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    Formation of Water-in-Carbon Dioxide Microemulsions with a Cationic Surfactant: A Small-Angle Neutron Scattering Study

    Year: 2000

    Journal: J. Phys. Chem. B. 104 ( 2000) 11094-11102, 20111221

    Authors: C. T. Lee, Jr., P. A. Psathas, K. J. Ziegler, K. P. Johnston, H. J. Dai, H. D. Cochran, Y. B. Melnichenko, and G. D. Wignall

    Organizations: Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712; Department of Chemical Engineering, University of Tennessee, Knoxville, Tennessee 37996; Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

    The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE-C(O)-NH-CH2-N+(CH3)3CH3COO-, 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 36Å for water-to-surfactant molar ratios (Wo) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60Å2/surfactant molecule at the water-CO2 interface, in reasonable agreement with the value of 72Å2 determined from the change in the droplet radius with Wo. The CO2-phobic functionality between the surfactant headgroup and perfluoropolyether tail reduces CO2 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) of approximately 1 kBT, along with the strong partitioning of the surfactant toward CO2 versus water, lead to the small, rigid, spherical water droplets in CO2.