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

Abstract

The effects of the structural factors of surfactants such as CO2-philic tail length and molecular structure on the formation of water-in-supercritical CO2 (W/scCO2) type microemulsions were examined at various temperatures and pressures for fluorinated Aerosol-OT (AOT) analogue surfactants, sodium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4), sodium bis(1H,1H,7H-dodecafluoroheptyl)-2-sulfosuccinate (di-HCF6), sodium bis(1H,1H,9H-hexadecafluorononyl)-2-sulfosuccinate (di-HCF8), sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), and sodium bis((1H,1H,2H,2H-heptadecafluorodecyl)oxyethylene)-2-sulfosuccinate (8FS(EO)4). With the all surfactants, a transparent single phase of water, a surfactant, and scCO2 mixture, or a W/scCO2 microemulsion phase, was formed under certain conditions. With an increase in the water composition, the microemulsion phase became a turbid W/scCO2 macroemulsion phase or completely separated into two phases. On the other hand, when pressure (CO2 density) or temperature was increased, the macroemulsion phase turned into a microemulsion phase. At CO2 densities <0.85 g/cm3 and temperatures <65 °C, di-HCF4 was unable to form a microemulsion. With an increase in fluorocarbon chain length (n) of di-HCFn, a microemulsion with a higher water-to-surfactant molar ratio (W0c, 8 for di-HCF6 and 12 for di-HCF8 at CO2 density 0.85 g/cm3 and 35 °C) was formed. This suggests that the water/scCO2 interfacial activity increases with an increase in fluorocarbon chain length, resulting in the formation of stable W/scCO2 microemulsion. The superior ability of 8FS(EO)2 to stabilize W/scCO2 microemulsion with W0c = 32 was found to be lowered with a replacement of the terminal CF3− by HCF2− of the fluorocarbon chains and an addition of oxyethylene units between the CO2-philic fluorocarbon chains and the hydrophilic sulfosuccinate group.