Single-Crystal Plasticity Defies Bulk-Phase Mechanics in Isoniazid Cocrystals with Analogous Coformers

The crystal structures of four INZ cocrystals with analogous crystal coformers were probed to understand the relationships among molecular packing, H-bonding dimensionality, single-crystal plasticity, and bulk mechanical behavior. These structurally analogous coformers coherently directed H-bonds by "philic" functionalities (-OH and -COOH) and vdW interactions by a "phobic" scaffold (- C₆H_5-n, where n = 0, 1, 3). In comparison to INZ:2HBA and INZ:4HBA, INZ:BA and INZ:GA exhibited higher plasticity and, hence, better tableting performance due to larger bonding area and higher tensile strength. The rank order of apparent yield pressure and incipient plasticity quantified from "in-die" Heckel analysis of the bulk phase, INZ:2HBA > INZ:4HBA > INZ:GA > INZ:BA, however, does not match that of nanomechanical hardness and elastic modulus, INZ:BA > INZ:2HBA > INZ:4HBA > INZ:GA. The discrepancy may be attributed to the anisotropy in crystal mechanical properties, where the stiffness of the dominant crystal faces probed with nanoindentation may grossly deviate from the bulk mechanical behavior. Therefore, nanomechanical attributes are more predictive of more isotropic molecular crystals, such as 3D H-bonded or interlocked structures, in comparison to those exhibiting gross structural anisotropy, such as crystals with distinct molecular layers that favor facile slip. Hence, the accurate prediction of bulk behavior on the basis of nanomechanical characterization requires the incorporation of crystal shape and packing as well as knowledge of facet-specific mechanical properties. Moreover, the prediction of bonding strength on the basis of molecular packing is still warranted when the crystallographic molecular slip may cause a deviation in the proposed relationship.