Water-mediated weakening of inter-ionic interactions in aqueous mixtures of ionic liquid: An investigation combining quantum chemical calculations and molecular dynamics simulations

Abstract

Quantum chemical calculations and molecular dynamic simulations have been performed to understand the structure and energetics of ionic liquid (IL) in presence of water. Addition of water is found to induce charge flow from the anion to cation, reducing the strength of the electrostatic interactions between the ion-pair without changing the interionic distance. Composition dependent isotropic (α0) and anisotropic (αaniso) components of the total polarizability do not show any dependence on the number of water molecules present in the (IL + water) binary mixture. Vibrational density of states for ions obtained from simulated velocity auto correlation function do not show any composition dependence. These provide much-needed explanation for the insensitivity of spectral shift to water concentration reported in OHD-RIKES measurements with IL + water mixtures. Continuous and structural H-bond lifetimes predict a decrease of both with water concentration. This can be understood in terms of concomitant reductions in inter-ionic electrostatic interactions and solution viscosity.