Dielectric Relaxation of Cationic
Surfactants in Aqueous Solution. 2. Solute Relaxation.
C. Baar, R. Buchner*, and W. Kunz
J. Phys. Chem. B
Abstract
The complex dielectric permittivity spectra of octyl-, dodecyl-, and
hexadecyltrimethylammonium bromide and of dodecyltrimethylammonium chloride
reveal two low frequency relaxation processes centered around 80 MHz 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
concentrations 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 these solutions, determined in the range 0.09
<=n/GHz <=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
KA about 26. This mechanism contributes also significantly above the
cmc. The surprising presence of the 80 MHz relaxation process also at
submicellar concentrations of C8TAB may hint for a premicellar
aggregation of surfactant ions.
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| (c) 2001 Josef Duschl |
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