Mixed micelle formation between anionic surfactant and α,ω-type cationic surfactant in aqueous solutions: Sodium dodecyl sulfate and N,N′-1,12-dodecanediylbis(triethylammonium bromide) system

Abstract

Mixed micellar properties of the aqueous mixtures of sodium dodecyl sulfate (SDS) and α,ω-type cationic surfactant N,N′-1,12-dodecanediylbis(triethylammonium bromide)(C 12Et 6), at an ionic strength 0.1 mol dm −3 NaBr have been investigated. Mixtures of SDS and C 12Et 6 form clear homogeneous solutions unlike typical anionic-cationic surfactant mixture systems. Mixed micelle formation has been observed over the entire composition range investigated. The compositions in the mixed adsorbed film and mixed micelle at the CMC are evaluated by applying the thermodynamic equations derived by Motomura to the results of surface tension measurements. The behaviors of both phases are fairly similar and the compositions are almost nearly equivalent molar mixtures over a large range of mixing ratios. This indicates that there exists a strong synergism between anionic and cationic surfactants, which is due to electrostatic attraction between oppositely charged head groups of surfactant molecules. The size of the mixed micelles, on the other hand, depends remarkably on the mixing ratio, and shows a maximum at the nearly equivalent molar mixing ratio, suggesting that the composition of the mixed micelles becomes nearly equal to the mixing ratio as the total surfactant concentration increases. The microenvironment polarities at interfacial and interior regions of the mixed micelles are found to depend on the mixing ratio, associated with the composition and size of the mixed micelles.