Crystallization at Solvent Interfaces Enables Access to a Variety of Cocrystal Polymorphs and Hydrates

A crystal growth technique, interfacial cocrystallization, is demonstrated to be a simple and effective method for preparing multicomponent crystal forms. The technique is based on the generation of a liquid–liquid interface between two immiscible solutions of cocrystal-forming compounds, and its utility is demonstrated through the preparation of polymorphs and hydrates of caffeine cocrystals, involving three different hydroxy-2-naphthoic acids, including the formation of some with unexpected compositions.