Role of different counterions and size of micelle in concentration dependence micellar structure of ionic surfactants

Small-angle neutron scattering experiments have been carried out on aqueous micellar solutions of cationic surfactants cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl) and dodecyltrimethylammoinum bromide (DTABr) as a function of varying surfactant concentration. It is found that while CTABr shows sphere-to-rodlike transition of micelles with the increase in the surfactant concentration, the micelles of CTACl and DTABr remain more or less spherical even at very high surfactant concentrations. We explain the propensity of the above surfactants to form different structures in terms of the counterion condensation. The counterion condensation on the micelle decreases as the hydrated size of the counterion increases or the size of the micelle decreases.