Self-Assembly and Rheological Properties of a Pseudogemini Surfactant Formed in a Salt-Free Catanionic Surfactant Mixture in Water

The surface and bulk properties of bola-type dicarboxylic acid (sebacic acid, SA) and zwitterionic surfactant tetradecyldimethylamine oxide (C(14)DMAO) mixtures in aqueous solutions were studied. Surface tension measurements indicate a pronounced synergistic effect between SA and C14DMAO. In bulk aqueous solutions, rich phase behavior was observed with a varied SA-to-C(14)DMAO ratio (rho) and a total surfactant concentration. Typically at rho = 0.5, a novel pseudogemini surfactant (C-14-S-C-14) forms, driven by electrostatic interaction and hydrogen bonding. The C-14-S-C-14/H2O system exhibits rich phase behavior induced by the transition of aggregates. With increasing concentration of C-14-S-C-14, one can observe a viscous L-1 phase, an L-1/L-alpha two-phase region where a birefringent L-alpha phase is on the top of an L-1 phase, a single L-alpha phase, and finally a mixture of an L-alpha phase and a precipitate. Microstructures formed in the L-alpha phases were determined by freeze fracture transmission electron microscopy (FF-TEM) and cryogenic-transmission electron microscopy (cryo-TEM) observations. Polymorphic aggregation behavior was observed with the formation of a variety of bilayer structures including unilamellar vesicles, onions, and open and hyperbranched bilayers. Rheological measurements showed that the L-alpha phases are viscoelastic and sensitive to temperature where a quick loss of viscoelasticity was observed at elevated temperature.