Cooperative and molecular dynamics of
alcohol/water mixtures: The view of dielectric spectroscopy
Takaaki Sato* and Richard Buchner a
J. Mol. Liq 117 (2005) 23-31
* Division of Pure and Applied Physics, Graduate School of Science
and Engineering,
Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
a Institut für Physikalische und Theoretische Chemie,
Universität Regensburg, D-93040 Regensburg, Germany
Abstract
This contribution reviews our recent investigations into the dielectric
relaxation behavior of methanol/water (MW), ethanol/water (EW),
1-propanol/water (1PW), and 2-propanol/water (2PW) mixtures. The analysis of
the complex permittivity spectra measured in the frequency range 0.1 <=n/GHz <=89 reveals that in the alcohol-rich region of
~0.3<=XA<=1.0, XA is the mole fraction of alcohol,
a three-step relaxation model is most appropriate for the description of the
spectra whereas at low XAthe intermediate process becomes too small
to be resolved. The dominating low-frequency Cole-Cole dispersion (j=1) is
assigned to the cooperative dynamics of the H-bond system where the motions of
alcohol and water molecules cannot be distinguished. Its timescale is largely
governed by the number density of H-bond acceptor and donor sites but steric
effects also contribute. Two additional Debye terms (j=2 and j=3) with
relaxation times of t2 ~10-20ps and
t3 ~1-2ps are required to reproduce the
high-frequency part of the spectrum. These small-amplitude dispersion steps can
be assigned to the motion of singly H-bonded alcohol monomers at the ends of
the chain structure (j=2) and to the flipping motion of free OH (j=3). The
increase of the amplitude De2 and the simultaneous decrease of the
(effective) dipole-dipole correlation factor with decreasing XAin
~0.5<=XA<=1.0 suggests insertion of water molecules into the
zigzag structure of H-bonded alcohol chains inducing a reduction of the average
chain length and an increase of the number of end-standing alcohol molecules
that can contribute to the t2-mode.
The excess activation free energy,DGE , enthalpy, DHE, and entropy, DSE, of the cooperative relaxation time,
t1, and their partial molar quantities,
DGiE,
DHiE, and
DSiE,
DHiE (i=alcohol, A, or
water, W) are discussed. Above the boundary concentration Xb(MW:
Xb ~0.30; EW: 0.18; 1PW: 0.14; 2PW: 0.15), DHAE and DSAE remain nearly zero,
indicating that alcohol molecules in the mixtures already form a zigzag chain
structure similar to pure liquids but branched by inserted water molecules. The
two pertinent maxima of DHAE and DSAE in the water-rich region
at X1 and X2 (MW: X1~0.045; EW: 0.04; 1PW:
0.03; 2PW: 0.03; MW: X2~0.12; EW: 0.08, 1PW: 0.06; 2PW: 0.07) are
connected with the hydrophobic hydration of alcohol monomers (X1)
and small multimers (X2) predominating at these concentrations.
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