Errington J R, Debenedetti P G
Department of Chemical Engineering, Princeton University, New Jersey 08544-5263, USA.
Nature. 2001 Jan 18;409(6818):318-21. doi: 10.1038/35053024.
In contrast to crystalline solids--for which a precise framework exists for describing structure--quantifying structural order in liquids and glasses has proved more difficult because even though such systems possess short-range order, they lack long-range crystalline order. Some progress has been made using model systems of hard spheres, but it remains difficult to describe accurately liquids such as water, where directional attractions (hydrogen bonds) combine with short-range repulsions to determine the relative orientation of neighbouring molecules as well as their instantaneous separation. This difficulty is particularly relevant when discussing the anomalous kinetic and thermodynamic properties of water, which have long been interpreted qualitatively in terms of underlying structural causes. Here we attempt to gain a quantitative understanding of these structure-property relationships through the study of translational and orientational order in a models of water. Using molecular dynamics simulations, we identify a structurally anomalous region--bounded by loci of maximum orientational order (at low densities) and minimum translational order (at high densities)--in which order decreases on compression, and where orientational and translational order are strongly coupled. This region encloses the entire range of temperatures and densities for which the anomalous diffusivity and thermal expansion coefficient of water are observed, and enables us to quantify the degree of structural order needed for these anomalies to occur. We also find that these structural, kinetic and thermodynamic anomalies constitute a cascade: they occur consecutively as the degree of order is increased.
与晶体固体不同——晶体固体有精确的框架来描述其结构——对液体和玻璃中的结构有序性进行量化已被证明更加困难，因为即使这些体系具有短程有序性，但它们缺乏长程晶体有序性。使用硬球模型体系已取得了一些进展，但要准确描述像水这样的液体仍然困难，在水中，定向吸引力（氢键）与短程排斥力共同作用，决定了相邻分子的相对取向及其瞬时间距。在讨论水的反常动力学和热力学性质时，这一困难尤为突出，长期以来人们一直从潜在的结构原因对这些性质进行定性解释。在此，我们试图通过研究水的模型中的平移有序性和取向有序性，来定量理解这些结构 - 性质关系。利用分子动力学模拟，我们确定了一个结构反常区域——由最大取向有序性（在低密度时）和最小平移有序性（在高密度时）的轨迹界定——在这个区域中，有序性在压缩时降低，且取向有序性和平移有序性强烈耦合。这个区域涵盖了观察到水的反常扩散率和热膨胀系数的整个温度和密度范围，并使我们能够量化这些反常现象发生所需的结构有序程度。我们还发现，这些结构、动力学和热力学反常现象构成了一个级联：随着有序程度的增加，它们依次出现。
