Friction Coefficients of Ions in Aqueous Solution at 25 °C

The diffusion coefficients of ions and of uncharged solutes in aqueous solution at 25 °C and at infinite dilution are studied by computer simulation using the SPC/E model for water and solute−water potentials employed in previous work (Koneshan, S.; et al. J. Phys. Chem. 1998, 102, 4193−4204). The mobilities of the ions calculated from the diffusion coefficients pass through a maximum as a function of ion size, with distinct curves and maximums for positive and negative ions in qualitative agreement with experiment. We aim to understand this at a microscopic level in terms of theoretical studies of the friction coefficient ζ, which is related to diffusion coefficient by the Stokes−Einstein relation. This provides one method of calculating ζ, but it can also be obtained in principle from the random force autocorrelation function which is the starting point of molecular theories of the friction. Molecular and continuum theories divide the friction into hydrodynamic and dielectric components calculated in di...