Surface Behavior of Octanoic Acid and Perfluorooctanoic Acid Surfactants

A combined experiment and molecular simulation approach is used to address the behavior of surfactants in ethanol-water solutions.  In particular, the effect of replacing an alkane chain with a fluorinated chain is studied.  The experiments demonstrate that the effectiveness of ocatanoic acid (OA) as a surfactant decreases with increasing ethanol concentration, eventually vanishing at ethanol concentrations of 60% wt. and above. On the other hand it was observed that perfluoro octanoic acid (PFOA) was effective in decreasing the surface tension of ethanol-water solutions through all concentrations (from 0% to 100% wt. of ethanol).  Molecular dynamics simulations were used to understand this behavior.  First, the simulations were validated by obtaining surface tensions in reasonable agreement with experiment.  Then, to understand the physical basis of the behavior, free energy landscapes, were determined as a function of distance from the surface, for OA and PFOA through molecular dynamics. The simulation results demonstrated that the surface activity of PFOA is higher than OA in ethanol-water solvents, and successfully predicted that the free energy driving force pushing OA to the surface becomes insignificant at ethanol concentrations above 70%.