FLUORINATED, PROTONATED, AND MIXED SURFACTANT SOLUTIONS - A SMALL-ANGLE NEUTRON-SCATTERING STUDY

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.