Dynamics of ionic and hydrophobic solutes in water-methanol mixtures of varying composition.

We have carried out a series of molecular-dynamics simulations of water-methanol mixtures containing either an ionic or a neutral atomic solute to investigate the effects of composition of the mixture on the diffusion of these solutes. Altogether, we have considered 17 different systems of varying composition ranging from pure water to pure methanol. The diffusion coefficients of ionic solutes are found to show nonideal behavior with variation of composition of the solvent mixture. The extent of nonideality of the solute diffusion is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and methanol molecules in these mixtures and is attributed to the enhanced stability of the hydrogen bonds and formation of interspecies complexes in the mixtures. The neutral solute shows characteristics of hydrophobic solvation and its diffusion decreases monotonically with increase of methanol concentration. The present simulation results are compared with those of experiments wherever available.