Structural Basis for Mechanical Anisotropy in Polymorphs of a Caffeine–Glutaric Acid Cocrystal

Insights into structure–mechanical property correlations in molecular and multicomponent crystals have recently attracted significant attention owing to their practical applications in the pharmaceutical and specialty fine chemicals manufacturing. In this contribution, we systematically examine the mechanical properties of dimorphic forms, Forms I and II, of a 1:1 caffeine–glutaric acid cocrystal on multiple faces using nanoindentation to fully understand their mechanical anisotropy and mechanical stability under an applied load. The higher hardness, H, and elastic modulus, E, of stable Form II has been rationalized based on its corrugated layers, higher interlayer energy, lower interlayer separation, and presence of more intermolecular interactions in the crystal structure compared to metastable Form I. Our results show that mechanical anisotropy in both polymorphs arises due to the difference in orientation of the same two-dimensional structural features, namely, the number of possible slip systems, and the strength of the intermolecular interactions with respect to the indentation direction. The mechanical properties results suggest that the 1:1 caffeine–glutaric acid cocrystal, metastable form (Form I) could be a suitable candidate with desired tablet performance to that of stable Form II. Overall, it demonstrates that the multiple faces of nanoindentation are critical to determine mechanical anisotropy and structure-mechanical property correlations. Further, the structural–mechanical property correlations aid in the selection of the best solid phase for macroscopic pharmaceutical formulation.