Comparative Relaxation Dynamics of Glucose and Maltitol

Purpose To demonstrate the utility of differential scanning calorimetry (DSC) for determining activation energy landscape in amorphous pharmaceutical systems throughout the sub-T _g and T _g regions.Materials and Methods DSC was employed to determine the effective activation energies (E) of the relaxation in sub-T _g and T _g regions as well as the sizes of cooperatively rearranging regions in glassy maltitol and glucose.Results It has been found that in the sub-T _g region E decreases with decreasing T reaching the values ∼60 (glucose) and ∼70 (maltitol) kJ mol⁻¹ that are comparable to the literature values of the activation energies for the β-relaxation. In the T _g region E decreases (from ∼250 to ∼150 kJ mol⁻¹ in maltitol and from ∼220 to ∼170 kJ mol⁻¹ in glucose) with increasing T as typically found for the α-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.