Fluorescence-probe study of vesicle and micelle formations in a binary cationic surfactants system

A phase diagram of a dodecyltrimethylammonium bromide (DTAB)–didodecyldimethylammonium bromide (DDAB) system was obtained by a combination of two fluorescence probes, auramine and pyrene. In the DTAB-rich region, a vesicle was formed at first, and then a vesicle–micelle transition occurred at a higher concentration, near the critical micelle concentration of DTAB. On the other hand, in the region with a DDAB-rich composition, a small micelle appeared at lower concentrations, and a micelle–vesicle transition occurred at a higher concentration. It was found that the polarization was at a maximum at the DDAB-rich composition, and the addition of a small amount of DTAB to the DDAB vesicle caused the molecular packing at the vesicle membrane to tighten. However, the hydrophobic chain length of the single-tailed surfactant did not have a significant influence on the mixed vesicle membrane at a DDAB-rich composition.