Thermodynamic representation of ternary liquid–liquid equilibria near-to and far-from the plait point

Common classical expressions for the molar excess Gibbs energy of mixing gE do not contain a contribution from composition fluctuations that are significant in the vicinity of the plait point for a ternary system. We propose a correction to gE based on reasonable phenomenological assumptions. This correction requires three ternary constants, but two are obtained from stability relations provided that we know the composition of the plait point. While the method proposed here is not predictive, it provides a consistent thermodynamic procedure for calculating the liquid–liquid phase diagram of a ternary system with a plait point. The proposed method is illustrated for three ternaries. When calculations are based on the classical expressions for gE alone, calculated results are in serious disagreement with experiment near critical conditions. Inclusion of the proposed correction for gE provides dramatic improvement.