Alkyl Chain Symmetry Effects in Mixed Cationic–Anionic Surfactant Systems

Abstract

We study symmetry effects of hydrophobic tails in mixed oppositely charged surfactant systems. Equal hydrophobicity of the participating surfactants is obtained either (1) by equal hydrocarbon surfactants chain length or (2) by unequal surfactants chain length when one chain is perfluorinated. Three systems were studied: symmetric system of type (1) or (2), and an asymmetric system. Some unusual structural changes are observed in a hydrocarbon–fluorocarbon mixed single-chain oppositely charged surfactant system in the dilute region. Both the cationic hydrocarbon surfactant, dodecytrimethylammoniumchloride (DoTAC), and the anionic fluorocarbon surfactant, sodium perfluorononanoate (SPFN), are separately soluble in water, forming micellar solutions at high water concentration followed by the formation of liquid crystal phases at low water content. Upon mixing the two surfactants in an equimolar ratio at low total surfactant concentration (C< 5 wt%), a stable, dense, flow-birefringent phase is formed at the bottom of the test tube, and a clear phase is formed at the top. Cryo-transmission electron microscopy (cryo-TEM) micrographs of the dense phase show dispersed lamellar droplets having a very smalldspacing, and fairly monodispersed ∼100 nm single-walled vesicles in the supernatant. The latter is supported by NMR self-diffusion measurements. This structural behavior is studied at different molar ratios of the two surfactants. A micelle–vesicle transformation is indicated by NMR self-diffusion measurements. The same behavior is found for symmetry of type (1). The asymmetric system DoTAC–sodium nonanoate (SN) shows an isotropic phase in all molar ratios up to 40 wt% of surfactant, where both imaging and rheological measurements indicate the existence of elongated micelles in the equimolar solution. It was found that the micellar size distribution is coupled with the molar ratio only in the SN-rich part.