Importance of Solvent Selection for Stoichiometrically Diverse Cocrystal Systems: Caffeine/Maleic Acid 1:1 and 2:1 Cocrystals

Abstract

Phase diagrams of cocrystals often show a highly unsymmetrical nature. The solvent has an important impact on the overall aspect of these diagrams. In this paper, we show how the solvent affects the composition of the stoichiometric solid phase nucleated. Suitable conditions for nucleation and growth of a single 2:1 caffeine/maleic acid cocrystal are obtained in ethyl acetate, showing comparable solubility toward both caffeine and maleic acid. Through a full kinetic screen, we were able to identify, for the first time, reproducible conditions for the spontaneous crystallization of the 2:1 phase in solution. Furthermore, during the screening experiments, a hithertho unknown form of the 1:1 cocrystal phase was encountered. Structural X-ray diffraction analyses of both the 2:1, as well as the 1:1 polymorphic phases, show an out of plane maleic acid compound. The carboxylic acid groups are oriented in such a manner to promote intermolecular formation of hydrogen bonded synthons.