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

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.