PHENOMENOLOGY OF POLYMORPHISM .2. CRITERIA FOR OVERALL (P, T) MONOTROPY - APPLICATIONS TO MONOCHLOROACETIC ACID AND TO HYDRAZINE MONOHYDRATE

The most general case of monotropy, (p, T) overall monotropy, is described by taking both pressure and temperature variables into account as high-pressure enantiotropy, which involves a high-pressure stable domain for a room-pressure metastable solid modification, cannot be a priori excluded. Criteria for ascertainment of overall monotropy are established by comparing the melting curve slope of the stable form with those of the solid-to-solid transition curves concerned with it and of the fusion curves of the metastable varieties. These comparisons allow also to unambiguously discern between overall monotropy and high-pressure enantiotropy. In applying such criteria the use of straight lines for describing melting and transition equilibria is retained. Their slopes are then calculated by means of the Clapeyron equation also in the case high-pressure results lack, as illustrated by two examples on monochloroacetic acid and hydrazine monohydrate, respectively, whose monotropic behaviour is not only critically accounted for, but also extended to pressures higher than the atmospheric one. © 1993 Academic Press, Inc.