Droplet interactions in water-in-carbon dioxide microemulsions near the critical point: A small-angle neutron scattering study

Droplet interactions in water-in-carbon dioxide (W/C) microemulsions formed with a perfluoropolyether-based surfactant for droplet volume fractions from 0.05 to 0.10 are studied with small-angle neutron scattering (SANS) to understand the mechanism of microemulsion stability. The water-to-surfactant ratio (W-o) is fixed at 12.5. Droplet interactions increase as the upper critical solution pressure is approached with decreasing pressure at constant. temperature, increasing temperature at constant pressure, or increasing droplet: volume fraction. These interactions are quantified in terms of the structure factor at zero momentum vector, S(0), and the correlation length, xi, for an Omstein-Zernicke structure factor, or the square-well depth for the structure factor of a square-well potential. Near the critical solution pressure, the interaction strength (il) approaches the value predicted for a hard-sphere fluid with a van der Waals attractive term. The observed interaction strength between droplets is larger in W/C relative to water-in-oil microemulsions due to stronger tail-tail interactions resulting from the weak solvation by CO2. Overcoming these larger droplet interactions is the key challenge in forming stable W/C microemulsions. Pressure and volume fraction have a negligible effect on the droplet size; however, drop size decreases with temperature.