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 τs ∼ 5 ns reduced its magnitude with increasing the concentration of surfactants (CD). On the other hand, the magnitude of a fast relaxation mechanism with the relaxation time τf ∼ 0.5 ns was proportional to CD. 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.

We investigated the viscoelasticity of aqueous threadlike micellar solutions of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and additives, sodium p-toluenesulfonate (NapTS) and sodium salicylate (NaSal). The dependence of the rheological longest relaxation time (τ m ) on a molar ratio of pTS − relative to that of Sal − in the interior of threadlike micelles was mainly discussed. The value of τ m was dependent on the ratio and also on the total concentration (C * S ) of free pTS − and Sal − anions in the bulk aqueous phase at C * S < 30 mmol kg −1 . On the other hand, the value of τ m in the solutions with the molar ratio in the micellar interior higher than 0.5 exhibited the same C * S dependence at C * S > 50 mmol kg −1 as observed in a threadlike micellar system formed by CTAB and NaSal. These imply that the entanglement relaxation mechanism in the threadlike micellar system is influenced by both the microstructure of the threadlike micelles and the C *

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.

Aqueous solutions of a small size electrolyte, sodium p-toluenesulfonate (NapTS), were examined for dielectric relaxation behavior. Dissociated pTS- shows pronounced dielectric relaxation behavior in the aqueous solutions well-described with a Debye-type relaxation function. The value of the dielectric relaxation time is much longer than that of water molecules and is very close to the rotational relaxation time determined from the value of a spin−lattice relaxation time (T 1, 1H NMR). This means that pTS- keeps significantly large dipole moment (μ) and its rotational relaxation mode is detected by the dielectric relaxation measurements. Moreover, the concentration dependence of dielectric relaxation data suggests that 2−3.5 water molecules are tightly hydrated to pTS- in aqueous solution. We also carried out ab initio quantum chemical calculations to obtain the optimized geometries of individual pTS-, pTS- dihydrate, and pTS- trihydrate. The magnitudes of μ evaluated to be 7.3, 6.1, and 6.2 D, respectively, at the optimized geometries are significantly large and are crucial evidence that pTS- keeps large μ in aqueous solution.