Formation, growth and dissociation of clathrate hydrate crystals in liquid water in contact with a hydrophobic hydrate-forming liquid

Variations in macroscopic morphology of hydrate crystals formed to intervene between a liquid-water phase and a liquid-hydrochlorofluorocarbon (HCFC) phase have been observed. The HCFC used was R-141b (CH3CCl₂F) which is known to form a structure II clathrate hydrate at temperatures lower than 281.5 K, the triple liquid-water/hydrate/liquid-R-141b equilibrium temperature T_tri, under the atmospheric pressure. Each experiment was commenced by immersing an R-141b drop in a quiescent pool of water, which was either an ordinary pure water having no prior contact with R-141b or a water presaturated with R-141b at a temperature higher than T_tri. With presaturated water held at a large subcooling (∼6.5 K) below T_tri, we have distinguished two stages of hydrate-crystal growth, which are greatly different from each other both in crystal-growth morphology and in time span. The primary stage is characterized by lateral growth of a thin, fine-grained polycrystalline layer along the surface of each R-141b drop for a period of several tens of seconds. The secondary stage begins typically with a delay of some 10 min and lasts for a few tens of hours; it is characterized by radial growth of plate-like crystals standing upright on the outer surface of the drop-enclosing hydrate shell formed in the primary stage, thereby protruding into the liquid-water phase. A subsequent temperature rise within the range below Ttri causes a dissolution of plate-like crystals, leaving the hydrate shell apparently unchanged. The second stage is never observable with pure water and/or at a small subcooling (≲2 K). A qualitative interpretation of these observations is presented.