Small angle neutron scattering studies on micellar systems part 2. Mixed systems of ammonium decanoate and ammonium perfluoro-octanoate

Abstract

Mixtures of ammonium decanoate (AmDec) and ammonium perfluoro-octanoate (APFO), in ammonium chloride: ammonium hydroxide buffer at pH 8.8 and an ionic strength of 0.1, were examined by small angle neutron scattering with varying proportions of the two surface active agents. The results indicated that mixed micelles were formed and it is suggested from analysis of the data that these were cylindrical in shape at APFO: AmDec ratios of 2:1,1:1 and 1:2. At 2:1 and 1:1 the micelles were found to contain more APFO than expected on the basis of ideal mixing of the surface active agents in the micelle. At a ratio of 1:9 the miceUes were found to be spherical but larger than those formed from AmDec alone. A speculative model is proposed for the mixed micelle which still, however, allows for segregation between hydrocarbon and fluorocarbon chains within the micelle.

Mixed Surfactant Systems

“…Binary mixed micelles of hydrocarbon surfactants studied by light scattering include nonionic-nonionic (54), anionic-nonionic (54)(55)(56), cationic-nonionic (54) and zwitterionic-anionic (32) mixtures. Mixed fluorocarbon-hydrocarbon surfactant systems have been studied by light scattering (56), and small angle neutron scattering studies have been made on both mixed micelles of anionic-nonionic (57) and cationic-nonionic (58) hydrocarbon surfactants and anionic-anionic fluorocarbon-hydrocarbon surfactant mixtures (59). A fluorescence probing technique has also been used to investigate micelle sizes in both anionic-nonionic hydrocarbon surfactant mixtures (60), and fluorocarbon-hydrocarbon anionic-nonionic (60,61), nonionic-anionic (61), nonionic-nonionic (61) and anionicanionic (60) surfactant systems.…”

“…Mixtures of fluorocarbon and hydrocarbon surfactants have been extensively studied (47,48,50,56,(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69)(70)(71)(72)(73) and are especially interesting because of the potential immisibility of the two types of hydrophobic groups in mixed micelles and the possibility of demixing. The different types of fluorocarbon-hydrocarbon mixed surfactant systems studied have included anionic-anionic (47,48,50,59,60,62,(64)(65)(66)(67)(68)(69), cationic-cationic (63), anionic-cationic (66,69), cationic-anionic (66), and anionicnonionic (56,60), and nonionic-nonionic (64,69) surfactant mixtures.…”

“…The different types of fluorocarbon-hydrocarbon mixed surfactant systems studied have included anionic-anionic (47,48,50,59,60,62,(64)(65)(66)(67)(68)(69), cationic-cationic (63), anionic-cationic (66,69), cationic-anionic (66), and anionicnonionic (56,60), and nonionic-nonionic (64,69) surfactant mixtures.…”

“…Evidence for this includes significant observed positive deviations from ideal mixing in the behavior of mixed CMCs determined from surface tension (62)(63)(64)(65)(67)(68)(69), ultra-filtration experiments (47,48), and the results of NMR studies of self-diffusion (50). However, the results of neutron scattering experiments on at least one anionic-anionic system indicate that mixed micelles do form (59). The problem of mixed systems of fluorocarbon and hydrocarbon surfactants and demixing is addressed in several other chapters in this volume (43,52,74,75).…”

“…The evidence for demixing in fluorocarbon-hydrocarbon surfactant systems is not unanimous however, with small angle neutron scattering results showing that at least some anionic-anionic mixtures form mixed micelles (59). As pointed out by Ottewill and co-workers (59), small angle neutron scattering measures spatial correlations and thus may provide the most direct means to observe micelle compositions.…”