Langmuir Monolayers of a Hydrogenated/Fluorinated Catanionic Surfactant: From the Macroscopic to the Nanoscopic Size Scale

Langmuir monolayers of the hydrogenated/fluorinated catanionic surfactant cetyltrimethylammonium perfluorooctanoate at the air/water interface are studied at room temperature. Excess Gibbs energies of mixing, DeltaG(E), as well as transition areas and pressures, were obtained from the surface pressure-area isotherm. The DeltaG(E) curve indicates that tail-tail interactions are more important than head-head interactions at low pressures and vice versa. Atomic force microscopy and molecular dynamics simulations allowed a fine characterization of the monolayer structure as a function of the area per molecule at mesoscopic and nanoscopic size scales, respectively. A combined analysis of the techniques allow us to conclude that electrostatic interactions between the ionic head groups are dominant in the monolayer while hydrophobic parts are of secondary importance. Overall, results obtained from the different techniques complement to each other, giving a comprehensive characterization of the monolayer.