POTENTIAL OF MEAN FORCE OF THE SODIUM-CHLORIDE ION-PAIR IN DIMETHYL-SULFOXIDE - A CONSTRAINED MOLECULAR-DYNAMICS STUDY

The molecular dynamics simulations of the Na+-Cl- ion pair in DMSO have been performed to obtain the potential of mean force (PMF) and the mean force between the Na+ and the Cl- ions at room temperature. The reaction field method has been used to estimate the effect of the long-range interactions. The results obtained are compared with the reported potential of mean force of Na+-Cl- ion pair in water. The mean force potential in DMSO shows that the contact ion pair (CIP) is much stabler than the solvent-separated ion pair (SSIP), whereas in water, the SSIP is as stable as the CIP. The barrier for crossing from the contact ion pair to the solvent-separated ion pair in DMSO is much larger than the corresponding value in water, whereas the barriers for crossing from the SSIP to the CIP are similar in both these solvents. The density profiles of all the interaction sites around the ion pair at the first minimum in the PMF have also been presented.