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

Surfactant mixtures of the nonionic tetradecyldimethylamine oxide (TDMAO) and the cationic tetradecyltrimethylammonium bromide ((TTA)Br) have been investigated by means of small-angle neutron scattering (SANS), static light scattering, and conductivity measurements. Upon increasing 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 approximation and mean spherical approximation), and from this analysis the 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 with more than 30 mol % (TTA)Br; i.e., for higher ionic content the additional counterions become effectively bound to the micellar surface. In contrast to that, the electric conductivity still increases continuously in that range, however, much more slowly than in the range before the onset of the counterion binding. The bound counterions still contribute to the electrical conductance to some degree, since other effects like the rise of monomer concentration and the decrease of the micellar size with rising (fTA)Br content of the mixtures are not sufficient to explain the increase of conductivity observed experimentally.