Wei-Hsien Hsieh, Wen-Ting Cheng, Ling-Chun Chen, Hong-Liang Lin and Shan-Yang Lin
Two fundamental tools in thermal analysis [differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA)] using five heating rates and one DSC-Fourier Transform Infrared (DSC-FTIR) microspectroscopy using one heating rate, were used to determine the thermal characteristics and dehydration kinetics of glucose (Glc) monohydrate, maltose (Mal) monohydrate and trehalose (Tre) dihydrate in the solid state. Non-isothermal dehydration kinetics of these three sugar excipients was investigated using a model-free isoconversional Flynn–Wall–Ozawa integral method by TGA technique at different heating rates. The apparent activation energy of the dehydration kinetics was determined as: 215.7 ± 33.1, 364.9 ± 49.8 and 207.7 ± 49.4 kJ/mole for Glc monohydrate, Mal monohydrate and Tre dihydrate, respectively. The thermal-responsive changes for several specific FTIR bands in the three-dimensional FTIR spectral contour profile were observed within 50~136°C for Glc monohydrate and >95°C for Mal monohydrate in the dehydration process by the one-step DSC-FTIR microspectroscopic technique. However, two unique FTIR peaks at 1640 and 1687 cm-1 due to the bending vibrational mode of solid-like water and liquid water in the molecules of Tre dihydrate were gradually changed in the range of temperatures between 69 and 81°C during the thermal-induced dehydration process from DSC-FTIR microspectroscopic contour profile.
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