Yujing Chenab, Yukihiro Ozakib and Mirosław A. Czarnecki*c
aSchool of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
bSchool of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
cFaculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland. E-mail: miroslaw.czarnecki@chem.uni.wroc.pl; Fax: +48-71-3282348
First published on 27th September 2013
The molecular structure and hydrogen bonding of ethylene glycol (EG) and EG–water mixtures in the liquid phase were studied by using near-infrared (NIR) spectroscopy. The spectra were evaluated using a two-dimensional (2D) correlation approach, moving-window 2D correlation analysis and chemometric methods. The minor changes for the CH stretching bands indicate that the structures of pure liquid EG and EG–water mixtures are determined by the intermolecular hydrogen bonding through the OH groups. The analysis of the ν2 + ν3 combination band of water reveals that in EG-rich solutions the molecules of water are predominantly bonded with two molecules of EG and this cooperative hydrogen bonding is stronger than that in bulk water. Further increase in the water content leads to formation of small water clusters around OH groups of EG. Comparing results for the binary mixtures of water with different organic solvents one can conclude that the total amount and distribution of the polar groups are the most important factors determining the solubility of water in the organic phase. The distribution of these groups depends on the length and structure of the hydrocarbon chain. Due to high population and relatively uniform distribution of the OH groups of EG water has unlimited solubility in liquid EG.
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Fig. 1 Optimized structures of EG with (a) and without (b) intramolecular hydrogen bonding obtained from DFT calculations. |
In contrast, vapour-phase overtone spectra of EG are dominated by two conformers that have weak intramolecular hydrogen bonding with red-shifts of 30 and 45 cm−1.7Ab initio conformational analysis shows that the lowest energy conformers of EG have gauche OCCO arrangements and are stabilized by intramolecular hydrogen bonding.8–11 However, in the lowest energy 1:
1 complex with water this bonding is disrupted, and EG forms two intermolecular hydrogen bonds of equal strength.10,11 DFT studies coupled with many-body analysis in EG–(H2O)n (n = 1–3) complexes evidence that the structure in which a water molecule bridges two OH groups of EG is more stable than the other structures.12 The quantum chemical conformational analysis reveals that the population of EG molecules with no intramolecular hydrogen bonding increases from 17 to 25% upon solvation.13 In contrast, an ATR-IR spectroscopic study of neat EG proves that the entire population of OH groups is involved in the inter- and intramolecular hydrogen bonding.14 The temperature raise leads to an increase in the population of the dimeric species at the expense of the species with the intramolecular hydrogen bonding. Recent ab initio and DFT studies have shown that EG–water interaction is predominant over the EG–EG and water–water interactions.15 It has been found that the second molecule of water does not create a hydrogen bond with the first molecule of water, but breaks a hydrogen bond between two EG molecules. The second hydrogen bond in the EG dimer remains intact. The water–water interaction occurs after addition of three or more molecules of water.
Though the presence of the intramolecular hydrogen bond in vicinal diols has not been convincingly evidenced as yet, this bond plays an important role in the stabilization of the structure of 1,3- and 1,4-diols.16,17 Our recent studies did not reveal the presence of the intramolecular hydrogen bond in diluted CCl4 solutions of 1,2-propanediol (12PD), while it was clearly manifested in the spectra of 1,3-propanediol (13PD) solutions.18 An increase in the concentration of 13PD shifts the equilibrium towards the formation of the intermolecular hydrogen bonding at the expense of the intramolecular one. The addition of water leads to faster thermal disruption of the associates of propanediols in the liquid phase. This means that the creation of hydrogen bonding between diol and water weakens the diol–diol interactions, as observed for EG–water complexes.15 The optimized structures of EG–water complexes shown in ref. 15 (Fig. 2) have at least one free OH group originating from the singly-bonded water. On the other hand, in propanediol–water mixtures these groups were not observed.18 The presence or absence of the free OH groups is of great importance for elucidation of the correct structure of the mixtures. NIR spectroscopy is a particularly useful tool for examination of the state of water since the ν2 + ν3 band appears in the region free from absorption of the other functional groups.19,20 The absorptivities of NIR bands are much weaker than those in the IR spectra, and more convenient path lengths can be used. Besides, the bands originating from different hydrogen-bonded species are better separated in the overtone region.21–23
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Fig. 2 NIR spectra of neat EG at 10 °C (blue) and 90 °C (red) together with the second derivative spectra at 10 °C (green) and 90 °C (magenta). To appear in this scale, the second derivative spectra were multiplied by 2000. |
Here we report studies on the effect of temperature and concentration on molecular structure and hydrogen bonding of EG and EG–water binary mixtures in the liquid phase. Particular attention was paid to the state of water in the mixture and the effect of water content on the structure of EG–water mixture. The structure of the liquid phase as well as the interaction between water and solvent determine the solubility of water in the organic phase. Hence, the comparison of the results for binary mixtures of water with various organic solvents is expected to provide new information on the relationship between the solubility and the chemical structure of the organic solvent. To enhance the spectral resolution and obtain more detailed information from NIR spectra we applied the generalized 2D correlation analysis,24–26 the moving window approach27 and chemometric methods like principal component analysis (PCA),28 evolving factor analysis (EFA)29 and multivariate curve resolution-alternating least squares (MCR-ALS) analysis.30
The moving-window 2D correlation analysis was performed following the method proposed by Thomas and Richardson,27 and the window length was set to three spectra. This method is based on the partitioning of the entire data set into smaller subsets (windows) and sequentially calculating 2D correlation spectra for each of the data subsets. Next, the diagonal of the synchronous spectra (power spectrum) are plotted versus the average temperature of the window. The power spectrum represents the overall extent of intensity changes at individual wavenumbers. In this way the moving-window 2D correlation analysis yields detailed information on the spectral changes at particular wavenumbers as a function of the temperature.
Vibration | Position [cm−1] | Assignment |
---|---|---|
Bulk EG at 10/90 °C | ||
2ν(OH) | 6285/6339 | EG–EG, strongly bonded |
2ν(OH) | −/6870 | EG–EG, weakly bonded |
2ν(OH) | 7022/7030 | Free |
EG–water mixture (X = 0.5) at 10/90 °C | ||
ν2 + ν3 | 5164/5207 | EG–H2O–EG + H2O–H2O, H2O–H2O, bonded |
ν1 + ν3 | 6846/6981 | EG–H2O–EG + H2O–H2O, H2O–H2O, bonded |
EG–water mixture with a water content of 5%/50%/95% at 25 °C | ||
ν2 + ν3 | 5149/5166/5182 | EG–H2O–EG, H2O–H2O, bonded |
2ν(OH) | 6273/6273/6275 | EG–EG, bonded |
ν1 + ν3 | −/6850/6881 | EG–H2O–EG, H2O–H2O, bonded |
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Fig. 3 Asynchronous 2D correlation spectrum of neat EG from 10 to 90 °C. Positive peaks were shaded. |
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Fig. 4 Moving-window 2D correlation spectrum of neat EG from 10 to 90 °C. |
Fig. 5 shows the NIR spectra of the EG–water equimolar mixture and bulk water at 10 and 90 °C. The temperature rise shifts the position of the ν2 + ν3 band of water in the mixture from 5164 to 5207 cm–1, while the corresponding band of bulk water is shifted from 5175 to 5228 cm–1. A similar trend was observed for the ν1 + ν3 band of water in the mixture that shifts from 6850 cm–1 to 6981 cm–1 upon temperature rise from 10 to 90 °C. For comparison, in bulk water this band shifts from 6869 to 7040 cm–1. Clearly, in this temperature range the molecules of water in the equimolar mixture are involved in stronger hydrogen bonding as compared with those in bulk water, and this difference is more distinct at higher temperatures. This means that water–water hydrogen bonding weakens faster than that of EG–water upon temperature rise. The results obtained for the mixtures of water with propanediols18 and aliphatic alcohols20,36 also reveal that the interaction between water and diol or alcohol is stronger than that of water–water. The ν2 + ν3 band of water has a high frequency component that was resolved in the second derivative spectra (not shown). At elevated temperatures this band has the same position (5255 cm–1) both in the mixture and in bulk water, suggesting that the temperature increase leads to the formation of similar species in both systems. From the position of this peak one can conclude that the water–water interactions within these clusters are relatively weak. The moving window 2D correlation analysis (Fig. 6) reveals that the high frequency band of water is only slightly shifted with the temperature rise, whereas the low frequency band reveals a more distinct shift. Besides, the extent of spectral changes for strongly bonded species increases with the temperature, whereas for weakly bonded water one can observe the opposite tendency. This means that the thermal disruption of strongly bonded species appears more easily at higher temperatures. In contrast, the population of weakly hydrogen bonded species increases predominantly at lower to moderate temperatures.
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Fig. 5 NIR spectra of water in EG–water equimolar mixture at 10 °C (blue) and 90 °C (red) together with the spectra of bulk water at 10 °C (magenta) and 90 °C (green). The spectra of bulk water were normalized against the intensities of the spectra of water in the mixture. |
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Fig. 6 Moving-window 2D correlation spectrum of EG–water equimolar mixture from 10 to 90 °C. |
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Fig. 7 NIR spectra of EG–water mixture from pure EG to pure water with a step of 5% (weight) at 25 °C. The arrows indicate the direction of increasing water content. |
The second derivative spectrum (Fig. 8a) reveals that the ν2 + ν3 band has two components. The position of the higher frequency band (5253 cm–1) does not vary with the water content, and is the same as that in the second derivative spectrum of bulk water. In contrast, the low frequency band is blue-shifted from 5162 to 5191 cm–1 (Δν = 29 cm–1). Also in the asynchronous spectrum constructed from the spectra of the mixtures (Fig. 8b) the ν2 + ν3 band is resolved into two components located at 5149 and 5255 cm–1. As can be seen (Fig. 8b), similar splitting occurs for the ν1 + ν3 band. The lower frequency band was assigned to water involved in the cooperative interaction with EG, while the other bands result from water–water interactions. Hence, it is clear from Fig. 8a that increasing water content leads to growth in the population of the clusters of water, partially at the expense of the EG–water species.
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Fig. 8 Normalized (against water content) second derivative spectra of EG–water mixtures at a water content of 25% (blue), 50% (magenta), 75% (green) and 100% (red) (a). The asynchronous 2D correlation spectrum (b) constructed from the spectra of EG–water mixtures shown in Fig. 7. Positive asynchronous peaks were shaded. |
PCA and EFA analysis (not shown) suggest the presence of three independent components contributing to the concentration-dependent changes in the spectra of EG–water mixtures. Fig. 9 displays the concentration and spectral profiles obtained from MCR-ALS. For this purpose we used the als function from PLS-Toolbox and the initial concentration profiles were estimated from EFA (evolvfa function). The spectral component decreasing in intensity is very similar to the spectrum of pure EG, and was assigned to EG. This similarity confirms our earlier conclusion that addition of water has a small effect on the structure of organic phase.18,20,36 Recently, analogous result was obtained from neutron diffraction studies of glycerol–water mixtures.37 It was shown that glycerol–glycerol hydrogen bonding was largely unperturbed by the presence of water in the mixture. The spectral profile of the component increasing in intensity (Fig. 9a) is the same as that of the spectrum of bulk water. Therefore, this component was assigned to water involved in water–water interactions. The third component at first increases in intensity up to about 50%, and then decreases to zero intensity in pure water (Fig. 9a). The ν2 + ν3 combination band in this spectral profile (Fig. 9b) is 22 cm–1 red-shifted as compared with the corresponding band in the spectrum of bulk water evidencing that this component participates in stronger interactions than those in bulk water. Hence, this spectral profile was assigned to water engaged in interaction with EG. The spectral changes for both components of water are unlike, giving rise to the asynchronicity observed in Fig. 8b.
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Fig. 9 Concentration profiles of water (blue, green) and EG (red) (a) together with the corresponding spectral profiles of water (blue, green) and EG (red) (b) obtained from MCR-ALS of the concentration-dependent spectra of EG–water mixture at 25 °C. |
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Fig. 10 Optimized structures of 4![]() ![]() ![]() ![]() |
A MD study has shown that the structure of liquid EG is dominated by three-dimensional networks of hydrogen bonded molecules.39 This results from the fact that each molecule of EG can act as a double proton donor and a double proton acceptor, forming up to four hydrogen bonds. The presence of the three-dimensional structure in liquid EG leads to relatively homogeneous distribution of the OH groups through the entire volume. This forces a homogeneous distribution of water forming small clusters around OH groups of EG. As a result, water is fully miscible with EG. The size of water clusters depends on the concentration of EG: the higher EG concentration the smaller the water clusters. This conclusion is supported by a MD study of glycerol–water liquid mixtures showing that the increase of glycerol content causes reduction in the size of water clusters.40
Dielectric measurements of the short chain alcohols, from methanol to propanol, demonstrated the preference for formation of long chain linear associates in the liquid phase.41 This causes less homogeneous distribution of the OH groups as compared with liquid EG, and the molecules of water may form larger clusters. The water–water-interaction in these clusters is stronger as compared with that in the EG–water mixture, and hence the ν2 + ν3 band of water in alcohols occurs near 5200 cm–1.20,36 For comparison, the corresponding band in bulk water is red-shifted only by 16 cm–1. Still the alcohol–water interaction is strong enough to prevent the phase separation. The interaction of the hydrophobic parts has a minor impact on the structure of short chain alcohols in the liquid phase. A neutron diffraction study of 7:
3 mole ratio methanol–water solution has shown that the properties of this mixture are influenced mainly by the polar interaction of water with the OH group of methanol.42 In longer chain alcohols (n > 3) there is significant increase of the hydrophobic interactions, and the tendency for creation of the micelle-like associates.43 As a result, the distribution of the OH groups in the liquid phase is not uniform, and the interaction between molecules of water and these associates is less favourable. Thus, the molecules of water tend to form large clusters, and the alcohol–water interaction is too weak to prevent the phase separation. Branching in the vicinity of the OH group reduces the extent of self-association of alcohol. The associates of these alcohols are smaller and the OH groups are easier to access for interaction with water. Besides, the interaction between the hydrophobic parts of tert-butyl alcohol is weaker than that in n-butyl alcohol. As a result, the solubility of water in tert-butyl alcohol is unlimited, whereas in n-butanol it is limited. It seems that the free OH groups of alcohols or other solvents do not decide on the extent of solubility of water in the organic phase. Also the presence or absence of the intramolecular bonding has a minor impact on the structure of the condensed phase, as concluded from MD studies.37 Comparing the present results with those previously obtained for binary mixtures of water with different organic solvents20,36 one can conclude that the solubility of water in the organic phase is determined primarily by the total number and distribution of the polar groups. In turn, the distribution of these groups in the organic phase depends on the size and structure of the hydrocarbon chain. This suggestion was confirmed using 17O NMR studies of dynamic hydration of diols and alcohols in aqueous solutions.44
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