Water selectively solvates a solute in ionic liquid and water mixtures: dependence on ionic liquid type

Ionic liquids (ILs), which differ from conventional salts in terms of weaker electrostatic interactions, possess lower melting points and are gaining traction as “third solvents” alongside water and organic solvents. Recently, some ILs have been reported to have a pronounced ability to preserve the structure and function of proteins upon the addition of a small amount of water. To clarify the underlying mechanism of protein preservation in IL/water mixtures, in this study, we investigated the solvation structure around a solute in aqueous solutions of three ILs with different protein-stabilizing abilities, viz., choline dihydrogen phosphate ([ch][dhp]), choline bromide ([ch][Br]), and 1-butyl-3-methylimidazolium chloride [C₄mim][Cl]. Small-angle X-ray scattering studies revealed that these ILs formed water-rich solvation layers around a silica particle used as a model solute. The [ch][dhp]/water system, which has the most effective protein-stabilizing ability, was found to have the thickest solvation layer with a water content of 97–87 %. The thickness of the solvation layer increased as the water content was decreased, reaching approximately three layers of water molecules. Infrared spectral studies revealed that the hydrogen bonds between the water molecules in the solvation layer were broken to some extent. Molecular dynamics simulations indicated that water molecules aggregated in the binary mixture of [ch][dhp] and water. This result suggests that the aggregated water selectively approached the solute surface to form a water-rich solvation layer in the [ch][dhp]/water system. In contrast, the [ch][Br]/water and [C₄mim][Cl]/water systems had less pronounced water-rich layers. The strong correlation between solvation layer formation and protein stabilization suggests that the protein-stabilizing effect of IL/water systems is achieved through the formation of water-rich solvation layers, which is supported by the observed IL concentration dependence of the solvation layer thickness.