Formation of water-in-carbon dioxide microemulsions with a cationic surfactant: A small-angle neutron scattering study

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 degreesC) and pressures (87.3-415 bar). Spherical droplets are observed by SANS with radii ranging from 16 to 36 Angstrom for water-to-surfactant molar ratios (W-o) from 9.5 to 28. Pored analysis of the SANS data indicates an area of approximately 60 Angstrom (2)/surfactant molecule at the water-CO2 interface, in reasonable agreement with the value of 72 Angstrom (2) determined from the change in the droplet radius with W-o. 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) over bar) of approximately 1 k(B)T, along with the strong partitioning of the surfactant toward CO2 versus water, lead to the small, rigid, spherical water droplets in CO2.