Molecular mechanisms of nucleation and growth of clathrate hydrates

Clathrate hydrates are a crystalline phase of water in which the hydrogen bond network forms polyhedral cages that can trap nonhydrogen bonding molecules such as methane and carbon dioxide. Methane clathrate hydrates occur naturally on the ocean floor and are so abundant that it is estimated they contain more energy than the rest of the world's hydrocarbon reserves combined. As a result, methods to promote and inhibit the formation of clathrates are highly sought after. However, a detailed understanding of the microscopic mechanism of clathrate nucleation remains elusive. Experimental techniques are unable to resolve the structure of clathrate nuclei. Simulations provide an alternative, but nucleation is a stochastic event and the computational cost to observe nucleation events can be prohibitive. In this work we studied the mechanism of clathrate hydrate nucleation with molecular simulations using an efficient coarse-grained model mW that represents water as a single site that is encouraged to form "hydrogen-bonded" configurations without the use of hydrogen atoms. The coarse-grained model allows the observation of many more nucleation events than is possible with atomistic models. Using the coarse-grained water model and guest potentials, we studied the nucleation of clathrate hydrates at supercooled conditions. The nucleation mechanism we observed is a multistep mechanism with an amorphous intermediate that we call "the blob." The first step in the nucleation is the densification of guest molecules in solution to form long-lived solvent-separated configurations. These persist in solution and give rise to the formation and dissolution of individual cages. Once "the blob" is large enough, it eventually forms persistent polyhedral cages. The critical nucleus is defined by the solvent-separated guest molecules and polyhedral water cages. The clathrate at this point is amorphous and lacks the symmetry of the crystalline clathrate, but is made of the same building blocks. We find that an amorphous clathrate seed is able to nucleate the formation of crystalline clathrates. We develop order parameters for the densification of guest molecules in solution, the identification of polyhedral cages, and the crystallinity of the clathrate phase. These order parameters will be useful for describing the reaction coordinate of clathrate nucleation at low driving force.