Dielectric Relaxation Behavior of Cationic Micellar Solutions:  2

Dielectric relaxation behavior was examined, in the frequency range from 1 MHz to 20 GHz, for an aqueous micellar system of cetyltrimethylammonium bromide (CTAB) with the concentration ( C D ) from 2 to 1000 m mol kg -1 . The system exhibited three dielectric relaxation modes. The fastest mode, with a relaxation time around 1 ns, had relaxation strength proportional to C D up to C D values of 100 m mol kg -1, and showed a constant relaxation strength independent of C D above that concentration. The fastest relaxation mode attributed to the rotational relaxation mode of ionic pairs formed between CTA + and Br - in spherical micelles is considerably depressed in contact areas on the micellar surface of two micelles in contact with each other at C D values higher than 100 m mol kg -1 . This is likely the reason for the leveling-off in the magnitude of the fastest relaxation at high C D and the appearance of the slowest relaxation mode, which has a relaxation time of about 20 ns and increases in strength with increasing C D . The intermediate relaxation mode (with a relaxation time around 8 ns, and strength which decreases with increasing with C D ) is attributed to the migration of electrically bound Br - ions around spherical micelles. The average number of Br - ions bound into an ionic cloud covering a micelle was estimated to be less than unity (from the values of direct current electric conductance and Br - concentration determined by use of a Br - selective ion electrode), and a small number of micelles possessed ionic clouds which showed dielectric relaxation behavior.