Properties of Surface and Micelle in a pH‐Mediated Ternary Surfactant Mixture in Salt Solution

Abstract

We have investigated the mixing behavoir of a pH‐mediated ternary surfactant mixture at constant ratio of dodecyldimethylamine oxide (DDAO) and Triton X‐100 (9:1). From the equilibrium surface tension measurements at different pHs, the critical micelle concentration (cmc) data were obtained as functions of the pH. Values of the cmc and composition of the micelles were predicted using the regular solution approximation. To some extent, the experimental cmc values agree with the predicted cmc. The average degree of ionization of dodecyldimethylamine oxide in the mixed surfactant systems was estimated using potentiometric titrations. The surface electric potential of the micelles (Ψo) was determined using two methods, one by hydrogen ion titration and the other by the dissociation constants of an acid‐base indicator. In a high degree of ionization of DDAO in the micelles phase (am), Ψo estimated from acid‐base indicator is much higher than that from hydrogen ion titration. In the protonated dodecyldimethylamine oxide/TX‐100 binary surfactant system, Ψo estimated from hydrogen ion titration was as high as 89 mV. The micellar aggregation numbers evaluated by the steady‐state fluorescence probe method increase with pH except at pH=5.03. At pH=5.03, the maximum micelle aggregation number was observed.