Thermal behavior and decomposition kinetics of rifampicin polymorphs under isothermal and non-isothermal conditions

The thermal behavior of two polymorphic forms of rifampicin was studied by DSC and TG/DTG. The thermoanalytical results clearly showed the differences between the two crystalline forms. Polymorph I was the most thermally stable form, the DSC curve showed no fusion for this species and the thermal decomposition process occurred around 245 degrees C. The DSC curve of polymorph II showed two consecutive events, an endothermic event (T-peak = 193.9 degrees C) and one exothermic event (T-peak = 209.4 degrees C), due to a melting-process followed by recrystallization, which was attributed to the conversion of form II to form I. Isothermal and non-isothermal thermogravimetric methods were used to determine the kinetic parameters of the thermal decomposition process. For non-isothermal experiments, the activation energy (E-a) was derived from the plot of Log beta vs I/T, yielding values for polymorph form 1 and II of 154 and 123 kJ mol(-1), respectively. In the isothermal experiments, the E-a was obtained from the plot of Int is I/T at a constant conversion level. The mean values found for form I and form II were 137 and 144 kJ mol(-1), respectively.