Surfactant aggregation in CO2/heptane solvent mixtures.

The effect of improving "solvent quality" of pure liquid CO(2) with a heptane cosolvent on the phase behavior and micellization of commercially available surfactants has been explored using high-pressure small-angle neutron scattering (HP-SANS). The nonionic C(12)E(5) was found to be highly soluble in both pure CO(2) and the solvent blends, but no aggregation was detected by HP-SANS for any of the compositions studied, even up to 12 vol % surfactant. On the other hand, improving CO(2) solvent quality by adding heptane above 30 vol % promoted solubility and aggregate formation with normal sodium bis(ethylhexyl)sulfosuccinate (AOT). The solvent quality index Hildebrand solubility parameter, used to predict surfactant aggregation in pure hydrocarbon solvents (Langmuir, 2008, 24 (21), 12235-12240) has been tested here for CO(2)-heptane mixtures. The results show how solubility and efficiency of AOT, a commercially viable, well-known, and commonly used surfactant, can be boosted in alkane-containing CO(2)-rich fluids compared to pure CO(2) alone.