(319f) Characterization of Xenon Hydrates Formed At High Driving Forces and Advancements in Hydrate Equilibrium Predictions

Clathrate hydrates are inclusion compounds composed of water and “guest” molecules.  The water molecules form a crystalline packing of polyhedral cages with their hydrogen bond network.  The guest molecules become trapped, or enclathrated, in the cage-like polyhedra giving hydrates their inclusion properties.  Guest molecules are typically small and hydrophobic, including methane, ethane, and carbon dioxide.  In the gas and oil industry, hydrates are often regarded as a nuisance as their formation can plug subsea pipelines; however, hydrates have recently gained interest as viable options for energy storage, water desalination, and carbon dioxide sequestration.  While much is known about the thermophysical properties and crystal structure of hydrates at moderate conditions, less is known on the behavior of hydrates formed at high-driving forces conditions. In this study, we present our findings of xenon hydrates formed in situ at high driving forces in a differential scanning calorimeter (DSC), which reveals an ill-defined equilibrium temperature often displaying multiple dissociations, or endotherms characteristic of the melting of multiple solid structures.  This apparent violation of the Gibbs phase-rule has been seldom observed with other guests, which raises the question of how well the xenon-hydrate crystal structure is actually defined.  In this study, we probe the unusual dissociation behavior of xenon hydrates using DSC, powder x-ray diffraction, and nuclear magnetic resonance and revisit the van der Waals-Platteeuw model to incorporate guest-guest interactions for more realistic phase predictions.