Dielectric study of the α and β processes in supercooled ethylene glycol oligomer–water mixtures

Abstract

Broadband dielectric measurements for 65 wt % ethylene glycol oligomer (EGO)-water mixtures with one to six repeat units of EGO molecules were performed in the frequency range of 10 microHz-10 GHz and the temperature range of 128-298 K. In the case of the water-EGO mixtures with one and two repeat units of the EGO molecule (small EGO), the shape of the dielectric loss peak of the primary process is asymmetrical about the logarithm of the frequency of maximum loss above the crossover temperature, T(C). The asymmetric process continues to the alpha process at a low frequency, and an additional beta process appears in the frequency range higher than that of the alpha process below T(C). In contrast, the water-EGO mixtures with three or more repeat units of the EGO molecule (large EGO) show a broad and symmetrical loss peak of the primary process above T(C). The symmetric process continues to the beta process, and an additional alpha process appears in the frequency range lower than that of the beta process below T(C). These different scenarios of the alpha-beta separation related to the shape of the loss peak above T(C) are a result of the difference in the cooperative motion of water and solute molecules. The solute and water molecules move cooperatively in the small EGO-water mixtures above T(C), and this cooperative motion leads to the asymmetric loss peak above T(C) and the alpha process below T(C). For the large EGO-water mixtures, the spatially restricted motion of water confined by solute molecules leads to the symmetric loss peak above T(C) and the beta process below T(C).

Broadband Dielectric Spectroscopic, Calorimetric, and FTIR‐ATR Investigations of D‐Arabinose Aqueous Solutions

“…Moreover, the different temperature dependences of the relaxation time of process-II at temperatures above and below the T g of arabinose-water binary systems are more or less the same as the relaxation times observed in various aqueous solutions. [7][8][9][10][11][14][15][16][17][18][19][20][21][22] These similar properties suggest that the relaxation process-II observed in d-arabinose-water systems is due to the reorientational motion of water in these mixtures. Recently, Kaminski et al [29] have shown that pure d-arabinose possesses two kinds of relaxation processes, namely a and g. For each water concentration, the relaxation time of process-II is closer to the g-process of anhydrous d-arabinose (Figure 6 b), but the interesting point is that their dielectric strengths have much higher magnitudes as compared to the g-process of darabinose.…”

“…Thus, much attention has been paid in the literature to the dielectric behavior of water dynamics in several aqueous mixtures, such as carbohydrates, [7][8][9][10][11] proteins, [12,13] and polymers. [14][15][16][17][18][19][20] Among the various glass-forming materials, the study of the dynamical behavior of carbohydrates (mono-, di-, and polysaccharides) possesses an essential interest, as they belong to an interesting group of materials able to undergo a glass transition both in the anhydrous form and in aqueous solutions. Although an enormous amount of work has been devoted to the understanding of its occurrence in supercooled systems, many unresolved issues [21] are still left to be clarified.…”

Dynamics and Glass Transition of Aqueous Solutions of Molecular Liquid, Polymer, and Protein Studied by Broadband Dielectric Spectroscopy

“…In solutions containing solutes with a low molecular weight, such as alcohols (molecular weight, M W 100), the observed relaxation curve was asymmetric with respect to the loss peak above the separation temperature T S [39,40,55,[59][60][61], as shown in Fig. 9.3a.…”

“…Here, we call it ν-relaxation [55], ν being the initial letter of νερo, which means water in Greek. It was shown that ν-relaxation is the primary dielectric relaxation of water observed in various aqueous solutions at around room temperature, and the shape of the ν-relaxation spectrum depends on the molecular weight of the solute [39,40,55,[58][59][60][61][62].…”

“…The solutes used were glycerol [35,36], ethylene glycol [37,38], ethylene glycol oligomer [39][40][41], poly(ethylene glycol) [36,42,43], propylene glycol and its oligomers [36,44], poly(propylene glycol) [45], propanol [46], PVP [36,43,[47][48][49], poly (vinyl methyl ether) (PVME) [50,51], fructose [52], and other various materials [38,43,53], which have different molecular structures and a wide range of T g values. Figure 9.2 shows dielectric loss spectra observed for a 65 wt% pentaethylene glycol-water mixture at various temperatures as an example of water mixtures.…”