Chapter 13 - Applications of Thermodynamics to Phase Equilibria Studies of Pure Substances

Abstract

This chapter uses thermodynamic relationships to describe phase equilibria of pure substances. To differentiate among the varieties of phase equilibria that occur, a classification of phase transitions is proposed, which is based upon the behavior of the chemical potential of the system as it passes through the phase transition. In a first order transition, two phases co-exist at the transition temperature. From the Gibbs phase rule, if thermodynamic equilibrium is maintained, these phases can only co-exist at a single temperature (for a fixed pressure). At that temperature there are two values for the entropy, enthalpy, and volume. Properties associated with continuous or critical phase transitions behave similarly as one approaches the critical point. In addition, the exponents measured for a given property, such as heat capacity or the order parameter, are found to be the same within experimental error in a wide variety of substances. Certain experimental techniques, including neutron and optical scattering, show that properties, like the order parameter, fluctuate significantly about the average in a bulk sample as one approaches the critical temperature. Continuous phase transitions can result from many different types of effects, but they all have many common characteristics. Modern theories of phase transitions have shown that the classification scheme fails at orders higher than one.