Dielectric Relaxation Processes in
Ethanol/Water Mixtures
T. Sato and R. Buchner
J. Chem. Phys. A108 (2004) 5007-5015
Abstract
We have determined the complex dielectric spectra of ethanol/water mixtures at
25°C for the nine molar fractions of ethanol, XEA=0.04, 0.08,
0.11, 0.18, 0.3, 0.5, 0.7, 0.9, and 1.0 in the frequency range of 0.1
<=n/GHz <=89 using TDR in 0.1 <=n/GHz <=25 and wave guide interferometers in 13
<=n/GHz <=89. At 0.3 <=XEA
<=1.0, a three-step relaxation model turns out to be most appropriate.
Besides a Cole-Cole relaxation for the dominating low-frequency process (j=1),
assigned to the cooperative dynamics of the H-bond system, which exhibits a
pronounced increase of its relaxation time, t1, when going from XEA=0 to 1, two
additional Debye terms (j=2 and j=3) with the relaxation times of
t2~10ps and t3~1-2ps are required to reproduce the
high-frequency part of the spectrum. In view of the relaxation mechanisms of
pure liquids, these high-frequency processes can respectively be assigned to
the motion of singly H-bonded ethanol monomers at the ends of the chain
structure (j=2) and the flipping motion of free OH (j=3). The unusual increase
of the amplitude De2 with decreasing XEA in ~0.5
<=XEA <=1.0 strongly suggests insertion of water molecules
into the zigzag structure of winding H-bonded ethanol chains resulting in a
reduction of the average chain length and an increase of the number of
end-standing ethanol molecules that can contribute to the t2-mode. At XEA< 03,
t1 rapidly approaches t3 and De2 → 0 so that the intermediate ethanol
monomer process (j=2) becomes inseparable while the fast switching process with
t3~1ps can always be resolved. The
analysis of the effective dipolar correlation factor, geff, revealed
that the parallel arrangement of the dipole vectors of ethanol molecules is
fairly disturbed by the presence of small amounts of water. Water has a strong
perturbation effect on the ethanol hydrogen-bonding chain structure in the
ethanol-rich region of 0.3 <=XEA <=1.0. |
