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.

The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE-C(O)-NH-CH 2 -N + (CH 3 ) 3 CH 3 COO -, is reported over a range of temperatures (25-90°C) and pressures (87.3-415 bar). Spherical droplets are observed by SANS with radii ranging from 16 to 36 Å for water-to-surfactant molar ratios (W o ) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60 Å 2 /surfactant molecule at the water-CO 2 interface, in reasonable agreement with the value of 72 Å 2 determined from the change in the droplet radius with W o . The CO 2 -phobic functionality between the surfactant headgroup and perfluoropolyether tail reduces CO 2 penetration of the tails, resulting in a smaller area/surfactant than in the case of an anionic perfluoropolyether surfactant [Langmuir 1997, 13, 3934]. A relatively rigid film, with a mean film rigidity (2K + K h ) of approximately 1 k B T, along with the strong partitioning of the surfactant toward CO 2 versus water, lead to the small, rigid, spherical water droplets in CO 2 .

We have developed a parallel-plate rheometer in a magnetic-disk drive configuration constructed of optically transparent materials and operating with a very small gap for measurements at very high shear rates. The friction force at the disk–slider interface has been measured as a function of sliding speed while the film thickness was monitored in situ using a capacitance technique. The shear rate is calculated from the film thickness and the sliding speed. A thin film can be applied on the disk, which allows very high-shear-rate measurements at low sliding speeds with negligible viscous heating. Both disk and slider have been made of optically transparent material to allow optical measurements simultaneously with the rheological measurements. In the present mode, the apparatus is set up for simultaneous rheometery and birefringence measurements on a thin film of polymer lubricant. Rheology and birefringence measurements were made on a perfluoropolyether lubricant over a range of strain rate from 103 s−1 to greater than 106 s−1 with 800 nm, 400 nm, and 200 nm film thicknesses.