Center for Hydrate Research, Chemical & Biological Engineering Department, Colorado School of Mines, Golden, CO 80401, USA.
Phys Chem Chem Phys. 2011 Nov 28;13(44):19951-9. doi: 10.1039/c1cp21899a. Epub 2011 Oct 14.
Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.
人们对描述笼形水合物形成机制的兴趣跨越了多个科学和技术应用领域。在这里，我们报告了通过完全混合和无序的甲烷-水二相流体系统进行的多次自发甲烷笼形水合物成核和生长的分子动力学模拟结果。在一系列热力学条件、模拟几何形状和尺寸下，一组七个笼型约占成核固体中形成的所有笼型的 95%。这组笼型包括普遍存在的 5(12)笼、5(12)6(n)子集（其中 n 的范围为 2-4）和 4(1)5(10)6(n)子集（其中 n 的范围也为 2-4）。这些笼型之间的转变通过水分子对的插入/去除和旋转发生，并且可能阐明实验中观察到的固-固结构重排的机制。在所有成核轨迹中，笼型的相对丰度存在一定的一致性。在成核固体中，5(12)笼总是两种最丰富的笼型之一，通常也是最丰富的笼型。在所有模拟中，具有相同水分子数的 5(12)6(n)笼型的数量多于其 4(1)5(10)6(n)对应物。在这些一致特征中，某些笼型比例和成核固体的长程有序性存在一定的随机性。即使在比较在相同条件下进行的模拟时，一些轨迹会产生多个相邻 sI 单位细胞的条带和超过 5nm 的长程有序，而其他轨迹则只产生孤立的 sI 单位细胞和很少的长程有序。与长程有序的成核固体相比，具有少量长程有序的成核固体中，5(12)6(2)/5(12)和 5(12)6(3)/4(1)5(10)6(2)笼型比例更高。在高驱动力下形成多个相邻的 sI 水合物单位细胞表明，除了涉及通过无定形中间体形成的主流水合物成核假说之外，还存在替代或补充假说。
