Study of Salt Effects on the Micelle-Monomer Exchange Process of Octyl-, Decyl-, and Dodecyltrimethylammonium Bromide in Aqueous Solutions by Means of Ultrasonic Relaxation Spectroscopy.

The effects of added salt on the micelle-monomer exchange process of octyl- and decyltrimethylammonium bromide were investigated by means of ultrasonic relaxation techniques. Using the Aniansson-Wall model, the micellar distribution width sigma and the dissociation rate constant k(-1) of the micelle-monomer exchange process were determined as functions of carbon number and of added salt concentration in aqueous solutions of a series of tetraalkylammonium bromide-NaBr systems and discussed on the basis of the Aniansson-Wall-Teubner model. From the rate constant of the micelle-monomer exchange process, the total energy change for the transfer of the alkyl chain of the surfactant ion from the interior of the micelle to a position where its terminal methyl groups is at the surface of the micelles was estimated and compared with those calculated by the Gouy-Chapmann model of the electric double-layer theory. Copyright 2000 Academic Press.