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Structural Approach to Ice Growth (and Nucleation) in Liquid Water

机译：液态水中冰增长（和成核）的结构方法

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Crystal growth is commonly conceptualized with a simple structural model, the stacking of molecules or atoms represented by cubes, or sometimes spheres. The growth process is by adding single molecules at kinks in steps on otherwise flat ("smooth") interfaces, or anywhere on surfaces that are thermodynamically "rough." However, the strong preference for tetrahedral bonding between water molecules dictates the much more open structure of ice (1h) crystals. The present study was motivated by curiosity about whether the ice crystal structure might directly influence its growth process. More specifically, why do ice crystals growing from slightly supercooled water form facets only at {0001}, the basal orientation, and form rounded interfaces otherwise, but with the fastest growth on the secondary prism interface, {11 (2) over bar0}? The approach here is to examine a model of a molecule-by-molecule growth process that forces growth of ice's structure, its particular, tetrahedrally connected network of H-bonded water molecules. For a water molecule to become completely fixed in the ice structure, it is necessary and sufficient that it has two H-bonds to molecules of a single ice crystal. Starting growth on any of the three, low-index interfaces (the basal and the primary and secondary prism interfaces), the first molecule to bond to the ice structure can form only one bond to the crystal. That locates it in the ice structure, but its other three bond orientations need not be compatible with the structure. At either of the prism interfaces, a second water molecule can bond to both the first one and to another molecule in the original surface, so that the pair of molecules is then held completely in the structure, including all of the actual and potential bond orientations. However, in initiating a new layer of molecules at a basal interface, a second molecule bonded to the first cannot also bond in the ice structure and therefore has no reason to be in ice. It is possible that this hindrance to the initiation of a new basal layer may explain why the basal orientation is unique in providing the only growth facet on ice growing from slightly supercooled, liquid water. The more conventional explanation of the basal growth facets involves classical nucleation theory applied to layer formation, but at the low supercooling, that would require a large size for the critical embryo.
机译：晶体生长通常用简单的结构模型概念化，堆叠由立方体或有时是球体表示的分子或原子。生长过程是通过在另外的平面（“平滑”）界面的步骤中以扭结添加单个分子，或在热力学上“粗糙的表面上的任何位置”。然而，水分子之间的四面体键合的强烈偏好决定了冰（1H）晶体的更远开放结构。本研究通过好奇心，关于冰晶结构是否可能直接影响其生长过程。更具体地说，为什么从{0001}，基础取向和形成圆形界面的均匀水形外部生长的冰晶否则，否则，但在次级棱镜界面上的增长最快，{11（2）上方的Bar0}？这里的方法是检查分子 - 分子生长过程的模型，其强制冰结构的生长，其特定的，其特定的H键合水分子网络。对于水分子完全固定在冰结构中，需要并且足以使其对单个冰晶的分子具有两个H键。开始增长三个，低指数界面（基础和初级和二次棱镜界面中的任何一种，第一个键合到冰结构的分子可以仅形成晶体的一个键。它位于冰结构中，但其其他三个债券取向不需要与结构不兼容。在任一棱镜界面中，第二水分子可以粘合到原始表面中的第一一个和另一个分子中，从而将一对分子完全保持在结构中，包括所有实际和潜在的债券取向。然而，在启动基底界面的新分子层中，与第一个键合的第二分子也不能在冰结构中键合，因此没有理由在冰中。这种障碍可以解释为什么基础取向在为从稍微过冷，液态水中的冰上提供唯一的增长面是独特的。基础生长面的更常规说明涉及施加到层形成的经典成核理论，但在低过冷处，这将需要大尺寸的临界胚胎。

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