Aqueous ion transport properties and water reorientation dynamics from ambient to supercritical conditions
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Ion transport properties including the friction coefficient, Walden product (product of conductivity and viscosity), and the limiting equivalent 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 coordination shell compared with the bulk, and the results are incorporated into a hydrodynamic expression for the ionic friction coefficient. Along the coexistence curve of water, the effective Stokes-Einstein radius implied by the model is relatively constant. However, for Cl-, K+, and Rb+, this radius increases at typical supercritical water conditions, where the motion of the first shell water molecules is coupled more closely to that of the ion. For Na+, the coupling is already quite strong at higher solvent densities. The increment to the friction coefficient in excess of the bare ion Stokes-Einstein result contributes a larger fraction of the total in supercritical water at typical densities (up to 0.29 g/cm3) than it does in higher density subcritical water, as a result of electrostriction. The limiting equivalent conductance 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 et al. (J. Phys. Chem. 1995, 99, 11612-11625).
