279181 Clathrate Hydrate Formation in Emulsions Subjected to Shear Flow
Clathrate hydrate formation is a phase transition from fluid to solid associated with forming cages of host (water) molecules around a guest molecule, typically of hydrocarbon form, with examples including methane, propane, and cyclopentane. The last of these, cyclopentane, forms hydrates at atmospheric pressure and temperatures above the freezing point of water; these features make it rather convenient for study of the influence of the hydrate-forming reaction on the properties of an emulsion containing hydrate formers in the organic phase; we study cyclopentane hydrate formation in a water-in-oil emulsion stabilized by surfactants, focusing on sodium mooleate (an oil soluble surfactant). Our interest is in the rheology of the mixtures, and how bulk flow effects this reaction process.
Hydrate formation is relatively unusual in the reaction generating a solid phase at a fluid-fluid boundary, and this spatial localization of the reaction will be a focus in our considerations here. In particular, it wil be shown that the formation at an interface results in a pronounced influence of surface-active agents upon the reaction rate and the resulting morphology of the reactant crystals. We will describe novel hollow conical crystals and their relationship to the surfactant loading; additionally, the influence on morphology of the cystalline stucture by the surfactant will be analyzed, and correlated to observations of the evolution of rheological properties as the reaction to form hydrate progresses. An interesting feature in this system is the role of flow-induced stresses. We observe that the reaction can be shutdown by mass transfer limitations, as the hydrate shell formed at the interface isolates the water from the organic phase. As a consequence, the role of flow is not simple mixing, but also involves fracturing of shells of hydrate around water drops allowing surface wetting of existing hydrate crystals. Reaction rate jumps suddenly when this phenomenon occurs, as evidenced by the rapid release of energy associated with the exothermic hydrate formation.