Potentials of mean force of sodium chloride ion pair in dimethyl sulfoxide–methanol mixtures

Abstract

The constrained molecular dynamics technique has been used to simulate solutions of sodium chloride ion pair in pure methanol (MeOH), pure dimethyl sulfoxide (DMSO) and DMSO–MeOH mixtures with MeOH mole fractions (xMeOH) of 0.25, 0.50 and 0.75. The potentials of mean force (PMFs) of the sodium chloride ion pair have been computed for all the above compositions. The PMFs for the pure solvents indicate that contact ion pairs (CIPs) are more stable than the solvent separated ion pairs (SSIPs). In the mixtures, contact ion pairs (CIPs) dominate the PMF and solvent assisted ion pairs (SAIPs) persist in all the compositions. As the mole fraction of DMSO increases, the stability of CIP increases. In all three solvent mixtures, the Na+Cl− ion pair is preferentially solvated by MeOH molecules. These results have been confirmed by dynamical ion pair trajectories for long times. A study of the running coordination numbers in the CIP and SAIP/SSIP configurations shows that the number of DMSO and methanol molecules around the ion pair does not change significantly with composition beyond xMeOH=0.50.