Water/Supercritical CO₂ Microemulsions with Mixed Surfactant Systems

Abstract

Phase behavior was investigated for water/supercritical CO 2 (W/scCO2) microemulsions stabilized with sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO) 2) mixed with various guest surfactants. Only for the mixtures with fluorocarbon-hydrocarbon hybrid anionic surfactants (FC6-HC n), the maximum water-to-surfactant molar ratio (W0(c)) was larger than that estimated from linear interpolation of the W0(c) values for pure 8FS(EO) 2 and pure guest surfactant. Fourier transform infrared (FT-IR) measurement for the microemulsion revealed that the mixing of 8FS(EO) 2 with FC6-HC n can prevent a phase transition from the microemulsion to the liquid crystal even in the presence of excess water. It was also found from the measurement of water/scCO 2 interfacial tension that the area occupied per surfactant molecule was markedly increased by the mixing with FC6-HC n. The loose molecular packing, probably due to a microsegregation of 8FS(EO) 2 and FC6-HC n, is consistent with the enhanced stability of the microemulsion upon surfactant mixing.

Nanostructures in Water-in-CO2 Microemulsions Stabilized by Double-Chain Fluorocarbon Solubilizers

“…[3][4][5][6][7][8][9] However, most commercial and known hydrocarbon surfactants are insoluble and inactive in scCO2 systems 3 . In this regard, it became apparent that conventional surfactant-design theory cannot be applied to W/CO2 systems directly, and that CO2-philicity is not directly comparable to 4 oleo-philicity.…”

Shape Transitions in Supercritical CO2 Microemulsions Induced by Hydrotropes

“…Unfortunately, these surfactants are highly fluorinated and therefore expensive and environmentally unfriendly 25,26 . To overcome this problem, low fluorine/hydrocarbon replacements have recently been developed [27][28][29] .…”

Characterization of Water/Supercritical CO2 Microemulsion by UV-visible Spectroscopy and Dynamic Light Scattering

“…However, only a few have succeeded in developing a W/scCO 2 mE with W 0 c > 0 (W 0 c :water-to-surfactant molar ratio corrected by subtracting the moles of water soluble in CO 2 ). In a previous work [20][21][22][23][24][25][26][27] , we synthesized fluorinated Aerosol-OT (AOT) analogue surfactants and examined their ability to stabilize W/scCO 2 mEs. We found that a fluorinated AOT-analogue surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2 -sulfosuccinate (8FS(EO) 2 ), successfully stabilized a transparent single-phase Winsor-IV W/scCO 2 mE (IV mE) with a W 0 c value of 32.…”

“…The W 0 c value of 32 is extremely large compared to those previously reported for other W/scCO 2 mE systems . On the other hand, the IV mE became a turbid W/scCO 2 macroemulsion (E) with decreasing pressure, and then it turned into a Winsor-I-or Winsor-II-mE-like two-phase (2Ø) when magnetic stirring was stopped [20][21][22][23][24][25][26][27][28] . In this case, the Winsor-I and -II mean scCO 2 /W mE with a separated excess scCO 2 and W/scCO 2 mE with a separated excess water, respectively 28) .…”

“…A high-pressure vessel with an optical window and a moving piston inside the vessel was used to observe the changes in the phase with varying pressure and temperature but constant composition. A detailed description of the experimental apparatus and procedures of the measurements can be found in the literatures [20][21][22][23][24][25][26][27] . In order to examine the solution properties of the aqueous core in an 8FS(EO) 2 /W/scCO 2 mE, UV-visible absorption spectrum measurements were carried out using MO, a double-beam spectrometer (Hitachi High-Technologies, Co., U-2810), and a quartz window cell (volume: 1.5 cm 3 ), which was connected to the experimental apparatus.…”