Surfactant effects on the crystal growth of clathrate hydrate at the interface of water and hydrophobic-guest liquid

Visual observations of the effect of surfactants on clathrate hydrate crystal growth at the interface of water and cyclopentane has been studied. Surfactants used in the present study are sorbitan monooleate (Span 80), naphthenic acid, and polypropylene glycol. The surfactants were each used at a mass fraction of 0.001% and 0.01%. All the surfactants were soluble in cyclopentane. The crystalline morphology and crystal growth behavior of the hydrate at the interface were found to be substantially affected by the addition of all surfactants. The size of the individual hydrate crystals in the surfactant system was larger than those in the pure cyclopentane system. The observations showed a distinct variation in the behavior of cyclopentane hydrate crystal growth depending on the chemical species of the surfactants, their concentration, and ΔT_sub, which is defined as the difference between the equilibrium temperature and the experimental temperature. For the system with surfactants, the growing hydrate crystals did not engulf the interface, because the crystal grains that have grown detached from the interface and fallen into the water. From the observations, it is clear that hydrate crystal production increased in comparison to systems without surfactants due to the interface area being preserved. These observations explain the physical mechanism of two surfactant effects, where one is the prevention of hydrate agglomeration (interface area preservation) and the other is the promotion of hydrate production.