Stabilization of carbon dioxide-in-water emulsions with silica nanoparticles

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-CO2 interface. The emulsion stability also increased with an increase in the particle concentration, CO2 density, and shear rate. The dominant destabilization mechanism was creaming, whereas flocculation, coalescence, and Ostwald ripening played only a minor role over the CO2 densities investigated. The ability to stabilize these emulsions with solid particles at CO2 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 CO2.