In Situ Studies of the Mass Transfer Mechanism across a Methane Hydrate Film Using High-Resolution Confocal Raman Spectroscopy

Hydrate films typically form at gas-water interfaces where the concentrations of guest and host molecules are the highest. Once formed, the films provide a significant mass transfer barrier to further hydrate formation. However, controversy exists about whether it is the transport through the crystal film of the guest or the host (water) molecule that controls further hydrate growth. In this study, methane hydrate films were formed at a gas-water interface, and the gas and water phases were then replaced by isotopic tracers. The concentration profiles of the tracers across the hydrate films were studied over time using confocal Raman spectroscopy in order to determine the relative mobility of guest and host molecules within hydrate film. The films were found to contain gas-filled pores. which provided pathways for gas migration. These pores annealed over time, increasing the mass transfer resistance. The results indicate that the host Water molecules are the most mobile species in the hydrate phase, and that hydrate growth is controlled by the movement of water within the hydrate film.