Interfacial Properties of Branch-Tailed Fluorinated Surfactants Yielding a Water/Supercritical CO₂ Microemulsion

Abstract

We have examined the interfacial properties of several fluorinated surfactants in a water/CO2 mixture with a pendant drop tensiometer and revealed the relationships between the interfacial properties, the surfactant structure, and the microemulsifying power. We employed the following Aerosol-OT analogue surfactants that have two fluorinated tails: bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4), 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). To discuss the effect of the fluorocarbon/hydrocarbon ratio in single surfactant molecules, water/CO2 interfacial tension (IFT) of a hybrid surfactant with one fluorocarbon and one hydrocarbon tail, that of a surfactant with a single fluorinated tail, and that of a hydrocarbon surfactant, Aerosol-OT (AOT), were examined. The hybrid surfactant employed was sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), and the single-tailed surfactant was perfluoropolyether ammonium carboxylate (PFPECOONH4, CF3CF2(CF2OCF(CF3))4COONH4). All of the fluorinated AOT analogue surfactants exhibited an excellent level of activity at the water/CO2 interface compared with other fluorinated surfactants and AOT. With a larger hydrocarbon chain number in the CO2-philic tails (i.e., from 0 to 2), the IFT of the AOT analogue surfactants was increased. The area occupied by one surfactant molecule at the water/CO2 interface, A, and the critical microemulsion concentration, cmicroc, were determined and used to examine the water-to-surfactant molar ratio within a reversed micelle, W0c, of the surfactants. The surfactants that form W/scCO2 microemulsions with a large W0c were found to lower the interfacial tension efficiently irrespective of increases in temperature. To achieve the most desirable W0C, the surfactant needs not only a high CO2-philicity of the tails but also a high Krafft point, properties which induce a low hydrophilic/CO2-philic balance.

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

“…The results of the FT-IR measurements and visual observations revealed the formation of a transparent and single-phase W/scCO 2 mE, that is, a mE of the Winsor-IV type (IV mE) 28) , as observed [20][21][22][23][24][25][26][27][28][29] . However, the presence of the W/scCO 2 mE in the 2Ø region was not confirmed.…”

“…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.…”

“…The W 0 c parameter is generally used to express a water-to-surfactant molar ratio in a reversed micelle in scCO 2 .The solubility of water in scCO 2 is taken into account as [20][21][22][23][24][25][26][27] : , , were loaded in a variable-volume high-pressure optical cell. Then, water or an aqueous MO solution (0.1 or 0.3 wt%) was added into the optical cell through a six-port valve until the clear IVmE solution became a turbid macroemulsion (E) solution or a precipitated hydrated surfactant.…”