Molecular dynamics simulations of K+–Cl− ion pair in polar mixtures of acetone and water: Preferential solvation and structural studies

Abstract

Molecular dynamics simulation studies have been performed for a series of polar mixtures of acetone and water containing a K+–Cl− ion pair. We have obtained the potentials of mean force (PMFs) for 15 mixtures spanning the whole composition range from 0% to 100% acetone. The PMFs indicate the presence of a stable contact ion pair (CIP), a solvent assisted ion pair (SAIP) and a solvent separated ion pair (SSIP) in all the compositions with acetone mole fraction (xacetone)≤0.50. However, for xacetone>0.50 and xacetone≤0.90, PMFs suggest the presence of only a stable CIP and SAIP. For xacetone>0.90, PMFs show the existence of only a stable CIP. While the stability of CIPs in mixtures increases with xacetone, the reverse trend is observed for SAIPs. The determination of thermodynamic properties suggests that entropy favours the CIPs and SAIPs in all the mixtures. The analysis of radial distribution functions using Kirkwood–Buff (KB) integrals explains the preferential solvation of the ion pair. We observed that in all the mixtures, K+–Cl− ion pair is preferentially solvated by water. Running coordination numbers of solvent molecules around the ion pair and preferential binding parameters also support the above observations. The dynamical aspects have been explored by calculating self-diffusion constants and hydrogen bond dynamics.