Compatibility study of theophylline with excipients using thermogravimetry supported by kinetic analysis

Thermogravimetry was applied to a compatibility study between theophylline and excipients such as glicocol, glucose, sorbitol, sucrose, cellulose and arabic gum. The technique is extremely useful in studies involving non-isothermal kinetic with a Flynn and Wall model. Based on the thermogravimetric data obtained, the activation energy (Ea) of decomposition for theophylline, excipients and their binary mixtures was determined using four different heating rates. With an increase in heating rate, the decomposition of samples under study was shifted to higher temperatures in conjunction with an increase in the rate of mass loss. The activation energy of theophylline decreased with an increase in conversion degree. It is valid to assume that the values of Ea of theophylline mixtures with glicocol and cellulose are similar to the Ea values of theophylline alone. Hence, it may be assumed that glicocol and cellulose are compatible with theophylline, findings confirmed by DSC, FTIR and PXRD. The results obtained show that kinetic parameters calculated using a thermogravimetric curve can be employed to improve TG interpretation and to predict more accurately the compatibility/incompatibility of theophylline with excipients.