Role of chain length and electrolyte on the micellization of anionic fluorinated surfactants in water

The micellization of ionic surfactants in aqueous media reflects a balance between the hydrophobic attraction of surfactant tails and the electrostatic repulsion of surfactant headgroups. We investigate here the effect on surfactant micellization of the hydrophobic chain length and electrolyte composition, by focusing on the fluorinated surfactants sodium perfluorohexanoate (NaPFHx) and sodium perfluorooctanoate (NaPFO) in aqueous solution without/with NaCl, and analyzing experimental data from surface tension, pyrene fluorescence, small-angle neutron scattering (SANS), and viscosity techniques. The CMC of NaPFHx in water was 200 mM, much higher than that of NaPFO, 30 mM, due to the additional two -CF₂- groups of the latter. Upon addition of 0.25 M NaCl to the aqueous solution, the CMC of NaPFHx and NaPFO decreased by about 50% and 80%, respectively. The NaPFHx and NaPFO micelles are both ellipsoid in shape. NaPFHx forms smaller micelles compared to NaPFO: the association number for NaPFHx in D₂O was 15 compared to 25 for NaPFO. Upon NaCl addition, the micelle shape became more elongated. This is the first report on the NaPFHx micelle structure. The information presented here supports the re-formulation and replacement of long-chain fluorinated surfactants with relatively safer long-chain surfactants.