Behaviour of Mixtures of Different Surfactant Types in Solution and on Interfaces

The formation of mixed micelles and mixed adsorption layers in aqueous solutions of homologous and non‐homologous surfactants was investigated according to various complementary methods. – Mixed solutions of homologous surfactants show ideal mixing behaviour in the mixed micelles and mixed monolayers, in which the homologue with the longer chain prevails. – Mixed micelles and mixed monolayers of anionic and nonionic surfactants, however, show a strong deviation of the ideal behaviour within the meaning of an enhanced association and surface activity. The cause is the decrease of the electric repulsion of the ionic groups of the anionic surfactants by the ethylene oxide chains of the nonionic surfactants which are incorporated in the micelles and the monolayers. In the range of small concentrations mainly micelles of the nonionic component are formed at first. With increasing concentration there occurs an enhanced incorporation of the anionic surfactant in the micelle even below the critical micelle concentration of the pure anionic surfactant. Investigations on the equilibrium adsorption on hydrophobic interfaces clearly contribute to the problematic nature of non‐homologous surfactants.