APPLICATION OF STOKES-LAW TO IONS IN AQUEOUS-SOLUTION

A comparison of the so-called Stokes radii of ions in aqueous electrolyte solutions with the corresponding ion crystal radii gives order-of-magnitude agreement but with a tantalizing discrepancy: some Stokes radii are found to be smaller than the corresponding crystal radii, contrary to expectation for hydrated ions. Although the extrapolation of continuum hydrodynamics to the atomic domain would seem to be a prime source of this discrepancy, the close agreement with experiment given by the Stokes-Einstein diffusion constant for uncharged molecules (using the slip boundary condition) encourages the view that the discrepancy for ions may result less from the assumption of continuum hydrodynamics than from some neglected interaction with the solvent. Within this context, this paper reexamines the "stick" vs "slip" boundary conditions for Stokes' law with specific reference to ions in aqueous solution. Further, a new concept is introduced that involves an enhanced electrical force acting on an ion as a consequence of local dielectric saturation of the solvent environment - which, so to speak, allows the applied external field to "shine through". Inclusion of this force enhancement, together with the slip boundary condition, considerably improves the predictions of Stokes' law. © 1990 American Chemical Society.