Role of Cocrystal and Solution Chemistry on the Formation and Stability of Cocrystals with Different Stoichiometry

The objective of this study is to identify factors and conditions governing the formation and stability of cocrystals with different stoichiometry. Carbamazepine−4-aminobenzoic acid (CBZ−4ABA) cocrystals were chosen as the model system. A 1:1 CBZ−4ABA cocrystal was discovered by the reaction crystallization method. This cocrystal is characterized by carboxylic acid···acid and amide···amide homosynthons. The stability of 2:1 and 1:1 cocrystals is shown to be dependent on ligand solution concentration. The cocrystal richer in ligand component is more stable at higher ligand solution concentrations. Solubility of cocrystals and crystalline drug is explained by 1:1 solution complexation, and both cocrystals follow solubility product behavior. Mathematical models based on cocrystal and solution chemistry were fit to experimental data to generate phase solubility and triangular phase diagrams.