Differential Scanning Calorimetry to Crystallize Metastable Polymorphs to Construct Free-Energy Temperature Diagrams

摘要

Complete characterization of a polymorphic system as monotropic or enantiotropicrnrequires preparation and isolation of the metastable forms. Metastable anhydrous forms often arerndifficult to isolate by crystallization from solvents because higher soluble meta-stable formsrnoften readily recrystallize as the least soluble most stable form. Super-cooled melts, on the otherrnhand, often crystallize as metastable anhydrous polymorphs according to Ostwald's Law ofrnStages to minimize the entropy difference between the crystallized solid and the amorphous melt.rnA differential scanning calorimeter is useful to control crystallization temperatures ofrnsuper cooled melts, but more important, can quantify heats of crystallization, establish times torncomplete crystallization and detect solid state conversions to other forms of lower free energy.rnSuper-cooled melts often provide metastable materials that when reheated in the calorimeter giverngood estimates for their melting points and latent heats of fusion. Free-energy temperaturerndiagrams constructed from calorimetry data reveal whether each polymorphic pair is monotropicrnor enantiotropic, as well as, real or virtual transition temperatures.