Dodecyldimethylamine Oxide Micelles in Solutions without Added Salt

Critical micelle concentrations (cmc) of dodecyldimethylamine oxide (DDAO) were determined at 25°C in solutions in the absence of salt at different pH values by four methods: surface tension, dye solubilization (Sudan III), pH-concentration break points, and counterion (Cl-) activity. With the aid of the cmc values, the degree of ionization of DDAO micelle alphaM was determined as a function of pH from the hydrogen ion titration. The cmc increased sharply in a narrow range of alphaM from 0.35 to 0.40. This destabilization of micelles was also noted on the anomalous change of pK on the hydrogen ion titration curve. The counterion activity was measured potentiometrically, and the degree of the counterion binding theta was determined as a function of alphaM. The value of theta showed a sharp increase with alphaM also around 0.4. The line width of NMR absorption peak of 35Cl ions was well correlated with alphaM. The surface electric potentials psi0 of DDAO micelles were evaluated from both the hydrogen ion titration and the acid-base indicators, and the former value was larger than the latter one. A procedure is proposed to compare the stabilities of the micelles having different alphaM values. The result indicates much larger dependences on alphaM than those uncorrected for different ionic strengths at the cmc. Copyright 1998 Academic Press. Copyright 1998Academic Press