Water-in-carbon dioxide emulsions: Formation and stability

Stable water-in-carbon dioxide (WIC) emulsions, for either liquid or superc ritical CO2 containing up to 70 vol % water, are formed with various molecu lar weight perfluoropolyether ammonium carboxylate surfactants. Water dropl et sizes ranging from 3 to 10 mu m were determined by optical microscopy. F rom conductivity measurements, an inversion to C/W emulsions results from a decrease in CO2 density or salinity at constant pressure, a decrease in su rfactant molecular weight, or an increase in temperature. Emulsions become more stable with a change in any of these formulation variables away from t he balanced state, which increases interfacial tensions and interfacial ten sion gradients, enhancing Marangoni-Gibbs stabilization. This type of stabi lity is enhanced with an increase in the molecular weight of the surfactant tails, which increases the thickness of the stabilizing films between drop lets. W/C emulsions formed with the 7500 molecular weight surfactant were s table for several days.