Compund formation in phenobarbitone-urea systems

Detection of cocrystal formation based on binary phase diagrams using thermal analysis

PURPOSE

Although a number of studies have reported that cocrystals can form by heating a physical mixture of two components, details surrounding heat-induced cocrystal formation remain unclear. Here, we attempted to clarify the thermal behavior of a physical mixture and cocrystal formation in reference to a binary phase diagram.

METHODS

Physical mixtures prepared using an agate mortar were heated at rates of 2, 5, 10, and 30 °C/min using differential scanning calorimetry (DSC). Some mixtures were further analyzed using X-ray DSC and polarization microscopy.

RESULTS

When a physical mixture consisting of two components which was capable of cocrystal formation was heated using DSC, an exothermic peak associated with cocrystal formation was detected immediately after an endothermic peak. In some combinations, several endothermic peaks were detected and associated with metastable eutectic melting, eutectic melting, and cocrystal melting. In contrast, when a physical mixture of two components which is incapable of cocrystal formation was heated using DSC, only a single endothermic peak associated with eutectic melting was detected.

CONCLUSION

These experimental observations demonstrated how the thermal events were attributed to phase transitions occurring in a binary mixture and clarified the relationship between exothermic peaks and cocrystal formation.

Oleate salt formation and mesomorphic behavior in the propranolol/oleic acid binary system

Thermal analysis of propranolol/oleic acid mixtures prepared by solvent evaporation enabled construction of the binary system phase diagram. This allowed both physical and chemical interactions to be identified, including complex formation at the equimolar composition. An incongruent melting complex with a characteristic reaction point was identified in excess oleic acid compositions, a common property of fatty acid/fatty acid salt binary systems. The equimolar complex was confirmed to be propranolol oleate using infrared spectroscopy. Wide-angle X-ray powder diffractometry demonstrated that propranolol oleate possessed long-range positional order ( approximately 25 A d spacing) accompanied by a degree of disorder over shorter d spacings. Such a pattern suggested mesophase formation, explaining the unctuous nature of propranolol oleate at room temperature. Accurate measurement of the long-range d spacing was achieved using small-angle X-ray scattering, permitting differentiation of the three different phases identified (phase I: 25.4 A, phase II: 24.6 A, phase III: 25.4-25.5 A). The implications of drug fatty acid salt formation and also mesomorphism in pharmaceutical systems are discussed.