Conformal Ionic Mixtures

The excess free energy of mixing of molten reciprocal salt solutions (A+, B+, X—, Y—) has been calculated up to the second order by the perturbation method of Reiss, Katz, and Kleppa. The result is a power series containing integral powers of the ion fractions (concentrations). This demonstrates that mixtures of two salts of the same valence type cannot be considered as a mixture of a salt with a dielectric solvent. The first term in the series is proportional to the free-energy change for the reaction AX+BY→AY+BX and is the same as the theory of Flood, Fo/rland, and Grjotheim. The results of the symmetric approximation based upon the quasilattice model are consistent with the first term and one of the second-order terms. The remaining second-order terms are related to the excess free energy of mixing in the binary systems AX—AY, BX—BY, AX—BX, AY—BY and are of a form first suggested by Fo/rland. This theory, thus, gives a relatively rigorous justification to aspects of previous theories of reciprocal salt solutions which were based upon restrictive models and assumptions.