Dielectric relaxations and the phenomena of glass transition in supercooled D-xylose aqueous mixtures

Abstract

D-xylose aqueous solutions at various concentrations of water (0 ≤ x ≤ 40 %) were investigated using broadband dielectric spectroscopy and differential scanning calorimeter in the wide temperature range (150–300 K). Two resolvable relaxations were observed in all studied D-xylose aqueous solutions. The slower relaxation process (process-I) observed above the glass transition temperature (Tg), usually referred to as the structural relaxation process, is responsible for the collective movement of the D-xylose and water mixture. The faster one (process-II) observed below Tg is mainly related to the water molecular relaxation inside the D-xylose matrix. The temperature dependence of the average dielectric relaxation time and dielectric strength of both observed processes (I & II) were analyzed for all concentrations of water (x) in D-xylose. It was observed that the dielectric strength of process-II in the D-xylose water mixture increased sharply above critical concentration, x = 25 % shows moderate characteristics as compared to its isomers viz. D-ribose, D-arabinose and D-lyxose. Also the activation energies were found to be almost water content independent above critical concentration, x = 25 %. This suggests that this dynamic process was dominated by water-water interaction. Moreover, the Tg of bulk water was estimated at 133 ± 1 K and 138 ± 1.2 K from the extrapolation of DSC Tg and dielectric Tg, respectively of the D-xylose aqueous mixture using the Gordon-Taylor equation. This gives further support to the commonly accepted Tg value of bulk water.