Mechanism for the Formation of Micellar Aggregates Prepared from Two Oppositely Charged Surfactants

This study aims to provide experimental evidence supporting a mechanism that rationalizes how micellar aggregates (MAs) form in aqueous solution. This mechanism postulates that when surfactant mixtures generate MAs, the surfactant in excess forms the same surfactant micelles (ssMic) that cap the exposed hydrophobic edges of the MAs to stabilize them and halt their growth. To this end, mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) were prepared at overall surfactant concentrations of 50 and 100 mM with a known molar fraction (fDTAB) of DTAB, and pyrene fluorescence was applied to determine their critical micellar concentration (CMC or C*). C* was used to determine the molar composition in terms of the molar fraction (xDTAB) of DTAB in the MAs generated by the SDS/DTAB mixtures. xDTAB was always different from fDTAB, implying that one surfactant in the SDS/DTAB mixtures was in excess, was not involved in the MAs, and formed ssMic, since its concentration was above its CMC. A simple mass balance led to the determination of the theoretical concentration of [ssMic]th. Analysis of the fluorescence decays generated by pyrene dissolved in aqueous solutions of SDS/DTAB mixtures provided the concentration [MA] of the micellar aggregates. The good match found between [ssMic]th and [MA] indicated that ssMic must control the growth of MAs, as suggested by the proposed mechanism. The agreement found between [ssMic]th and [MA] for our two series of experiments was also observed for two published series of experiments conducted with pyrene excimer formation (PEF) and small-angle neutron scattering (SANS). Thus, four series of experiments conducted by three different laboratories with two different techniques led to the same conclusions supporting the proposed mechanism, at least for SDS/DTAB mixtures. Future studies will aim to assess whether this mechanism can be extended to other systems of amphiphilic molecules.