Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven Accumulation of Water Molecules at Interfaces

Abstract

This work investigates the influence of uncharged interfaces on the distribution of water molecules in three aqueous dialkylimidazolium-based ionic liquid mixtures at various water concentrations. The results are based on atomistic molecular dynamics (MD) simulations supported by sum-frequency generation (SFG) experiments. All outcomes highlight an entropically driven accumulation of water molecules in front of interfaces with slight, but technologically relevant differences. Our findings reveal that the local water density depends crucially on the water mole fraction, local ordering effects, and the molecular structure of the ionic liquids (ILs). We unravel the influence of hydrophobicity/hydrophilicity and bulkiness of the ions, as well as the effect of water in defining the role of the ILs as a main solvent, a cosolvent or cosolute. The outcome of this study allows the definition of reliable criteria for beneficial water-IL combinations in view of distinct applications.