Thermodynamic Model and Sum Rules for Three-Phase Coexistence near the Tricritical Point in a Liquid Mixture

Abstract
Two sum rules for order-parameter susceptibilities are derived for the classical theory of the three-phase region near a tricritical point in ordinary liquid mixtures. The classical theory can also be fitted in a quantitative way to the composition data of Lang and Widom near the tricritical point in the mixture ethanol + benzene + water + ammonium sulfate.