Predicting Entropic Effects of Water Mixing with Ionic Liquids Containing Anions of Strong Hydrogen Bonding Ability: Role of the Cation.

Ionic liquids (ILs) such as choline dihydrogen phosphate exhibit an extraordinary solubilizing ability for proteins such as cytochrome C when mixed with 20 wt % water. Most widely used imidazolium-based ionic liquids coupled with dihydrogen phosphate do not exhibit the same solubilizing properties, suggesting that a multifunctional cation such as choline might play a key role in enhancing these properties of ionic liquid mixtures with water. In this theoretical work, we compare intermolecular interactions between the water molecule and ionic liquid ions in two ion-paired clusters of choline- and 1-butyl-3-methyl-imidazolium-based ionic liquids coupled with acetate, dihydrogen phosphate, and mesylate. Gibbs free energy (GFE) of solvation of water in these ionic liquids was calculated. Incorporation of a water molecule into ionic liquid clusters was accompanied by negative GFEs of solvation in both types of cations. These results were in good agreement with previously reported experimental GFEs of solvation of water in ILs. Compared to imidazolium-based clusters, strong interionic interactions of choline ionic liquids resulted in more negative GFEs due to their smaller deformation upon the addition of a water molecule, with dihydrogen phosphate and mesylate predicting the lowest GFEs of -30.1 and -43.5 kJ/mol-1, respectively. Lower GFEs of solvation of water in choline-based clusters were also accompanied with smaller entropic penalties, suggesting that water easily incorporates itself into the existing ionic network. Analysis of the intramolecular bonds within the water molecule showed that the choline hydroxyl group donates electron density to the neighboring water molecule, leading to additional polarization. The predicted infrared spectra of clusters of ionic liquids with water showed a pronounced red shift due to strongly polarized O-H bonds, in excellent agreement with the experimentally measured infrared spectra of ionic liquid mixtures with water. Increased polarization of water in choline-based ionic liquids undoubtedly creates more effective solvents for stabilizing biological molecules such as proteins.