Conformal ionic solution theory for additive ternary molten–ionic systems

Abstract

Excess free energies of mixing (ΔAEm) for additive ternary molten ionic systems (AX–BX–CX or AX–AY–AZ) have been calculated from conformal ionic solution (CIS) theory. The CIS theory applies rigorously to ionic systems containing cations (or anions) of different size, whereas quasichemical theory, which is based on a lattice model, cannot be rigorously applied to such systems. Nevertheless, most of the terms calculated from CIS theory have counterparts in quasichemical theory except for asymmetric terms proportional to products of mole fractions of components i and j of the form X2iXj. Such asymmetric terms are known to be significant in molten salt systems. In the expression for ΔAEm calculated to third order [Eq. (20)], all of the coefficients can be evaluated from data on the subsidiary binaries. In the expression for ΔAEm calculated to fourth order [Eq. (33)], one can show that the only term which cannot be evaluated from data on the binaries is probably negligible. Thus the theory permits the calculation of properties of ternaries solely from a knowledge of properties of binaries.