharmaceuticals

Background: The transdermal delivery of biopharmaceuticals like proteins, peptides, and nucleic acids is particularly challenging due to their large molecular size and susceptibility to degradation. Conventional solvent-based systems often fail to provide adequate penetration or stability [85] [86].

Experimental Protocol:

• Ionic Liquid Selection and Synthesis: Lipid- or choline-derived ILs (e.g., choline geranate) are typically synthesized via metathesis reactions or acid-base neutralization. The cations and anions are selected based on desired properties like biocompatibility and enhancement efficacy [85].
• Formulation Development: The active pharmaceutical ingredient (API), such as insulin or siRNA, is incorporated into the IL. This IL-drug solution is then integrated into advanced nanocarrier systems, such as:
  • IL-in-oil microemulsions: Where the IL containing the drug is dispersed as micro-droplets in a continuous oil phase.
  • Ethosomes/Transethosomes: Lipid vesicles modified with ILs to enhance deformability and skin penetration [85].
• In Vitro Permeation Studies: The formulation is applied to excised human or animal skin (e.g., porcine skin) mounted in a Franz diffusion cell. The receptor chamber is filled with phosphate-buffered saline (PBS) at 37°C to maintain skin viability. Samples from the receptor medium are analyzed at predetermined intervals using HPLC or ELISA to quantify drug permeation [83].
• In Vivo Efficacy Testing: For diabetic models, the IL-based formulation containing insulin is applied topically to the skin of shaved animals. Blood glucose levels are monitored over time to assess the formulation's ability to achieve prolonged glycemic control [85] [86].

Results and Efficacy Validation: IL-based formulations have demonstrated significant success in preclinical studies. For instance, IL-enabled TDDS have facilitated the delivery of insulin, achieving prolonged glycemic control in diabetic models. Similarly, IL-based systems have delivered siRNA and mRNA, eliciting potent anti-tumor responses in nucleic-acid immunotherapy, thereby validating their efficacy for macromolecular delivery [85] [86].

Diagram 1: Workflow for validating IL-based transdermal delivery.

Ionic Liquids in Biomass Pretreatment

Mechanism of Biomass Deconstruction

Lignocellulosic biomass, a renewable feedstock for biofuels and chemicals, is highly recalcitrant due to the complex interlinked matrix of cellulose, hemicellulose, and lignin. Pretreatment is a critical first step to disrupt this structure and facilitate enzymatic hydrolysis [88] [89].

Ionic liquids pretreat biomass by:

• Dissolving key components: Certain ILs, like cholinium lysinate, effectively dissolve lignin and hemicellulose, reducing biomass recalcitrance [89] [90].
• Disrupting crystalline cellulose: ILs break the extensive hydrogen-bonding network in native cellulose, making it more accessible to hydrolytic enzymes [89] [87].
• Fractionation: ILs allow for the selective extraction and recovery of individual biomass components, enabling biorefining [87].

Case Study: Comparative Pretreatment of Oilcane

Background: A recent study directly compared the efficacy of hydrothermal (HT), soaking in aqueous ammonia (SAA), and ionic liquid (IL) pretreatments on transgenic lipid-accumulating sugarcane (oilcane) and its non-modified counterpart [90]. This case study provides quantitative data on sugar yields, ethanol production, and lipid recovery.

Experimental Protocol:

• Feedstock Preparation: Stems of non-modified sugarcane (CP88-1762) and transgenic oilcane lines (1565 & 1566) were juiced. The resulting bagasse was hammer-milled to a particle size of 1–2 cm and dried [90].
• Pretreatment Methods:
  • Hydrothermal (HT): Bagasse (10% w/v solid loading) was treated in a stainless-steel reactor at 180°C for 10 minutes. The mixture was dried after quenching [90].
  • Soaking in Aqueous Ammonia (SAA): Bagasse was treated with 18% ammonium hydroxide at 75°C for 3.5 hours in sealed pressure tubes, followed by extensive drying [90].
  • Ionic Liquid (IL): Bagasse was pretreated with 10% (w/w) cholinium lysinate at 140°C in a reactor, with a 15% (w/w) biomass loading [90].
• Enzymatic Hydrolysis: Pretreated biomass underwent fed-batch enzymatic hydrolysis at high solid loading to produce fermentable hydrolysates [90].
• Fermentation: The hydrolysates were fermented to assess ethanol production potential. Ethanol titer, yield, and productivity were measured [90].
• Lipid Analysis: Lipid extraction and analysis were performed on both raw and pretreated bagasse samples to determine fatty acid content and recovery [90].

Table 2: Comparative Efficacy of Biomass Pretreatment Methods on Oilcane [90]

Performance Metric	Hydrothermal (HT)	Soaking in Aqueous Ammonia (SAA)	Ionic Liquid (IL)
Total Sugar Yield (g L⁻¹)	213.10	253.73	154.20
Ethanol Titer (g L⁻¹)	64.47	100.62	52.95
Ethanol Productivity (g L⁻¹ h⁻¹)	0.53	2.08	0.36
Lipid Recovery	Higher retention	Reduced in bagasse	Reduced in bagasse
Key Inhibitors	Acetic acid	Lower inhibitor level	Residual cholinium lysinate

Results and Efficacy Validation: The study demonstrated that all three methods are industrially viable, but with distinct trade-offs. SAA pretreatment yielded the highest sugar concentration and most fermentable hydrolysate, leading to the greatest ethanol titer and productivity. IL pretreatment, while effective at deconstructing the biomass, resulted in lower sugar yields and ethanol productivities, likely due to the inhibitory effect of residual cholinium lysinate on downstream enzymes and fermenting microbes. Furthermore, ammonia and IL pretreatments reduced the total fatty acid content in the bagasse compared to hydrothermal pretreatment, a critical factor for lipid recovery in oilcane processing [90].

The Scientist's Toolkit: Essential Research Reagents and Materials

The following table details key reagents, their functions, and considerations for researchers designing experiments in IL-based transdermal delivery or biomass pretreatment.

Table 3: Essential Research Reagents and Materials

Reagent/Material	Function in Research	Specific Application Notes
Cholinium Lysinate	Biocompatible IL for biomass pretreatment and drug delivery [85] [90]	Effective for lignin dissolution; may inhibit enzymes/fermentation if not adequately removed [90].
Franz Diffusion Cell	In vitro apparatus to study drug permeation kinetics through excised skin [83]	Standard for evaluating transdermal formulations; uses skin from human donors or animal models (e.g., porcine) [82] [83].
Lipid-Derived Ionic Liquids	Multifunctional enhancers for transdermal delivery [85]	Can act as solvent and permeation enhancer; improve stability of biologics like insulin [85] [86].
Ethosomes/Transethosomes	Nanocarrier systems for enhanced skin delivery [85] [83]	Lipid vesicles often combined with ILs to improve deformability and deep skin penetration [85].
Parr Reactor	High-pressure/temperature vessel for biomass pretreatment [90]	Essential for conducting hydrothermal and IL pretreatments at controlled scales [90].
Cellulolytic Enzymes	Catalyst for hydrolyzing pretreated cellulose to fermentable sugars [88] [90]	Cocktails containing cellulases and hemicellulases; performance is highly dependent on pretreatment efficacy [90].

Diagram 2: Workflow for validating IL-based biomass pretreatment.

Cross-Disciplinary Challenges and Future Perspectives

The application of ILs in both transdermal delivery and biomass pretreatment faces shared challenges that guide future research directions. A primary challenge is the need to balance high efficacy with biocompatibility and environmental sustainability. In TDDS, this involves designing hypoallergenic ILs and formulations that minimize skin irritation [84]. In biomass processing, it necessitates developing ILs with reduced toxicity and improved biodegradability [89] [87].

Cost-effectiveness and recyclability are also critical for industrial adoption, especially in large-scale biomass pretreatment where ILs are more expensive than traditional solvents. Advances in IL recycling techniques—such as antisolvent precipitation, membrane separation, and distillation—are crucial to close the material loop and improve economic viability [89] [87]. Future research will continue to focus on the rational design of task-specific ILs with optimized properties for each application, whether it's enhancing skin permeability or selectively dissolving lignin, ultimately validating their efficacy as versatile solvents of the future.

Conclusion

The development of ionic liquids represents a paradigm shift in solvent science, moving from initial curiosity to a cornerstone of green and sustainable chemistry. The key takeaway is the evolution from first-generation ILs with notable toxicity to sophisticated, biocompatible third-generation ILs derived from natural sources like choline and amino acids. These advancements have unlocked profound implications for biomedical and clinical research, particularly in enhancing drug solubility and creating effective transdermal delivery systems. Future progress hinges on the continued design of biodegradable, non-toxic ILs, the resolution of scale-up and cost challenges, and the development of robust regulatory frameworks. As research intensifies, ionic liquids are poised to become indispensable in creating next-generation pharmaceuticals and bioprocesses, solidifying their role as truly versatile and sustainable designer solvents.

References

• 1. Ionic liquids: a brief history - PMC [https://pmc.ncbi.nlm.nih.gov/articles/PMC5988633/]
• 2. Applications of ionic in the chemical industry - Chemical... liquids [https://pubs.rsc.org/en/content/articlehtml/2008/cs/b006677j]
• 3. Ionic liquid [https://en.wikipedia.org/wiki/Ionic_liquid]
• 4. Role and Recent Advancements of Ionic Liquids in Drug ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC9963759/]
• 5. Chapter 1. Important Developments in the History of Ionic ... [https://www.nist.gov/publications/chapter-1-important-developments-history-ionic-liquids-academic-curiosity-commercial]
• 6. Thirty years of ionic liquids: a personal perspective [https://www.degruyterbrill.com/document/doi/10.1515/pac-2025-0440/html?recommended=sidebar&srsltid=AfmBOoqXjWiNyvhSlDE4vID0m-5S3LrgR3jzncX6bczBOu8kpkdi8IYx]
• 7. A review on the evolution of ionic liquids: Sustainable ... [https://www.sciencedirect.com/science/article/pii/S2589234725000892]
• 8. Ionic Liquids in Pharmaceutical and Biomedical Applications [https://www.mdpi.com/1999-4923/16/1/151]
• 9. An Overview on the Role of Ionic Liquids and Deep ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11946670/]
• 10. A Review of the State of the Art on Ionic Liquids and Their ... [https://www.mdpi.com/1996-1073/18/15/4053]
• 11. Glycerol-derived ionic liquids: a new family of high-potential ... [https://pubs.rsc.org/en/content/articlehtml/2025/su/d5su00393h]
• 12. Ionic liquids: An innovative approach in drug delivery [https://www.sciencedirect.com/science/article/pii/S2590238524002376]
• 13. Solvate ionic liquids: past, present and future [https://pubs.rsc.org/en/content/articlelanding/2025/ta/d5ta01406a]
• 14. Ionic liquids revolutionizing biomedicine: recent advances and ... [https://pubs.rsc.org/en/content/articlehtml/2023/cs/d3cs00510k]
• 15. Recent Advances in Biocompatible Ionic Liquids in Drug ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10145603/]
• 16. Ionic liquids as biocompatible stabilizers of proteins - PMC [https://pmc.ncbi.nlm.nih.gov/articles/PMC5988623/]
• 17. Ionic liquids revolutionizing drug delivery [https://www.sciencedirect.com/science/article/abs/pii/S0168365925009691]
• 18. Ionic liquid-based transdermal drug delivery systems for ... [https://pubs.rsc.org/en/content/articlehtml/2025/cc/d5cc03935e]
• 19. Development, Characterization, and Evaluation of CAGE ... [https://pubmed.ncbi.nlm.nih.gov/41082503/]
• 20. Ionic Liquid Market Size, Trends, Share & Forecast Report ... [https://www.mordorintelligence.com/industry-reports/ionic-liquid-market]
• 21. A novel system based on a task-specific pyrrolinium ... [https://pubs.rsc.org/en/content/articlelanding/2025/nj/d5nj01661d]
• 22. Thirty years of ionic liquids: a personal perspective [https://www.degruyterbrill.com/document/doi/10.1515/pac-2025-0440/html?recommended=sidebar&srsltid=AfmBOopA2eXQPO5Q_kKbmWoILGyzA2Nhoo1TQcHIuhMRMWy2nrYZ2wHs]
• 23. Cationic alkyl chain length and nanoaggregate form of ... [https://www.nature.com/articles/s41467-025-62206-x]
• 24. Ionic liquid toxicity prediction and mechanism research ... [https://www.sciencedirect.com/science/article/abs/pii/S0009250925014903]
• 25. Expanding the chemical space of ionic liquids using ... [https://chemrxiv.org/engage/chemrxiv/article-details/6909c083113cc7cffffc2961]
• 26. Advances in ionic liquid recycling for lignocellulosic biomass ... [https://pubs.rsc.org/en/content/articlehtml/2025/gc/d5gc03051j?page=search]
• 27. Challenges and opportunities on the utilisation of ionic ... [https://www.nature.com/articles/s44296-024-00015-x]
• 28. Process intensification of the ionoSolv pretreatment [https://pmc.ncbi.nlm.nih.gov/articles/PMC8319198/]
• 29. A Review: Using Ionic Liquids for Lignin Extraction from ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12196028/]
• 30. Efficient extraction of carboxylated nanocellulose from ... [https://link.springer.com/article/10.1007/s10570-024-06319-4]
• 31. The evolution of Lignin's role in enzymatic Hydrolysis [https://www.sciencedirect.com/science/article/abs/pii/S0961953425005100]
• 32. Ionic liquids as potentially hazardous pollutants: Evidences ... [https://www.sciencedirect.com/science/article/pii/S0304389422011463]
• 33. A review on assessment of ionic liquids in extraction of lithium ... [https://pubs.rsc.org/en/content/articlehtml/2025/ra/d4ra08429b]
• 34. the expanding role of ionic liquids in dermatological drug ... [https://pubmed.ncbi.nlm.nih.gov/40945807/]
• 35. Ionic Liquids Boost Skin Drug Delivery [https://www.dermatologytimes.com/view/ionic-liquids-boost-skin-drug-delivery]
• 36. Performance and Synthesis of Ionic Liquid [https://www.crt.tf.fau.eu/forschung/arbeitsgruppen/komplexe-katalysatorsysteme-und-kontinuierliche-verfahren/performance-und-synthese-von-ionischen-flussigkeiten/]
• 37. Review Ionic liquids synthesis and applications: An overview [https://www.sciencedirect.com/science/article/abs/pii/S0167732219333719]
• 38. Electrochemical carbon capture processes for mitigation of CO ... [https://pubs.rsc.org/en/content/articlehtml/2022/cs/d2cs00443g]
• 39. The life-or-death race to improve carbon capture - C&EN [https://cen.acs.org/environment/greenhouse-gases/capture-flue-gas-co2-emissions/99/i26]
• 40. Carbon dioxide capture and green conversion to clean ... [https://link.springer.com/article/10.1007/s42114-024-00955-x]
• 41. Electrochemical Engineer @ RenewCO2 [https://befjobs.breakthroughenergy.org/companies/renewco2/jobs/32424237-electrochemical-engineer]
• 42. An insight into pharmaceutical challenges with ionic liquids [https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2024.1454247/full]
• 43. Recent Advances in Biocompatible Ionic Liquids in Drug ... [https://www.mdpi.com/1999-4923/15/4/1179]
• 44. a critical review of amino acid-based ionic liquids versus ... [https://www.sciencedirect.com/science/article/pii/S2772422025000394]
• 45. Diversity of Potential (Bio)Technological Applications ... [https://www.mdpi.com/2076-3417/15/3/1515]
• 46. Revisiting Ionic Liquid Structure-Property Relationship [https://pmc.ncbi.nlm.nih.gov/articles/PMC7589445/]
• 47. Thirty years of ionic liquids: a personal perspective [https://www.degruyterbrill.com/document/doi/10.1515/pac-2025-0440/html?recommended=sidebar&srsltid=AfmBOorS4-F3OTfOH8T4QE1hhK_EWzUiVYDe_SQZsq6nsdJkez5wnZzp]
• 48. Emerging impacts of ionic liquids on eco-environmental ... [https://pubs.rsc.org/en/content/articlelanding/2021/cs/d1cs00946j]
• 49. Mechanistic insights into the cytotoxic and apoptotic effects ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12455699/]
• 50. A comprehensive dataset on cytotoxicity of ionic liquids [https://www.nature.com/articles/s41597-024-04190-3]
• 51. Benzyl-Functionalised Imidazolium Ionic Liquids as Lead ... [https://pubmed.ncbi.nlm.nih.gov/41284710/?utm_source=FeedFetcher&utm_medium=rss&utm_campaign=None&utm_content=1NosRw4HLZNZZTZqMNWWphauvW4RIAfGeq9QmvGYx9qcTUosbe&fc=None&ff=20251124231905&v=2.18.0.post22+67771e2]
• 52. Biocompatible ionic liquids and their applications in ... [https://pubs.rsc.org/en/content/articlelanding/2020/gc/d0gc02387f]
• 53. Scale-up challenges for ionic liquid processes: economic ... [https://eureka.patsnap.com/report-scale-up-challenges-for-ionic-liquid-processes-economic-models]
• 54. Ionic liquids-mediated technologies for metals recycling ... [https://www.sciencedirect.com/science/article/abs/pii/S1383586625026875]
• 55. Research progress of ionic liquids in recycling waste ... [https://www.sciencedirect.com/science/article/abs/pii/S1383586625034471]
• 56. Ionic Liquids Market Size to Hit USD 136.18 Million by 2034 [https://www.precedenceresearch.com/ionic-liquids-market]
• 57. Imidazolium-based ionic liquids as corrosion inhibitors for ... [https://www.sciencedirect.com/science/article/pii/S2238785424000668]
• 58. How to prevent industrial drive corrosion: 5 strategies [https://www.controleng.com/how-to-prevent-industrial-drive-corrosion-5-strategies/]
• 59. The complete guide to corrosion protection in industrial ... [https://dstchemicals.com/resources/knowledge/the-complete-guide-to-corrosion-protection-in-industrial-settings]
• 60. Ways to Mitigate Metal Corrosion in Your Industrial Plant [https://www.knowltonindustrialsteel.com/ways-to-mitigate-metal-corrosion-in-your-industrial-plant]
• 61. Interphase engineering of reactive metal surfaces using ... [https://www.nature.com/articles/s41529-017-0016-z]
• 62. Corrosion behaviour of ionic liquids [https://pubs.rsc.org/en/content/articlelanding/2005/gc/b419320m]
• 63. Four Ways to Prevent Corrosion in Industrial Components [https://htscoatings.com/blogs/our-craft-our-culture/four-ways-to-prevent-corrosion-in-industrial-components]
• 64. Role and Recent Advancements of Ionic Liquids in Drug ... [https://www.mdpi.com/1999-4923/15/2/702]
• 65. Ionic Liquids in Pharmaceutical and Biomedical Applications [https://pmc.ncbi.nlm.nih.gov/articles/PMC10818567/]
• 66. Recent Developments on Ionic Liquids and Deep Eutectic ... [https://link.springer.com/article/10.1007/s11095-022-03315-w]
• 67. Some Solvents [https://greensciencepolicy.org/harmful-chemicals/some-solvents/]
• 68. Green solvents and their role in modern sample ... [https://link.springer.com/article/10.1007/s44371-025-00325-6]
• 69. New recycled polyethylene terephthalate imidazolium ionic ... [https://www.nature.com/articles/s41598-025-15433-7]
• 70. Green Solvents Market Research 2025-2035 [https://finance.yahoo.com/news/green-solvents-market-research-2025-084800250.html]
• 71. Aqueous Ionic Liquid Process for Chemical Recycling of ... [https://ipo.lbl.gov/2025/06/16/2021-190-aqueous-ionic-liquid-process-for-chemical-recycling-of-plastics/]
• 72. Recent progress and future perspectives of ionic liquid ... [https://www.sciencedirect.com/science/article/pii/S2468025724002838]
• 73. Green chemistry: Six key trends to watch [https://www.cas.org/resources/cas-insights/green-chemistry-trends]
• 74. Green Analytical Approaches and Eco-Friendly Solvents [https://www.orientjchem.org/vol41no2/green-analytical-approaches-and-eco-friendly-solvents-advancing-industrial-applications-and-environmental-sustainability-a-comprehensive-review/]
• 75. Green and sustainable solvents of the future: Deep eutectic ... [https://www.sciencedirect.com/science/article/abs/pii/S0167732223004798]
• 76. Ionic liquids as environmental hazards – Crucial data in ... [https://www.sciencedirect.com/science/article/abs/pii/S0304389420318859]
• 77. Ionic Liquids: Transforming Sustainability in the U.S. Market [https://roco.global/ionic-liquids-us-market-sustainable-innovation/?srsltid=AfmBOopcnnxYTc5ff_q5YeMGvULsslRNmNh-5WWUKR8qOYaszS9_uHBS]
• 78. Deep eutectic solvent [https://en.wikipedia.org/wiki/Deep_eutectic_solvent]
• 79. Deep Eutectic Solvent - an overview [https://www.sciencedirect.com/topics/materials-science/deep-eutectic-solvent]
• 80. Exploring ionic liquids and deep eutectic solvents for ... [https://www.nature.com/articles/s44296-024-00042-8]
• 81. Challenging DESs and ILs in the valorization of food waste [https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1270221/full]
• 82. Advances in Transdermal Drug Delivery Systems and Clinical ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12195946/]
• 83. Transdermal Drug Delivery Systems: Methods for Enhancing ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12300492/]
• 84. Review article Transdermal drug delivery system [https://www.sciencedirect.com/science/article/pii/S2352952025000143]
• 85. Ionic Liquid-Based Transdermal Drug Delivery Systems for ... [https://pubs.rsc.org/en/content/articlelanding/2025/cc/d5cc03935e]
• 86. Ionic liquid-based transdermal drug delivery systems for ... [https://pubmed.ncbi.nlm.nih.gov/40985481/]
• 87. Exploring the versatility of ionic liquids in biomass utilization [https://www.sciencedirect.com/science/article/abs/pii/S0167732225014278]
• 88. Recent developments and emerging methodologies in the ... [https://www.sciencedirect.com/science/article/pii/S294983922500080X]
• 89. Advances in ionic liquid recycling for lignocellulosic ... [https://pubs.rsc.org/en/content/articlelanding/2025/gc/d5gc03051j]
• 90. Evaluating the industrial potential of emerging biomass ... [https://pubs.rsc.org/en/content/articlehtml/2025/se/d5se00519a]