Glass transition phenomena and dielectric relaxations in supercooled d-lyxose aqueous solutions

Abstract

Differential scanning calorimeter and broadband dielectric spectroscopy in a broad range of temperatures (150-300 K) were employed to study the d-lyxose aqueous mixture at different hydration levels. Two relaxation processes were observed in all investigated d-lyxose aqueous mixtures. A relaxation process (process-I) usually known as the primary relaxation mode which is accountable for the collective motion of d-lyxose aqueous solution, was observed above the glass transition temperature (T_g). Below T_g, another process designated as process-II was found which is mainly related to the water molecule relaxation inside the d-lyxose matrix. The average relaxation times as a function of temperature and dielectric strengths of both observed relaxation processes (I & II) were analyzed for all hydration levels in d-lyxose. It was identified that the relaxation amplitude of process-II in the d-lyxose aqueous mixture was increased drastically and their activation energies were found to be approximately independent of the content of water above critical concentration, x_c = 0.28. This suggests that the dynamical process observed above x_c was dominated by the presence of water clusters. In the current aqueous mixture, the critical content of water (x_c) is slightly higher as compared to previously reported aqueous mixtures, indicating a more cooperative nature of water molecules with a d-lyxose matrix. Additionally, the T_g of pure water was estimated at 128 ± 5.8 K from the extrapolation of DSC T_g data of the d-lyxose aqueous solution by using the well-known Gordon-Taylor equation. Our current result gives further support to the well-accepted glass transition (T_g) of pure water.

Keywords: Aqueous mixture; Dielectric relaxation; Differential scanning calorimeter; Glass transition; Water dynamics; d-lyxose.