Crystal Growth of Ionic Semiclathrate Hydrate Formed at the Interface between CO₂ 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 CO₂ 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_TBAB = 0.10 and w_TBAB = 0.40, where w_TBAB 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_TBAB = 0.40. The individual crystals that constitute the polycrystalline hydrate film were observed, and the morphology was classified according to the system subcooling ΔT_sub (ΔT_sub ≡ T_eq - T_ex, where T_eq is the equilibrium temperature and T_ex is the experimental temperature). In all ΔT_sub at w_TBAB = 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_sub was not observed. (Figure Presented).