Ion-solvation and ion-pair equilibria in electrolyte solutions

Abstract

In a concentrated acid or salt solution the amount of solvent molecules bound in the ionic solvation layers is an appreciable fraction of the total amount of solvent available. Some of the bound solvent molecules are released upon contact-ion-pair formation. The participation of the solvent is taken into account in the calculation of ion-pair formation by explicitly balancing the mixing entropy change with the standard and excess energy changes. The latter are taken from the mean spherical approximation theory for ion-ion interaction, augmented with a term for electrostatic ion-ion-pair interaction. A least-squares global optimization applied to a large body of available literature data on conductivities of univalent electrolytes over the concentration range up to saturated solutions is used for obtaining consistent values for ionic hydration numbers, standard free energies of ion-pair formation and electrostatic interaction parameters. With these values the measured conductivity data can be reproduced over the whole concentration range with an accuracy approaching that of the measurements themselves. As an example, the results for HCl and CsCl are presented and compared.