How water manifests the structural regimes in ionic liquids

Ionic liquids (ILs) are being considered as greener alternatives to the conventional organic solvents. However, highly viscous nature of ILs often limits their applications. Hence studies on IL/water binary mixtures have received tremendous attention. These mixtures exhibit much lower viscosity, but almost similar density, compressibility and other properties as that of the neat ILs, up to certain water content. Hence, determining the IL-water ratio till which the solution behaves like IL and subsequently changes to a state of solute IL dissolved in continuous water phase is of paramount importance. Noting the very different and characteristic behaviours of neat ILs and pure water over a temperature range, herein, we measured the various thermophysical properties of the binary mixtures of tetramethylguanidinium benzoate/water and tetramethylguanidinium salicylate/water with water content varying from 20 wt% to 95 wt% for a temperature range of 298 K to 343 K. The results show that similar to neat ILs, the measured densities and compressibility of these mixtures display a linear change, and viscosity decreases rapidly as temperature is increased for water content up to 50 wt%. At higher water concentrations, the measured density and compressibility exhibit nonlinear behaviour and the decrease in viscosity with increased temperature is minute, mimicking the behaviour of bulk water. MD simulations were carried out to explain the experimental observations. Simulation results show a greater temperature-induced disintegration of IL ion-water interactions in dense systems, explaining the rapid decay of the properties with temperature. The results also exhibit the presence of a neat, IL-like, H-bond mediated expanded structure in concentrated solution versus a collapsed IL structure in dilute solution.