Surfactant Effects on Hydrate Crystallization at the Water-Oil Interface: Hollow-Conical Crystals

Abstract

Clathrate hydrates are icelike crystalline compounds with small guest molecules trapped inside the cages of hydrogen bonded water molecules. Clathrate hydrate crystal growth is studied for the specific case of the guest molecule cyclopentane. Cyclopentane hydrate formation is visualized at a millimeter-scale water drop. Crystal formation takes place at the water-organic interface and has been shown in prior work to occur in a three-step sequence of nucleation, lateral surface growth, and radial growth. This study describes cyclopentane hydrate crystal characteristics during the lateral surface growth and demonstrates the effect of the oil-soluble surfactant sorbitan monooleate (Span 80) on the hydrate crystal growth. A faceted polycrystalline hydrate shell is formed around the water drop in the absence of surfactant. A unique hollow-conical crystal is observed at Span 80 concentrations greater than 0.01% by volume in cyclopentane; the critical micelle concentration is 0.03% Span 80. The conical crystals have a polygonal base, usually hexagonal, which is pinned at the water-cyclopentane interface; growth occurs at this base and drives the previously formed crystal into the water phase. Morphology is dependent upon the growth taking place at the interface where both components of hydrate (i.e., water and cyclopentane) are present at large concentration. The hollow-conical crystals grow to over 100 mu m in linear dimensions in all directions. The morphology described is seen at a range of Span 80 and cyclopentane concentrations. A hypothesis is proposed to relate crowding of surfactant molecules at the interface to the observed hollow-conical crystal shape.