Dynamical structure of water in dioxane aqueous solution
by low-frequency Raman scattering
Yasunori Tominaga and Sachiko Miyoshi Takeuchia)
Department of Physics, Faculty of Science, Ochanomizu University, 2-1-1, Otsuka, Bunkyo-ku, Tokyo 112,
Japan
~Received 23 October 1995; accepted 16 February 1996!
The low-frequency Raman spectra of dioxane aqueous solutions have been analyzed from a
dynamical aspect of water structure. The reduced Raman spectra x 9 ( n̄ ) of the dioxane aqueous
solutions are well explained by a superposition of three characteristic modes of water and one
Gaussian mode ~;60 cm21! of dioxane. The concentration dependence of x 9 ( n̄ ) spectra shows that
the 190 cm21 mode of water disappears below about 0.8 molar fraction of water. This molar fraction
corresponds to the ratio of four water molecules to one dioxane molecule. This means that the
tetrahedral-like structure of water which is formed by five water molecules is almost destroyed
below about 0.8 molar fraction. Thus the basic idea of the five water molecules model of liquid
water has been confirmed from Raman spectroscopic point of view. © 1996 American Institute of
Physics. @S0021-9606~96!51319-9#
I. INTRODUCTION
There have been many investigations on liquid water by
various spectroscopic experiments.1 The most impressive result of them was carried out by x-ray diffraction study.2 The
result indicates that in liquid water the average numbers of
nearest neighbor oxygen atoms were 4.4 for each oxygen
atom. This means that the average structure of water can be
considered as a tetrahedrally coordinated pentamer which is
formed by about five water molecules through the hydrogen
bonds.
It must be noted that the dynamical structure of liquid
water is also very important as well as the static average
structure, because the hydrogen bonds between water molecules are not permanent and the hydrogen bonds are continuously created and destroyed. However the details of dynamical structure of liquid water are not still fully
understood.
Raman spectroscopy has been usually employed to investigate the dynamical structure of water and aqueous solutions for a long time. In the high-frequency spectral region
above 300 cm21, intramolecular vibration spectra of water
molecule are measured and these spectra are analyzed and
discussed by many researchers.3–8
On the other hand in the low-frequency region intermolecular vibration bands which are due to the interaction between water molecules through the hydrogen bonds are
observed.5,9–23 In this low-frequency region there appear a
stretching-like band around 190 cm21 ~S band! and a
bending-like band around 70 cm21 ~B band! among water
molecules based on a five-molecules cluster model.5,19 The
spectral profiles of these modes in aqueous solutions have
been widely reported.24–33
Recently it has been found that besides the above two
broad bands one relaxation mode appears as a central coma!
Present address: Matsushita communication Industrial Co. Ltd., 4-3-1
Tsunashima-higashi, Kouhoku-ku, Yokohama 223, Japan.
J. Chem. Phys. 104 (19), 15 May 1996
ponent below 50 cm21.9,16–20,23 This relaxation mode is due
to the creation and annihilation process of hydrogen bond
among water clusters.
In the present work based on the tetrahedrally coordinated pentamer structure in liquid water, depolarized Raman
spectra below 250 cm21 in the aqueous solution of dioxane
were measured and analyzed to clarify the dynamical structure of water.
Since dioxane molecules cannot make the hydrogen
bond with themselves, liquid dioxane does not make large
clusters with neighboring dioxane molecules. In addition, dioxane molecules can be mixed with water uniformly and are
considered to break the water pentamer structure. Accordingly, to investigate the dynamical structure of liquid water it
is effective to choose this dioxane aqueous solution.
II. EXPERIMENT
The specimen is the 1,4 dioxane which was purchased
from Wako Pure Chemical Industries, Co. Ltd. The aqueous
solution was prepared by mixing the dioxane and de-ionized
distilled water. To remove fine dust, the dioxane aqueous
solution was filtered with 0.2 mm Millipore filters before it
was contained in a 13134 cm3 fused silica cell.
Raman scattering spectra were obtained by a four-slit
double-grating spectrometer ~Jobin-Yvon HG-2000M!. The
exciting light source was a NEC Ar-ion laser operating at
488 nm with a power from 100 to 300 mW. An apparent
local heating due to the laser light was not observed. A rightangle-scattering geometry is always adopted in the present
light-scattering experiments. The depolarized Raman spectra
were measured with the configuration of X(VH)Y , where the
XY plane is horizontal and X denotes the direction of incident light and Y denotes the direction of scattered light. The
typical spectral resolution was 1.5 cm21 for the spectral region from 250 to 50 cm21 and 4.0 cm21 for the spectral
region from 2250 to 250 cm21. The scattered light signals
were detected by a photomultiplier in conjunction with
0021-9606/96/104(19)/7377/5/$10.00
© 1996 American Institute of Physics
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Y. Tominaga and S. M. Takeuchi: Dynamical structure of water in dioxane
FIG. 1. The low-frequency Raman spectra of dioxane aqueous solutions as
a function of molar fraction of water. The top spectrum is distilled water and
the bottom spectrum is dioxane. The temperature is 293 K.
FIG. 2. The reduced Raman spectra x 9 ( n̄ ) of dioxane aqueous solutions as
a function of molar fraction of water. The solid curves are the fitting spectra
described in the text.
photon-counting electronics. All spectra were recorded at
20.0 °C~293 K!. The accuracy of temperature control is
within 60.1 °C.
the Raman frequency shift which is denoted by cm21, and
the n̄ i ~in cm21! is the frequency of incident laser light. The
K is the instrumental constant. The frequency region of n̄ .0
corresponds to Stokes component and that of n̄ ,0 corresponds to anti-Stokes component. The ( n̄ i – n̄ ) 24 represents a
correction of scattering efficiency.
Using Eq. ~1! we can reduce the measured Raman spectra I( n̄ ) into x 9 ( n̄ ) spectra. Figure 2 shows the obtained
x 9 ( n̄ ) spectra of dioxane aqueous solutions as a function of
concentration. These spectra are derived from X(VH)Y Raman spectral intensity I( n̄ ) in the frequency region from
2250 to 250 cm21. The solid curves are the fitting spectra
discussed later. It is remarkable that the broad 190 cm21
peak disappears from the spectral profiles below 0.68 molar
fraction of water in dioxane aqueous solution.
In the case of liquid water we referred to one Debye-type
relaxation mode and two damped harmonic oscillator modes
to explain the low-frequency Raman profile.23 On the other
hand in dioxane aqueous solutions besides the above three
characteristic modes an additional Gaussian mode is needed
to explain the spectral profile of the imaginary part of the
dynamical susceptibility x 9 ( n̄ ).
III. RESULTS AND ANALYSIS
Figure 1 shows low-frequency X(VH)Y depolarized Raman spectra I( n̄ ) of distilled water, dioxane, and dioxane
aqueous solutions as a function of concentration in the frequency region from 2250 to 250 cm21. Because of rapid
increasing of central component, it is not easy to analyze the
low-frequency Raman spectrum directly. To obtain the spectral profile of low-frequency Raman spectra more clearly, the
reduced form of spectral intensity is often used.19–22,34,35
In the present paper we refers to a slightly different reduced form of spectral intensity, that is, the imaginary part of
dynamical susceptibility x 9 ( n̄ ) which is obtained from Raman spectral intensity I( n̄ ) dividing by Bose thermal factor.
The x 9 ( n̄ ) is given by
x 9 ~ n̄ ! 5K ~ n̄ i 2 n̄ ! 24 @ n ~ n̄ ! 11 # 21 I ~ n̄ ! ,
21
~1!
where n( n̄ )5@exp(hc n̄ /kT)21] ; T represents the absolute temperature, and c is the light velocity. The n̄ (5 n /c) is
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Y. Tominaga and S. M. Takeuchi: Dynamical structure of water in dioxane
With increasing dioxane concentration the central component below 10 cm21 becomes gradually strong. This central component may include both the dynamical relaxation
component and the elastic scattering component. Since two
components cannot be distinguished clearly in the present
experiments, both contributions are approximated by one
modified Debye relaxation form. This modified Debye relaxation form improves a high-frequency response36 which is
especially effective for dioxane aqueous solution in the frequency region above about 100 cm21. Because the highfrequency tail of a simple Debye-type relaxation form excess
the observed spectral response.
Strictly speaking it is well known that a simple Debye
form must be basically broken down in the higher frequency
region compared with the characteristic relaxation time. The
imaginary part of the complex susceptibility of the dioxane
aqueous solution system is represented as
H
g 0 g 1 ~ g 0 1g 1 ! n̄
~ n̄ 2 n̄ G ! 2
x 9 ~ n̄ ! 5A 1 2
1A
exp
2
G
2
2s2
~ g 0 1 n̄ 2 !~ g 1 1 n̄ 2 !
1A 2
1A 3
J
n̄ 22 g 2 n̄
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FIG. 3. The reduced Raman spectra x 9 ( n̄ ) of 0.95 molar fraction of water.
The spectral components are also shown. The B band is a bending-like
mode of water, the S band is a stretching-like mode of water, and the
Gaussian mode is an additional dioxane mode. The central component is the
modified Debye-type relaxation mode described in the text.
~ n̄ 22 2 n̄ 2 ! 2 1 ~ n̄ g 2 ! 2
n̄ 23 g 3 n̄
~ n̄ 23 2 n̄ 2 ! 2 1 ~ n̄ g 3 ! 2
,
~2!
where the first term is a modified Debye relaxation form, the
second term is an additional Gaussian mode of dioxane, and
the third and fourth terms are two damped harmonic oscillators of water. The g 1 ~in cm21! corresponds to the spectral
half width of Debye-type relaxation mode and g 0 ~in cm21!
is the high frequency cutoff frequency. In the present analysis we put g 0 '100 cm21. The n̄ G ~in cm21! and s ~in cm21!
are the center frequency and variance of the Gaussian mode,
respectively. The n̄ j ( j52,3) and g j ( j52,3) are the characteristic frequencies and damping constants of intermolecular
vibration modes represented with cm21, respectively. The
A G , A 1 , A 2 , and A 3 are the strength of each mode.
Using Eq. ~2!, the reduced Raman spectrum x 9 ( n̄ ) was
fitted by a nonlinear least-squares method. In the present
fitting procedure, we fitted 6250 cm21 spectra to determine
the above parameters. The best fitted x 9 ( n̄ ) spectra are
shown by solid curves in Fig. 2.
Figure 3 shows a typical reduced spectrum x 9 ( n̄ ) of 0.95
molar fraction and each component of the best fitted curve.
We can see one modified Debye-type relaxation mode ~the
central component!, two damped oscillator modes, and one
additional Gaussian mode.
Figure 4 shows the concentration dependence of the
characteristic frequencies ( n̄ 2 , n̄ 3 ) and their damping constants (g 2 ,g 3 ). The n̄ 3 and g 3 are the parameters of the 190
cm21 S band. The n̄ 2 and g 2 are the parameters of the 70
cm21 B band. The values of n̄ 3 and n̄ 2 in liquid water are
about 190 and 68 cm21, respectively. In liquid water these
values are slightly higher than the x 9 ( n̄ ) peak positions at
about 170 and 50 cm21, respectively. It is remarkable that
the values of n̄ 3 and g 3 drastically increase near 0.8 molar
fraction with increasing dioxane concentration. Near 0.8 mo-
lar fraction the g 3 is almost divergent. On the other hand the
n̄ 2 and g 2 are almost constant as a function of concentration.
Figure 5 shows the concentration dependence of the center frequency n̄ G and its variance s of the Gaussian mode. In
pure water ~molar fraction51.0! there is no Gaussian mode.
We can see that the n̄ G has a maximum value and the s has
a minimum value at around 0.8 molar fraction.
IV. DISCUSSION
We have analyzed the low-frequency x 9 ( n̄ ) spectra of
dioxane aqueous solutions as a superposition of the modified
Debye relaxation form which has a high-frequency cutoff,36
two damped harmonic oscillator modes and additional one
Gaussian mode ~;60 cm21! of dioxane. By using this modified Debye form, the spectral fitting of higher frequency tail
of x 9 ( n̄ ) is much improved.
From Fig. 4 the most significant result is that the damping constant of S-band ~;190 cm21! g 3 diverges around 0.8
molar fraction with increasing concentration of dioxane. The
characteristic frequency n̄ 3 also rapidly increases around 0.8
molar fraction with increasing concentration of dioxane.
However, this behavior of n̄ 3 is not so definite compared
with the divergence of damping constant g 3 because the
spectral profile becomes so broad on S band that the characteristic frequency has some ambiguities at the large damping
constant.
Since 0.8 molar fraction means that the mole ratio of
water against dioxane is 4:1, the pentamer structure of water
which is formed by five water molecules is almost destroyed
at 0.8 molar fraction if the dioxane and water are uniformly
mixing. Thus, from Raman spectroscopic point of view, the
above result indicates the basic idea that the S band is a
characteristic mode of the pentamer structure of water and
this mode characterizes the dynamical structure of water.
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Y. Tominaga and S. M. Takeuchi: Dynamical structure of water in dioxane
FIG. 4. The concentration dependence of the characteristic frequencies ~upper figure! and the damping constants ~lower figure! of both B band and S
band.
From Fig. 5 one can see that the mode parameters of
Gaussian mode also show significant behavior at 0.8 molar
fraction. However, at this stage we cannot assign the origin
of this Gaussian mode of dioxane. One important point is
that this dioxane mode cannot be fitted by an ordinary
damped harmonic oscillator form. Thus the Gaussian form is
essential to fit the spectral profile of this dioxane mode.
From the dielectric relaxation measurement of dioxane
aqueous solutions,37 the relaxation strength which is normalized by water concentration rapidly decreases with decreasing water concentration above 0.83;0.80 molar fraction and
shows almost constant below 0.80 molar fraction. The normalized relaxation strength is proportional to the amount of
dipole moment of water clusters. This result means that in
the dioxane aqueous solution the water clusters are destroyed
below 0.83;0.80 molar fraction. The 0.83 and 0.80 molar
fraction correspond to 5:1 and 4:1 mole ratio of water against
dioxane, respectively. Thus the dipole moment in distilled
water is due to the cluster of six or five water molecules.
FIG. 5. The concentration dependence of the center frequencies ~upper figure! and the variances ~lower figure! of the Gaussian mode of dioxane.
In ice Ih structure the hexagonal structure of six water
molecules and the tetrahedral structure of five water molecules can coexist. In liquid water there can exist a certain
temporal structure like ice in short time by hydrogen bond
among water molecules.5,19 Therefore the present Raman result and the above dielectric relaxation result are consistent
with the idea that the dynamical structure of liquid water is
temporal tetrahedral structure of five water molecules in
short time.
ACKNOWLEDGMENTS
This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and
Culture. We also thank to Microcalorimetry Research Center, Osaka University for providing us the chromelconstantan thermocouple.
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Y. Tominaga and S. M. Takeuchi: Dynamical structure of water in dioxane
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