Dynamical structure of water in dioxane aqueous solution by low ...

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corresponds to the ratio of four water molecules to one dioxane molecule. This means that ... The specimen is the 1,4 dioxane which was purchased from Wako ...
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 (¯ n ) of the dioxane aqueous solutions are well explained by a superposition of three characteristic modes of water and one n ) spectra shows that Gaussian mode ~;60 cm21! of dioxane. The concentration dependence of x 9 (¯ 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 (¯ n ) 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 n i ~in cm21! is the frequency of incident laser light. The the ¯ K is the instrumental constant. The frequency region of ¯ n .0 n ,0 correcorresponds to Stokes component and that of ¯ sponds to anti-Stokes component. The (¯ ni –¯ n ) 24 represents a correction of scattering efficiency. Using Eq. ~1! we can reduce the measured Raman specn ) into x 9 (¯ n ) spectra. Figure 2 shows the obtained tra I(¯ ¯ x 9 ( n ) spectra of dioxane aqueous solutions as a function of concentration. These spectra are derived from X(VH)Y Ran ) in the frequency region from man spectral intensity I(¯ 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 n ). dynamical susceptibility x 9 (¯

III. RESULTS AND ANALYSIS

Figure 1 shows low-frequency X(VH)Y depolarized Raman spectra I(¯ n ) 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 (¯ n ) which is obtained from Ran ) dividing by Bose thermal factor. man spectral intensity I(¯ The x 9 (¯ n ) is given by

x 9 ~¯ n ! 5K ~¯ n i 2¯ n ! 24 @ n ~¯ n ! 11 # 21 I ~¯ n !, 21

~1!

where n(¯ n )5@exp(hc¯ n /kT)21] ; T represents the abson (5 n /c) is lute temperature, and c is the light velocity. The ¯

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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 !¯ n n 2¯ nG!2 ~¯ x 9 ~¯ n ! 5A 1 2 1A exp 2 G 2 2s2 n 2 !~ g 1 1¯ n2! ~ g 0 1¯

J

¯ n 22 g 2¯ n 1A 2 2 n 2 2¯ n 2 ! 2 1 ~¯ ng2!2 ~¯ ¯ n 23 g 3¯ n 1A 3 2 , 2 2 n 3 2¯ n ! 1 ~¯ ng3!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 analyn G ~in cm21! and s ~in cm21! sis we put g 0 '100 cm21. The ¯ are the center frequency and variance of the Gaussian mode, n j ( j52,3) and g j ( j52,3) are the characrespectively. The ¯ teristic 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. n ) was Using Eq. ~2!, the reduced Raman spectrum x 9 (¯ 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 (¯ n ) spectra are shown by solid curves in Fig. 2. n ) of 0.95 Figure 3 shows a typical reduced spectrum x 9 (¯ 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 n 2 ,¯ n 3 ) and their damping concharacteristic frequencies (¯ ¯ stants (g 2 ,g 3 ). The n 3 and g 3 are the parameters of the 190 n 2 and g 2 are the parameters of the 70 cm21 S band. The ¯ cm21 B band. The values of ¯ n 3 and ¯ n 2 in liquid water are 21 about 190 and 68 cm , respectively. In liquid water these values are slightly higher than the x 9 (¯ n ) peak positions at about 170 and 50 cm21, respectively. It is remarkable that the values of ¯ n 3 and g 3 drastically increase near 0.8 molar fraction with increasing dioxane concentration. Near 0.8 mo-

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FIG. 3. The reduced Raman spectra x 9 (¯ n ) 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.

lar fraction the g 3 is almost divergent. On the other hand the ¯ n 2 and g 2 are almost constant as a function of concentration. Figure 5 shows the concentration dependence of the center frequency ¯ n G and its variance s of the Gaussian mode. In pure water ~molar fraction51.0! there is no Gaussian mode. n G has a maximum value and the s has We can see that the ¯ a minimum value at around 0.8 molar fraction. IV. DISCUSSION

We have analyzed the low-frequency x 9 (¯ n ) 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 n ) is much improved. of x 9 (¯ 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 n 3 also rapidly increases around 0.8 characteristic frequency ¯ molar fraction with increasing concentration of dioxane. n 3 is not so definite compared However, this behavior of ¯ 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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