Super-Efficient Surfactant for Stabilizing Water-in-Carbon Dioxide Microemulsions

The fluorinated double-tailed glutarate anionic surfactant, sodium 1,5-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)2), was found to stabilize water-in-supercritical CO2 microemulsions with high water-to-surfactant molar ratios (W0). Studies were carried out here to obtain detailed information on the phase stability and nanostructure of the microemulsions by using a high-pressure UV−vis dye probe and small-angle neutron scattering (SANS) measurements. The UV−vis spectra, with methyl orange as a reporter dye, indicated a maximum attainable W0 of 60 at 45 and 75 °C, and SANS profiles indicated regular droplet swelling with a linear relationship between the water core nanodroplet radius and W0. This represents the highest water solubilization reported to date for any water-in-CO2 microemulsion. Further analysis of the SANS data indicated critical packing parameters for 8FG(EO)2 at the microemulsion interface >1.34, representing approximately 1.1 times the value for common aerosol...