Dielectric relaxation of cationic surfactants in aqueous solution. 2. Solute relaxation

The complex dielectric permittivity spectra of octyl-, dodecyl-, and hexadecyltrimethylammonium bromide and of dodecyltrimethylammonium chloride reveal two low-frequency relaxation processes centered around 80 and 250 MHz. These are assigned to the diffusional relaxation of free and bound counterions, respectively, around the charged micelles. It is found that at low concencentrations the model of Grosse is able to rationalize simultaneously the amplitudes and relaxation times of both relaxation processes. The similar model of Pauly and Schwan, which is restricted to the relaxation of the bound counterions, fits the parameters of the 250 MHz process over the entire concentration range. Both models were used to determine the radius and surface conductivity of the micelles. Volume fractions determined from these radii are in good agreement with literature data and with values obtained from the analysis of the water relaxation processes. This shows that the complex permittivity spectra of theses solutions, determined in the range 0.09 less than or equal to v/GHz less than or equal to 89, can be assigned to microscopic processes in a consistent way. Additionally, the data suggest that for octyltrimethylammonium bromide solutions below the critical micelle concentration, cmc, the 250 MHz relaxation is due to the tumbling motion of solvent-separated ion pairs formed with an association constant of K-A approximate to 26. This mechanism contributes also significantly above the cmc. The surprising presence of the 80 MHz relaxation process also at submicellar concentrations of C(8)TAB may hint for a premicellar aggregation of surfactant ions.