(356c) Molecular Dynamics Studies of Structure ΙΙ Hydrate Nucleation Using Advanced Sampling Techniques

Clathrate hydrates are solid, non-stoichiometric combinations of guest molecules entrapped in a solid water lattice. The guest molecules are generally light gases, such as methane or carbon dioxide. Clathrate hydrates are potentially an important energy resource; some estimates suggest there is more energy trapped in hydrate form than in all other fossil fuel sources on earth. Hydrates also represent a major flow assurance hazard in the petroleum industry. Currently active research on applications of clathrate hydrates ranges from energy and flow assurance to gas separations and transportation. Many such applications of clathrate hydrates require a firm understanding of the mechanism of hydrate formation.

The formation of a new hydrate phase occurs from a supersaturated liquid solution, through a process called nucleation. In nucleation, the formation of the new phase must overcome an activation barrier. The nucleation process is inherently a molecular-level phenomenon, occurring on length- and time-scales that are difficult to probe with experimental techniques. The length and time-scales of hydrate nucleation appear well-suited to be studied through molecular simulation. However, in most cases the associated activation barrier is large. Therefore, the nucleation is a rare event â?? the waiting time between nucleation events is orders-of-magnitude longer than the nucleation events themselves. Correspondingly, in straightforward simulations most of the computational power is expended waiting for the nucleation event to happen, rather than simulating the nucleation events of interest. In order to study hydrate nucleation with molecular dynamics simulations, we use an advanced sampling technique known as forward flux sampling (FFS).

We study the nucleation of sII clathrate hydrates from a supersaturated liquid solution with FFS. The water and guest molecules are described by a coarse-grained model which uses monatomic water and was developed by Jacobson et al.¹ In this presentation, we report results from large-scale FFS calculations comprising over 190,000 individual molecular dynamics trajectories. A committor probability analysis is used in conjunction with FFS to elucidate the nucleation mechanism. We estimate the reaction coordinate with a least squares estimation technique developed for FFS by Borrero and Escobedo.² Elements of the labile cluster hypothesis, local structuring hypothesis, and blob-mechanism are observed at different stages of the nucleation. We discuss the ability of several proposed order parameters to capture nucleation of sII hydrates. Our results highlight the various factors at play in clathrate hydrate nucleation.

(1) Jacobson, L. C.; Hujo, W.; Molinero, V. J. Phys. Chem. B 2010, 114 (43), 13796â??13807.

(2) Borrero, E. E.; Escobedo, F. A. J. Chem. Phys. 2007, 127 (16), 164101.