Surfactant-modified CO2-water interface:: A molecular view

An important goal is the design of economically viable amphiphiles capable of forming and stabilizing water domains in bulk CO2. To enhance the understanding of this class of systems, we present in this work the results from atomistic molecular dynamics computer simulations of a perfluoropolyether ammonium carboxylate surfactant monolayer at the high-pressure CO2\water interface. The system is modeled including all internal degrees of freedom for the surfactant anion, and considering all atoms explicitly. At 318 K and 23 MPa, and 84 Angstrom(2)/molecule surface coverage, the calculated decrement in interfacial tension due to the presence of the monolayer agrees with the experimental value previously obtained in our laboratories. We show that the surfactant monolayer retains most of the structural features commonly observed in conventional hydrocarbon amphiphiles at the oil\water interface. However, CO2 penetrates the surfactant monolayer to a larger extent than conventional hydrocarbon solvents. Correspondingly, CO2 is also capable of solvating the fluorinated tail-group throughout. The result is a thick, fairly structured monolayer, comparable to analogous hydrocarbon surfactants at the oil\water interface, a surprising observation given the much lower surfactant surface coverage. On the aqueous side, in contrast, the carboxylic carbon is well solvated, but very little contact between water and the tail-group is observed past the CF2 adjacent to the head-group. It is also observed that most of the ammonium counterions are associated with the head-groups.