Departamento de Química Inorgánica, Analítica y Química Física/INQUIMAE, Facultad de Ciencias Exactas y Naturales , Universidad de Buenos Aires, Ciudad Universitaria , Pab. II , Buenos Aires C1428EHA , Argentina.
Departamento de Física de la Materia Condensada, Centro Atómico Constituyentes , Comisión Nacional de Energía Atómica , Av. General Paz 1499 , San Martin , 1650 Buenos Aires , Argentina.
J Phys Chem B. 2018 May 10;122(18):4880-4890. doi: 10.1021/acs.jpcb.8b00784. Epub 2018 Apr 26.
In this study, the solid-vapor equilibrium and the quasi liquid layer (QLL) of ice Ih exposing the basal and primary prismatic faces were explored by means of grand canonical molecular dynamics simulations with the monatomic mW potential. For this model, the solid-vapor equilibrium was found to follow the Clausius-Clapeyron relation in the range examined, from 250 to 270 K, with a Δ H of 50 kJ/mol in excellent agreement with the experimental value. The phase diagram of the mW model was constructed for the low pressure region around the triple point. The analysis of the crystallization dynamics during condensation and evaporation revealed that, for the basal face, both processes are highly activated, and in particular cubic ice is formed during condensation, producing stacking-disordered ice. The basal and primary prismatic surfaces of ice Ih were investigated at different temperatures and at their corresponding equilibrium vapor pressures. Our results show that the region known as QLL can be interpreted as the outermost layers of the solid where a partial melting takes place. Solid islands in the nanometer length scale are surrounded by interconnected liquid areas, generating a bidimensional nanophase segregation that spans throughout the entire width of the outermost layer even at 250 K. Two approaches were adopted to quantify the QLL and discussed in light of their ability to reflect this nanophase segregation phenomena. Our results in the μVT ensemble were compared with NPT and NVT simulations for two system sizes. No significant differences were found between the results as a consequence of model system size or of the working ensemble. Nevertheless, certain advantages of performing μVT simulations in order to reproduce the experimental situation are highlighted. On the one hand, the QLL thickness measured out of equilibrium might be affected because of crystallization being slower than condensation. On the other, preliminary simulations of AFM indentation experiments show that the tip can induce capillary condensation over the ice surface, enlarging the apparent QLL.
在这项研究中，通过使用单原子 mW 势的巨正则分子动力学模拟，研究了暴露基底面和初级棱柱面的冰 Ih 的固-汽平衡和准液相层 (QLL)。对于该模型，在 250 至 270 K 的范围内，固-汽平衡遵循克劳修斯-克拉珀龙关系，ΔH 为 50 kJ/mol，与实验值非常吻合。构建了 mW 模型在三相点附近的低压区域的相图。对冷凝和蒸发过程中的结晶动力学的分析表明，对于基底面，两个过程都是高度激活的，特别是在冷凝过程中形成了立方冰，产生了堆积无序冰。在不同温度和相应的平衡蒸汽压力下研究了冰 Ih 的基底面和初级棱柱面。我们的结果表明，所谓的 QLL 区域可以解释为发生部分熔化的固体最外层。在纳米长度尺度上的固体岛屿被相互连接的液体区域包围，产生二维纳米相分离，甚至在 250 K 时，横跨整个最外层的整个宽度。采用两种方法来量化 QLL，并根据它们反映这种纳米相分离现象的能力进行讨论。我们在μVT 系综中的结果与两个系统大小的 NPT 和 NVT 模拟进行了比较。由于模型系统大小或工作系综的原因，结果之间没有发现显著差异。然而，为了再现实验情况而执行μVT 模拟具有某些优势。一方面，由于结晶比冷凝慢，非平衡状态下测量的 QLL 厚度可能会受到影响。另一方面，AFM 压痕实验的初步模拟表明，针尖可以在冰表面上诱导毛细冷凝，从而扩大表观 QLL。
