Oxygenated hydrocarbon ionic surfactants exhibit CO₂ solubility

Several oxygenated hydrocarbons, including acetylated sugars, poly(propylene glycol), and oligo-(vinyl acetate), have been used to generate CO₂-soluble ionic surfactants. Surfactants with vinyl acetate tails yielded the most promising results, exhibiting levels of CO₂ solubility comparable to those associated with fluorinated ionic surfactants. For example, a sodium sulfate with single, oligomeric vinyl acetate (VAc) tails consisting of 10 VAc repeat units was 7 wt % soluble in CO₂ at 25 °C and 48 MPa. Upon introduction of water to these systems, only surfactants with the oligomeric vinyl acetate tails exhibited spectroscopic evidence of a polar environment that was capable of solubilizing the methyl orange into the CO₂-rich phase. For example, a single-phase solution of CO ₂, 0.15 wt % sodium bis(vinyl acetate)8 sulfosuccinate, and water, at water loading (W) values ranging from 10 to 40 at 25 °C and 34.5 MPa, exhibited a methyl orange peak at 423 nm. This result indicated that the core of a reverse micelle provided a microenvironment with a polarity similar to that of methanol. Quantum chemical calculations indicate that the acetylated sugars may be too hydrophilic to readily form reverse micelles, whereas the VAc-based surfactants appear to have the correct balance of hydrophilic and hydrophobic forces necessary to form reverse micelles.