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

The complex dielectric permittivity spectra of octyl-, dodecyl-, and hexade cyltrimethylammonium bromide and of dodecyltrimethylammonium chloride revea l two low-frequency relaxation processes centered around 80 and 250 MHz. Th ese are assigned to the diffusional relaxation of free and bound counterion s, respectively, around the charged micelles. It is found that at low conce ncentrations the model of Grosse is able to rationalize simultaneously the amplitudes and relaxation times of both relaxation processes. The similar m odel of Pauly and Schwan, which is restricted to the relaxation of the boun d counterions, fits the parameters of the 250 MHz process over the entire c oncentration range. Both models were used to determine the radius and surfa ce conductivity of the micelles. Volume fractions determined from these rad ii are in good agreement with literature data and with values obtained from the analysis of the water relaxation processes. This shows that the comple x permittivity spectra of theses solutions, determined in the range 0.09 le ss than or equal to v/GHz less than or equal to 89, can be assigned to micr oscopic processes in a consistent way. Additionally, the data suggest that for octyltrimethylammonium bromide solutions below the critical micelle con centration, cmc, the 250 MHz relaxation is due to the tumbling motion of so lvent-separated ion pairs formed with an association constant of K-A approx imate to 26. This mechanism contributes also significantly above the cmc. T he surprising presence of the 80 MHz relaxation process also at submicellar concentrations of C(8)TAB may hint for a premicellar aggregation of surfac tant ions.