Crystal growth of ionic semiclathrate hydrate formed in CO₂ gas + tetrabutylammonium bromide aqueous solution system

This article reports the visual observations of the formation and growth of ionic semiclathrate hydrate crystals in the system of aqueous solution of tetrabutylammnonium bromide (TBAB) and CO₂ gas. The experimental thermodynamic conditions are (i) the temperature from 286 to 290 K for w _TBAB = 0.40 and (ii) 282 to 288 K at w_TBAB = 0.10, under the common pressure of 2.3 or 3.4 MPa, where w_TBAB denotes the mass fraction of TBAB in the aqueous solution. The lower limit of the temperature was set to avoid the formation of simple TBAB hydrate containing no CO₂. The hydrate crystals first formed in the bulk of the aqueous liquid as well as at the gas/liquid interface. The former crystals settled downward and the latter grew to form a hydrate film covering the interface. Both of the hydrate crystals then grew into the aqueous liquid. The morphology of hydrate crystals grown in the aqueous liquid varied distinctly depending on the system subcooling ΔT_sub. ΔT_sub ≡ T_eq - T _ex, where T_eq is the equilibrium temperature and T _ex is the system temperature. When ΔT_sub is smaller than ∼2.0 K, hydrate crystals with wedge morphology were observed. At the range of ∼3.1 K > ΔT_sub > ∼2.0 K, the wedge crystals were replaced by columnar shaped crystals. At ΔT_sub > ∼3.5 K, the hydrate crystals with needle and sword-like morphology were observed. The size of the individual hydrate crystals decreased with increasing ΔT_sub.