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解决水的玻璃化转变温度争议?

Resolving the controversy on the glass transition temperature of water?

机构信息

CNR-IPCF, Dipartimento di Fisica, Largo Bruno Pontecorvo 3, I-56127, Pisa, Italy.

出版信息

J Chem Phys. 2011 Sep 14;135(10):104504. doi: 10.1063/1.3633242.

Abstract

We consider experimental data on the dynamics of water (1) in glass-forming aqueous mixtures with glass transition temperature T(g) approaching the putative T(g) = 136 K of water from above and below, (2) in confined spaces of nanometer in size, and (3) in the bulk at temperatures above the homogeneous nucleation temperature. Altogether, the considered relaxation times from the data range nearly over 15 decades from 10(-12) to 10(3) s. Assisted by the various features in the isothermal spectra and theoretical interpretation, these considerations enable us to conclude that relaxation of un-crystallized water is highly non-cooperative. The exponent β(K) of its Kohlrausch stretched exponential correlation function is not far from having the value of one, and hence the deviation from exponential time decay is slight. Albeit the temperature dependence of its α-relaxation time being non-Arrhenius, the corresponding T(g)-scaled temperature dependence has small steepness index m, likely less than 44 at T(g), and hence water is not "'fragile" as a glassformer. The separation in time scale of the α- and the β-relaxations is small at T(g), becomes smaller at higher temperatures, and they merge together shortly above T(g). From all these properties and by inference, water is highly non-cooperative as a glass-former, it has short cooperative length-scale, and possibly smaller configurational entropy and change of heat capacity at T(g) compared with other organic glass-formers. This conclusion is perhaps unsurprising because water is the smallest molecule. Our deductions from the data rule out that the T(g) of water is higher than 160 K, and suggest that it is close to the traditional value of 136 K.

摘要

我们考虑了实验数据,这些数据涉及(1)在玻璃化转变温度 T(g)接近 136 K 的玻璃形成水的水溶液中,从上方和下方的动力学;(2)在纳米级大小的受限空间中;(3)在高于均相成核温度的体相中。总的来说,从数据中考虑的弛豫时间范围几乎跨越了 15 个数量级,从 10(-12)到 10(3) s。通过等温谱中的各种特征和理论解释的辅助,这些考虑使我们能够得出结论,即未结晶水的弛豫是高度非协同的。其 Kohlrausch 拉伸指数函数的指数β(K)接近于 1,因此与指数时间衰减的偏差很小。尽管其α-松弛时间的温度依赖性不是 Arrhenius 型的,但相应的 T(g)标度温度依赖性的陡峭指数 m 较小,在 T(g)处可能小于 44,因此水作为玻璃形成体并不“脆弱”。在 T(g)处,α-和β-松弛的时间尺度分离较小,在较高温度下变得更小,并且在 T(g)上方不久就合并在一起。从所有这些性质和推断来看,水作为玻璃形成体的非协同性很高,其协同长度尺度较短,并且与其他有机玻璃形成体相比,可能具有较小的构象熵和热容变化。由于水是最小的分子,这个结论可能并不令人惊讶。我们从数据中推断出,水的 T(g)不高于 160 K,并表明它接近传统的 136 K 值。

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