Sergey Vyazovkin1, Ion Dranca. 1. Department of Chemistry, University of Alabama at Birmingham, 901 S. 14th Street, Birmingham, AL 35294, USA. vyazovkin@uab.edu
Abstract
PURPOSE: To demonstrate the utility of differential scanning calorimetry (DSC) for determining activation energy landscape in amorphous pharmaceutical systems throughout the sub-Tg and Tg regions. MATERIALS AND METHODS: DSC was employed to determine the effective activation energies (E) of the relaxation in sub-Tg and Tg regions as well as the sizes of cooperatively rearranging regions in glassy maltitol and glucose. RESULTS: It has been found that in the sub-Tg region E decreases with decreasing T reaching the values approximately 60 (glucose) and approximately 70 (maltitol) kJ mol(-1) that are comparable to the literature values of the activation energies for the beta-relaxation. In the Tg region E decreases (from approximately 250 to approximately 150 kJ mol(-1) in maltitol and from approximately 220 to approximately 170 kJ mol(-1) in glucose) with increasing T as typically found for the alpha-relaxation. From the heat capacity measurements the sizes of cooperatively rearranging regions have been determined as 3.1 (maltitol) and 3.3 (glucose) nm. CONCLUSIONS: DSC can be used for evaluating the energy landscapes. The E values for maltitol are somewhat greater than for glucose due to the added impeding effect of the bulky substitute group in maltitol. The comparable sizes of the cooperatively rearranging regions suggest a similarity of the heterogeneous glassy structures of the two compounds.
PURPOSE: To demonstrate the utility of differential scanning calorimetry (DSC) for determining activation energy landscape in amorphous pharmaceutical systems throughout the sub-Tg and Tg regions. MATERIALS AND METHODS: DSC was employed to determine the effective activation energies (E) of the relaxation in sub-Tg and Tg regions as well as the sizes of cooperatively rearranging regions in glassy maltitol and glucose. RESULTS: It has been found that in the sub-Tg region E decreases with decreasing T reaching the values approximately 60 (glucose) and approximately 70 (maltitol) kJ mol(-1) that are comparable to the literature values of the activation energies for the beta-relaxation. In the Tg region E decreases (from approximately 250 to approximately 150 kJ mol(-1) in maltitol and from approximately 220 to approximately 170 kJ mol(-1) in glucose) with increasing T as typically found for the alpha-relaxation. From the heat capacity measurements the sizes of cooperatively rearranging regions have been determined as 3.1 (maltitol) and 3.3 (glucose) nm. CONCLUSIONS: DSC can be used for evaluating the energy landscapes. The E values for maltitol are somewhat greater than for glucose due to the added impeding effect of the bulky substitute group in maltitol. The comparable sizes of the cooperatively rearranging regions suggest a similarity of the heterogeneous glassy structures of the two compounds.