Geometric Origins of Surfactant Effectiveness in Mixed Solvent Systems

The surface activity of perfluorooctanoic acid (PFOA) and its hydrocarbon analog octanoic acid (OA) in ethanol-water solutions is studied with a combined experimental and molecular dynamics simulation approach. Experiments show that PFOA is more effective at reducing surface tension in ethanol water solvents than OA. The surface tension at the CMC is lower with PFOA than with OA, and PFOA is effective at lowering the surface tension in solutions with high ethanol content (up to 60%) while OA is not. We use molecular dynamics simulations to determine the underlying basis for these differences. The tendency of PFOA and OA molecules to partition to the surface at infinite dilution is evaluated using free energy profiles. Features of these free energy profiles at infinite dilution are shown to correlate with the experimental surface tension results. PFOA's superior surfactant properties are attributed to PFOA having a larger girth than OA, which is largely a consequence of the C-F bond being longer than the C-H bond. We expect the relationship between surfactant girth and effectiveness in reducing surface tension in mixed solvent systems to be general.