Micellar behavior of mixtures of sodium dodecyl sulfate and dodecyldimethylamine oxide in aqueous solutions

Studies on adsorption characteristics and micellization of mixtures containing cationic/nonionic surfactant dodecyldimethylamine oxide (DMDAO) and anionic surfactant sodium dodecyl sulfate (SDS) with and without NaCl as studied by surface tension, viscosity, dynamic light scattering (DLS) and small angle neutron scattering (SANS) showed strong synergisms and micellar growth. A large negative interaction parameter, β, using Rubing's theory for above mixed system was estimated. Sphere-to-rod transition driven by ion-dipole interactions between the dissimilar head groups in mixtures of the two surfactants (total concentration 25-100 mM) was well reflected in DLS and SANS measurements. These results are consistent with the concept that nonidealities of mixing in micelles of oppositely charged mixed surfactant systems are primarily of electrostatic origin with specific interactions.