Purpose: The work aims at investigating the correlation of water sorption potential with different measures of molecular mobility in an annealed amorphous model compound (trehalose). Methods: Amorphous trehalose, prepared by freeze-drying, was annealed at 100 C (17 C < T g) for up to 120 h. Global molecular mobility was studied using a broadband dielectric spectrometer in the frequency range of 106-10-2 Hz. Enthalpic recovery was measured by differential scanning calorimetry and water sorption profiles were obtained using an automated vapor sorption balance. Results: As a function of annealing time, there was an increase, both in average α-relaxation time and enthalpic recovery and a decrease in the amount of sorbed water. A strong linear correlation was observed between the water sorption potential and the dielectric relaxation time, indicating a common underlying mechanism of the effect of annealing time on these properties. Enthalpic recovery, which is widely used as a measure of structural relaxation, did not correlate well with the extent of water sorption. Conclusions: The α-relaxation time can be used as a predictor of the water sorption potential of amorphous trehalose. It will be of interest and value to develop such predictive models for other amorphous compounds of pharmaceutical interest.
Bibliographical noteFunding Information:
We are grateful to Brad L. Givot (3M, St. Paul, MN) for all his help in the dielectric measurements and 3M (St. Paul, MN) for providing us access to the dielectric spectrometer. We thank Vishard Ragoonan, Ph.D. and Khushboo Kothari for their comments. Partial support was provided by the William and Mildred Peters Endowment Fund.
- enthalpic recovery
- molecular mobility
- water sorption