Na+ Cl− ion pair association in water-DMSO mixtures: Effect of ion pair model potentials

Potentials of Mean Force (PMF) for the Na + Cl − ion pair in water–dimethyl sulfoxide (DMSO) mixtures for three DMSO mole fractions have been computed using constrained Molecular Dynamics (MD) simulations and confirmed by dynamical trajectories and residence times of the ion pair at various inter-ionic separations. The three ion-ion direct potentials used are 12-6-1, exp-6-1 and exp-8-6-1. The physical picture that emerges is that there is a strong contact ion pair (CIP) and strong to moderate solvent separated ion pair (SSIP) in these solutions. Analysis of local ion clusters shows that ions are dominantly solvated by water molecules. The 12-6-1 potential model predicts running coordination numbers closest to experimental data. Graphical Abstract The effect of the differences between the 12-6-1, Huggins-Mayer (exp-6-1) and Fumi-Tosi (exp-6-8-1) ion-ion potential models on the solvation structure of the sodium chloride ion pair in water-DMSO binary mixtures have been studied.