The relaxation processes of water mixtures of glycerol, ethylene glycol, ethylene glycol oligomers with two to six repeat units, poly(ethylene glycol) 400 and 600, fructose, and propanol have been studied by broadband dielectric spectroscopy at different water contents in the frequency range 10 μHz-20 GHz and in the temperature range 300-80 K without water crystallization. The results show that, in the vicinity of the glass transition temperature of the mixtures, two kinds of water exist. Part of the water behaves as excess water retaining its inherent mobility and appearing as a separate relaxation process (named here the ν-process) at frequencies higher than the structural α-process at subzero temperatures. Another part of the water moves cooperatively with solute molecules and contributes to the α-process.

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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).

Complex permittivity was measured in the frequency range from 10 MHz to 20 GHz at 25°C for water mixtures of 22 aliphatic alcohols. The molecular structures of these alcohols systematically changed with the number of carbon atoms and hydroxyl groups, and their positions in the molecules. The asymmetric shape of the frequency dependence of the dielectric loss for the primary relaxation process was observed for each mixture. The broadness of the asymmetric dielectric loss depends on the water content, and the broadest dielectric loss was observed in the water mole fraction range of 0.65 < x w < 0.85. There is a strong correlation between the broadness of dielectric loss and the number of carbon atoms in the alcohol molecule. Deviations of observed relaxation times from those estimated for ideal mixtures depend on the number of carbon atoms except for the mixtures of water and alcohols with large alkyl groups, which form a micelle-like structure. These experimental results are interpreted on the basis of a model of three kinds of cooperative domains coexisting in the mixtures.

Articles you may be interested inHydrogen network fluctuations of associating liquids: Dielectric relaxation of ethylene glycol oligomers and their mixtures with water J. Chem. Phys. 125, 084507 (2006); 10.1063/1.2338315Broadband dielectric study of the glass transition in poly(ethyleneglycol)-water mixture Relationship between the primary and secondary dielectric relaxation processes in propylene glycol and its oligomers Dielectric measurements of water mixtures of ethylene glycol oligomer ͑EGO͒ with 1-6 repeat units were carried out in the frequency range of 100 MHz-30 GHz at 25°C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO-water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO-water cluster for smaller EGO-water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO-water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters.