Department of Civil and Environmental Engineering, George Washington University, Washington, DC 20052, USA.
Phys Chem Chem Phys. 2011 Nov 28;13(44):19807-13. doi: 10.1039/c1cp22167a. Epub 2011 Oct 11.
Homogeneous ice nucleation from supercooled water was studied in the temperature range of 220-240 K through combining the forward flux sampling method (Allen et al., J. Chem. Phys., 2006, 124, 024102) with molecular dynamics simulations (FFS/MD), based on a recently developed coarse-grained water model (mW) (Molinero et al., J. Phys. Chem. B, 2009, 113, 4008). The calculated ice nucleation rates display a strong temperature dependence, ranging from 2.148 ± 0.635 × 10(25) m(-3) s(-1) at 220 K to 1.672 ± 0.970 × 10(-7) m(-3) s(-1) at 240 K. These rates can be fitted according to the classical nucleation theory, yielding an estimate of the effective ice-water interface energy γ(ls) of 31.01 ± 0.21 mJ m(-2) for the mW water model. Compared to experiments, our calculation underestimates the homogeneous ice nucleation rate by a few orders of magnitude. Possible reasons for the discrepancy are discussed. The nucleating ice embryo contains both cubic ice Ic and hexagonal ice Ih, with the fraction of each structure being roughly 50% when the critical size is reached. In particular, a novel defect structure containing nearly five-fold twin boundaries is identified in the ice clusters formed during nucleation. The way such defect structure is formed is found to be different from mechanisms proposed for the formation of the same defect in metallic nanoparticles and thin film. The quasi five-fold twin boundary structure found here is expected to occur in the crystallization of a wide range of materials with the diamond cubic structure, including ice.
采用正向通量抽样法（Allen 等人，J. Chem. Phys.，2006，124，024102）与分子动力学模拟（FFS/MD）相结合的方法，在 220-240 K 的温度范围内研究了过冷水的均相成核。该方法基于最近开发的粗粒化水模型（mW）（Molinero 等人，J. Phys. Chem. B，2009，113，4008）。计算得到的成核率随温度呈现强烈的依赖性，在 220 K 时范围为 2.148 ± 0.635×10(25) m(-3) s(-1)，在 240 K 时范围为 1.672 ± 0.970×10(-7) m(-3) s(-1)。这些速率可以根据经典成核理论进行拟合，从而得出 mW 水模型的有效冰-水界面能γ(ls)的估计值为 31.01 ± 0.21 mJ m(-2)。与实验相比，我们的计算低估了均相成核率几个数量级。讨论了产生差异的可能原因。成核的冰胚包含立方冰 Ic 和六方冰 Ih，当达到临界尺寸时，每种结构的比例约为 50%。特别是，在成核过程中形成的冰簇中鉴定出一种包含近五重孪晶边界的新型缺陷结构。发现这种缺陷结构的形成方式与在金属纳米粒子和薄膜中形成相同缺陷的机制不同。预计这里发现的准五重孪晶边界结构将出现在具有金刚石立方结构的广泛材料的结晶中，包括冰。
