AQUEOUS ION-TRANSPORT PROPERTIES AND WATER REORIENTATION DYNAMICS FROM AMBIENT TO SUPERCRITICAL CONDITIONS

Ion transport properties including the friction coefficient, Walden pr oduct (product of conductivity and viscosity), and the limiting equiva lent conductance are predicted in water at elevated temperatures using a semicontinuum model. Molecular dynamics computer simulation is used to determine water rotational reorientation times in the first coordi nation shell compared with the bulk, and the results are incorporated into a hydrodynamic expression for the ionic friction coefficient. Alo ng the coexistence curve of water, the effective Stokes-Einstein radiu s implied by the model is relatively constant. However, for Cl-, K+, a nd Rb+, this radius increases at typical supercritical water condition s, where the motion of the first shell water molecules is coupled more closely to that of the ion. For Naf, the coupling is already quite st rong at higher solvent densities. The increment to the friction coeffi cient in excess of the bare ion Stokes-Einstein result contributes a l arger fraction of the total in supercritical water at typical densitie s (up to 0.29 g/cm(3)) than it does in higher density subcritical wate r, as a result of electrostriction. The limiting equivalent conductanc e increases approximately linearly with decreasing solvent density in the supercritical regime, in qualitative accord with the experimental extrapolations of Quist and Marshall (J. Phys. Chem. 1968, 72, 684-703 ) and in contrast to the plateau with decreasing density inferred from much more recent experiments by Zimmerman er al. (J. Phys. Chern. 199 5, 99, 11612-11625).