How water manifests the structural regimes in ionic liquids

Area of Science:

• Physical Chemistry
• Materials Science

Background:

• Ionic liquids (ILs) offer greener solvent alternatives but suffer from high viscosity.
• IL/water mixtures reduce viscosity while retaining IL-like properties up to a critical water concentration.

Purpose of the Study:

• Determine the critical IL-water ratio where mixture properties transition from IL-dominated to water-dominated.
• Investigate thermophysical properties of tetramethylguanidinium benzoate/water and tetramethylguanidinium salicylate/water mixtures.

Main Methods:

• Experimental measurement of density, compressibility, and viscosity for IL/water mixtures (20-95 wt% water) from 298 K to 343 K.
• Molecular Dynamics (MD) simulations to elucidate experimental observations and structural behavior.

Main Results:

• Mixtures up to 50 wt% water showed linear density/compressibility and rapid viscosity decrease with temperature, similar to neat ILs.
• Higher water concentrations ( >50 wt%) exhibited nonlinear density/compressibility and minimal viscosity change with temperature, mimicking bulk water.
• MD simulations revealed temperature-induced disintegration of IL ion-water interactions and distinct structures in concentrated vs. dilute solutions.

Conclusions:

• Established the critical water concentration threshold for IL/water mixture property transitions.
• Provided molecular-level insights into the structure-property relationships of IL/water mixtures using MD simulations.
• Highlighted the importance of water content for tailoring IL/water mixture properties for specific applications.