Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X₂ ≤ 1, where X₂ is the mole fraction of DDAB in the DTAB-DDAB mixture, while vesicle particles were formed directly from monomer solution at 0 < X₂ < 0.1. Furthermore, the transition from vesicle to micelle was found at 0 < X₂ < 0.4 at higher concentrations. An addition of DTAB to DDAB solution lowered considerably the DDAB concentration of the vesicle formation, which is attributable to asymmetric distribution of DTAB molecules between inner and outer monolayers of the vesicle bilayer. The shape and size of aggregates were obtained from surface tension, cryo-TEM, and light scattering data.