Solvation structure of sodium chloride (Na+–Cl−) ion pair in dimethyl sulfoxide–acetonitrile mixtures

Abstract

Constrained molecular dynamics method has been used to compute the potentials of mean force (PMFs) of Na+-Cl− in dimethyl sulfoxide (DMSO)–acetonitrile (AN) binary mixtures. The PMFs are confirmed by calculating the residence times of the ion pair at various inter-ionic separations. Contact ion pairs (CIPs) are found to be more stable than the solvent shared ion pairs (SShIPs). The stabilities of CIPs generally increase with increase in the mole fraction of AN. The running coordination number analysis shows that the coordination number of Na+ in pure acetonitrile is 4.6, close to the experimental value by FTIR and refractometric study. The preferential solvation study through the excess coordination numbers shows that both Na+ and Cl− are preferentially solvated by DMSO, unlike the water–DMSO mixtures.