Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

Abstract

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 μm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO₂-philic. The stability of emulsions comprising equal weights of CO₂ and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO₂ density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO₂-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10⁶kT, even at CO₂ densities as low as 0.78 g ml⁻¹. At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.