Transition types in hydrocarbons and related substances

More than 300 pure organic substances have been studied in detail by calorimetric methods. More than one-third of these compounds exhibit some type of thermal anomaly in the condensed phase. In 1940 Deffet! listed nearly 1,200 organic sustances in which polymorphism had been detected by either thermal or non-thermal methods, or both. Obviously, phase transformations other than melting are among the more common phenomena encountered in physical and thermodynamic studies of organic substances. Despite an abundance of experimental facts accumulated in the last 50 years, and despite the efforts of many able scientists to interpret these facts, satisfactory theoretical explanations of phase transformations in organic solids are rare. Several recent reviews illustrate clearly the difficulties met in attempts to understand phase transformations either in terms of classical thermodynamic phase relations or of molecular and structural phenomena2-4. Theoretical treatments involving the use of statistical mechanics have yielded results in