The slow molecular mobility in amorphous captopril and captopril disulfide: A DSC and TSDC study

The slow molecular mobility in the amorphous pharmaceutical drug captopril is studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). The general kinetic features of the secondary relaxations and of the main relaxation were determined by TSDC. The dynamic fragility of captopril is obtained by DSC and TSDC, and the values compared with that obtained by dielectric relaxation spectroscopy (DRS). The local motional modes of the fast secondary relaxation were found to be slightly aging sensitive, even at temperatures lower than 100° below Tg, which was ascribed to the intermolecular hydrogen bond network that is reported to exist in the amorphous solid state of captopril. Captopril disulfide, a metabolite product of captopril, was synthetized and characterized by DSC and TSDC. Compared to captopril, it behaves as a stronger glass forming liquid.