The glass transition and dielectric secondary relaxation of fructose-water mixtures.
The glass transition and dielectric secondary relaxation of fructose-water mixtures.
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DOI：
10.1021/jp807038r
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发表时间：
2008-11
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通讯作者：
N. Shinyashiki;M. Shinohara;Y. Iwata;T. Goto;M. Oyama;S. Suzuki;W. Yamamoto;S. Yagihara;T. Inoue;S. Oyaizu;S. Yamamoto;K. Ngai;S. Capaccioli
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作者：
N. Shinyashiki;M. Shinohara;Y. Iwata;T. Goto;M. Oyama;S. Suzuki;W. Yamamoto;S. Yagihara;T. Inoue;S. Oyaizu;S. Yamamoto;K. Ngai;S. Capaccioli
在-70至45 ℃的温度范围内，在2 mHz至20 GHz的频率范围内，对果糖浓度在70.0和94.6 wt%之间的果糖-水混合物进行宽带介电测量。两个弛豫过程，α过程在较低的频率和二次β过程在较高的频率，观察到。α过程的介电弛豫时间在玻璃化转变温度T（g）下为100 s，通过差示扫描量热法（DSC）测定。β过程的弛豫时间和强度从低于T（g）的较弱温度依赖性变为高于T（g）的较强温度依赖性。在果糖-水混合物中的β过程在穿过混合物的T（g）时的行为的这些变化与在具有氢键分子液体、聚合物和纳米多孔系统的各种其它水性混合物中的水的次级过程中发现的相同。这些结果导致的结论是，初级α过程的果糖-水的混合物的水和果糖分子的合作运动的结果，和次级β过程是Johari-Goldstein过程的混合物中水。在T（g）附近和以上的温度下，在某些混合物中观察到α和β过程，并测定了它们的弛豫时间τ（α）和τ（β），τ（α）/τ（β）的比值与耦合模型的预测值雅阁。将τ（α）固定在100 s，τ（α）/τ（β）的比率随着混合物中果糖浓度的降低而降低。这一趋势也是一致的耦合模型从降低果糖浓度时的分子间耦合参数的减少预期。
Broad-band dielectric measurements for fructose-water mixtures with fructose concentrations between 70.0 and 94.6 wt% were carried out in the frequency range of 2 mHz to 20 GHz in the temperature range of -70 to 45 degrees C. Two relaxation processes, the alpha process at lower frequency and the secondary beta process at higher frequency, were observed. The dielectric relaxation time of the alpha process was 100 s at the glass transition temperature, T(g), determined by differential scanning calorimetry (DSC). The relaxation time and strength of the beta process changed from weaker temperature dependences of below T(g) to a stronger one above T(g). These changes in behaviors of the beta process in fructose-water mixtures upon crossing the T(g) of the mixtures is the same as that found for the secondary process of water in various other aqueous mixtures with hydrogen-bonding molecular liquids, polymers, and nanoporous systems. These results lead to the conclusion that the primary alpha process of fructose-water mixtures results from the cooperative motion of water and fructose molecules, and the secondary beta process is the Johari-Goldstein process of water in the mixture. At temperatures near and above T(g) where both the alpha and the beta processes were observed and their relaxation times, tau(alpha) and tau(beta), were determined in some mixtures, the ratio tau(alpha)/tau(beta) is in accord with that predicted by the coupling model. Fixing tau(alpha) at 100 s, the ratio tau(alpha)/tau(beta) decreases with decreasing concentration of fructose in the mixtures. This trend is also consistent with that expected by the coupling model from the decrease of the intermolecular coupling parameter upon decreasing fructose concentration.