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 ultrasoni c relaxation techniques. Using the Aniansson-Wall model, the micellar distr ibution 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 tetraalkyl ammonium bromide-NaBr systems and discussed on the basis of the Aniansson-W all-Teubner model. From the rate constant of the micelle-monomer exchange p rocess, 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 ter minal methyl groups is at the surface of the micelles was estimated and com pared with those calculated by the Gouy-Chapmann model of the electric doub le-layer theory. (C) 2000 Academic Press.