Rapid phase transitions binary mixtures

In this chapter we shall consider applications of stability theory to critical phenomena of liquid-vapor transitions and separation of binary mixtures. When the applied pressure and temperature are altered, systems can become unstable, causing their physical state to transform into another distinct state. For example, when the temperature of a two-component liquid mixture (such as hexane and nitrobenzene) changes, the mixture may become unstable to changes in the composition the mixture then separates into two phases, each rich in one of the components. In Chapters 18 and 19 we shall see that in far-from-equilibrium systems, loss of stability can lead to a wide variety of complex nonequilibrium states. We shall also look at how a system that can undergo internal transformations responds to a rapid change in temperature. This leads us to the concept of configurational heat capacity. 

A complete classification of smectic phases by texture is not always possible. It can happen that similar textures are observed with two liquid crystalline states separated by a phase transition.If so, an extremely useful and powerful tool for assessing the type of mesophase is the determination of the isobaric temperature-concentration diagrams of binary mixtures. According to Sackmann and Demus isomorphous liquid crystals are considered as equivalent and characterized by the same symbol. While uninterrupted miscibility establishes isomorphism, the converse is not necessarily true. Temperature-composition phase diagrams for liquid crystalline mixtures can be generated from thermal data or, because of the various optical features characteristic of each mesophase structure, from observations of microscopic textures of the mesophases between crossed polarizers. The latter method (also called the contact method allows great rapidity in the assessment of the phase diagram. 

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