Dynamics of counterions bound to linear micelles in solutions as studied by electric birefringence relaxation and high-frequency dielectric relaxation spectroscopies

The dynamics of counterions bound to linear micelles in an aqueous solution of the surfactant cetyltrimethylammonium bromide (CTAB) with sodium salicylate (NaSal) as added salts is investigated by frequency-domain electric birefringence relaxation spectroscopy (FEB method) and high-frequency (HF) dielectric relaxation spectroscopy. First, the influence of NaSal concentration C(s) on the linear micelle formation is studied by the FEB method. The contour length L of linear micelles is found to increase with increasing C(s) but for further increase of C(s) in excess of the CTAB concentration, L decreases, i.e. linear micelles break into shorter pieces. The FEB result also indicates that the diffusion constant of Sal^- ions bound to a linear micelle is nearly independent of C(s) with an average value smaller than that of free Sal^- ions. We also measured the dielectric increment Δε and the relaxation time τ, by HF dielectric relaxation, which showed the same concentration dependence as polyelectrolyte. These results indicate that the diffusion of Sal^- ions along the chain produces a charge density fluctuation on the chain and the other counterions, Br^- and Na⁺, are loosely bound to these charge fluctuating areas. The HF dielectric relaxation is attributed to the movement of these counterions localized in the electric potential trough on the chain. Copyright (C) 1998 Elsevier Science B.V.