Demixing in liquids: approaches based on simple equations of state

Abstract

The demixing transition in partially miscible liquid mixtures is investigated within the framework of the simple van der Waals (vdW)-type equations of state (EOS) approaches. Two approximations for the excluded volume free energy have been considered. One is a modified vdW expression (denoted as vdW here). The other is the well known Mansoori–Carnahan–Starling–Leland (MCSL) excess free energy for hard sphere fluid mixtures (MCSL). In order to adopt an universal representation, we suggest to rescale the physical quantities such as the temperature, pressure and density by the critical parameters of the liquid–vapour (L–V) transition of the monodisperse fluid. We compare phase diagrams from the two theories and the theoretical predictions with available simulation results of binary Lennard–Jones mixtures. For the systems studied, the two theoretical approaches give qualitatively similar L–V and liquid–liquid (L–L) phase diagrams. The MCSL EOS is able to predict in a semi-quantitative way the L–L phase diagram.

Keywords:

• liquid demixing
• thermodynamics
• phase diagrams

Acknowledgements

The computer resources from the PMMS centre at Metz University are gratefully acknowledged. Y.L. Li thanks the support by the National Natural Science Foundation of China under Grant No. 61164006/F030203.