Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
J Phys Chem B. 2010 Nov 4;114(43):13796-807. doi: 10.1021/jp107269q.
Understanding the microscopic mechanism of nucleation of clathrate hydrates is important for their use in hydrogen storage, CO(2) sequestration, storage and transport of natural gas, and the prevention of the formation of hydrate plugs in oil and gas pipelines. These applications involve hydrate guests of varied sizes and solubility in water that form different hydrate crystal structures. Nevertheless, molecular studies of the mechanism of nucleation of hydrates have focused on the single class of small hydrophobic guests that stabilize the sI crystal. In this work, we use molecular dynamics simulations with a very efficient coarse-grained model to elucidate the mechanisms of nucleation of clathrate hydrates of four model guests that span a 2 orders of magnitude range in solubility in water and that encompass sizes which stabilize each one a different hydrate structure (sI and sII, with and without occupancy of the dodecahedral cages). We find that the overall mechanism of clathrate nucleation is similar for all guests and involves a first step of formation of blobs, dense clusters of solvent-separated guest molecules that are the birthplace of the clathrate cages. Blobs of hydrophobic guests are rarer and longer-lived than those for soluble guests. For each guest, we find multiple competing channels to form the critical nuclei, filled dodecahedral (5(12)) cages, empty 5(12) cages, and a variety of filled large (5(12)6(n) with n = 2, 3, and 4) clathrate cages. Formation of empty dodecahedra is an important nucleation channel for all but the smallest guest. The empty 5(12) cages are stabilized by the presence of guests from the blob in their first solvation shell. Under conditions of high supercooling, the structure of the critical and subcritical nuclei is mainly determined by the size of the guest and does not reflect the cage composition or ordering of the stable or metastable clathrate crystals.
理解笼形水合物成核的微观机制对于其在氢气储存、二氧化碳封存、天然气的储存和运输以及防止油气管道中形成水合物堵塞方面的应用非常重要。这些应用涉及到大小和在水中溶解度不同的水合物客体，它们形成不同的水合物晶体结构。然而，水合物成核机制的分子研究主要集中在稳定 sI 晶体的单一类小疏水性客体上。在这项工作中，我们使用具有非常高效的粗粒模型的分子动力学模拟来阐明跨越水溶解度 2 个数量级的 4 种模型客体的笼形水合物成核机制，这些客体的尺寸涵盖了每种客体稳定的不同水合物结构（sI 和 sII，有空或没空十二面体笼）。我们发现，所有客体的笼形水合物成核的总体机制是相似的，涉及到形成液滴的第一步，液滴是笼形笼的诞生地，由溶剂分离的客体分子密集簇组成。疏水性客体的液滴比可溶性客体的液滴更稀少，寿命更长。对于每个客体，我们发现形成临界核的多个竞争通道，包括充满的十二面体（5(12)）笼、空的 5(12)笼以及各种充满的大笼（5(12)6(n)，其中 n = 2、3 和 4）。除了最小的客体之外，形成空的十二面体是所有客体的重要成核通道。空的 5(12)笼通过在其第一个溶剂化壳层中存在客体而稳定。在高过冷度条件下，临界和亚临界核的结构主要由客体的大小决定，而不反映稳定或亚稳笼形晶体的笼组成或有序性。
