Acid–Base Equilibrium of Dodecyldimethyl-Amine-<Emphasis Type="Italic">N</Emphasis>-Oxide Micelles in Water–Butanol Binary at 298 K

Dodecyldimethyl-amine-N-oxide (DDAO) is a zwitterionic surfactant that can be protonated in water to form DDAOH⁺ if the pH of the medium is appropriate. We proposed, in a previous study, an experimental approach to determine directly the ratio of ionization of the surfactant in the micelle and in the bulk, at a given acidity level, without the need of a presupposed model of protonic exchange. We also showed that the relations of Thermodynamics impose a relationship between the critical micelle concentration (cmc) and the level of protonation of DDAO in the micelle and in the bulk. In order to appreciate the general nature of our approach, we have now modified the protonic exchange equilibrium between DDAO and DDAOH⁺ by introducing butanol into the solution. We studied three mixed media (2, 4 and 6% alcohol by volume). For all these mixtures, we validated our experimental and theoretical approaches. Moreover, for all the hydroalcoholic mixtures considered and all pH values, the presence of butanol stabilizes the micelle leading to a decrease of the cmc. Similar to pure water (Lair et al. in Langmuir, 20:8490–8495, 2004), we showed that, in these media, the protonic exchange equilibrium between DDAOH⁺ and DDAO in their micellized forms cannot be characterized by an unique equilibrium constant independent of the pH. However, as the butanol content of the medium increases, the variations in apparent pK with pH decrease, indicating that the behaviour of the acid–base pair in the micelle is classical and corresponds to a stoichiometric proton exchange.