ULTRASONIC-ABSORPTION STUDIES OF SURFACTANT EXCHANGE BETWEEN MICELLES AND THE BULK PHASE IN AQUEOUS MICELLAR SOLUTIONS OF AMPHOTERIC SURFACTANTS

The kinetics of surfactant exchange between micelles and the bulk phase has been investigated by the ultrasonic relaxation absorption method in aqueous solutions of the two amphoteric surfactants: the n-octyl and n-decyl (dimethylammonio)propanesulfonates, referred to as 8DAPS and 10DAPS, respectively. Complementary determinations of critical micelle concentration, micelle aggregation number, and isothermal volume change upon micellization have been performed using spectrofluorometry (with pyrene as probe), ultrasonic absorption, time-resolved fluorescence quenching, and density measurements. The ultrasonic relaxation results support the model of Aniansson and Wall for the surfactant exchange. As for ionic and nonionic surfactants, the rate constant for the incorporation of amphoteric surfactants into the micelles is found to be above 10(9) M(-1) s(-1) indicating that this process is nearly diffusion-controlled. The spectrofluorometric results suggest that the pyrene molecules solubilized in the 8DAPS and 10DAPS micelles reside close to the positively charged nitrogen atom.