Preparation of a W/scCO₂ Microemulsion Using Fluorinated Surfactants

Abstract

Formation of water (W) in supercritical carbon dioxide (scCO2) (W/scCO2) type microemulsions was examined using four hybrid surfactants, the sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-alkanesulfonates (FC6-HCn, n = 2, 4, 6, and 8), which have a hydrocarbon chain of different length and a fluorocarbon chain in one molecule and an Aerosol-OT (AOT) analogue fluorinated twin tail type surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2). For comparison AOT was also used. The hybrid type surfactants (FC6-HCn) gave a transparent single phase, identified as a W/scCO2 microemulsion, with a water-to-surfactant molar ratio, W0c < 7, irrespective of hydrocarbon chain length. The fluorinated AOT analogue also yielded a transparent single phase, again identified as a W/scCO2 microemulsion, with a W0c value close to 32one of the highest ever reported. The aqueous core in the 8FS(EO)2 reversed micelle was examined by FT-IR spectra using D2O. The spectra revealed that the aqueous core swells on addition of water and shrinks with increase in pressure. The remarkable ability of 8FS(EO)2 to form a W/scCO2 microemulsion would be brought about by its high adsorption capacity and its excellent facility to lower the water/scCO2 interfacial tension, in addition to a low interaction and strong steric repulsion between its CO2-philic groups.