Conformal Solution Theory of Certain Molten Salts

Abstract

The conformal solution theory of molten salts developed by Reiss, Katz, and Kleppa is utilized to obtain formulas for the thermodynamic excess quantities for mixing at constant temperature and pressure. The RKK theory was developed for mixing at constant temperature and volume. For the special case that the anion-cation repulsive potential may be represented as a hard-sphere potential we have simplified the theoretical formulas by eliminating an integral involving a four-particle correlation function. Some of the integrals in the theoretical expressions are related to the thermodynamic properties of a reference system. Number estimates of the heat of mixing, based partially on a pseudo one-component fluid model and partially on a lattice model, compare favorably with experiment. According to the theory, the volume of mixing should vary as the square of the difference in the ionic diameters of the components salts. This is in contradiction to the empirical fit achieved by Powers, Katz, and Kleppa who found that the fourth power of the difference in diameters represented the data. We have shown, however, that the data agrees with our theory as well as it does with the proposed fit of RKK.