Nanostructural Reorganization Manifests in Sui-Generis Density Trend of Imidazolium Acetate/Water Binary Mixtures

Ionic liquids (ILs) are emerging as a novel class of solvents in chemical and biochemical research. Their range of applications further expands when a small quantity of water is added. Thus, the past decade has seen extensive research on IL/water binary mixtures. While the thermophysical properties of most of these mixtures exhibited the expected trend, few others have shown deviations from the general course. One such example is the increase in density of the 1-alkyl-3-methyl imidazolium acetate ([R<inf>n</inf>mim][Ac])-based ILs with the addition of low to moderate concentrations of water. Although such a unique trend was observed for imidazolium cations of different tail lengths and also from independent experiments, the molecular basis of this unique behavior remains unknown. In this study, we examine the nanostructural reordering in [R<inf>n</inf>mim][Ac] (n = 2-6) ILs due to added water by means of molecular dynamics simulations, and correlate the observed changes to the sui-generis density trend. Results suggest that the initial rise in density in these ILs mainly pertains to the water-induced increased spatial correlation among the polar components, where high basicity of the acetate anion plays a key role. At moderate water concentration, the density can rise further for ILs with longer cation tails due to hydrophobic clustering. Thus, while [emim][Ac]/water mixtures exhibit the density turnover at X<inf>w</inf> = 0.5, [bmim][Ac] and [hmim][Ac] show the turnover at X<inf>w</inf> = 0.7. The detailed understanding provided here could help the preparation of optimal IL/water binary mixtures for various biochemical applications. (Graph Presented).