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

Abstract

The complex dielec. permittivity spectra of aq. octyl- (OTAB), 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, resp., around the charged micelles. It is found that at low concns. 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 concn. range. Both models were used to det. the radius and surface cond. of the micelles. Vol. fractions detd. from these radii are in good agreement with literature data and with values obtained from the anal. of the water relaxation processes. This shows that the complex permittivity spectra of theses solns., detd. in the range 0.09 ≤ v/GHz ≤ 89, can be assigned to microscopic processes in a consistent way. Addnl., the data suggest that for OTAB solns. below the crit. micelle concn., cmc, the 250 MHz relaxation is due to the tumbling motion of solvent-sepd. ion pairs formed with an assocn. const. of KA ≈ 26. This mechanism contributes also significantly above the cmc. The surprising presence of the 80 MHz relaxation process also at submicellar concns. of OTAB may hint for a premicellar aggregation of surfactant ions.