Without salt, only mixing micelles exist in aqueous mixture of anionic surfactant sodium dodecyl benzenesulfonate (SDBS) and nonionic surfactant octylphenoxypolyethoxyethanol (TX-100). However, the addition of salt induces the spontaneous formation of vesicle, which makes the formation of vesicle much easier and simpler. The mechanism of the formation may be attributed to the compression of salt on the electric bilayer of the surfactant head groups, which reduces their area and increases the packing parameter P of the surfactant under the participation of TX-100, resulting in the formation of larger aggregates. The vesicle region is presented in a pseudo-ternary diagram of SDBS/TX-100/brine by visual observation and surface tension measurement, which exhibits a strip-shape. The existence of vesicles is demonstrated by TEM image using negative-staining method. The variations of vesicle size with the salinity and the surfactant concentration are determined using dynamic light scattering method. It is found that the vesicle size is independent on the surfactant concentration, but subject to the salinity of the solution.

Key words: spontaneous vesicle formation, salt inducement, negative-staining TEM, dynamic light scattering, mixing of single-tailed anionic/nonionic surfactant