Dielectric Relaxation of Cationic Surfactants in Aqueous Solution. 1. Solvent Relaxation

Complex dielectric permittivity spectra, ε(ν), in the frequency range 0.09 ≤ ν/GHz ≤ 89 were recorded for aqueous solutions of octyl-, dodecyl-, and hexadecyltrimethylammonium bromide and dodecyltrimethylammonium chloride at room temperature and concentrations c ≤ 1 mol dm^-3. Additionally, for selected solutions the temperature dependence of ε(ν) was examined in the range of 0 ≤ ϑ/°C ≤ 65 in order to obtain Eyring activation enthalpies and entropies of the relaxation times. It was found that a superposition of up to five Debye relaxation processes is required for a consistent fit of all spectra. The low-frequency dispersion steps with relaxation times τ₁ ≈ 1.5 ns and τ₂ ≈ 300 ps can be assigned to the diffusion of free and bound counterions surrounding the charged micelles. The remaining relaxation processes are due to the solvent. As in the pure solvent, a small contribution of fast mobile water molecules, τ₅ ≈ 1 ps, can be observed besides the dominating relaxation of the hydrogen bond network of bulk water, τ₄ ≈ 8 ps. Additionally, a slower relaxation process, τ₃ ≈ 25 ps, is detected which can be attributed to water molecules within the micelles and in the vicinity of the hydrophobic tail of monomeric octyltrimethylammonium ions. From the amplitudes of the water relaxation processes hydration numbers of the surfactant ions and micelles as well as the volume fraction of micelles were calculated.