H and 19 F NMR measurements on aqueous solutions of sodium perfluorooctanoate (SPFO) and sodium dodecanoate (SD) mixtures are reported. The surfactant concentration ranged from ∼0.3 to 10 times the critical micelle concentration (cmc = 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 (δ) was independent of the total concentration for both surfactants. Above the cmc, however, the δ values for 19 F 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.

The compositions of mixed micelles formed in aqueous solutions of sodium dodecanoate and sodium perfluorooctanoate at different total surfactant concentrations and at three (0.33, 0.53, and 0.73) sodium perfluorooctanoate mole fractions were determined by the small angle neutron scattering technique coupled with the external contrast method. At each concentration, measurements were performed as functions of the solvent H2O/D2O composition in order to determine the micellar neutron scattering densities. The method described has allowed, at least for the cases under study, a direct determination of the micellar compositions, which, in the past, had been particularly difficult and the object of considerable debate. At all concentrations considered and within the experimental error, one kind of mixed micelle was always observed; these micelles were always richer in the component present in solution in greater proportion. An overall qualitative agreement between the present results and literature predictions based on regular solution theory was found, although a significant difference was noticeable at large sodium perfluorooctanoate concentrations, suggesting that further tests of the theory are required in order to include subtle interaction effects due to differences in the chemical nature of the surfactants.

The structure of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and that of urea containing AOT reversed micelles has been investigated by small-angle neutron scattering (SANS) and Fourier transform infrared (FT-IR) spectroscopy at different AOT concentrations and urea/AOT molar ratios. For the AOT/n-heptane system, SANS data analysis indicates that AOT molecules form prolate ellipsoidal aggregates, which grow asymmetrically along the major axis upon increasing the surfactant concentration. For the urea/AOT/ n-heptane system, the SANS results are consistent with the hypothesis that urea is encapsulated as small-sized ellipsoidal hydrogen-bonded clusters within the hydrophilic micellar core of the AOT reversed micelles. The insertion of urea in the micellar core causes a significant increase of the aggregate size along the minor-axis direction. FT-IR data indicates that, quite independently from the urea and AOT concentrations, the encapsulation of urea clusters in the AOT micellar core involves some changes of the H-bonded structure characterizing pure solid urea. This structural change was rationalized in terms of the specific interactions between the urea NH2 and the AOT SO3groups in small-sized urea clusters. Moreover, the CO stretching mode analysis suggests that within the cluster the urea CO groups interact with the urea NH 2 groups whereas the AOT CO groups do not.

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