Water-in-CO2 Microemulsions Studied by Small-Angle Neutron Scattering

Aggregate structures in water-in-CO 2 microemulsions were studied by high-pressure small-angle neutron scattering (SANS). With liquid CO 2 at 15 °C, the partially fluorinated, di-chain surfactant bis(1 H,1 H,5 H -octafluoro- n -pentyl) sodium sulfosuccinate (di-HCF4) stabilized single-phase microemulsions at pressures above ∼400 bar. The maximum water loading ( w ) investigated was 30 ([water]/[di-HCF4]), representing formation of relatively large water droplets in the microemulsion. Between w = 5 and 30, the SANS data were consistent with a model for attractive polydisperse spherical droplets. A linear relationship between the water droplet radius ( R c ) and w was found, which gave an apparent head group area for the surfactant of 87 Å 2 at the water−CO 2 interface. In Winsor II type microemulsions the value of R c, measured in the presence of excess water, increased with pressure from 36 Å at 400 bar to 56 Å at 550 bar.