(754d) Molecular Dynamics Simulations of Clathrate Hydrate Nucleation Near Model Hydrophobic and Hydrophilic Surfaces

Clathrate hydrates are non-stoichiometric crystalline compounds composed of small guest molecules trapped within a solid water lattice. The guest molecules are frequently light gases, such as methane, carbon dioxide, propane, etc; as such, clathrate hydrates are commonly referred to as gas hydrates. These structures generally form at temperatures near the freezing point of water and at elevated pressures. Gas hydrates represent a large potential energy source; by some estimates there is more energy trapped in hydrate form than in all other fossil fuels combined. Additionally, gas hydrates are a serious flow assurance hazard in the petroleum industry. Other possible applications of gas hydrates include gas separations, water desalination, and gas transportation.

In many instances gas hydrates form in the presence of an interface. Though mechanism of gas hydrate nucleation has been studied using molecular simulations for well over the past decade, few studies have investigated hydrate nucleation near an interface. We perform extensive molecular dynamics simulations to study the nucleation of sII hydrates in the presence of model hydrophobic and hydrophilic interfaces. We use -OH and -CH₃ terminated self assembled monolayers (SAMs) as model surfaces to study the effect of chemistry alone since the SAM chains are flexible and therefore the surface has no fixed crystal lattice.

We find that the -CH₃SAM surface inhibits hydrate nucleation relative to the -OHSAM surface. A variety of factors including guest density, fluctuations in the size of sub-critical hydrate nuclei, and diffusion of the water and guest species influence the relative promotion or inhibition of hydrate formation. Interestingly, we never observe hydrate nucleation on the SAM surfaces. Furthermore, we find no structural changes in the interfacial water prior to nucleation. We attempt to decouple the factors described above by studying nucleation at different guest concentrations in both the -OHSAM and bulk (no SAM) systems. Our results emphasize the complexity of heterogeneous gas hydrate nucleation by demonstrating the interplay between the various factors that influence this phenomenon.