MICELLES AND AGGREGATES OF FLUORINATED SURFACTANTS

The formation of micelles and aggregates by three kinds of fluorinated surfactants has been studied by F-19 NMR, light scattering, microscopy, centrifugation, conductance measurement, and pH titration. The surfactants studied are perfluoroheptanoic acid, its salts, and its ethoxylated amides. By the use of F-19 NMR, the mean lifetime of the surfactant molecules exchanging between the monomeric state and the aggregated state was found to be about 10(-6)s for the salts, 10(-4)s for the acid, and 10(-1)s for the amides. The reason for the increase in the mean lifetime for the acid and the amides is the formation of aggregates above the critical micelle concentration. The aggregates have an average diameter of about 100-200 nm, which is much larger than ordinary micelles. They can be distinguished under a phase-contrast microscope and can be separated from the surfactant dispersion by centrifugation. Conductance measurements of perfluoroheptanoic acid indicate strong counterion binding in the aggregates, the formation of which is favored by a reduction of the net charge in the hydrophilic head groups. It was found that large aggregates start to form when about 20% of the sodium ions in the salt are replaced by protons. Judging from the characteristics of the F-19 NMR spectra of the nonionic fluorinated surfactants (ethoxylated amides) and the optical isotropy of the droplets viewed under a polarizing microscope, it is suggested that the aggregates may exist in a cubic phase.