Stable water-in-carbon dioxide (W/C) 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 μm 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.

The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE-C(O)-NH-CH 2 -N + (CH 3 ) 3 CH 3 COO -, is reported over a range of temperatures (25-90°C) and pressures (87.3-415 bar). Spherical droplets are observed by SANS with radii ranging from 16 to 36 Å for water-to-surfactant molar ratios (W o ) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60 Å 2 /surfactant molecule at the water-CO 2 interface, in reasonable agreement with the value of 72 Å 2 determined from the change in the droplet radius with W o . The CO 2 -phobic functionality between the surfactant headgroup and perfluoropolyether tail reduces CO 2 penetration of the tails, resulting in a smaller area/surfactant than in the case of an anionic perfluoropolyether surfactant [Langmuir 1997, 13, 3934]. A relatively rigid film, with a mean film rigidity (2K + K h ) of approximately 1 k B T, along with the strong partitioning of the surfactant toward CO 2 versus water, lead to the small, rigid, spherical water droplets in CO 2 .

The stability of water-in-CO2 (W/C) emulsions stabilized with poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) and PDMS-b-poly(acrylic acid) (PDMS-b-PAA) ionomer surfactants is reported as a function of surfactant architecture, pH, temperature, pressure, and droplet flocculation. For a given PDMS block length, the stability of the emulsion is correlated with the distance from the balanced state where the surfactant prefers the water and CO2 phases equally. When the pH starting at 3 is raised up to 5−6, the hydrophilic/CO2-philic balance of the surfactant increases, because of ionization of COOH, and the emulsion becomes more stable. At the pH of maximum stability, the emulsion becomes more stable with a decrease in the PDMS length, for a given ratio of block lengths, because of gelation of the flocculated 2−5 μm primary droplets. W/C emulsions are stable with respect to sedimentation for >24 h and are resistant to coalescence for more than 7 days. Because of gelation, the W/C emulsions are more stable than water/hexane emulsions (at ambient pressure) formed at the same conditions. The addition of 20% hexane to CO2 as a cosolvent reduced flocculation in some cases to zero.

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