Processes of dehydration and rehydration of raffinose pentahydrate investigated by thermal analysis and FT-IR/DSC microscopic system

Abstract

This study deals with the investigation of dehydration or rehydration process of raffinose pentahydrate by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) microspectroscopy equipped with thermal analyzer. Raffinose pentahydrate was compressed on one KBr pellet (1 KBr method) or sealed within two KBr pellets (2 KBr method) for FT-IR determination. Thermogram results from DSC indicate that three endothermic peaks at 56, 73 and 85 °C were observed, likely corresponding to the loss of one, two, and 2 mol of water respectively, and corresponding to specific weight loss in the TGA curves. The total weight loss during TGA analysis was about 15.43% and was almost equal to the loss of 5 mol of water from raffinose pentahydrate. The thermal-dependent FT-IR spectra for raffinose pentahydrate revealed that the peak intensity at 1651 cm −1 was reduced gradually with temperature up to 50 °C, decreased significantly above 50 °C, and disappeared completely above 100 °C, due to the evaporation of water. The peak at 1651 cm −1 was shifted to 1639 cm −1 from 48 °C for 2 KBr sample and maintained its peak position even heating to 150 °C. The former peak was assigned to the scissoring vibration mode of the crystal water in raffinose pentahydrate, the latter peak was due to the free liquid water dehydrated from the raffinose pentahydrate but which was still sealed within two KBr pellets. The 62 °C-preheated raffinose sample could rehydrate to pentahydrate under 30 °C, 70%RH isothermal condition for 180 min via shifting the IR peak from 1645 to 1651 cm −1 with time. Once three water molecules were dehydrated from raffinose pentahydrate by preheating sample above 81 °C, its rehydration process seemed to be difficult. The extent of dehydration for raffinose pentahydrate might play a key role to influence the process of rehydration for the preheated raffinose samples.