Solution-mediated phase transition of pharmaceutical compounds: Case studies of acetaminophen and aspirin

Polymorphism is crucial in organic solid-state chemistry because different polymorphs exhibit different physical properties. Thus, the control of crystal polymorphism is a long-standing issue. We have been engaged in crystal polymorph control research, mainly targeting pharmaceutical compounds. Three processes to control the poly-morphism must be considered: nucleation, crystal growth, and phase transition. Finding a metastable phase by nucleation control techniques is the first step. The next crucial step is controlling phase transition and keeping the metastable phase as long as possible. Understanding the phase transition from metastable to stable phase is essential, especially to realize the high stability metastable phase. Herein we will show the phase transition processes of acetaminophen and aspirin as model materials.