Fundamentals

This chapter presents the thermodynamic fundamentals of polymorphism. The Gibbs Phase Rule is reviewed, followed by its application to polymorphic systems. The concept of energy/temperature diagrams is introduced, and then applied to enantiotropic and monotropic systems. The alternative representation of phase diagrams in terms of pressure and temperature is also given. The Burger–Ramburger Rules, derived from the energy temperature diagrams and analytical data (e.g., hot stage, DSC, infrared spectra) are then represented: the heat-of-transition rule, heat-of-fusion rule, entropy fusion rule, heat-capacity rule, density rule, and infrared rule. The importance of kinetic factors is introduced, followed by the structural fundamentals required to compare polymorphic forms. The chapter closes with recommendations for the presentation of graphic comparisons of polymorphic structures.