Role of solvent permittivity in estimation of electrolyte activity coefficients on the basis of the mean spherical approximation

A modified version of the model for electrolyte solutions based on the restricted version of the mean spherical approximation is applied to estimate mean ionic activity coefficients for aqueous solutions of the alkali metal halides. The modification involves using the actual dielectric permittivity of the solution rather than the permittivity of the pure solvent. In the case of sodium and potassium salts, fits between theory and experiment are possible over wide concentration ranges up to 2.5 M using one adjustable parameter, namely, the single effective ionic diameter. Limitations of the model, especially with respect to systems with weak ion pairing, are discussed.