Protic Ionic Liquids and Their Surprising Effects on Human Cells
Imagine a class of liquids so versatile they could revolutionize industries from pharmaceuticals to energy, all while being kinder to our environment. This is the promise of ionic liquids—remarkable salts that remain liquid at relatively low temperatures. Unlike traditional volatile solvents that evaporate into the air we breathe, ionic liquids are salts in liquid form that barely vaporize, reducing airborne pollution and inhalation risks.
Ionic liquids reduce airborne pollution due to their extremely low vapor pressure.
Used in pharmaceuticals, energy storage, catalysis, and separation processes.
Among these, protic ionic liquids (PILs) have recently emerged as particularly promising candidates. Created through a simple reaction between an acid and a base, PILs offer an even more environmentally friendly profile with potentially lower toxicity and higher biodegradability. But are these "green" solvents truly safe for human health? A groundbreaking 2019 study set out to answer this very question by examining how PILs affect human liver and skin cells, with surprising results that could shape the future of sustainable chemistry.
Think of table salt, which becomes liquid only at extremely high temperatures (around 800°C). Now imagine salts that remain liquid below 100°C, some even at room temperature. These are ionic liquids—liquid electrolytes composed entirely of ions (charged particles).
The secret to their liquid state lies in their bulky, irregularly shaped ions. Unlike the neat, compact structure of sodium chloride, the ions in ionic liquids don't pack together efficiently, preventing them from easily forming solids. This same structural feature allows scientists to "designer" their properties by swapping different cations and anions.
Not all ionic liquids are created equal. The key distinction lies in how they're formed:
This fundamental difference gives PILs significant advantages. They're generally simpler to produce, more biodegradable, and often less toxic than their aprotic counterparts.
| Property | Protic Ionic Liquids (PILs) | Aprotic Ionic Liquids (AILs) |
|---|---|---|
| Synthesis | Simple acid-base reaction | Complex multi-step synthesis |
| Biodegradability | Generally higher | Often lower |
| Toxicity | Typically lower | Can be significant |
| Cost | More economical | Often expensive |
To truly understand how PILs might affect the human body, researchers turned to two specifically chosen cell types:
This dual approach provided crucial insights into potential effects on both internal organs and external barriers—particularly important for workers who might handle these chemicals in industrial settings.
The research team synthesized 13 different PILs using a straightforward but precise method 1 . The process began with placing either monoethanolamine or diethanolamine in a specialized glass flask cooled in an ice bath.
This reaction continued for 24 hours at laboratory temperature, resulting in final viscous yellow liquids. But the process didn't stop there—the PILs underwent an additional 48-hour purification process under slight heating and reduced pressure to eliminate any residual reagents and atmospheric moisture.
| Cation | Anion | PIL Name | Abbreviation |
|---|---|---|---|
| 2-Hydroxy ethylammonium | Formate | 2-Hydroxy ethylammonium formate | 2-HEAF |
| 2-Hydroxy ethylammonium | Acetate | 2-Hydroxy ethylammonium acetate | 2-HEAA |
| 2-Hydroxy ethylammonium | Lactate | 2-Hydroxy ethylammonium lactate | 2-HEAL |
| 2-Hydroxy diethylammonium | Acetate | 2-Hydroxy diethylammonium acetate | 2-HDEAA |
| 2-Hydroxy diethylammonium | Benzoate | 2-Hydroxy diethylammonium benzoate | 2-HDEABe |
| 2-Hydroxy diethylammonium | Citrate | 2-Hydroxy diethylammonium citrate | 2-HDEACi |
The core of the experiment involved exposing the HepG2 and HaCaT cells to various concentrations of the 13 PILs and measuring cell survival. Researchers used standardized laboratory tests that measure cell viability—essentially determining what percentage of cells remained healthy after exposure compared to untreated cells.
The key metric was the IC50 value—the concentration of a substance that inhibits 50% of cell growth or causes 50% cell death. Lower IC50 values indicate higher toxicity, while higher values suggest lower toxicity.
The experimental results offered encouraging news for the future of PILs. The tested protic ionic liquids demonstrated remarkably low toxicity toward both human liver and skin cell lines 1 5 7 .
While specific numerical IC50 values weren't provided in the available excerpts, the authors explicitly described the toxicity as "remarkably low" across the studied in vitro systems. This consistent pattern across multiple PIL structures and two different human cell types suggests that protic ionic liquids may genuinely represent a safer alternative to traditional solvents and some aprotic ionic liquids.
The study also ventured beyond laboratory experiments to create a predictive computer model. Using a Group Contribution QSAR Model (Quantitative Structure-Activity Relationship), researchers successfully correlated the structural features of PILs with their cytotoxicity 1 . The strong correlation (R² = 0.9260) between predicted and experimental values means scientists may eventually be able to forecast the toxicity of new PILs before even synthesizing them, potentially accelerating the development of truly green solvents.
PILs demonstrated remarkably low toxicity toward human liver and skin cells.
Strong correlation (R² = 0.9260) between predicted and experimental toxicity values.
| Mechanism | Effect on Cells |
|---|---|
| Membrane disruption | Alters lipid distribution and membrane integrity |
| Mitochondrial dysfunction | Interferes with cellular energy production |
| Oxidative stress | Generates reactive oxygen species that damage cellular components |
| Protein/enzyme interference | Disrupts normal cellular functions |
| DNA damage | Causes genetic material fragmentation |
The fact that the tested PILs showed low toxicity despite these potential mechanisms suggests their specific chemical structures may avoid triggering these harmful pathways.
The initial enthusiasm for ionic liquids as universally "green" solvents faced a reality check when studies began revealing their potential ecological and health impacts. Research has shown that ionic liquids can:
This understanding has shifted the perspective from assuming all ionic liquids are environmentally friendly to carefully evaluating each new candidate—exactly what the featured study accomplished with protic ionic liquids.
Initial enthusiasm for ionic liquids as "green" alternatives to traditional solvents.
Studies reveal some ionic liquids display significant toxicity and environmental persistence.
Increased focus on protic ionic liquids as potentially safer alternatives.
Study shows remarkably low cytotoxicity of PILs in human liver and skin cells.
Continued research examining thiocyanate-containing PILs' effects on human dermal fibroblasts 2 .
The investigation into protic ionic liquids represents more than just an isolated laboratory study—it embodies the evolving understanding of what truly constitutes "green" chemistry. The promising results from the HepG2 and HaCaT cell experiments suggest that carefully designed PILs could deliver on the early promise of ionic liquids as environmentally safer alternatives to traditional solvents.
Perhaps most importantly, the successful correlation between experimental results and computational models points toward a future where we can design safety into chemicals from their very conception, rather than discovering toxicity after widespread use.
| Material/Reagent | Function in the Study |
|---|---|
| HepG2 cell line | Models human liver response to chemical exposure |
| HaCaT cell line | Represents human skin contact and barrier function |
| Monoethanolamine/Diethanolamine | Provides cations for PIL synthesis |
| Carboxylic acids (formic, acetic, lactic, etc.) | Provides anions for PIL synthesis |
| Cell viability assays | Measures cytotoxic effects on human cells |
| Vibrational tube densimeter | Characterizes physicochemical properties of synthesized PILs |
Human liver carcinoma cell line used for toxicity screening.
Human keratinocyte cell line modeling skin exposure.
Computational approach predicting toxicity from molecular structure.
The journey toward truly sustainable solvents continues, but protic ionic liquids have undoubtedly emerged as compelling candidates worthy of both optimism and careful scientific scrutiny.
The promising results from cellular studies combined with predictive computational models represent a significant step forward in designing safer chemicals for industrial applications. As research continues to expand, the scientific community moves closer to realizing the full potential of these remarkable liquids without compromising human health or environmental integrity.