Dynamics of aqueous solutions of ions and neutral solutes at infinite dilution at a supercritical temperature of 683 K

We discuss the results of a molecular dynamics (MD) study of the residence times of water in the primary hydration shells of ions and uncharged solute s at infinite dilution in supercritical water at 683 K, as well as the solv ation dynamics and diffusion coefficients of these species. The SPC/E model is used to represent water in this paper and its companion, where structur al aspects of the same systems are discussed. The residence times at 683 K are found to be lower than those under ambient conditions and only weakly d ependent on the size and sign of the charge on the ions. In contrast to ear lier studies at room temperature (298 K at a solvent density of 0.997 g cm( -3)), the solvation dynamics of sodium and chloride ions at 683 K and a sol vent density of 0.35 g cm(-3) are very similar to each other and indicate a solvent relaxation time of 0.8 ps. The ion diffusion coefficients are larg er in magnitude at 683 K and less differentiated by size and charge sign at low solvent density (less than or equal to0.35 g cm(-3)) than they are at room temperature. In the low-density range, an uncharged solute of the same size as an ion diffuses 2-9 times faster than the corresponding charged sp ecies, with the smaller solutes moving much faster than the larger ones. In creasing the solvent density to 0.997 g cm(-3) at 683 K decreases the diffu sion coefficients of ions and uncharged solutes. At this density, the diffu sion coefficients pass through distinct maxima as a function of size at bot h 298 and 683 K. An explanation of the diffusion coefficients of charged an d uncharged species at low solvent density is sought in terms of a semicont inuum theory. The importance of the time scale of the solvent density fluct uations in controlling diffusion as distinct from the spatial extent of the se fluctuations (determined by the correlation length) is emphasized. These observations suggest that the mechanisms of solute diffusion at supercriti cal conditions and at room temperature may be fundamentally different.