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 bro mide (CTAB) with the concentration (Co) from 2 to 1000 m mol kg(-1). The sy stem 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 mo kg(-1), and showed a constant relaxation strength independent of CD above that concentration. The fastest relaxation mode att ributed to the rotational relaxation mode of ionic pairs formed between CTA (+) and Br- in spherical micelles is considerably depressed in contact area s 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 leve ling-off in the magnitude of the fastest relaxation at high CD and the appe arance of the slowest relaxation mode, which has a relaxation time of about 20 ns and increases in strength with increasing CD. The intermediate relax ation mode (with a relaxation time around 8 ns, and strength which decrease s with increasing with CD) is attributed to the migration of electrically b ound Br- ions around spherical micelles. The average number of Br- ions bou nd into an ionic cloud covering a micelle was estimated to be less than uni ty (from the values of direct current electric conductance and Br- concentr ation determined by use of a Br- selective ion electrode), and a small numb er of micelles possessed ionic clouds which showed dielectric relaxation be havior.