Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance: Variations in crystal morphology

This paper reports on our interpretation of our visual observations of the variations in macroscopic morphology of hydrate crystals growing in liquid water saturated with a guest substance prior to the hydrate formation. The observations were made in a high-pressure cell charged with liquid water and gaseous CO₂. They revealed distinct variations in the morphology of hydrate crystals depending on the system subcooling δT_sub, the temperature deficiency inside the cell from the triple CO ₂-hydrate-water equilibrium temperature under a given pressure. When δT_sub ≲ 3K, a hydrate film first grew along the CO ₂-water interface; then hydrate crystals with dendritic morphology grew in large numbers into the liquid-water phase from that hydrate film. When δT_sub ≲ 2K, the dendritic crystals were replaced by skeletal or polyhedral crystals. We present a non-dimensional index for such variations in hydrate crystal morphology. This is based on the idea that this morphology depends on the growth rate of hydrate crystals, and their growth rate is controlled by the mass transfer of the hydrate-guest substance (CO ₂ in the present experiments), dissolved in the bulk of liquid water, to the hydrate crystal surfaces. The morphology variations observed in the present and previous studies are related to this index.