Effects of CO₂-philic Tail Structure on Phase Behavior of Fluorinated Aerosol-OT Analogue Surfactant/Water/Supercritical CO₂ Systems

The effects of the structural factors of surfactants such as CO 2 -philic tail length and molecular structure on the formation of water-in-supercritical CO 2 (W/scCO 2 ) type microemulsions were examined at various temperatures and pressures for fluorinated Aerosol-OT (AOT) analogue surfactants, sodium bis(1 H,1 H,5 H -octafluoropentyl)-2-sulfosuccinate (di-HCF4), sodium bis(1 H,1 H,7 H -dodecafluoroheptyl)-2-sulfosuccinate (di-HCF6), sodium bis(1 H,1 H,9 H -hexadecafluorononyl)-2-sulfosuccinate (di-HCF8), sodium bis(1 H,1 H,2 H,2 H -heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO) 2 ), and sodium bis((1 H,1 H,2 H,2 H -heptadecafluorodecyl)oxyethylene)-2-sulfosuccinate (8FS(EO) 4 ). With the all surfactants, a transparent single phase of water, a surfactant, and scCO 2 mixture, or a W/scCO 2 microemulsion phase, was formed under certain conditions. With an increase in the water composition, the microemulsion phase became a turbid W/scCO 2 macroemulsion phase or completely separated into two phases. On the other hand, when pressure (CO 2 density) or temperature was increased, the macroemulsion phase turned into a microemulsion phase. At CO 2 densities <0.85 g/cm 3 and temperatures <65 °C, di-HCF4 was unable to form a microemulsion. With an increase in fluorocarbon chain length ( n ) of di-HCF n, a microemulsion with a higher water-to-surfactant molar ratio ( W 0 c, 8 for di-HCF6 and 12 for di-HCF8 at CO 2 density 0.85 g/cm 3 and 35 °C) was formed. This suggests that the water/scCO 2 interfacial activity increases with an increase in fluorocarbon chain length, resulting in the formation of stable W/scCO 2 microemulsion. The superior ability of 8FS(EO) 2 to stabilize W/scCO 2 microemulsion with W 0 c = 32 was found to be lowered with a replacement of the terminal CF 3 − by HCF 2 − of the fluorocarbon chains and an addition of oxyethylene units between the CO 2 -philic fluorocarbon chains and the hydrophilic sulfosuccinate group.