Slow Molecular Dynamics in Three Organic Salts of Active Pharmaceutical Ingredients

The slow molecular mobility in three ionic liquids (ILs) that are active pharmaceutical ingredients (APIs) was studied by thermally stimulated depolarization currents (TSDC). One liquid readily crystallizes on cooling and its thermal behavior was analyzed. The other two liquids were studied in the temperature regions of the glassy state, the glass transition, and the metastable liquid. The kinetic parameters associated with the different modes of motion in these APIs were determined from the output of the TSDC experiments. Differential scanning calorimetry (DSC) was used to characterize the glass transition and analyze the thermal behavior in the metastable liquid region. The analysis of the glass transition relaxation showed that these APIs are strong glass-formers. TSDC also showed the presence in both glass-forming APIs of a relaxation in the metastable liquid, which has some features of a liquid-liquid transition and was characterized by a high intensity and a moderate cooperativity. No secondary relaxations were detected.