Department of Chemistry, The University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
J Chem Phys. 2019 Sep 21;151(11):114707. doi: 10.1063/1.5119823.
Methane hydrates can be preserved at ambient pressure, beyond their region of thermodynamic stability, by storing them at temperatures from 240 to 270 K. The origin of this anomalous self-preservation is the formation of an ice coating that covers the clathrate particles and prevents further loss of gas. While there have been several studies on self-preservation, the question of what is the mechanism by which ice nucleates on the decomposing clathrate hydrates has not yet been fully explained. Here, we use molecular simulations, thermodynamic analysis, and nucleation theory to investigate possible scenarios for the nucleation of ice: heterogeneous nucleation at the clathrate/vapor or clathrate/liquid interfaces and homogeneous nucleation from supercooled water. Our results indicate that clathrates cannot heterogeneously nucleate ice and that ice nucleation is due to the cooling of water at the decomposing clathrate/liquid interface, which suffices to trigger homogeneous ice nucleation. We find that the (111) face of the sII structure clathrate can bind to the (111) plane of cubic ice or the basal plane of hexagonal ice through domain matching, resulting in a weak binding that-while insufficient to promote heterogeneous ice nucleation-suffices to produce epitaxy and alignment between these crystals. We use thermodynamic relations, theory, and the contact angles of ice at the (111) sII clathrate/liquid interface to determine-for the first time-the interfacial free energy of this most favorable ice-clathrate interface, 59 ± 5 mJ/m. We discuss the implications of our results for the feasibility of heterogeneous nucleation of gas clathrates at ice/vapor interfaces.
在环境压力下，通过将甲烷水合物储存在 240 至 270 K 的温度下，可以使其在热力学稳定区域之外得以保存。这种异常自我保存的原因是形成了覆盖笼形水合物颗粒的冰涂层，防止了气体的进一步损失。尽管已经有几项关于自我保存的研究，但关于冰在分解的笼形水合物上成核的机制问题尚未得到充分解释。在这里，我们使用分子模拟、热力学分析和成核理论来研究冰成核的可能情景：在笼形水合物/蒸气或笼形水合物/液体界面上的异质成核以及过冷水中的均相成核。我们的结果表明，笼形水合物不能异质成核冰，并且冰成核是由于分解的笼形水合物/液体界面处的水冷却所致，足以引发均相成核。我们发现，sII 结构笼形水合物的（111）面可以通过域匹配与立方冰的（111）面或六方冰的基面结合，从而产生弱结合，虽然不足以促进异质冰成核，但足以产生这些晶体之间的外延和对齐。我们使用热力学关系、理论和冰在（111）sII 笼形水合物/液体界面上的接触角来首次确定这种最有利的冰-笼形水合物界面的界面自由能，为 59 ± 5 mJ/m。我们讨论了我们的结果对在冰/蒸气界面上气体笼形水合物异质成核可行性的影响。
