Hydrogen Bonding in Aqueous Ethanol Solutions Studied by Raman Spectroscopy Sergey Burikov*a, Tatiana Dolenkoa, Masashi Hojob, Svetlana Patsaevaa, Victor Yuzhakova Dept. of Physics, Moscow State University, Leninskie Gory, G-991, Moscow, Russia, 119991 b Dept. of Science, Kochi University, Akebono-cho, Kochi, 7808520, Japan
a
ABSTRACT The results of Raman spectroscopy research of aqueous ethanol solutions with various mixing ratios are presented. Analysis of behaviour of Raman spectra with change of ethanol concentration from 0 to pure ethanol are given. The analysis of contour of OH-groups valence band provided information about changes in Hydrogen bonding due to increase of ethanol concentration. Obtained results showed that maximal strength of H-bonding in aqueous ethanol solution corresponded to ethanol concentration 15…20 % w/w. The strengthening of H-bonding is caused by formation of ethanol hydrates with clathrate-like structure. These results were supported by application of MCR-ALS method. Keywords: Raman scattering, valence vibrations, Hydrogen bonds, clathrate-like structure
1. INTRODUCTION Numerous studies on aqueous ethanol systems reveal that such solutions are non-ideal and this has been interpreted in terms of alcohol hydrates or a clathrate-like structure formation [1-4]. The area of clathrate hydrate research is anew receiving attention because of increasing interest to clathrate chemistry [5], examination of clathrate hydrates as a possible energy resource [6], their astrophysical implications [7]. Clathrate hydrate studies are also important in assessing geohazards to deep-water exploration and development [8]. In [1] it was proposed that water molecules surrounding tert-butanol molecules form a clathrate-like structure by Hydrogen-bonded network and stabilize the cluster. This effect is usually called the ‘hydrophobic hydration’. Evidence for the existence of clathrate hydrates in a number of alcohols is provided using dielectric relaxation technique and differential scanning calorimetry (DSC) [2], the stoichiometric ratio for clathrates is suggested to be around 5-6 water molecules per one alcohol molecule. The structure and composition of alcohol hydrates remained controversial even for better studied water-ethanol systems in solid state. Two kinds of ethanol hydrate solid were confirmed to exist [3] in water-ethanol systems: Et·4.67H2O in solutions frozen just after mixing ethanol with water, and Et·4.75H2O in solutions frozen after their storage for a few days at room temperature. Using DSC technique three ethanol hydrates in water-ethanol systems were found Et·2Н2О, Et·3Н2О and Et·4.75Н2О [4]. The last one is described as a semi-clathrate, i.e. clathrate in which the ethanol hydroxyl group is linked by Hydrogen bonding to the surrounding water framework. Hydrogen bonding plays a key role in the structural, physical, and chemical properties of liquids such as water, alcohols and in macromolecular structures like proteins. A water molecule can form up to four Hydrogen bonds, which are continually being broken, and new bonds are being formed on a picoseconds time scale [9]. Hydrogen bonding plays essential role in formation of structure of aqueous ethanol solutions [10,11]. A molecule of ethanol can form up to two Hydrogen bonds. Water and ethanol molecules can also interact via Hydrogen bonding. In the works [10,11] the factors that could affect the Hydrogen-bonding structure of water-ethanol have been investigated on the basis of proton nuclear magnetic resonance (1H NMR) chemical shifts of the OH of water-ethanol and Raman OH stretching spectra. Not only acids (H+ and HA: undissociated acids) but also bases (OH- and A-: conjugate-base anions from weak acids) strengthened the Hydrogen-bonding structure of water-ethanol. The nature of molecular association in aqueous ethanol solutions is essential to understanding the structural basis of the physical-chemical properties of alcohol solutions. Despite decades of research on alcohol hydration, however, the understanding of structure of alcohol solutions is still incomplete. *
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Vibrational spectroscopy is an important tool for understanding Hydrogen bonding in water and aqueous solutions [9,12,13]. In the previous studies [14,15] of aqueous ethanol solutions by Raman scattering we have observed the transformations of non-homogeneously OH-valence band with increase of ethanol concentration and interpreted them as strengthening of Hydrogen bonding. In the work [16] IR spectra obtained for aqueous ethanol solutions were analysed using Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method. The results showed that these mixtures could be described by a mixture model consisting of four species: ethanol, water, and two hydrates. In this study we investigated aqueous ethanol solutions with various mixing ratios ranging from pure water to pure ethanol using Raman spectroscopy. The contour analysis of non-homogeneously broadened OH valence band allowed observing manifestation of changes in Hydrogen bonding in solutions with ethanol concentration increasing. Additionally we used the results of MCR-ALS method to confirm our conclusions.
2. RESULTS 2.1 Experiment Aqueous ethanol solutions were prepared from purified ethyl alcohol and bi-distilled water. The purity of alcohol and water was controlled by absence of fluorescence excited by UV light. The ethanol concentration in the prepared solutions was changed from 0 to pure ethanol with 2-5 % w/w increment. Raman spectra were excited using Ar-laser radiation (wavelength 488 nm, power 450 mW) and registered by means of CCD camera with spectral resolution of 2 cm-1. The Raman spectrometer is described in details in [15]. The spectra were corrected for the laser radiation power and acquisition time. Further data processing of spectra within the region of CH and OH stretching bands included normalization of spectra to the integrated intensity of both CH and OH bands. 2.2 Raman spectroscopy of aqueous ethanol solutions
Figure 1. Raman scattering spectra of water and ethanol solutions within wavenumber range 200-4000 cm-1. Figure 1 shows Raman scattering spectra measured for water and aqueous ethanol solutions with different ethanol concentrations within wavenumber range 200-4000 cm-1. As it follows from obtained results, along with rising ethanol
concentration in solution the position and width of ethanol lines in Raman spectra are practically constant, while their amplitudes are increasing. Amplitudes of water bending and OH stretching bands are monotonously decreasing with rising of ethanol concentration in the solution. For Raman bands of stretching vibrations of CH- and OH-groups besides changes in intensity we have observed remarkable changes in the spectral contour shape. In this study special attention was devoted to behaviour of the stretching OH band in water-ethanol solutions due to the fact that the contour of this band is essentially affected by Hydrogen bonding. Figure 2 presents Raman spectra of water and ethanol solutions in the wavenumber region 2600–3800 cm-1. In this Figure, the stretching lines of CH-groups apparently seen in the region 2800-3000 cm-1 overlay the extremely wide and non-homogeneously broadened band of OH groups of ethanol and water molecules spreading from 2900 to 3800 cm-1. Upon increasing ethanol concentration in the solution the OH band undergoes changes not only in its integral intensity, but also in its contour shape and the ratio of intensities taken at low-frequency (around 3200 cm-1) to high-frequency (around band maximum) regions.
Figure 2. Raman scattering spectra of water and ethanol solutions with various ethanol concentrations within region of CH and OH stretching bands.
Figure 3. Ratio of Raman intensities taken at wavenumbers 3200 and 3420 cm -1 as a function of ethanol concentration in solution.
To quantify changes in spectral contour shape for OH band we used the ratio of amplitudes taken at frequencies of vibrations of OH-groups linked with strong and with weak H-bonds [12,13]. To characterize changes in the Raman spectra we took the ratio of Raman intensities at 3200 and 3420 cm-1. The dependence of these ratios on ethanol concentration for Raman spectra are shown in Figure 3. As it is seen from Figure 3, the ratio takes maximal values around ethanol concentration 10-20 % w/w. We interpret these results in terms of strengthening of Hydrogen bonding in the solution at certain ethanol concentration. This conclusion is supported by the measurements of IR spectra of the same aqueous ethanol solutions [16,17]. 2.3 MCR-ALS Component analysis of CH and OH stretching bands Multivariate curve resolution-alternating least squares (MCR-ALS) analysis [18] is a chemometric method, and recently it has been used to study molecular association in alcohol solutions [19]. The results [19] showed that the structure of water-methanol mixtures could be described by a mixture model consisting of four species, namely, methanol, water, and two complexes, methanol/water (1:1) and methanol/water (1:4). In [16,17] MCR-ALS analysis was applied to IR spectra of water-ethanol solutions. It was found that four components are adequate to describe the concentration dependencies of water-ethanol systems by a mixture model consisting of ethanol, water, and two hydrates, water-rich and ethanol-rich. The composition of ethanol hydrates was determined as EtOH·5.4H2O and EtOH·1.3H2O [16]. Another approach to determine the composition of the ethanol-water hydrates from Raman and IR spectra was used in [17]. We calculated integrated intensities of CH or OH band divided by their sum area, consequently, ICH/(ICH+IOH) and IOH/(ICH+IOH), for MCR-resolved spectra as well as for Raman spectra of ethanol solutions with known concentrations. Then the concentration dependency of either ICH/(ICH+IOH) or IOH/(ICH+IOH) was used to interpolate water/ethanol ratio for MCR-resolved hydrates.. The hydrate numbers of the resolved components determined in this way were 5 (Raman) and 4.3 (mid-IR) for water-rich hydrate and 1 (Raman) and 2.4 for the second hydrate. Resuming we conclude that composition of the water-rich hydrate (hydrate I) is close to EtOH·5H2O and for another hydrate (hydrate II) water/ethanol ratio lies between 1 and 2.
Figure 4. MCR-ALS resolved components in Raman spectra of aqueous ethanol solutions with various ethanol concentrations. In this study the results of application of MCR-ALS method to the Raman CH and OH stretching bands in water-ethanol mixtures were used to analyse the shape of resolved spectral components. The resolved spectra are shown in Figure 4 for four components: water, two types of ethanol hydrate and ethanol. Two of them are almost identical to Raman scattering
spectra of pure water and pure ethanol. However, other the spectra of ethanol hydrate resolved by MCR-ALS, differ in shape from the Raman scattering spectra of aqueous ethanol solutions measured at room temperature. The spectrum for hydrate I in Figure 4 shows higher ratio of intensities taken at low-frequency (~ 3200 cm-1) and high-frequency (around band maximum) regions. If we take into consideration that the low-frequency region corresponds to vibrations of strongly H-bonded OH groups, and the region around 3450 cm-1 is caused by vibrations of weakly H-bonded OH groups, we conclude that Hydrogen bonding in water-rich hydrate is strengthened compared to that of pure water. In contrast to hydrate I, from the shape of resolved spectrum for another ethanol hydrate (hydrate II in Figure 4) we conclude that H-bonding between hydroxyl groups in this structure is weaker than in pure water. This type of ethanol hydrates most likely represents water-ethanol chain associates, where water molecules have lees than 4 Hydrogen bonds. With ethanol concentration exceeding 20 % w/w in solution the number of such ethanol hydrates is rising followed by weakening of Hydrogen bonding in solution. Consequently, continuous decrease of ratio of intensities (both in Raman and IR spectra) taken at low-frequency and high-frequency regions is observed.
3. DISCUSSION On the basis of modern conceptions of water structure and the theory of non-electrolytes solutions, we explain the results received in this work in the following way. At ethanol concentration from 10 to 20% w/w in water there are enough ethanol molecules to break and even to destroy the spatial network of water molecules. When alcohol molecules penetrate the spatial network of water molecules, rearrangement of water structure takes place, as alcohol molecules do not fit the size of cavities formed by water molecules within the ice-like network. Penetration of ethanol molecules into the network of water molecules takes place in such a way that polar hydroxyl OH groups of ethanol displace water molecules from the network, and more extensive hydrophobic ethylic groups enter the cavities formed by water molecules. Hydrophobic interactions of ethanol and water do not cause significant changes in the positions and bandwidths of low-frequency vibrational lines of water-ethanol solutions. Hydroxyl groups of ethanol and OH groups of water molecules interact via Hydrogen bonding, causing changes in the shape of stretching bands of OH groups. In this process, ice-like network of water molecules is transformed into dodecahedric one, with larger cavities among the nodes. In the other words, at ethanol concentrations 10-20 % w/w alcohol and water molecules form a "guest-host" clathratelike structure, which properties essentially differ from that of both pure solvents. Such clathrate-like structures in alcohol solutions attract attention of researchers nowadays and present particular interest of further studies.
4. CONCLUSIONS The results presented in this paper prove the fact of essential structural rearrangement taking place in water-ethanol solutions at concentrations 15-20 % w/w, accompanied by Hydrogen bonds strengthening, and the fact of clathrate-like structures being formed in solutions. Analysis of behaviour of quantitative characteristics of OH stretching band in Raman and IR spectra along with increasing ethanol concentration showed that Hydrogen bonding in solutions at ethanol concentration around 15 % w/w is strengthened compared to that of pure water. At concentrations exceeding 15-20 % w/w the strengthening of Hydrogen bonding is continuously decreasing towards concentrated ethanol solutions. Component analysis using MCR-ALS method applied to CH and OH stretching bands of Raman spectra revealed four components in the solutions: pure water, water-rich hydrate EtOH·5H2O, ethanol-rich hydrate EtOH·1÷2H2O and pure ethanol. The increased intensity around 3200 cm-1 in the contour of MCR-resolved component (namely, water-rich hydrates) also confirms that H-bonding in this hydrate is more strong that in pure water. This result corroborates Hydrogen bonds strengthening in water-ethanol solutions at specific ethanol concentration, as well as formation of "guest-host" clathrate-like structures as well.
5. ACKNOWLEDGEMENTS The authors thank Prof. Dale W. Schaefer and Dr. Naiping Hu (Department of Chemical and Materials Engineering, University of Cincinnati, USA) for performed MCR-ALS analysis and helpful discussion of results. The authors thank
the OVAL Ltd Company for financial support.
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