Microstructure in D₂O/n-hexaethylene glycol monododecyl ether (C₁₂E₆)/CO₂ mixtures determined by small-angle neutron scattering

Phase behavior and small-angle neutron scattering (SANS) experiments were performed on mixtures of D₂O/ CO₂/n-hexaethylene glycol monododecyl ether (C₁₂E₆) as a function of temperature (25-31°C), pressure (63.1-90.7 bar), and CO₂ composition (0-3.9 wt %). The addition of carbon dioxide shifts the liquid-liquid miscibility gap present in C₁₂E₆/D₂O mixtures to lower temperatures. Parameters extracted from model fits of the SANS spectra indicate that, while micellar structure remains essentially unchanged, critical concentration fluctuations increase as the phase boundary and plait point are approached. The variations of micelle structure, scattering contrast, and particle volume fraction with pressure, temperature, and carbon dioxide concentration are consistent with only a minimal amount of carbon dioxide entering the micelles under all conditions.