Aggregation Behavior of Brij-35/Perfluorononanoic Acid Mixtures

Abstract

The mixed system of a non-ionic hydrocarbon surfactant, polyoxyethylene (23) lauryl ether (Brij-35), and a perfluorinated surfactant, perfluorononanoic acid, was investigated by a combination of methods. The critical micellar concentrations (CMC) have been determined over a wide range of sample compositions, by fluorescence and UV-visible spectrophotometry using pyrene and N-(4-nitrophenyl) perfluorononanamide respectively as molecular probes. The values of the CMC obtained were considerably different with the two techniques employed. Measurements of 19F nuclear magnetic resonance chemical shift of the same mixtures showed two breaks in the plots of Ddf vs. molar fraction of the perfluorinated surfactant. Conductivity as well as surface tension measurements also showed two breaks. The behavior is attributed to the formation of mixed micelles which change their composition when the fraction of the fluorinated compound increases and some segregation of the fluorinated compound takes place at high total surfactant concentration.