Thermodynamics and Statistical Mechanics of Phase Transitions

Any substance of fixed chemical composition, water H2O, for example, can exist in homogeneous forms whose properties can be distinguished, called states. Water exists as a gas, a liquid, or a solid, ice. These three states of matter (solid, liquid, and gas) differ in density, heat capacity, etc. The optical and mechanical properties of a liquid and a solid are also very different. By applying high pressures to a sample of ice (several kilobars), several varieties of ice corresponding to distinct crystalline forms can be obtained (Fig. 1.1). In general, for the same solid or liquid substance, several distinct arrangements of the atoms, molecules, or particles associated with them can be observed and will correspond to different properties of the solid or liquid, constituting phases. There are thus several phases of ice corresponding to distinct crystalline and amorphous varieties of solid water. Either an isotropic phase or a liquid crystal phase can be obtained for some liquids, they can be distinguished by their optical properties and differ in the orientation of their molecules (Fig. 1.2). Experiments thus demonstrate phase transitions or changes of state. For example: a substance passes from the liquid state to the solid state (solidification); the molecular arrangements in a crystal are modified by application of pressure and it passes from one crystalline phase to another. Phase transitions are physical events that have been known for a very long time. They are encountered in nature (for example, condensation of drops of water in clouds) or daily life; they are also used in numerous technical systems or industrial processes; evaporation of water in the steam generator of a nuclear power plant is the physical process for activating the turbines in electric generators, and melting and then solidification of metals are important stages of metallurgical operations, etc.