Dipartimento di Fisica E. Amaldi, Università degli Studi Roma Tre, Roma, Italy.
Phys Chem Chem Phys. 2011 Nov 28;13(44):19773-9. doi: 10.1039/c1cp22029b. Epub 2011 Oct 3.
Broadband dielectric spectroscopy data on water confined in three different environments, namely at the surface of a globular protein or inside the small pores of two silica substrates, in the temperature range 140 K ≤ T ≤ 300 K, are presented and discussed in comparison with previous results from different techniques. It is found that all samples show a fast relaxation process, independently of the hydration level and confinement size. This relaxation is well known in the literature and its cross-over from Arrhenius to non-Arrhenius temperature behavior is the object of vivid debate, given its claimed relation to the existence of a second critical point of water. We find such a cross-over at a temperature of ~180 K, and assign the relaxation process to the layer of molecules adjacent and strongly interacting with the substrate surface. This is the water layer known to have the highest density and slowest translational dynamics compared to the average: its apparent cross-over may be due to the freezing of some degree of freedom and survival of very localized motions alone, to the onset of finite size effects, or to the presence of a calorimetric glass transition of the hydration shell at ~170 K. Another relaxation process is visible in water confined in the silica matrices: this is slower than the previous one and has distinct temperature behaviors, depending on the size of the confining volume and consequent ice nucleation.
本文呈现并讨论了在 140 K ≤ T ≤ 300 K 的温度范围内，三种不同环境中（即球形蛋白质表面或两种二氧化硅基质的小孔内）水的宽频介电谱数据，这些环境与先前来自不同技术的结果进行了比较。结果表明，所有样品均显示出快速弛豫过程，而与水合水平和限制尺寸无关。该弛豫在文献中广为人知，其从 Arrhenius 到非 Arrhenius 温度行为的交叉是一个活跃的争论主题，因为其与水的第二个临界点的存在有关。我们在180 K 的温度下发现了这种交叉，并将弛豫过程归因于与基质表面相邻且强烈相互作用的分子层。与平均水平相比，这种水层具有最高的密度和最慢的平移动力学，其明显的交叉可能是由于某些自由度的冻结和仅存在非常局部的运动、有限尺寸效应的出现，或者在170 K 时水合壳的量热玻璃化转变所致。在二氧化硅基质中限制的水中还可以看到另一个弛豫过程：该过程比前一个过程慢，并且具有不同的温度行为，这取决于限制体积的大小和随后的成核。
