NMR study of the transformation of perfluorinated surfactant solutions

The formation of micelles and aggregates in D2O solutions of the homologous series of surfactant cesium perfluoroalkanoates CnF2n+1COO-Cs+ (n = 5, 7, 9) and the other salts (Na+, NH4(+), and Rb+) of perfluorooctanoic acids were studied at 30 and 50 degrees C by using some NMR techniques. The critical micelle concentrations (cmc) were determined by F-19 chemical shift measurements. The studies of Cs salts have shown that the effect of each CF2 group on micelle formation is roughly equivalent to that of 1.5 CH2 groups. Except for the n = 5 system at 50 C-degrees, second breaks other than cmc at higher concentration were observed for alpha-CF2 groups of all Cs salts, which suggest the transformation into a larger micelle, most likely ellipsoidal. In the case of omega-CF3 groups these second breaks were not observed. Support for the F-19 results was furnished by Cs-133 chemical shift studies. The perfluorooctanoate surfactants with the Cs+ as a counter ion showed the behavior that was different from the other salts of the same chain length. This could be explained in terms of electrostatic and hydrophobic interactions between the counter ions and the micelles. Additionally, the molecular motions in monomer and micellar states are discussed by means of the F-19 relaxation times of the fluorine nuclei in the micelles.