Solvation structure and dynamics of potassium chloride ion pair in dimethyl sulfoxide-water mixtures

Structure and dynamics of potassium chloride ion pair in mixtures of dimethyl sulfoxide (DMSO) and water have been investigated using constrained molecular dynamics. We have obtained the potentials of mean force (PMFs) between the K+ and Cl− ions at 298 K over a wide concentration range of these mixtures. The results obtained are compared for relative stabilities of contact ion pair (CIP), solvent assisted ion pair (SAIP) and solvent separated ion pair (SSIP) in the mixtures with varying compositions. The stability of the CIPs decreases significantly as the mole fraction of water increases. The study of running coordination numbers at the first solvation shell suggests preferential solvation of ions by water in all the compositions and there is hardly any participation by DMSO molecules in the first solvation shell of the Cl− ion. The extent of association of potassium chloride ion pair in water, DMSO and their mixtures is determined by calculating the association constants. We have calculated the diffusion constants of ions and solvent molecules in the first solvation shell around K+ and Cl− in all the compositions. The diffusion constants of the shell molecules are the smallest in the DMSO mole fraction ranging from 0.2 to 0.5.