1H and 19F NMR investigation on mixed hydrocarbon-fluorocarbon micelles

H-1 and F-19 NMR measurements on aqueous solutions of sodium perfluorooctanoate (SPFO) and sodium dodecanoate (SD) mixtures are reported. The surfactant concentration ranged from similar to 0.3 to 10 times the critical micelle concentration (cmc congruent to 0.03 mol L-1). The cmc of the SD/SPFO/water mixed system obtained from NMR data was in good agreement with that previously obtained by conductivity measurements. Below the cmc, the experimental chemical shift (6) was independent of the total concentration for both surfactants. Above the cmc, however, the 6 values for 19F varied linearly with concentration, whereas the values for the hydrogenated surfactant deviated from linearity. These observations indicate that below the cmc each monomer is not affected by the presence of the others. Above the cmc, on increasing the total concentration, the chemical shift trends indicate that the fluorinated chains begin to aggregate, forming islands among hydrocarbon chain domains. Since the extended chain of the fluorinated surfactant is shorter than the inner micelle radius, some methyl groups of the longer SD must be segregated within the micelle. This patchwork distribution, involving an intramicellar phase separation, prevents the computation of the micelle composition; however, NMR data give information complementary to that obtained by a previous SANS study indicating the existence of mixed micelles having the same composition. Information on the structure of micelles and on the mean distribution of the two components in the system are obtained by SANS, while the NMR technique suggests details on the chemical environment of a single monomer and on the structural organization of the molecules within a micelle. Thus, the patchwork model here proposed is able to explain apparently conflicting data obtained from different techniques.