Thermoanalytical investigation of drug-excipient interaction - Part I. Piroxicam, cellulose and chitosan as starting materials

Cellulose, chitosan and piroxicam were investigated by TG and DSC at heating up to 215 degrees C, and by X-ray powder diffraction before and after the heating. Dehydration of cellulose and chitosan comes to the end near 160 degrees C. Thermal decomposition of chitosan starts at the final stage of its dehydration, and the mass losses after these two reactions overlap with one another. Enthalpy of dehydration is 47.1 +/- 2.4 kJ mol(-1) of water for cellulose and 46.2 +/- 2.0 kJ mol(-1) for chitosan. Thermal decomposition of chitosan is an exothermic process. Crystal structure of cellulose after heating remains unchanged, but that of chitosan contracts. Piroxicam melts at 200.7 degrees C with the enthalpy of melting 35 kJ mol(-1). Heat capacity of the liquid phase is greater than that of the solid phase by approximately 100 J mol(-1) K-1. Cooled back to ambient temperature, piroxicam remains glassy for a long time, crystallizing slowly back into the starting polymorph.