(286a) Nucleation Mechanism of Clathrate Hydrates of Soluble Guest Molecules

Clathrate hydrates are a solid phase composed of guest molecules entrapped in a lattice of polyhedral water cages. The guest molecules are generally light gases, such as methane or carbon dioxide. Currently active research on applications of clathrate hydrates ranges from energy and flow assurance to gas storage. Many such applications of clathrate hydrates require a firm understanding of the mechanism of hydrate formation.

The birth of the hydrate phase occurs from a supersaturated liquid solution of water and guest molecules through an activated process called nucleation. Hydrate nucleation is inherently a molecular-level phenomenon, occurring on length- and time-scales that are difficult to probe with current experimental techniques. Though the length and time-scales of hydrate nucleation appear well-suited to study through molecular simulation, in most cases the associated activation barrier is large, and nucleation is a rare event in simulations. 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. To overcome these challenges, we use a technique called forward flux sampling (FFS) to generate over one thousand nucleation trajectories.

We focus on the nucleation of clathrate hydrates of soluble guest molecules. The water and guest molecules are described by the coarse-grained monatomic water and associated guest models. Accurate committor probabilities were calculated for over 150 configurations generated with FFS. One and two parameter reaction coordinate models were then created from several common order parameters and variants thereof. In this talk, we present results indicating that order parameters that rely on water structure are generally more effective at describing the liquid-to-solid transition than order parameters relying on guest structure. An analysis of over 200 configurations near the transition state reveals that the transition state consists of a small number of empty, face-sharing cages surrounded by guest molecules. These surrounding guest molecules begin to form a network of connected precursors to occupied cages. Motivated by these findings, we discuss results from further simulations which reveal that two or more face-sharing cages significantly affect the structure of the surrounding guest molecules. We conclude by investigating how our proposed nucleation mechanism applies to mixed hydrate systems with two types of guest molecules.