A Small-Angle Neutron Scattering Study of Water in Carbon Dioxide Microemulsions

Abstract

Phase behavior and small angle neutron scattering (SANS) experiments were performed on mixtures of D2O/CO2/ammonium carboxylate perfluoropolyether (PFPE) surfactant as a function of pressure (192−287 bar) and D2O composition (0.8−2.0 wt %) at 35 °C. SANS measurements provided direct evidence of water-swollen inverted micelles in CO2. At a constant PFPE concentration of 2.1 wt % in CO2, the microemulsion core radius increased from 20 to 36 Å as the D2O concentration increased from 0.8 to 2.0 wt %. For a constant water concentration, parameters extracted from model fits of the SANS spectra indicated that as the phase boundary was approached on reducing pressure, micellar structure remained essentially unchanged, while critical fluctuations increased.

Properties of surfactant films in water-in-CO2 microemulsions obtained by small-angle neutron scattering

“…The details of surfactant synthesis, purification and characterization can be found elswhere 6 20 , and dsurfactant=1.7 g cm -3 as a typical fluorinated compound 21 . From the mass density and bound coherent scattering lengths of the isotopes, the SLD of each component can be calculated and constrained for fitting (ρsurfactant=3.5×10 -6 Å -2 , ρD2O=6.3×10 -6 Å -2 , ρCO2=1.6×10 -6 Å -2 when dCO2 =0.68 g cm -3 , and 2.3×10 -6 Å -2 when dCO2 =0.92 g cm -3 ) It should be noted that the SLD for the water cores of microemulsion droplets is varied through H2O/D2O mixing, and the actual values are noted in appropriate sections below.…”

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

“…16,17 To obtain more economic and environmentally friendly W/CO2 µEs, the minimum fluorine content necessary to render a surfactant CO2-philic has been identified by using double-pentyl-tail surfactants with different fluorination levels. It was found that at least two fluorinated carbons (CF3CF2-) are required to stabilize µEs.…”

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

“…A number of studies have been directed toward the development of a surfactant system for stable formation of a W/scCO 2 mE, and more than 150 surfactants have been examined in these studies [7][8][9][10][11][12][13][14][15][16][17][18][19] . 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 ).…”