Dielectric Relaxation of Cationic
Surfactants in Aqueous Solution. 1. Solvent Relaxation.
C. Baar, R. Buchner*, and W. Kunz
J. Phys. Chem. B
Abstract
Complex dielectric permittivity spectra, e*(n), in the
frequency range 0.09 <=n/GHz <=89 were
recorded for aqueous solutions of octyl-, dodecyl-, and
hexadecyltrimethylammonium bromide and dodecyltrimethylammonium chloride at
room temperature and concentrations c <=1 mol / dm³. Additionally, for
selected solutions the temperature dependence of e*(n) was examined
in the range of 0 <=T/°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 t1 around 1.5 ns and
t2 around 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, t5 around 1 ps, can be observed besides the
dominating relaxation of the hydrogen bond network of bulk water,
t4 around 8 ps. Additionally, a slower
relaxation process, t3 around 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.
|
| (c) 2001 Josef Duschl |
 |
|