Arjun A, Bolhuis Peter G
van 't Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, 1090 GD Amsterdam, The Netherlands.
J Chem Phys. 2021 Apr 28;154(16):164507. doi: 10.1063/5.0044883.
Carbon dioxide and water can form solid clathrate structures in which water cages encapsulate the gas molecules. Such hydrates have sparked much interest due to their possible application in CO sequestration. How the solid structure forms exactly from the liquid phase via a homogenous nucleation process is still poorly understood. This nucleation event is rare on the molecular timescale even under moderate undercooling or supersaturation conditions because of the large free energy barrier toward crystallization, rendering a brute force simulation of hydrate nucleation unfeasible for moderate undercooling or supersaturation. Here, we perform transition interface sampling simulations to quantify the homogenous nucleation rate for CO hydrate formation using accurate atomistic force fields at 500 bars for three different temperatures between 260 and 273 K. Collecting more than 100 000 pathways comprising roughly two milliseconds of simulation time, we computed a nucleation rate in the amorphous phase of ∼10 nuclei s cm for a temperature of 260 K and a rate of ∼10 nuclei s cm for a temperature of 265 K. For a temperature of 273 K, we find that the hydrate forms an sI crystalline phase with a rate of order of ∼10 nuclei s cm. We compare these rates to classical nucleation theory estimates as well as experiments, and to nucleation rate estimates for methane hydrates and discuss possible causes of the observed differences. Our findings shed light on the kinetics of this important clathrate and should assist in future hydrate formation investigation.
二氧化碳和水能够形成固体笼形结构，其中水笼包裹着气体分子。这类水合物因其在二氧化碳封存方面的潜在应用而引发了广泛关注。然而，对于从液相通过均匀成核过程究竟如何精确形成固体结构，目前仍知之甚少。由于结晶过程存在巨大的自由能垒，即使在适度过冷或过饱和条件下，这种成核事件在分子时间尺度上也是罕见的，这使得对适度过冷或过饱和情况下水合物成核进行强力模拟变得不可行。在此，我们进行过渡界面采样模拟，以使用精确的原子力场在500巴压力下、260至273K之间的三个不同温度下量化二氧化碳水合物形成的均匀成核速率。通过收集超过100000条包含约两毫秒模拟时间的路径，我们计算出在260K温度下非晶相中的成核速率约为10个核·秒⁻¹·厘米⁻³，在265K温度下约为10个核·秒⁻¹·厘米⁻³。对于273K的温度，我们发现水合物以约10个核·秒⁻¹·厘米⁻³的速率形成sI晶相。我们将这些速率与经典成核理论估计值以及实验结果进行比较，同时也与甲烷水合物的成核速率估计值进行比较，并讨论了观察到的差异的可能原因。我们的研究结果揭示了这种重要笼形物的动力学过程，应有助于未来水合物形成的研究。
