Langmuir 1993,9,1193-1200 1193 Fluorinated, Protonated, and Mixed Surfactant Solutions: A Small-Angle Neutron Scattering Study E. Caponetti,' D. Chillura Martino, M. A. Floriano, and R. Triolo Dipartimento di Chimica Fisica, Universith di Palermo, V. Archirafi 26,90123 Palermo, Italy Received May 4,1992. In Final Form: February 9,1993 Contrast matching experiments have been performed on aqueoussolutionsof sodiumperfluorooctanoate, 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 syetems indicated the coexistenceof 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-definedcontrast 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 confiied by fitting the experimental patterns with a model baaed on the above-mentionedhypothesie: the structure function has been calculated by means of the rescaled mean spherical approximation usin a screened Coulombic potential plus hard sphere repulsion; the particle form factor has been calculatedusing several different models. Among the models tested, a core plus shellprolateellipsoid modelgave the best fib. 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. Introduction With an increase in the concentration of surfactant molecules, made with hydrophilic and hydrophobic por- tions, in aqueous solution, a sudden change in most of the solution properties can be observed at a critical micellar concentration (cmc). This change is related to the formation of aggregates in which the contact between the hydrophobic portion of the molecules and water is minimized, while, at the same time, the hydrophilic interaction is maximized. These aggregates are highly dynamic entities, though stable enough to be detected using different techniques. Micellar solutionsformed from amixture of two or more surfactants are of considerable interest from both practical and fundamental viewpoints. Mixtures of ionic and nonionic Surfactants have been studied by Bucci et al.lt2 Other authors considered mixtures of surfactants differing in the length of the hydrocarbon chain- or having different head groups6or isomeric ~ o n t e n t . ~ So far, most studies on mixed surfactants have focused on the cmc of the system in order to discuss the miscibility of compo- Mixed systems of hydrocarbon and fluorocarbon sur- factants are of considerable interest because of the difference in their thermodynamic parameters of micel- lization. The main focus is on whether hydrocarbon and fluorocarbon Surfactants combine to form mixed or ho- mogeneous micelles; the result could depend on the chain length, the head group, and the counterion. So far the behavior of these systams has been explained in terms of nents.2A8.9 nonideal mixing, which determines the coexistence of two kinds of mixed micelles, one rich in hydrocarbon, the other in fluorocarbon s u r f a ~ t a n t s . ~ J ~ J ~ These conclusion have been reached by studying these systems with various methods,*15 but the structural studies, reported in the literature, are surprisingly scarce. In this paper we describe a study on a mixed surfactant system formed by sodium perfluorooctanoate (C7F15' COONa) andsodiumdodecanoate(CllH&OONa). c7F15' COONa was chosen since it has been studied by different authors.16J7 The protonated surfactant was chosen be- cause it is known that the cmc values of fluorocarbon surfactants are close to those of hydrocarbon surfactants whose carbon atoms chain is 1.5 times longer;lg this has been attributed to the difference in the free energy of transferring the 4F2- and 4H2- from the aqueous environment to the micellar state. It is interesting to note that the volume of the two molecules is about the same, 364 A3 for the hydrogenatedsurfactant and 384 A3 for the perfluorinated one. We have used small-angle neutron scattering (SANS) coupled with the external contrast variation technique. It is well-known that in "two-phase" systems, such as a micellar solution, both inter- and intraparticle scattering contribute to the measured SANS intensity. As a con- sequence, important information about particles size and shape, the distribution of particles sizes, and the nature and the strength of interparticle interactions can be inferred. Moreover, because of the substantial difference in coherent scattering length densities between hydro- _ _ _ _ ~ ~ ~ (1) Rubmgh, D. N. In Solution Chemistry of Surfactants; Mittal, K. (2) Bucci, 5.; Fagotti, C.; De Giorgio, V.; Piazza, R. Langmuir 1991,7, (3) Malliaris, A.; Binana-Limbele, W.; Zana, R. J. Colloid Interface L., Ed.; Plenum: New Yolk, 1979; p. 337. 824. 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