Competitive binding of counterions at the surface of mixed ionic/nonionic micelles: Application of the ion exchange formalism

The competition between bromide and chloride for the binding at the surface of mixed micelles composed of tetradecyltrimethylammonium and Brij 35 surfactants has been studied by fluorescence quenching techniques. The results obtained conform to the pseudophase ion-exchange formalism over a wide range of micelle compositions and allow for the determination of the selectivity coefficients. It was found that the competitive binding is well described by a uniqueK(Br/Cl) = 6.3 ± 1 selectivity coefficient, irrespective of the micelle's composition within the range considered (mole fraction of the ionic component in the micelle from 1 to 0.4). The results indicate that the factors involved in determining the selective binding are almost totally related to the nature of the exchanging counterions and very little dependent on the characteristics of the micelle surface and superficial charge density.