Model dependence of solvent separated sodium chloride ion pairs in water-DMSO mixtures

Constrained molecular dynamics simulations have been used to investigate ion pairing in water - DMSO mixtures. The potentials of mean force between the sodium - chloride ion pair are constructed by estimating the mean forces between the ion pair at various interionic separations and then integrating the mean forces. Two compositions of the solvent mixture with DMSO mole fractions of 0.21 and 0.35 are considered. Two model potentials for water and DMSO have been considered. One of the main observations is that the contact ion pair which is dominantly present in both the individual solvents is conspicuously absent in the mixture compositions studied. While solvent separated ion pairs dominate in all the mixture compositions, there is a presence of a second solvent separated ion pair in the water-DMSO mixture of composition with mole fraction of DMSO = 0.21. The potentials of mean force are verified by dynamical trajectories of the ion pair. The dynamics of the solvation shells has also been investigated in detail.