Interaction between zwitterionic and anionic surfactants: spontaneous formation of zwitanionic vesicles

The physicochemical properties, such as critical micelle concentration (cmc), surface tension at cmc (γ(cmc)), and surface activity parameters of the mixtures of a new amino acid-based zwitterionic surfactant, N-(n-dodecyl-2-aminoethanoyl)-glycine (C(12)Gly) and an anionic surfactant, sodium dodecyl sulfate (SDS) at different molar fractions, X(1) (= [C(12)Gly]/([C(12)Gly] + [SDS])) of C(12)Gly were studied. A synergistic interaction was observed between the surfactants in mixtures of different X(1). The self-organization of the mixtures at different molar fractions, concentrations, and pH was investigated. Fluorescence depolarization studies in combination with dynamic light scattering, and transmission electron microscopic and confocal fluorescence microscopic images suggested the formation of bilayer vesicles in dilute solutions of SDS rich mixtures with X(1) ≤ 0.17 in the pH range 7.0 to 9.0. However, the electronic micrographs showed structures with fingerprint-like texture in moderately dilute to concentrated C(12)Gly/SDS mixture at X(1) = 0.50. The vesicles were observed to transform into small micelles upon lowering the solution pH and upon increase of total surfactant concentration in mixtures with X(1) ≤ 0.17. However, decrease of SDS content transformed vesicles into wormlike micelles. The structural transitions were correlated with bulk viscosity of the binary mixtures.