Dielectric Relaxation Behavior of Cationic Micellar Solutions: 2

Abstract

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 (CD) 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 CD up to CD values of 100 m mol kg-1, and showed a constant relaxation strength independent of CD 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 CD 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 CD and the appearance of the slowest relaxation mode, which has a relaxation time of about 20 ns and increases in strength with increasing CD. The intermediate relaxation mode (with a relaxation time around 8 ns, and strength which decreases with increasing with CD) 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.