Phase Equilibrium in Real Mixtures

Abstract

In this chapter, we concentrate on these aspects of phase equilibrium in two-component systems that cannot be explained by the ideal mixture model. For instance, large deviations from Raoult’s law may lead to liquid–vapour phase diagrams with a minimum or maximum on the liquid and vapour composition lines, and the mixture of such a composition is called an azeotrop. Existence of azeotropic mixtures have important consequences for separation of the components by distillation. Another example is partial miscibility of liquids or solids, which means that in a certain composition range the mixture cannot exist as a single phase. It often happens that the components are partially miscible only below a certain temperature called the upper critical solution temperature and above that temperature they mix in all proportions. Systems with the lower critical solution temperature or with both critical temperatures also exist. To describe deviations of mixtures from the ideal behaviour, we use a model called the simple solution.