Water-in-Carbon Dioxide Emulsions with Poly(dimethylsiloxane)-Based Block Copolymer Ionomers

The stability of water-in-CO 2 (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 CO 2 phases equally. When the pH starting at 3 is raised up to 5−6, the hydrophilic/CO 2 -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 CO 2 as a cosolvent reduced flocculation in some cases to zero.