Vapor-Liquid Equilibrium and the Modified Leung-Griffiths Model
58This chapter is a review of vapor-liquid equilibria (VLE) of binary mixtures and their correlation with the Leung-Griffiths model as modified by Moldover and Rainwater. The model is specifically designed for an extended critical region, from the mixture critical pressure down to one half that pressure, and explicitly incorporates scaling-law critical exponents. The various possible types of mixture phase diagrams are reviewed, with emphasis on the class 1 mixture that has a continuous critical locus. Criteria are established for thorough measurement of VLE in the extended critical region, and a comprehensive bibliography is presented of 129 thoroughly measured mixtures composed of 73 pure fluids; the distribution of critical points of those fluids is examined and it is demonstrated that most mixtures of two of these fluids are class 1. The mathematical structure of the model is briefly reviewed and a systematic procedure is established with guidelines on the optimal number of adjustable parameters based on a quantitative measure (α2m) of fluid dissimilarity. Parameters are listed and analyzed for 42 successful nonazeotropic mixture fits to date. The utility of the model is discussed for several related problems including ternary mixtures, liquid-liquid equilibrium, explicit asymptotic expansions, azeotropy, a novel technique for critical density measurement, and interfacial tension.