Structural and dynamical origins of ionic mobilities in supercritical water

We report the results of simulations of the ionic mobility of Na+ and Cl- i n supercritical water at 673 K, including solvent densities below those pre viously considered in simulation or experimental data. By considering these results along with earlier published analyses, we find that the spatially inhomogeneous solvation structure around the ions and solvent dynamics are strongly coupled in determining transport rates. The appearance of a platea u in the infinite-dilution conductivity over a wide range of intermediate s olvent densities is a result of a subtle balance of excess (dielectric) fri ction and a nonlinear variation in the viscous friction. The result is stro ngly influenced by the inhomogeneous solvent density around the ions. but c annot be rationalized on the basis of only structural criteria. A reduced e ffective ionic radius is introduced that is inversely proportional to the W alden product and can be trivially evaluated from experimental conductivity results. It is shown that when represented in this way, conductivity data smoothly and continuously vary with solvent density over the entire density range and are much more readily interpreted. In particular, this effective ionic size exhibits a maximum at a density of ca. 0.2 g/cm(3), providing a natural division between high- and low-density solvents. At higher densiti es. the structure of the first hydration shell of the ions is only weakly d ependent on solvent density, while at lower densities, entropic forces incr easingly lead to the loss of this primary solvation shell. These results ar e consistent with the view that, with decreasing solvent density down to th is natural division, an increasing imbalance between ion-water and water-wa ter interactions produces an increasingly rigid ionic solvation shell and t hus an increasing friction on the ion.