Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2017 Nov 21;147(19):194508. doi: 10.1063/1.5004177.
We investigate collective molecular motion and the self-diffusion coefficient D of water molecules in the mobile interfacial layer of the secondary prismatic plane (112¯0) of hexagonal ice by molecular dynamics simulation based on the TIP4P/2005 water potential and a metrology of collective motion drawn from the field of glass-forming liquids. The width ξ of the mobile interfacial layer varies from a monolayer to a few nm as the temperature is increased towards the melting temperature T, in accordance with recent simulations and many experimental studies, although different experimental methods have differed in their precise estimates of the thickness of this layer. We also find that the dynamics within this mobile interfacial ice layer is "dynamically heterogeneous" in a fashion that has many features in common with glass-forming liquids and the interfacial dynamics of crystalline Ni over the same reduced temperature range, 2/3 < T/T < 1. In addition to exhibiting non-Gaussian diffusive transport, decoupling between mass diffusion and the structural relaxation time, and stretched exponential relaxation, we find string-like collective molecular exchange motion in the interfacial zone within the ice interfacial layer and colored noise fluctuations in the mean square molecular atomic displacement 〈u〉 after a "caging time" of 1 ps, i.e., the Debye-Waller factor. However, while the heterogeneous dynamics of ice is clearly similar in many ways to molecular and colloidal glass-forming materials, we find distinct trends between the diffusion coefficient activation energy E for diffusion D and the interfacial width ξ from the scale of collective string-like motion L than those found in glass-forming liquids.
我们通过基于 TIP4P/2005 水势和从玻璃形成液体领域得出的集体运动度量的分子动力学模拟，研究了水在六方冰的二次棱柱平面（112¯0）的可动界面层中的集体分子运动和自扩散系数 D。随着温度向熔融温度 T 升高，可动界面层的宽度 ξ 从单层增加到几个纳米，这与最近的模拟和许多实验研究一致，尽管不同的实验方法对该层的厚度有不同的精确估计。我们还发现，这种可动界面冰层内的动力学是“动态异质的”，具有许多与玻璃形成液体和相同过冷温度范围内结晶 Ni 的界面动力学共同的特征。除了表现出非高斯扩散输运、质量扩散和解耦与结构弛豫时间以及拉伸指数松弛之外，我们还发现界面区内在冰界面层内存在线状集体分子交换运动，以及在平均平方分子原子位移〈u〉中存在有色噪声波动经过 1 ps 的“笼化时间”后，即德拜-沃勒因子。然而，虽然冰的异质动力学在许多方面与分子和胶体玻璃形成材料明显相似，但我们发现扩散系数 D 的激活能 E 和界面宽度 ξ 之间的扩散系数 D 的趋势与玻璃形成液体中的趋势明显不同。
