Bi Yuanfei, Li Tianshu
Department of Civil and Environmental Engineering, George Washington University , Washington, D.C. 20052, United States.
J Phys Chem B. 2014 Nov 26;118(47):13324-32. doi: 10.1021/jp503000u. Epub 2014 Jun 5.
Understanding the nucleation of hydrate is the key to developing effective strategies for controlling methane hydrate formation. Here we present a computational study of methane hydrate nucleation, by combining the forward flux sampling (FFS) method and the coarse-grained water model mW. To facilitate the application of FFS in studying the formation of methane hydrate, we developed an effective order parameter λ on the basis of the topological analysis of the tetrahedral network. The order parameter capitalizes the signature of hydrate structure, i.e., polyhedral cages, and is capable of efficiently distinguishing hydrate from ice and liquid water while allowing the formation of different hydrate phases, i.e., sI, sII, and amorphous. Integration of the order parameter λ with FFS allows explicitly computing hydrate nucleation rates and obtaining an ensemble of nucleation trajectories under conditions where spontaneous hydrate nucleation becomes too slow to occur in direct simulation. The convergence of the obtained hydrate nucleation rate was found to depend crucially on the convergence of the spatial distribution for the spontaneously formed hydrate seeds obtained from the initial sampling of FFS. The validity of the approach is also verified by the agreement between the calculated nucleation rate and that inferred from the direct simulation. Analyzing the obtained large ensemble of hydrate nucleation trajectories, we show hydrate formation at 220 K and 500 bar is initiated by the nucleation events occurring in the vicinity of water-methane interface, and facilitated by a gradual transition from amorphous to crystalline structure. The latter provides the direct support to the proposed two-step nucleation mechanism of methane hydrate.
了解水合物的成核过程是制定有效控制甲烷水合物形成策略的关键。在此，我们通过结合正向通量采样（FFS）方法和粗粒度水模型mW，对甲烷水合物成核进行了计算研究。为便于FFS在研究甲烷水合物形成中的应用，我们基于四面体网络的拓扑分析开发了一种有效的序参量λ。该序参量利用了水合物结构的特征，即多面体笼，并且能够在允许形成不同水合物相（即sI、sII和非晶态）的同时，有效地区分水合物与冰和液态水。将序参量λ与FFS相结合，可以在直接模拟中自发水合物成核变得过于缓慢而无法发生的条件下，明确计算水合物成核速率并获得成核轨迹的系综。发现所获得的水合物成核速率的收敛性关键取决于从FFS的初始采样中获得的自发形成的水合物晶种的空间分布的收敛性。计算得到的成核速率与直接模拟推断的成核速率之间的一致性也验证了该方法的有效性。通过分析所获得的大量水合物成核轨迹系综，我们表明在220K和500bar条件下，水合物的形成是由水 - 甲烷界面附近发生的成核事件引发的，并由从非晶态到晶体结构的逐渐转变所促进。后者为所提出的甲烷水合物两步成核机制提供了直接支持。
