MD simulation studies of selective solvation in methanol-water mixtures: An effect of the charge density of a solute

MD simulations have been performed for NaCl, NaI, and fictitious solutions of discharged counterparts of Na+ and Cl- ions to investigate an effect of the charge density of a solute on its solvation in methanol-water mixtures. Solvent-solvent interactions have been described in terms of flexible models, whereas solutes have been considered as charged or uncharged Lennard-Jones spheres. An analysis of solvation shells has been based on radial distribution functions, angular distributions, coordination numbers, and residence times of solvent molecules. Preferential solvation of anions by methanol molecules becomes less pronounced with decreasing charge density of a solute and vanishes for the discharged chloride ion. In contrast to preferential hydration of Na+ in water deficit solvents, its uncharged counterpart Na-0 is preferentially solvated by methanol molecules over the whole range of solvent composition. Results for NaCl solution have been compared with those obtained with ab initio ion-solvent potentials and the same model of solvent molecules. The type of ion-solvent potential has small effect on the structure and composition of ionic shells, but its influence on the persistence of the coordination shells is more noticeable.