Cocrystal dissociation and molecular demixing in the solid state.

A new cocrystal containing caffeine and theophylline was found to dissociate on heating, with caffeine and theophylline molecules spontaneously demixing and recrystallizing as separate phases, in a solid-solid transition likely driven by an increase in entropy. The morphology and composition of the resulting crystals was determined by transmission electron microscopy.