van 't Hoff Institute for Molecular Sciences, University of Amsterdam, PO Box 94157, 1090 GD Amsterdam, The Netherlands.
J Phys Chem B. 2020 Sep 17;124(37):8099-8109. doi: 10.1021/acs.jpcb.0c04582. Epub 2020 Sep 8.
The crystallization of methane hydrates via homogeneous nucleation under natural, moderate conditions is of both industrial and scientific relevance, yet still poorly understood. Predicting the nucleation rates at such conditions is notoriously difficult due to high nucleation barriers, and requires, besides an accurate molecular model, enhanced sampling. Here, we apply the transition interface sampling technique, which efficiently computes the exact rate of nucleation by generating ensembles of unbiased dynamical trajectories crossing predefined interfaces located between the stable states. Using an accurate atomistic force field and focusing on specific conditions of 280 K and 500 bar, we compute for nucleation directly into the sI crystal phase at a rate of ∼10 nuclei per nanosecond per simulation volume or ∼10 nuclei per second per cm, in agreement with consensus estimates for nearby conditions. As this is most likely fortuitous, we discuss the causes of the large differences between our results and previous simulation studies. Our work shows that it is now possible to compute rates for methane hydrates at moderate supersaturation, without relying on any assumptions other than the force field.
甲烷水合物在自然、温和条件下通过均相成核结晶具有工业和科学意义，但仍未被充分理解。由于成核势垒高，预测这种条件下的成核速率非常困难，除了需要准确的分子模型外，还需要增强采样。在这里，我们应用了过渡界面采样技术，该技术通过生成无偏动力学轨迹的集合来有效地计算核的精确速率，这些轨迹跨越位于稳定状态之间的预定界面。我们使用精确的原子力场，针对 280 K 和 500 巴的特定条件，在 sI 晶体相中直接进行成核计算，每个模拟体积的成核速率约为每纳秒 10 个核或每立方厘米每秒 10 个核，与附近条件的共识估计值一致。由于这很可能是偶然的，我们讨论了我们的结果与之前模拟研究之间存在较大差异的原因。我们的工作表明，现在可以在不依赖于除力场以外的任何假设的情况下，计算中等过饱和度下水合物的速率。
