Effect of the concentration and composition on the size and shape of micelles of sodium oleate–cetyltrimethylammonium bromide mixtures

The dependence of the aggregation number (Nagg) on composition and concentration of sodium oleate–cetyltrimethylammonium bromide mixed micelles as revealed by cyclic voltammetry experiments shows a complex relationship with the total concentration and the composition of surfactant mixtures. This behavior is related to the evolution with composition of the HLB values and with the composition and the excess free energy of mixed micellization, and is explained by the inclusion of the double bonds of oleate chains in the micellar Stern layers. The increase in size probably leads to a reduction of the micelle surface available for the polar headgroups, causing a reduction in the proportion of double bonds in the hydrocarbon–water interface and a change in the mixed micelle composition. Therefore, the generally held supposition that the composition of mixed micelles does not change with concentration seems rather unrealistic.

The dependence of the aggregation number (Nagg) on the mole fraction of CTAB (αCTAB) and the total concentration (C).Figure optionsDownload high-quality image (121 K)Download as PowerPoint slideResearch highlights
► NaOl–CTAB mixtures do not precipitate at any composition.
► The presence of CHCH groups in the chain generates a complex behavior in mixed micelles.
► At all compositions micelles rapidly grow with concentration.
► At all concentrations there is a maximum of Nagg at about 60% CTAB.
► Techniques based on Nagg and composition independence on concentration may be wrong.