Crystal Growth of Ionic Semiclathrate Hydrate Formed at the Interface between CO Gas and Tetra-n-butylammonium Bromide Aqueous Solution

Formation and growth of ionic semiclathrate hydrate crystals on the surface of a liquid droplet of tetra-n-butylammonium bromide (TBAB) aqueous solution exposed to CO gas have been visually observed. Experiments were conducted at a temperature range between 280 and 290 K under the pressure of 2.3 MPa at w = 0.10 and w = 0.40, where w is defined as the mass fraction of TBAB in the aqueous solution. It was found that the hydrate crystals initially grew in the liquid phase, instead of growing at the gas/liquid interface. Then the hydrate grew to form a polycrystalline film covering the droplet only at w = 0.40. The individual crystals that constitute the polycrystalline hydrate film were observed, and the morphology was classified according to the system subcooling ΔT (ΔT ≡ T - T, where T is the equilibrium temperature and T is the experimental temperature). In all ΔT at w = 0.40, hydrate crystals with step-shaped and thin polygonal-shaped morphologies were observed. The difference in the size of the individual hydrate crystals due to the difference in ΔT was not observed. (Figure Presented).