Molecular Interpretation of Mechanical Behavior in Four Basic Crystal Packing of Isoniazid with Homologous Cocrystal Formers

Conformation of homologous cocrystal formers (hCCFs, (HOOC-(CH₂)_n-COOH, n = 1 to 6 and 8)) led to differential intermolecular interactions with Isoniazid (INZ) forming four types of basic molecular packing. These molecular packing types are defined based on their H-bonded basic structural motifs. Their mechanical behavior was systematically evaluated using nanoindentation and correlating them to "in-die" Heckel analysis, "out-of-die" bulk compaction, and stress-strain relationship. Counterintuitively, the known structural feature crystallographic slip planes exhibited relatively lower plasticity and plastic energy in INZ:SUC (succinic acid), and higher elastic modulus (E), mechanical hardness (H), and apparent mean yield pressure. Similar behavior was observed for isostructural crystal packing of INZ:ADP (adipic acid). On the other hand, superior plasticity was achieved in INZ:GLT (glutaric acid) and INZ:MLN (malonic acid), leading to a larger bonding area. However, its tabletability was lower. Conversely, stiffer molecular crystals INZ:SUC and INZ:ADP provided higher tensile strength having higher E, H and apparent mean yield pressure. Despite being low symmetry molecular solids, substantial correlation was found with anticipation that the preferred orientation of molecular planes provides a close approximation of their bulk compression and consolidation behavior. This study demonstrated that molecular level crystal structure governs the linkage between particle level nanomechanical attributes and bulk level deformation behavior.