Optimum Tail Length of Fluorinated Double-Tail Anionic Surfactant for Water/Supercritical CO₂ Microemulsion Formation

Abstract

The effect of surfactant tail structure on the stability of a water/supercritical CO2 microemulsion (W/scCO2 muE) was examined for various fluorinated double-tail anionic surfactants of different fluorocarbon chain lengths, F(CF2)n (n = 4, 6, 8, and 10), and oxyethylene spacer lengths, (CH2CH2O)(m/2) (m = 2 and 4). The phase behavior of the water/surfactant/CO2 systems was studied over a wide range of CO2 densities from 0.70 to 0.85 g/cm(3) (temperatures from 35 to 75 degrees C and pressures up to 500 bar) and corrected water-to-surfactant molar ratios (W0c). All of the surfactants yielded a W/scCO2 muE phase, that is, a transparent homogeneous phase with a water content larger than that permitted by the solubility of water in pure CO2. With increasing W0c, a phase transition occurred from the muE phase to a macroemulsion or a lamella-like liquid crystal phase. The maximum W0c value was obtained at a tail length of 12-14 A, indicating the presence of an optimum surfactant tail length for W/scCO2 muE formation.

Economical and Efficient Hybrid Surfactant with Low Fluorine Content for the Stabilisation of Water-in-CO2 Microemulsions

“…For decades, extensive studies were devoted to define the effect of the surfactant tail architecture [13,15,24,50,51]. However, only few works have alluded to the effects of hydrophilic-headgroup modifications on surfactant performance in CO2 [36,38,39].…”

“…Following the similar lines [36][37][38], the trend of increasing solubilising power is also observed when sulfosuccinate is changed for sulfoglutarate. It has been previously noted that for a CO2-philic surfactant to have high solubilising power, structural disorder in the surfactant molecular structure is required to decrease the length-to-breadth ratio, which decreases the possibility of liquid-crystal-like formation [36,50]. Although wmax is larger for the nFG(EO)2 series [36][37][38], here, the sulfoglutarate version provides slightly higher solubilising power than the normal sulfosuccinates di-CF2 (wmax = 10) and hybrid CF2/AOT14 (wmax = 3).…”

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) .…”

Effect of surfactant headgroup on low-fluorine-content CO2-philic hybrid surfactants

“…Following the similar lines [36][37][38], the trend of increasing solubilizing power is also observed when sulfosuccinate is changed for sulfoglutarate. For a CO2-philic surfactant to have high solubilizing power, structural disorder in the surfactant molecular structure is required to decrease the length-to-breadth ratio, which decreases the possibility of liquid-crystal-like formation [36,50]. Although wmax is larger for the nFG(EO)2 series [36][37][38], here, the sulfoglutarate version provides slightly higher solubilizing power than the normal sulfosuccinates di-CF2 (wmax = 10) and hybrid CF2/AOT14 (wmax = 3).…”