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W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. 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 CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) 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 CO(2) 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. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) 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.

Nondilute nanoparticle dispersions were stabilized in liquid CO2 at 25 degrees C at pressures as low as the vapor pressure for greater than 30 min. By modifying hydrophilic silica with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxy silane, a cross-linked polymer shell was formed around the silica core. The presence of the shell led to weaker Hamaker interactions between approaching fluoro-silica composite particles and enabled dispersibility at weaker solvent conditions (low pressures) than for metals with larger Hamaker constants. Steric stabilization of the nanoparticles was provided by low-molecular-weight perfluorodecane side chains at the surface of the fluoro-silica composite shell. Compared to polymeric chains, the perfluorodecane side chains are more easily solvated and thus stabilize nanoparticle dispersions in CO2 at much lower pressures, even down to the vapor pressure., (Copyright 2004 American Chemical Society)

Stable carbon dioxide-in-water emulsions were formed with silica nanoparticles adsorbed at the interface. The emulsion stability and droplet size were characterized with optical microscopy, turbidimetry, and measurements of creaming rates. The increase in the emulsion stability as the silica particle hydrophilicity was decreased from 100% SiOH to 76% SiOH is described in terms of the contact angles and the resulting energies of attachment for the silica particles at the water-CO(2) interface. The emulsion stability also increased with an increase in the particle concentration, CO(2) density, and shear rate. The dominant destabilization mechanism was creaming, whereas flocculation, coalescence, and Ostwald ripening played only a minor role over the CO(2) densities investigated. The ability to stabilize these emulsions with solid particles at CO(2) densities as low as 0.739 g/mL is particularly relevant in practical applications, given the difficulty in stabilizing these emulsions with surfactants, because of the unusually weak solvation of the surfactant tails by CO(2).