Department of Chemical Engineering, University of California, Santa Barbara, California 93106-9010, USA.
J Chem Phys. 2019 Sep 7;151(9):094501. doi: 10.1063/1.5111545.
A tetrahedral structure resulting from hydrogen bonding is a hallmark of liquid water and plays a significant role in determining its unique thermophysical properties. This water feature has helped understand anomalous properties and physically interpret and model hydrophobic solvation thermodynamics. Tetrahedrality is well described by the geometric relationship of any central water molecule with two of its nearest neighbors in the first coordination shell, as defined by the corresponding "three-body" angle. While order parameters and even full water models have been developed using specific or average features of the three-body angle distribution, here we examine the distribution holistically, tracking its response to changes in temperature, density, and the presence of model solutes. Surprisingly, we find that the three-body distribution responds by varying primarily along a single degree of freedom, suggesting a remarkably simplified view of water structure. We characterize three-body angle distributions across temperature and density space and identify principal components of the variations with state conditions. We show that these principal components embed physical significance and trace out transitions between tetrahedral and simple-fluid-like behavior. Moreover, we find that the ways three-body angles vary within the hydration shells of model colloids of different types and sizes are nearly identical to the variations seen in bulk water across density and temperature. Importantly, through the principal directions of these variations, we find that perturbations to the hydration-water distributions well predict the thermodynamics associated with colloid solvation, in particular, the relative entropy of this process that captures indirect, solvent-mediated contributions to the hydration free energy.
氢键导致的四面体结构是液态水的标志特征，对确定其独特的热物理性质起着重要作用。这种水的特征有助于理解异常性质，并从物理上解释和模拟疏水性溶剂化热力学。四面体结构可以通过中心水分子与其第一配位壳中两个最近邻之间的几何关系来很好地描述，由相应的“三体”角定义。虽然已经使用三体角分布的特定或平均特征开发了序参数甚至全水模型，但在这里我们整体上研究了分布，追踪其对温度、密度和模型溶质存在的响应。令人惊讶的是，我们发现三体分布主要通过单一自由度变化来响应，这表明对水结构有一个非常简化的看法。我们在温度和密度空间中描绘了三体角分布，并确定了与状态条件相关的变化的主要成分。我们表明这些主成分具有物理意义，并追踪了四面体和简单流体行为之间的转变。此外，我们发现不同类型和大小的模型胶体质点的水合壳内三体角的变化方式与在密度和温度范围内观察到的 bulk water 的变化几乎相同。重要的是，通过这些变化的主要方向，我们发现对水合分布的扰动可以很好地预测与胶体溶剂化相关的热力学，特别是捕捉间接、溶剂介导对水合自由能贡献的该过程的相对熵。
