A small-angle neutron scattering study of water in carbon dioxide microemulsions

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.