Existence of a size-dependent diffusivity maximum for uncharged solutes in water and its implications

Recent studies suggest that there exists a size-dependent diffusivity maximum in binary mixtures interacting via Lennard-Jones potential when the size of one of the two components is varied (Ghorai, P. K.; Yashonath, S. J. Phys. Chem., 2005, 109, 5824). We discuss in the present paper the importance of the existence of a size-dependent maximum for an uncharged solute in liquid or amorphous solid water and its relation to the ionic conductivity maximum in water. We report molecular dynamics investigations into the size dependence of the self-diffusivity, D, of the uncharged solutes in water at low temperatures (30 K) with immobile as well as mobile water. We find that a maximum in self-diffusivity exists as a function of the size of solute diffusing within water at low temperatures but not at high temperatures. This is due to the relatively weak interactions between the solute and the water compared to the kinetic energy at room temperature. Previously, we have shown that a similar maximum exists for guests sorbed in zeolites and is known as the levitation effect (LE). Thus, it appears that the existence of a size-dependent maximum is universal and extends from zeolites to simple liquids to solvents of polyatomic species. We examine the implications of this for the size-dependent maximum in ionic conductivity in polar solvents known for over a hundred years. These results support the view that the size-dependent maximum seen for ions in water has its origin in the LE (see Ghorai, P. Kr.; Yashonath, S.; Lynden-Bell, R. M. J. Phys. Chem. 2005, 109, 8120).