Gas hydrates are crystalline solids comprising hydrogen bonded water molecules forming cages that encapsulate guest molecules such as methane, ethane and carbon dioxide. The reserves of methane hydrates are considered potential energy sources. In addition, clathrate hydrates have found several potential technological applications such as natural gas storage and transportation, CO<sub>2</sub> sequestration and separation of gas mixtures. In several of these applications it is necessary to control the gas hydrate nucleation, and growth mechanisms. This has been achieved through addition of other components – for example, nucleation of methane hydrate can be accelerated by addition of tetrahydrofuran. However, a fundamental understanding of the underlying phenomena remains elusive. We have used advanced sampling techniques to elucidate the mechanism of nucleation for soluble guest molecules. We find that water structuring precedes guest structuring in the nucleation of such gas hydrates. This differs from the mechanism of insoluble guest molecules where guest structuring is found to precede water structuring.<br/>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 hydrate 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. We find that the -CH3SAM surface inhibits hydrate nucleation relative to the -OHSAM surface. Interestingly, we never observe hydrate nucleation on the SAM surfaces, and find no structural changes in the interfacial water prior to nucleation. Our results emphasize the complexity of heterogeneous gas hydrate nucleation by demonstrating the interplay between the various factors that influence this phenomenon.

Homogeneous and heterogeneous nucleation of gas hydrates: Interplay of water structure, dynamics, and guest solubility

2019 Fall National Meeting

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