Mechanism of micelle formation in sodium dodecyl sulfate and cetyltrimethylammonium bromide

pH changes as a function of concentration for sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were observed by addition of 1 N HCl and 1 N KOH. pH values increased up to the critical micellar concentration (CMC) for the SDS/HCl system and decreased for the CTAB/KOH system. In the SDS/HCl and CTAB/KOH systems, the micellar phase had a fixed composition and was homogeneous and monodisperse above the CMC. However, in the SDS/KOH and CTAB/HCl systems, pH values increased continuously and gradually below and above the CMC, and the properties of the micellar phase changed as a function of concentration, giving rise to inhomogeneity and polydispersity.