Ultrasonic relaxation study of the micelle—Monomer exchange process in aqueous solutions of dodecyltrimethylammonium bromide in the presence of NaBr

Abstract

Ultrasonic absorption measurements have been carried out in aqueous. NaBr solutions of dodecyltrimethylammonium bromide in order to elucidate the effect of added salt on the ultrasonic relaxation process in micellar solutions. The measurements have been done over the frequency range 0.2–8 MHz and the concentration range of NaBr ⩽ 0.2 M at 25°C. The ultrasonic relaxation spectra for all solutions investigated show a single relaxation process, which is identified as a “fast” relaxation process due to exchange of a surfactant monomer between micelle and bulk solution. The mean dissociation rate constant, κ −1, and the volume difference associated with the exchange process, |Δ V s|, are determined by applying the relations derived by Aniansson and Wall and by Teubner, respectively. Two kinds of determination have been carried out, taking into account two possibilities: that monomer concentration of surfactant remains constant, or varies with total concentration above cmc. The latter possibility more correctly explains the present experimental results than the former, and the determined values of κ −1 and |Δ V s| decrease with increasing concentration of NaBr. The decreasing tendency of |Δ V s| is consistent with the results from thermodynamic studies in the literature.