The inhomogeneous structure of water at ambient conditions
The inhomogeneous structure of water at ambient conditions
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DOI：
10.1073/pnas.0904743106
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发表时间：
2009-09-08
影响因子：
11.1
通讯作者：
Nilsson, A.
中科院分区：
文献类型：
--
作者：
Huang, C.;Wikfeldt, K. T.;Nilsson, A.
小角度x射线散射（SAXS）用于在大约1 nm的物理长度尺度上证明环境水中密度波动的存在；随着温度的降低，这一现象保持不变，而幅度增大。相比之下，正常液体（如CCl4）的波动幅度没有随着温度的降低而增强，环境条件下分子动力学模拟中的水也是如此。基于x射线发射光谱和x射线拉曼散射数据，我们提出SAXS中的密度差对比是由于四面体结构和氢键扭曲结构之间的波动，分别与低密度水和高密度水有关。我们结合我们的实验观察，提出了一个水的模型，作为一个温度依赖，波动平衡之间的两种类型的局部结构驱动的不适应的要求，最小化焓（强近四面体氢键）和最大化熵（非定向氢键和无序）。目前的结果提供了实验证据，表明即使在从环境沸点到接近沸点的条件下，在深度过冷状态下氢键环境中预期的极端差异令人惊讶地仍然存在于散装水中。
Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of approximate to 1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.