Structural effects on dynamic and energetic properties of mixtures of ionic liquids and water

The aim of this study is to improve our understanding of the microscopic and macroscopic properties of mixtures of ionic liquids with water, in the context of working pairs for absorption heat cycles. We report a molecular dynamics study of dynamic properties (viscosity and diffusion coefficients), water solvation (free energy and local solvation environments) and hydrogen bonding in mixtures of six ionic liquids with water, at two concentrations xH2O = 0.104 and xH2O = 0.900. Three anions, methanesulfonate, dicyanamide and acetate; and two cations, N-ethylpyridinium and cholinium, were chosen due to their potential for water absorption and halogen-free structures. Simulation results capture the trends of experimental data, and were interpreted in terms of the molecular structures and interactions. The strength of hydrogen bonding is a major criterion determining the affinity of an ionic liquid towards water. In particular, the cholinium cations compete with water establishing hydrogen bonds with the acetate anions and this is not favourable in terms of water affinity. Dicyanamide anions lead to the systems with lower viscosity.