Corrected Debye-Huckel theory of salt solutions: Size asymmetry and effective diameters

A recently developed extension of the Debye-Huckel theory, the corrected Debye-Huckel theory, is applied to salt solutions as described in Monte Carlo (MC) simulations and experiment. On the basis of modem density functional analysis, the theory accounts for both long-range electrostatic interactions in a linear response approximation and short-range ion size effects by a generalized van der Waals analysis. In its simplest implementation, the corrected Debye-Huckel restricted primitive model (CDH-RPM) theory, all ions are taken to be of the same diameter d in which case the excluded volume effects vanish in the linear response domain. Comparison with MC simulations verify the a priori application of the CDH theory to RPM electrolytes over a wide range of concentrations. Here, we investigate the relationship between the appropriate effective diameter d and the real diameters d(1) and d(2) in simulated binary salt solutions. Moreover, in the experimental studies of salt solutions, the relationship between d and the crystallographic or hydrated ionic diameters was investigated. Calculations have been performed for 67 salts. The optimized diameters are found to be in accord with current understanding of hydration effects. The range of concentrations in which a good fit is obtained for mean ionic activity coefficients and osmotic coefficients is typically 0-1 M. Comparison is also made with the Specific Interaction theory and the Pitzer and Bromley models of salt solutions.