Geometrical investigation and classification of three-suffix margules binary mixtures including single and double azeotropy

Abstract

The present study investigates the sub-critical behavior of binary fluid mixtures that follow the three-suffix Margules model for the excess Gibbs free energy. It is a systematic and geometric treatment of these binary mixtures that allows a global view of the conditions leading to zeotropy, single azeotropy, and double azeotropy, and provides additional criteria for the mixture classification of Shulgin et al. (Ind. Eng. Chem. Res. 40 (2001) 2742–2747). These conditions are delimited on the Ac-Bc plane, where Ac and Bc are variables determined solely by Henry's constants and pure components' saturation pressures. Based on these mixture properties the azeotropic composition, excess Gibbs free energy shape and criteria for the liquid phase splitting are given, for fluid mixtures obeying Margules model. The study proceeds by mapping real mixture behavior as temperature changes. With varying temperature, the mixture traces a path that crosses regions of different mixture types. Special mention is made on mapping the behavior of mixtures crossing the Bancroft point. The study concludes comparing Margules' model to van Laar's. It is proved that the latter cannot predict double azeotropy.