Small-angle neutron scattering measurements on sodium dodecyl sulfate aqueous solutions have been performed in the presence of n-alcohols, from methanol to octanol, at different alcohol concentrations. By modeling the experimental intensities, it was possible to obtain structural information and to derive simultaneously the distribution of the alcohols between the aqueous and the micellar phases. It was found that short chain alcohols tend to remain in the aqueous phase and, by altering the solvent properties, induce a decrease in the aggregation number of sodium dodecyl sulfate micelles. On the other hand, alcohols with longer hydrocarbon chains were found to be present in both phases though favoring the micellar phase the longer the alkyl chain and the larger the concentration; this could be rationalized by assuming that the insertion of alcohol molecules in the micelle produced weaker repulsive interactions between the charged head groups of the surfactant molecules. For long chain alcohols, appreciably localized in the micellar phase, screening of the interaction among head groups leads to bigger micelles than those observed in the absence of alcohol: in these cases the alcohol/surfactant molar ratio reaches the value of 0.86, and hence the aggregates can be considered as mixed micelles. Sodium dodecyl sulfate micelles, at the examined concentration, were found to deviate from spherical symmetry and, when added with heptanol or octanol, assumed an ellipsoidal shape growing preferentially along the rotation axis.

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Contrast matching experiments have been performed on aqueous solutions of sodium perfluorooctanoate, sodium dodecanoate, and a mixture of the two surfactants. Moreover, sodium dodecanoate has been studied as a function of the concentration. Previous findings in several mixed fluorocarbon-hydrocarbon systems indicated the coexistence of two different kinds of micelles, one rich in hydrocarbon and the other in fluorocarbon surfactant; on the contrary, because of the existence of a unique and well-defined contrast match point, the present data indicate the formation of mixed micelles having the same composition and a very narrow size distribution, at least at the composition examined. This has been confirmed by fitting the experimental patterns with a model based on the above-mentioned hypothesis: the structure function has been calculated by means of the rescaled mean spherical approximation using a screened Coulombic potential plus hard sphere repulsion; the particle form factor has been calculated using several different models. Among the models tested, a core plus shell prolate ellipsoid model gave the best fits. The aggregation number found for the mixed micelles was intermediate between those of the two single surfactant micelles, while the degree of counterion dissociation was lower than each of them.

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.

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