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, ξ, for an Ornstein−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 (A) 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.