Langmuir
E.B. Abuin, E. A. Lissi, R. Nunez, and A. Olea
Measurements of surfactant ion (tetradecyltrimethylammonium) and counterion (bromide) activities in binary tetradecyltrimethylammonium bromide ('I"I'AB)-Brij 35 surfactant mixtures have been performed by using ion-specific electrodes. From the data obtained in the TTAB solutions in the absence of Brij 35, a TTAB critical micelle concentration (cmc) of 3.6 (h0.2) mM was evaluated. The degree of counterion binding on TTAB micelles was calculated from the concentration dependence of bromide activity above the cmc, &, and by applying the charged phase separation model to the surfactant and counterion activity data obtained, jjcpe. Values found were &, = 0.75 and p,.+ = 0.8. From the data obtained in the surfactant mixtures, the micellar composition and the counterion binding degree (&) of the mixed micelles have been evaluated. It was found that (1) in dilute solutions (total surfactant concentration lower than 5 mM) mixed micelles having a very low fraction of associated counterions are formed until a critical situation is reached where the behavior of the binding changes sharply. This critical situation was found to be reached at micellar compositions ranging from 0.16 to 0.41 TTAB mole fraction depending on the experimental conditions. A rationale for this fiiding is given in terms of micellar surface charge density requirements for the formation of a Stern layer. (2) At total surfactant concentrations from 0.015 to 0.1 M, counterion binding was found to be mainly determined by the composition of the micelle. The binding dependence with micellar composition is accounted for by a simple binding model which considers site-specific adsorption promoted by the micellar surface potential. Surface potentials employed were evaluated from the measured micellar effect on the acid-base equilibrium of phenol red. The application of the charged phase separation model to account for the counterion binding on the mixed micelles is also discussed.
