Zielke Stephen A, Bertram Allan K, Patey Grenfell N
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
J Phys Chem B. 2015 Jul 23;119(29):9049-55. doi: 10.1021/jp508601s. Epub 2014 Oct 6.
Heterogeneous ice nucleation at solid surfaces is important in many physical systems including the Earth's atmosphere. AgI is one of the best ice nucleating agents known; however, why AgI is such an effective ice nucleus is unclear. Using molecular dynamics simulations, we show that a good lattice match between ice and a AgI surface is insufficient to predict the ice nucleation ability of the surface. Seven faces modeled to represent surfaces of both β-AgI and γ-AgI, each having a good lattice match with hexagonal and/or cubic ice, are considered, but ice nucleation is observed for only three. Our model simulations clearly show that the detailed atomistic structure of the surface is of crucial importance for ice nucleation. For example, when AgI is cleaved along certain crystal planes two faces result, one with silver ions and the other with iodide ions exposed as the outermost layer. Both faces have identical lattice matches with ice, but in our simulations ice nucleation occurred only at silver exposed surfaces. Moreover, although hexagonal ice is often the only polymorph of ice considered in discussions of heterogeneous ice nucleation, cubic ice was frequently observed in our simulations. We demonstrate that one possible mechanism of ice nucleation by AgI consists of particular AgI surfaces imposing a structure in the adjacent water layer that closely resembles a layer that exists in bulk ice (hexagonal or cubic). Ice nucleates at these surfaces and grows almost layer-by-layer into the bulk.
在包括地球大气在内的许多物理系统中，固体表面的异质冰核形成都很重要。碘化银是已知最好的冰核剂之一；然而，碘化银为何是如此有效的冰核尚不清楚。通过分子动力学模拟，我们表明冰与碘化银表面之间良好的晶格匹配不足以预测该表面的冰核形成能力。我们考虑了七个建模的面，分别代表β - 碘化银和γ - 碘化银的表面，每个面与六方和/或立方冰都有良好的晶格匹配，但仅观察到三个面有冰核形成。我们的模型模拟清楚地表明，表面的详细原子结构对冰核形成至关重要。例如，当碘化银沿某些晶面裂解时会产生两个面，一个面银离子暴露在外层，另一个面碘离子暴露在外层。两个面与冰的晶格匹配相同，但在我们的模拟中，冰核形成仅发生在银离子暴露的表面。此外，尽管在异质冰核形成的讨论中，六方冰通常是唯一被考虑的冰的多晶型，但在我们的模拟中经常观察到立方冰。我们证明，碘化银冰核形成的一种可能机制是特定的碘化银表面在相邻水层中形成一种结构，该结构与大块冰（六方或立方）中存在的一层非常相似。冰在这些表面成核，并几乎逐层生长到主体中。
