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.