Dielectric Relaxation of Surfactant Micellar Solutions

Dielectric relaxation behavior of spherical micelles of cationic surfactants, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and cetyltrimethylammonium bromide, in aqueous solution was examined in the frequency range from 10 6 to 10 9 Hz. Each surfactant solution shows pronounced dielectric dispersions with two distinct relaxation times above the critical micelle concentrations. A slow relaxation mode with a relaxation time τ s ∼ 5 ns reduced its magnitude with increasing the concentration of surfactants ( C D ). On the other hand, the magnitude of a fast relaxation mechanism with the relaxation time τ f ∼ 0.5 ns was proportional to C D . The slow relaxation mode was attributed to the fluctuation of a counterion distribution around the spherical micelle or that of a distribution of dissociated cationic surfactant headgroups on the micellar surface. The fast relaxation mode was assigned to the rotational motion of ionic pairs consisting of surfactant cations and counteranions in the spherical micelle.