Solvation structures of sodium halides in dimethyl sulfoxide (DMSO)–methanol (MeOH) mixtures

A constrained molecular dynamics technique has been used to study the structures and dynamics of the solvation shells of three sodium halides, namely sodium chloride (Na+–Cl−), sodium bromide (Na+–Br−) and sodium iodide (Na+–I−) in DMSO–MeOH mixtures. In the case of Na+–Cl− and Na+–Br−, Na+ is preferentially solvated by DMSO and Cl− and Br− are preferentially solvated by methanol in the contact ion pair (CIP) state. In the solvent-assisted ion pair (SAIP) configuration, Na+ ions of Na+–Cl− and Na+–Br− are preferentially solvated by methanol and Cl− and Br− also show preferential solvation by methanol over DMSO. In the case of Na+–I−, the only preferential solvation is in the SAIP state for I− ion by methanol. These observations are supported by the calculated excess coordination numbers and spatial density maps. The heights of the transition states barriers for CIPs and SAIPs/solvent-shared ion pairs (SSHIPs) are significantly affected when the mole fraction of methanol (xMeOH) changes from 0.0 to 0.25 because of a significant increase in the methanol density around halides. From the analysis of angular distribution functions of DMSO and methanol around the cations and anions, it is seen that DMSO and methanol molecules are present in parallel dipolar orientations (with respect to cation–solvent vector) in the first coordination shell of these three ion pairs at the CIP and SAIP states. Methanol molecules are nearly in an antiparallel (with respect to ion–solvent vector) orientation around the three halide ions.