Phase behaviour of ternary mixtures: a theoretical investigation of the critical properties of mixtures with equal size components

Abstract

Conformal solution theory in conjunction with the one-fluid model and the Guggenheim equation of state are used to calculate the critical properties of binary and ternary mixtures characterised by molecules of identical size (identical h conformal parameters) but different energy parameters (different f conformal parameters). When the geometric mean combining rule is used to obtain contributions from unlike interactions, binary mixtures of equal size components generate Type I, II and III phase behaviour. The critical properties of ternary mixtures composed of binary subsystems of different phase behaviour types are examined. Results are presented at different temperatures and pressures covering both vapour–liquid and liquid–liquid equilibria. The results illustrate the diversity of phenomena exhibited by ternary mixtures and the relationship of the behaviour exhibited to the relative strength of dissimilar interactions between different pairs of molecules. A progressive transition between vapour–liquid and liquid–liquid equilibria and higher order critical phenomena are likely to be common features of the phase behaviour of ternary mixtures.