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

Abstract

The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE−C(O)−NH−CH2−N+(CH3)3 CH3COO-, 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 (Wo) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60 Å2/surfactant molecule at the water−CO2 interface, in reasonable agreement with the value of 72 Å2 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 + K̄) 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.