MIXTURES OF NONIONIC AND IONIC SURFACTANTS - THE EFFECT OF COUNTERIONBINDING IN MIXTURES OF TETRADECYLDIMETHYLAMINE OXIDE AND TETRADECYLTRIMETHYLAMMONIUM BROMIDE

Surfactant mixtures of the nonionic tetradecyldimethylamine oxide (TDM AO) and the cationic tetradecyltrimethylammonium bromide ((TTA)Br) hav e been investigated by means of small-angle neutron scattering (SANS), static light scattering, and conductivity measurements. Upon increasi ng the content of (TTA)Br in these mixtures, a transition from rodlike to spherical micelles is observed. The SANS curves were analyzed with models for the structure factor of charged colloids (random phase app roximation and mean spherical approximation), and from this analysis t he effective charge z of the aggregates was derived as a fit parameter . With rising ionic surfactant content, z at first increases linearly up to a threshold of 8-10 mol%. For higher ionic substitution it rises only more slowly and then remains practically constant for mixtures w ith more than 30 mol % (TTA)Br; i.e., for higher ionic content the add itional counterions become effectively bound to the micellar surface. In contrast to that, the electric conductivity still increases continu ously in that range, however, much more slowly than in the range befor e the onset of the counterion binding. The bound counterions still con tribute to the electrical conductance to some degree, since other effe cts like the rise of monomer concentration and the decrease of the mic ellar size with rising (fTA)Br content of the mixtures are not suffici ent to explain the increase of conductivity observed experimentally.