Influence of Crystal Packing on the Mechanism of Decomposition of the Acetonitrile-Solvated Cocrystal of Piroxicam and Succinic Acid

Variable-temperature single-crystal and powder X-ray diffraction techniques have been used to study the thermal and mechanical decomposition of the acetonitrile solvate of the cocrystal formed between piroxicam and succinic acid (PRXSA-ACN). The results show that the thermal expansion behavior of PRXSA-ACN is highly anisotropic and can be correlated with the crystal structure. Thermally induced desolvation of PRXSA-ACN led initially to the formation of the α-form of piroxicam and the 1:1 piroxicam/succinic acid cocrystal (PRXSA), and this can be rationalized on the basis of the crystal structure of PRXSA-ACN and its thermal expansion behavior. Subsequent decomposition of PRXSA produced amorphous succinic acid and the thermodynamically more stable β-form of piroxicam. The α- and β-forms coexisted up to the melting of the α-form, at which point the sample recrystallized to give the β-form of piroxicam. Mechanical treatment (light grinding) of PRXSA-ACN resulted in mild structural damage to the crystallinity, and this led to subsequent desolvation.