Water-in-carbon dioxide emulsions: Formation and stability

Stable water-in-carbon dioxide (WIC) emulsions, for either liquid or supercritical CO2 containing up to 70 vol % water, are formed with various molecular weight perfluoropolyether ammonium carboxylate surfactants. Water droplet sizes ranging from 3 to 10 mu m were determined by optical microscopy. From conductivity measurements, an inversion to C/W emulsions results from a decrease in CO2 density or salinity at constant pressure, a decrease in surfactant molecular weight, or an increase in temperature. Emulsions become more stable with a change in any of these formulation variables away from the balanced state, which increases interfacial tensions and interfacial tension gradients, enhancing Marangoni-Gibbs stabilization. This type of stability is enhanced with an increase in the molecular weight of the surfactant tails, which increases the thickness of the stabilizing films between droplets. W/C emulsions formed with the 7500 molecular weight surfactant were stable for several days.