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J. Phys. Chem. B 2006, 110, 12372-12379
Thermal Properties and Mixing State of Ethylene Glycol-Water Binary Solutions by Calorimetry, Large-Angle X-ray Scattering, and Small-Angle Neutron Scattering Masaru Matsugami,† Toshiyuki Takamuku,*,‡ Toshiya Otomo,§ and Toshio Yamaguchi| Department of Functional Molecular Science, The Graduate UniVersity for AdVanced Studies, Myodaiji, Okazaki 444-8585, Japan, Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga 840-8502, Japan, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Oho, Tsukuba 305-0801, Japan, and AdVanced Materials Institute and Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed: March 9, 2006; In Final Form: April 29, 2006
Thermal properties and mixing states of ethylene glycol (EG)-water binary solutions in the entire mole fraction range of EG, 0 e xEG e 1, have been clarified by using differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), and small-angle neutron scattering (SANS) techniques. The DSC curves obtained have shown that the EG-water solutions over the range of EG mole fraction 0.3 e xEG e 0.5 are kept in the supercooling state until ∼100 K, and those in the range of 0.6 e xEG e 0.8 are vitrified, and those in the ranges of 0 < xEG e 0.2 and 0.9 e xEG < 1 are crystallized. The radial distribution function (RDF) for pure EG obtained from the LAXS measurements has suggested that a gauche conformation of an EG molecule is favorable in the liquid. The RDFs for the EG-water solutions have shown that the structure of the binary solutions moderately changes from the inherent structure of EG to the tetrahedral-like structure of water when the water content increases. The SANS intensities for deuterated ethylene glycol (HOCD2CD2OH) (EGd4)-water solutions at xEG ) 0.4 and 0.6 have not been significantly observed in the temperature range from 298 to 173 K, showing that EG and water molecules are homogeneously mixed. On the other hand, the SANS intensities at xEG ) 0.2 and 0.9 have been strengthened when the temperature decreases due to crystallization of the solutions. On the basis of all the present results, a relation between thermal properties of EG-water binary solutions and their mixing states clarified by the LAXS and SANS measurements has been discussed at the molecular level.
Introduction Ethylene glycol (EG) is mixed with water at any ratio, and addition of EG to water and aqueous solutions can easily prevent crystallization with decreasing temperature. Hence, EG is often used as an inhibitor for freezing of water in industry. In the field of biology, the vitrifiability of EG is applied to cryopreservation of biological cells and tissues without significant damage. On the other hand, physicochemical properties of EG and aqueous solutions of EG have been investigated by many researchers with various techniques. In particular, there have been numerous reports on thermodynamic properties of EG-water solutions, such as excess molar volumes1-3 and enthalpies.4-6 The physicochemical properties of EG-water solutions should arise from the microscopic mixing state of EG and water molecules. A possible conformation of an EG molecule has been investigated by using computer calculations, such as ab initio.7-10 Most of the investigations have shown an intramolecular hydrogen bond between the hydroxyl groups of an EG molecule, i.e., a gauche OC-CO conformation is stabilized by hydrogen bonding. Bako´ et al. have made X-ray and neutron scattering * To whom all correspondence should be addressed. E-mail: takamut@ cc.saga-u.ac.jp. † The Graduate University for Advanced Studies. ‡ Saga University. § High Energy Accelerator Research Organization (KEK). | Fukuoka University.
measurements on pure EG and treated their data by using a Monte Carlo method to clarify the liquid structure.11 They have also examined conformation of the EG molecule in gas phase by using density-functional theory (DFT). Both X-ray and neutron scattering on liquid EG and the theoretical method on EG in gas phase have reached the same conclusion that an intramolecular hydrogen bond is formed in an EG molecule with a gauche OC-CO conformation. Moreover, a plausible structure of dimer EG by hydrogen bonding has been proposed from the DFT results. Many molecular dynamics (MD) investigations have also been made on pure EG.12-15 The X-ray and neutron scattering measurements by Bako´ et al.11 and the MD simulations14,15 on pure EG showed that an EG molecule is hydrogen bonded with three to four neighbor molecules. For EG-water binary solutions, NMR measurements have been made on EG-water solutions to determine the self-diffusion coefficients of both EG and water molecules16 and the rotational correlation time of H217O.17 It has been indicated that the dynamic hydration number (5.7) of EG, which is estimated from the correlation time determined, is comparable with that (5.8) for methanol. There have been several reports of MD simulation on EG-water solutions to elucidate their structure.18-20 Despite these efforts, the structure of EG-water solutions has not yet been clarified at the molecular level by using X-ray and neutron scattering techniques. In the present investigation, to clarify the relation between thermal properties of EG-water binary solutions and their
10.1021/jp061456r CCC: $33.50 © 2006 American Chemical Society Published on Web 06/03/2006
Mixing State of Ethylene Glycol-Water Solutions mixing state at the molecular level, differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), and small-angle neutron scattering (SANS) measurements have been made on the solutions over the entire EG mole fraction range of 0 e xEG e 1. First, the thermal properties of EG-water solutions in the range of 0 < xEG < 1 have been examined by DSC experiments. In addition, DSC measurements have been done on aqueous solutions of deuterated ethylene glycol (EGd4) in the entire mole fraction range of EGd4. Then, structures of pure EG and EG-water solutions in the range of 0 < xEG < 1 at 298 K have been elucidated by using a LAXS technique. In accordance with the results from the DSC experiments, SANS measurements at 173, 188, 208, 228, and 298 K have been made on EGd4-water solutions at xEGd4 ) 0.2, 0.4, 0.6, and 0.9 to clarify a change in concentration fluctuation of the solutions with decreasing temperature at a mesoscopic scale. Finally, a relation between thermal properties of EG-water binary solutions in the entire mole fraction range and mixing states of the solutions clarified from both LAXS and SANS measurements is discussed at the molecular level. Experimental Section Sample Solutions. Dehydrated ethylene glycol (Wako Pure Chemicals, grade for organic synthesis) was used without further purification for DSC and LAXS experiments. Doubly distilled water was used for all experiments. EGd4 (CDN Isotopes, D atom content of 99.1%) was used without further purification for DSC and SANS measurements. Sample solutions were prepared by weighing EG or EGd4 and distilled water to required mole fractions of ethylene glycol. DSC Measurements. DSC measurements were made on both EG-water and EGd4-water binary solutions in the mole fraction range of 0 < xEG < 1 by using a differential scanning calorimeter (SEIKO Instruments Inc., DSC220CU). First, cooling DSC curves for the sample solutions were recorded in the temperature range from 298 to 153 K, and then heating curves were measured up to 298 K. Both cooling and heating rates were controlled at 2 K/min. LAXS Measurements. LAXS measurements at 298 K were made on pure EG and EG-water solutions at xEG ) 0.01, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9. A rapid liquid X-ray diffractometer (BRUKER AXS, model DIP301) with an imaging plate (IP) (Fuji Film Co.) as a two-dimensional detector was used in the present LAXS experiments. Details of the X-ray diffractometer have been described elsewhere.21,22 Densities of the sample solutions were measured at 298 K by using a densimeter (ANTON Paar K.G., DMA60). X-rays were generated at a rotary Mo anode (Rigaku, RU-300) operated at 50 kV and 200 mA and then monochromatized by a flat graphite crystal to obtain Mo KR radiation (the wavelength λ ) 0.7107 Å). X-ray scattering intensities for a sample solution sealed in a glass capillary of 2 mm inner diameter (wall thickness 0.01 mm) were accumulated on the IP for 1 h. The observed range of the scattering angle (2θ) was 0.2° to 107°, corresponding to the scattering vector s ()4πλ-1 sin θ) of 0.03-14.2 Å-1. X-ray intensities for an empty capillary were also measured as background. Two-dimensional X-ray data, Iobsd(x,y), where x and y are horizontal and vertical coordinates, measured on the IP were integrated into one-dimensional data, Iobsd(θ), after correction for polarization as previously reported.21 The observed intensities for the samples and empty capillary were also corrected for absorption.22 The contribution of the sample solution alone was obtained by subtracting the intensities for the empty capillary
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12373 from those for the sample. The corrected intensities were normalized to absolute units by conventional methods.23-25 The structure function, i(s), was calculated by using eq 1 in ref 26. In the data treatment the stoichiometric volume V was chosen to contain one O atom from both EG and water in the solutions. The structure function was Fourier transformed into the radial distribution function, D(r), in a usual manner.26 To quantitatively analyze the X-ray data, a comparison between the experimental structure function and the theoretical one, which was calculated on a structure model for the short-range interactions with the first term of eq 5 in ref 26, was made by a least-squares refinement procedure by using eq 4 in ref 26. The present X-ray diffraction data were treated by programs KURVLR27 and NLPLSQ.28 SANS Measurements. SANS measurements at 173, 188, 208, 228, and 298 K were made on EGd4-water solutions at xEGd4 ) 0.2, 0.4, 0.6, and 0.9 by using SWAN spectrometer at a pulsed neutron facilities, KENS, of High Energy Accelerator Research Organization, KEK, Tsukuba, Japan. The momentum transfer Q () 4πλ-1sin θ) range covered with SWAN was 0.01-12 Å-1.29 The sample solutions were kept in a cell made of a Ti-Zr null matrix alloy with 26 mm in width, 28 mm in height, and 3 mm in a sample thickness with an indium-wire packing. The temperature of sample solution was controlled within (0.1 K on a cryostat. The neutron beam size at a sample position was 20 × 20 mm2, and the wavelength range used was λ ) 0.5-11.6 Å. The scattering intensities were accumulated for 3-4 h per sample. The observed intensities were corrected for background, absorption, and cell scattering. The transmission by a sample and a cell was measured with a 3He position sensitive detector placed at a beam stopper position. The correction for detector efficiencies and normalization to absolute units were made by dividing the intensities of each solution by those of vanadium.29 In the present investigation, however, the incoherent scattering for the sample solutions was not corrected because their densities at the low temperatures investigated, which were necessary to determine concentrations of atoms, could not be measured due to the limitation of the densimeter described above. All parameter values required for the above corrections were taken from the literature.30 Results and Discussion Thermal Properties. Figure 1 shows DSC curves for EG-water (solid lines) binary solutions at xEG ) 0.2, 0.4, 0.6, and 0.9. To examine an effect of deuteration for EG on the thermal properties, those of EGd4-water solutions (dashed lines) at the same mole fractions were also measured for SANS experiments on EGd4-water mixtures. The features of the DSC curves for undeuterated and deuterated solutions are not significantly different from each other, except that exo- and endothermic peaks observed for the deuterated solutions shift to the lower temperature range. Hereafter, exo- and endothermic peaks are characterized by the temperature at which the DSC curve begins to rise or fall with varying temperature. In the cooling curve for the EG-water solution at xEG ) 0.2 an exothermic peak at 215 K indicates freezing of the solution; the heat of freezing per gram of the solution was estimated to be 43.25 J/g from the area of the exothermic peak. This is much smaller than the sum (253.40 J/g) of each heat of freezing for EG and water if EG and water are individually frozen from the solution at xEG ) 0.2. It is suggested that peritectic or eutectic crystals of EG and water are formed from the solution. In the heating curve for the EG-water solution at xEG ) 0.2 both small and large endothermic peaks are observed, showing that the
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Figure 1. DSC curves for the EG-water (solid lines) and EGd4water (dashed lines) solutions at xEG ) 0.2, 0.4, 0.6, and 0.9. The intensity-enhanced cooling curve is inserted in the DSC curves for EG-water solution at xEG ) 0.6. DSC curves for both solutions at xEG ) 0.9 are shifted from the origin and factored by the values in parentheses, respectively, to avoid overlap of the plots.
Figure 2. Structure functions i(s) multiplied by s for EG, water,33 and EG-water solutions at various EG mole fractions. The dotted and solid lines represent experimental and theoretical ones. The latter is omitted in a range of s/Å-1 e 2.0 for clarity because the corresponding interactions were not taken into account in the present analysis.
peritectic or eutectic crystal melts in two steps. In the cooling curve for the EGd4-water solution at xEG ) 0.2 an exothermic peak is observed at a lower temperature (203 K) than that observed for the undeuterated solution. In the heating curve for the solution, only an endothermic peak appears. A similar DSC curve was observed for EG-water and EGd4-water solutions at xEG ) 0.1. In the DSC curves for EG-water and EGd4water solutions at xEG ) 0.4, no peaks are observed in the temperature range investigated. This shows that EG-water and EGd4-water solutions at xEG ) 0.4 are kept in the supercooling state until ∼170 and ∼150 K, respectively. The same behavior was observed for EG-water and EGd4-water solutions at xEG ) 0.3 and 0.5 until ∼100 K, i.e., EG-water and EGd4-water solutions in the range of 0.3 e xEG e 0.5 are kept in the supercooling state down to ∼100 K. In the intensity-enhanced cooling curve for the EG-water solution at xEG ) 0.6, a small kink is observed at 222 K. In the heating curve, on the other hand, a large exothermic peak appears, followed by a large endothermic peak. These features are typical behavior for glass transition of a solution; the solution is vitrified at 222 K in the cooling process, whereas the solution is crystallized in the heating process because EG and water molecules are reorientated at lattices with increasing temperature, but the crystal rapidly melts with further increasing temperature. Although a small kink arising from glass transition is not observed in the cooling curve for the EGd4-water solution at xEG ) 0.6, the heating curve for the solution bears resemblance with that for the undeuterated solution. Hence, the deuterated solutions may be vitrified with cooling. In the range of 0.6 e xEG e 0.8, similar DSC curves were observed in the present experiments. Thus, the EG-water and EGd4-water solutions in this xEG range are vitrified with cooling. For both undeuterated and deuterated EG-water solutions at xEG ) 0.9, a very sharp and large exothermic peak appears at ∼217 K in the cooling curves. In addition, a small exothermic peak is observed in the cooling curves for the undeuterated and deuterated EG-water solutions at xEG ) 0.9 when the temperature is 199 and 190 K, respectively. On the other hand, small and large endothermic peaks are observed in the heating curve for both solutions. These findings suggest that EG and water are individually frozen and fused in the solutions; the pair of large exothermic peaks in the cooling curve and the endothermic one in the heating curve show freezing and melting of EG, while
the small peaks arise from freezing and melting of water. In fact, the heats (8.82 and 5.72 kJ/mol, respectively) of freezing estimated from the large and small exothermic peaks are comparable with those (9.958 and 6.0078 kJ/mol, respectively) of freezing of pure EG and pure water.31 The present DSC experiments show that thermal properties of EG-water solutions can be classified into four regimes: the range of 0 < xEG e 0.2 where peritectic or eutectic crystal is formed from the solutions, the range of 0.3 e xEG e 0.5 where the solutions are kept in the supercooling state until ∼100 K, the range of 0.6 e xEG e 0.8 where the solutions are vitrified, and the range of 0.9 e xEG < 1 where EG and water are individually frozen from the solutions. The present results are comparable with a solid-liquid phase diagram of the ethylene glycol-water system previously reported;32 both present and previous results show existence of metastable solutions in the range of 0.3 e xEG e 0.5. LAXS Measurements. Figure 2 shows the structure function i(s) weighted by s for pure EG (xEG ) 1) and the EG-water solutions in the range of 0.01 e xEG e 0.9 at 298 K, together with that for pure water (xEG ) 0) measured in the previous investigation33 for comparison. Figure 3 indicates the corresponding RDFs in the D(r)-4πr2F0 form. In the RDF for pure EG, a large peak at 1.5 Å is attributed to the intramolecular interactions within an EG molecule, such as C-O (1.42 Å) and C-C (1.52 Å) bonds. A peak at 2.5 Å is assigned mainly to nonbonding C‚‚‚O interactions of the EG molecule. A shoulder at 2.8 Å arises from O‚‚‚O hydrogen bonds between EG molecules. Two large and broad peaks centered at 4.8 and 9 Å are assigned to intermolecular interactions between EG molecules; the former arises from the first and second neighbor interactions, whereas the latter is attributed to the third neighbor ones. The peaks at 2.8, 4.8, and 9 Å suggest ordering of EG molecules by hydrogen bonding as well as monohydroxyl alcohols such as methanol and ethanol.33-36 In the liquid various conformations for an EG molecule would be possible because of free rotation of the C-C bond. Many ab initio investigations revealed that a gauche OC-CO conformation for the EG molecule is favorable due to an intramolecular hydrogen bond between the hydrogen atom and the oxygen lone pair of the two hydroxyl groups.7-10 The structure parameters of an EG molecule in a gauche conformation were already reported in the previous X-ray and neutron scattering
Mixing State of Ethylene Glycol-Water Solutions
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12375 TABLE 1: Intramolecular Interactions for the EG Molecule in Gauche Conformation and Water Moleculea interaction
Figure 3. Radial distribution functions in the D(r)-4πr2F0 form for EG, water,33 and EG-water solutions at various EG mole fractions. The solid lines represent experimental values (original RDFs), the dashed lines are intermolecular RDFs (IRDFs) obtained by subtraction of intramolecular interactions within EG and water molecules from the original RDFs, and the dash dotted lines are residual curves after subtraction of theoretical values calculated by using optimized parameter values (Table 2) from the IRDFs.
r
103b
EG in Gauche Conformation O-H 0.91 2 C-H 1.11 1 C-O 1.42 1 C1-C2 1.52 1 C1,2‚‚‚H(O1,2) 1.94 7 O1,2‚‚‚H(C1,2) 1.95 7 H(O1,2)‚‚‚H(C1,2) 2.00 8 O1‚‚‚O2 2.99 12 C1,2‚‚‚O2,1 2.45 5 C1,2‚‚‚H(C2,1) 2.01 7 C2‚‚‚H(O1) 2.55 8 O1‚‚‚H(C2) 2.77 7 H(C1)‚‚‚H(C2) 2.24 5 H(C1)‚‚‚H(C2) 2.33 8 H(C1)‚‚‚H(C2) 2.85 8 H(O1)‚‚‚H(O2) 3.23 8 O-H H‚‚‚H
Waterb 0.970 1.555
2 10
n 2 4 2 1 2 4 2 1 2 4 1 1 1 1 1 1 2 1
a The distance r (Å), temperature factor b (Å2), and number n, whose uncertainties are (0.01 Å, (1 × 10-3 Å2, and null,33 respectively. b Reference 37.
Figure 5. Structure model of the EG chain by hydrogen bonding. The dotted lines represent hydrogen bonds. Figure 4. The results of model fitting by trans, cis, and gauche conformations of EG molecules in (a) s- and (b) r-spaces. In the s-space the dotted and solid lines give experimental and theoretical curves, respectively. The latter is omitted in a range of s/Å-1 e 2.0 for clarity. In the r-space the solid and dotted lines represent experimental and theoretical curves, and the dash dotted curves represent residual values obtained by subtracting the theoretical ones from the experimental ones.
investigation.11 Thus, to confirm the reliability of the present result, three models for trans, cis, and gauche OC-CO conformations were examined to fit the data in both s- and r-spaces. In the analysis, O-H and C-H interactions were also considered, and the structure parameters of the intermolecular O‚‚‚O hydrogen bonds were fixed: as distance r, 2.717 Å; temperature factor b, 10 × 10-3 Å2; number of hydrogen bonds n, 1.03. However, the structure parameters of the intramolecular interactions within an EG molecule were not optimized in a least-squares refinement procedure. Parts a and b of Figure 4 show the results of the model fitting by trans, cis, and gauche conformations for s- and r-spaces, respectively. Figure 4 a reveals that in the s-space the theoretical values (solid line) for the trans conformation can well explain the observed ones (dotted line), and those for the gauche conformation satisfactorily reproduce the observed ones. However, it is clearly shown that those for the cis conformation cannot reproduce the observed ones. In the r-space (Figure 4 b), the differences among the models for the three conformations
are clearer. The theoretical values for the gauche conformation best explain the observed ones among the three conformations because the residual curve (dash dotted line) is smooth in the r-range from 0 to 3.5 Å. On the other hand, a peak at 3.0 Å remains in the residual curve for the trans conformation, and a deep valley at 2.5 Å in the curve for cis conformation indicates excess intramolecular interactions. These features depend mainly on the distance of O‚‚‚O interaction within an EG molecule, e.g., 3.63 Å (trans) and 2.44 Å (cis). Therefore, the present analysis suggests that the gauche conformation of EG molecule would be most favorable in the liquid due probably to the intramolecular hydrogen bond as concluded in the previous investigations.7-11 The structure parameters of the gauche conformation are listed in Table 1 (the notation of atoms are indicated in Figure 5). In the gauche conformation, the distance of the nonbonding O‚‚‚O interaction was estimated to be 2.99 Å, which gives the dihedral angle OC-CO of 81°. The O‚‚‚O distance is comparable with those (2.86 and 2.82 Å, respectively) obtained for gGg′ and tGg′ conformations by the DFT.11 In Figure 3, the RDFs for the EG-water binary solutions reveal the change in the features with decreasing xEG (increasing water content) as follows. The intramolecular interactions at 1.5 and 2.5 Å decrease with the decrease in the EG content. On the other hand, the interactions of O‚‚‚O hydrogen bonds at 2.8 Å have gradually grown. The intermolecular interactions
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TABLE 2: All Optimized Parameter Values of the Interactions in Water, EG, and Their Mixtures Obtained by Least-Squares Fitsa xEG ) 0b
xEG ) 0.01
xEG ) 0.02
xEG ) 0.05
xEG ) 0.10
xEG ) 0.20
xEG ) 0.30
xEG ) 0.40
xEG ) 0.50
xEG ) 0.60
xEG ) 0.70
xEG ) 0.80
xEG ) 0.90
xEG ) 1
Linear Hydrogen Bond of Water-Water, EG-Water, and EG-EG O‚‚‚O r 2.826(2) 2.821(3) 2.818(3) 2.815(3) 2.808(3) 2.800(4) 2.791(4) 2.781(5) 2.772(5) 2.765(7) 2.749(7) 2.738(8) 2.722(10) 2.717(10) 103b 17 16 16 16 15.5 15 14.5 14 13.5 13 12.5 12 11 10 n 3.43(3) 3.46(5) 3.41(5) 3.34(5) 3.05(5) 2.72(5) 2.41(5) 2.12(5) 1.88(5) 1.71(6) 1.47(6) 1.34(6) 1.12(6) 1.03(6) O‚‚‚O r 3.35 103b 15 n 1.00
3.35 15 1.00
3.35 15 1.00
3.35 15 1.00
3.35 15 0.90
Interstitial Water Molecules 3.35 3.35 3.35 15 15 15 0.80 0.70 0.60
O‚‚‚O r 4.00 103b 90 n 3.0
4.00 90 3.0
4.00 90 3.0
4.00 90 3.0
4.00 90 2.4
Second Neighbor of Water-Water 4.00 4.00 4.00 90 90 90 2.0 1.7 1.0
O‚‚‚C r 103b n C‚‚‚O r 103b n C‚‚‚C r 103b n
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
First Neighbor of EG-EG and EG-Water 3.50 3.50 3.50 3.50 3.50 20 20 20 20 20 2.0 2.0 2.0 2.0 2.0 3.70 3.70 3.70 3.70 3.70 20 20 20 20 20 2.0 2.0 2.0 2.0 2.0 4.00 4.00 4.00 4.00 4.00 20 20 20 20 20 2.0 2.0 2.0 2.0 2.0
3.35 15 0.40
3.35 15 0.40
3.35 15 0.20
3.35 15 0.20
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
3.50 20 2.0 3.70 20 2.0 4.00 20 2.0
a The interatomic distance r (Å), the temperature factor b (Å2), and the number of interactions n. The values in parentheses are standard deviations of the last figure. The parameters without standard deviations were not allowed to vary in the calculations. b Reference 33.
between EG molecules centered at 4.8 and 9 Å are gradually weakened when xEG decreases, in particular, the 9 Å peak almost disappears at xEG ) 0.3. A new peak at 7 Å appears in the RDF for the mixture at xEG ) 0.2 and gradually grows with decreasing xEG. In the range of xEG < 0.1, the RDFs for the solutions are comparable with that for pure water; three peaks at 2.8, 4.5, and 7 Å, which are characteristic of the first, second, and third neighbor interactions in the tetrahedral-like structure of water by hydrogen bonding, are clearly observed in the RDFs. These changes in the RDFs for the EG-water solutions indicate that the structure of the binary solutions moderately changes from the inherent structure of EG to the tetrahedral-like structure of water when the water content increases. It is likely that EG molecules in the inherent structure of EG are gradually replaced with water because of the two hydroxyl groups for intermolecular hydrogen bonding and the small hydrophobic ethylene group of an EG molecule. To quantitatively clarify the structural change of the EGwater solutions with decreasing xEG, the structure parameters of intermolecular O‚‚‚O hydrogen bonds formed in the solutions were evaluated from the RDFs. As shown in Figure 3, first, the contribution of all the intramolecular interactions of EG molecule in the gauche conformation and water one,37 which are listed in Table 1, was subtracted from the original RDFs (solid lines) for the solutions to extract the intermolecular RDFs (IRDFs, dashed lines). In the IRDFs a peak at 2.8 Å is assigned to O‚‚‚O hydrogen bonds of EG-EG, EG-water, and waterwater molecules formed in the solutions. As seen, the 2.8 Å peak is gradually evolved with decreasing xEG, showing enhancement of O‚‚‚O hydrogen bonds in the solutions. Next, the peaks observed in the IRDFs in the range of 2 e r/Å e 4.5 were fitted by structure models of intermolecular interactions related to O‚‚‚O hydrogen bonds of EG-EG, EG-water, and water-water. In Figure 5, a plausible structure model of EG chain by hydrogen bonding is depicted with important interatomic distances. For intermolecular interactions among water molecules the tetrahedral-like pentamer model of water and the nonbonding molecule of interstitial water33 were adopted to fit the IRDF. The structure parameters of intermolecular interac-
tions between EG and water molecules by hydrogen bonding are the same as the interactions between the EG molecule and an oxygen atom of another EG in the hydrogen-bonded chain model. These structure parameters were slightly varied in a trialand-error manner to obtain a smooth residual curve in the range of 2 e r/Å e 4.5. Finally, a least-squares refinement procedure was employed on the structure function in the range of 4.9 e s/Å-1 e 14.2 by using the structure parameters modeled. The structure parameter values optimized are summarized in Table 2. Figure 2 shows that the theoretical values calculated by using the structure parameter values listed in Table 2 well reproduce the observed ones in the s-space, except for the range of s < 4.9 Å-1 not taken into account in the present analysis. As seen in Figure 3, smooth residual curves in the r-space over the range of 0 e r/Å e 4.5 are obtained by subtracting the theoretical peaks from the IRDFs, showing no other significant interactions in the r-range. The distance (2.717 ( 0.010 Å) of O‚‚‚O hydrogen bonds between EG molecules in pure EG is comparable with that (2.771 ( 0.004 Å) between methanol molecules.33 With decreasing xEG from 1 to 0, the O‚‚‚O hydrogen bonds are gradually elongated to that (2.826 ( 0.002 Å) for water.33 This tendency has been observed for aqueous monohydroxyl alcohol solutions, such as methanol,33 1-propanol,38 and 2-propanol,39 due to the increase in number of water molecules; the O‚‚‚O hydrogen bond among water molecules is longer than that between monohydroxyl alcohol molecules because a water molecule may be drawn by four hydrogen-bonded water molecules. The number (1.03 ( 0.06) of intermolecular O‚‚‚O hydrogen bonds per oxygen atom of an EG molecule for pure EG estimated from the present X-ray scattering measurement is less than that (1.5) of intermolecular O‚‚‚H hydrogen bonds determined from the previous neutron scattering one11 and the MD simulations.14,15 The present result may be influenced by overlap of the intramolecular O‚‚‚O hydrogen bonds due to its close distance. However, the number of intermolecular O‚‚‚O hydrogen bonds estimated from the present experiment is slightly beyond unity, implying that EG molecules form not only a chain structure by hydrogen bonding but also a three-
Mixing State of Ethylene Glycol-Water Solutions
Figure 6. Number of hydrogen bonds per molecule of alcohol and water for (a) EG-water, (b) methanol-water,33 and (c) ethanol-water36 solutions as a function of alcohol mole fraction. The standard deviations σ were indicated as error bars.
dimensional branched structure, as concluded from the previous investigations,11,14,15 because the number of unity for intermolecular O‚‚‚O hydrogen bonds means an infinite chain of EG molecules in the liquid if EG molecules only form chain structure by hydrogen bonding. In the above procedure, the numbers of O‚‚‚O hydrogen bonds for the EG-water binary solutions were estimated as a value per oxygen atom of both EG and water molecules. Since an EG molecule involves the two hydroxyl groups, the numbers estimated for the EG-water binary solutions were converted into those per molecule of both EG and water to directly compare with the numbers estimated for methanol-water33 and ethanol-water36 solutions. Figure 6 a shows the numbers of O‚‚‚O hydrogen bonds per molecule for the EG-water solutions as a function of EG mole fraction. In parts b and c of Figure 6, those for methanol-water and ethanol-water solutions are also plotted against methanol and ethanol mole fractions, xM and xE, respectively.33,36 The numbers of O‚‚‚O hydrogen bonds per molecule for the EG-water solutions monotonically increase with decreasing xEG. On the other hand, break points appear in those for methanol-water and ethanol-water solutions at xM ≈ 0.3 and 0.7 and xE ≈ 0.2, respectively, where structural transition in clusters mainly formed in the solutions occurs as concluded in the previous investigations.33,36 It is thus suggested that the structure of the EG-water solutions does not drastically change at a specific mole fraction with decreasing xEG, whereas structural transition from hydrogen-bonded chain of alcohol to the tetrahedral-like structure of water was significantly observed even for the smallest alcohol of methanol.33 Probably, EG molecules in the hydrogen-bonded structure of EG are moderately replaced by water molecules with increasing water content because the two hydroxyl groups of an EG molecule more strongly contribute to mixing with water molecules than the hydrophobic ethylene group. Thus, EG molecules are more homogeneously mixed with water than methanol. This may be related to the fact that the EG-water solutions are easily kept in the supercooling or glassy state in the wide range of 0.3 e xEG e 0.8, while methanol-water solutions are crystallized over the entire mole fraction range.40-42 SANS Measurements. Figure 7 shows Guinier plots of the SANS intensities for the EGd4-water binary solutions at xEGd4 ) 0.2, 0.4, 0.6, and 0.9 as a function of temperature, where the intensities ln[I(Q)] are plotted against the Q2 value.43 For all the solutions at 298 K remarkable SANS intensities cannot be observed, showing no significant concentration fluctuation in the solutions, i.e., EG and water molecules do not form each EG and water cluster in the solutions. However, the SANS
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12377
Figure 7. Guinier plots of the SANS intensities for the EGd4-water binary solutions at xEGd4 ) 0.2, 0.4, 0.6, and 0.9. The circles represent the experimental values and the solid lines theoretical ones. The values in parentheses are those shifted from the origin to avoid overlap of the plots.
Figure 8. Neutron diffraction patterns in the Q range from 1.5 to 3.5 Å-1 for the EGd4-water at xEGd4 ) 0.9 as a function of temperature.
intensities for the EGd4-water solutions at xEGd4 ) 0.9 are significantly strengthened with decreasing temperature. For the solution at xEGd4 ) 0.2 the small increases in the SANS intensities are observed at 208 and 188 K. On the other hand, the intensities for the EGd4-water solutions at xEGd4 ) 0.4 and 0.6 are not enhanced even at 188 and 173 K, respectively. These results may be related to the thermal properties of the EGwater solutions as described above; the EGd4-water solutions at xEGd4 ) 0.2 and 0.9 are crystallized at 203 and 217 K, respectively, while the solutions at xEGd4 ) 0.4 and 0.6 are kept in the supercooling or glassy state in the temperature range investigated. Figure 8 shows the neutron diffraction patterns in the range of 1.5 e Q/Å-1 e 3.5 for the EGd4-water solution at xEGd4 ) 0.9 as a function of temperature, which were simultaneously recorded on the detectors for the high Q range of the SWAN during the SANS measurements. As seen in this figure, Bragg peaks appear in the pattern at 228 K, whereas no peaks are observed at 298 K, indicating that crystals are formed in the solution. This is consistent with the DSC results; the very large exothermic peak at 217 K characteristic of freezing of the solution is observed in the DSC cooling curve (Figure 1). The Bragg peaks are grown with decreasing temperature; i.e., crystallization gradually progresses in the solution when the temperature is lowered. The lattice spaces were estimated from the Q values of the Bragg peaks observed in the diffraction
12378 J. Phys. Chem. B, Vol. 110, No. 25, 2006
Matsugami et al.
TABLE 3: Lattice Space (d/Å) Observed for EGd4-Water Solution at xEGd4 ) 0.9 at 188 K in Figure 8, and for Solid EG and Ice Ih
a
no.
EGd4-water
EGa
ice Ihb
1 2 3 4 5 6 7 8 9 10
4.13 3.76 3.08 2.79 2.61 2.46 2.31 2.22 2.09 1.94
4.06 3.72 3.05 2.85 2.63 2.43 2.32 2.20 2.06 1.98
3.90 3.66 3.40 2.67 2.25 2.07 1.95 1.92 1.89 1.72
Reference 44. b Reference 45.
TABLE 4: Radii of Gyration, RG (Å), for EGd4-H2O Solutions at xEGd4 ) 0.2, 0.4, 0.6, and 0.9 as a Function of Temperature T/K
xEGd4 ) 0.2
xEGd4 ) 0.4
xEGd4 ) 0.6
xEGd4 ) 0.9
298 228 208 188 173
3.73(16) 4.28(12) 7.51(12) 7.17(11)
4.21(10) 4.55(12) 4.55(11) 4.88(11)
2.69(15) 2.72(14) 2.57(12) 2.79(11) 2.75(17)
3.81(15) 9.11(15) 10.64(18) 23.09(30)
pattern at 188 K and listed in Table 3. For comparison, those for solid EG, which were calculated from X-ray diffraction data of single-crystal EG,44 and ice Ih45 are also listed in the table. As shown in Table 3, the lattice spaces for the EGd4-water solution are in good agreement with those for solid EG, but not those for ice Ih. It can be concluded that crystals of EG are formed in the EGd4-water solution at xEGd4 ) 0.9 and gradually grown in the solutions with decreasing temperature. This finding agrees with the above discussion in the section of thermal properties. Although the DSC results show that water is also crystallized from the EGd4-water solution at xEGd4 ) 0.9 when the temperature is lowered at 190 K, Bragg peaks arising from water ice cannot be observed in the neutron diffraction pattern at 188 K. This may be because of a small amount of water ice due to the lower water content. For the EGd4-water solution at xEGd4 ) 0.2 when the temperatures are 188 and 208 K, Bragg peaks could not be observed in neutron diffraction patterns though the DSC curve suggesting crystallization of the solution at 217 K. It is probable that crystallization of the solution at xEGd4 ) 0.2 is not significantly progressed as seen from the smaller exothermic peak than that for the solution at xEGd4 ) 0.9. The SANS intensities ln[I(Q)] for the EGd4-water solutions as a function of Q2 were fitted by using a least-squares refinement procedure through the Guinier equation
1 ln[I(Q)] ) - RG2Q2 + ln[I(0)] 3
(1)
where RG gives a radius of gyration for clusters formed in the solutions.43 As shown in Figure 7, the observed values are reproduced by the theoretical ones obtained from the leastsquares refinement procedure, except that those for the solutions at xEGd4 ) 0.9 and 188 K slightly deviate from the straight line, resulting probably from a multidispersive pattern of the solution.46 The RG values estimated for the EGd4-water solutions at various temperatures are listed in Table 4. The RG values for the EGd4-water solutions at xEGd4 ) 0.4 and 0.6 very slightly increase with decreasing temperature. This result together with the DSC results suggest that EG and water molecules in the
solutions at xEGd4 ) 0.4 and 0.6 do not form EG and water clusters; thus, EG and water molecules are homogeneously mixed in the solutions. This is the reason for the supercooling and glassy states of the solutions at low temperatures. On the other hand, the RG for the EGd4-water solution at xEGd4 ) 0.9 remarkably increases when the temperature is lowered. This can be understood from the thermal properties and the neutron diffraction patterns as follows. At 298 K EG and water molecules are homogeneously mixed with each other in solution. With decreasing temperature, however, EG and water molecules may be gradually separated at a microscopic scale. It is likely that EG-rich and water-rich parts are formed in the solution at xEGd4 ) 0.9 when the temperature decreases down to 228 K. Unfortunately, this microheterogeneous state of the solution above 228 K could not be observed by the present SANS experiments due to the limitation of heater power of the cryostat. First, the crystal EG is formed in the EG-rich patch at 228 K, as discussed in the section of thermal properties. Thus, the RG at 228 K becomes larger by a factor of 2.4 than that at 298 K. During a further decrease in the temperature to 208 K, EG and water patches are more significantly separated, and crystallization of EG is progressed. It leads to a further increase in the RG. At 190 K, eventually, water is also crystallized. Hence, the RG at 188 K rises to twice as much as that at 208 K. In Figure 7, the deviation of the observed SANS intensities from the straight line may be attributed to crystallization of EG and water at several dimensions. At 298 K, EG and water molecules are also homogeneously mixed in the solution at xEGd4 ) 0.2. The RG for the EGd4-water solution at xEGd4 ) 0.2 increases with lowering temperature but less significantly than those for the solution at xEGd4 ) 0.9. As shown in the DSC results, when the temperature decreases down to 203 K, the solution at xEGd4 ) 0.2 is frozen with the smaller exothermic peak than that at xEGd4 ) 0.9, implying that a peritectic or eutectic crystal is formed from the solution. It is likely that the heterogeneity of the solution at xEGd4 ) 0.2 is more moderately progressed with decreasing temperature than that at xEGd4 ) 0.9. This may result in the small increase in the RG for the EGd4-water solution at xEGd4 ) 0.2. Mixing State. At 298 K, in the entire EG mole fraction range EG and water molecules do not significantly aggregate to form EG and water clusters in binary solutions, as shown in the SANS results. This arises from the large hydrophilicity of EG molecules due to the two hydroxyl groups and the small ethylene group. As discussed in the LAXS results, the number of hydrogen bonds per molecule for the EG-water solutions monotonically increases with increasing water content. It is thus likely that EG molecules in the hydrogen-bonded structure in pure EG are gradually replaced with water molecules when the water content increases. On the other hand, structural change in clusters mainly formed in binary solutions significantly occurs even in methanol-water solutions. This difference between EG and methanol molecules may be caused by not only the number of the hydroxyl groups but also the gauche OC-CO conformation of an EG molecule. Thus, the distance (2.99 Å) of O‚‚‚O in the EG molecule due to the intramolecular hydrogen bond may be suitable to embed into the tetrahedral-like structure of water, where the distance of O‚‚‚O hydrogen bonds is 2.83 Å.33 It results in more homogeneous mixing of EG and water molecules than the smallest alcohol of methanol despite the ethylene group of the EG molecule. At the low temperatures below both melting points (260 and 273 K, respectively) of EG and water, in the wide range of 0.3 e xEG e 0.8, the EG-water solutions are easily kept in the
Mixing State of Ethylene Glycol-Water Solutions supercooling or glassy state. This is because EG and water molecules are homogeneously mixed with each other by hydrogen bonding. Hence, EG and water molecules inhibit enhancement of the network structure of water and hydrogenbonded structure of EG with decreasing temperature, respectively. In fact, the RG obtained from the SANS measurements for the EGd4-water solutions at xEG ) 0.4 and 0.6 only slightly increase when the temperature decreases, revealing the very low concentration fluctuation due to the homogeneous mixing of the solutions even at the low temperatures. On the other hand, in the range of xEG e 0.2, the content of EG molecules is not enough to prevent crystallization of water in the solutions at the low temperatures. It is possible that EG molecules are included into water ice, i.e., peritectic crystal may be formed from the solutions, as discussed from the DSC results. In the high EG content range of xEG g 0.9, water molecules hardly inhibit enhancement of hydrogen-bonded structures of EG molecules at low temperatures due to the small amount of water molecules. When the temperature decreases, crystal of EG is first formed from the solutions at ∼217 K, and then water is crystallized at ∼199 K, despite the lower freezing point of EG than that of water due to the larger amount of EG. In a narrow range of xEG g 0.9, the hydrophobicity of the EG molecule may significantly affect mixing with water molecules to exclude water molecules from the hydrogen-bonded structure of EG molecules when the temperature decreases. Acknowledgment. The authors are grateful to Dr. T. Kurisaki of Fukuoka University for his calculation of the lattice spaces of solid ethylene glycol from X-ray diffraction data of its single crystal. This work was supported partly by Grantsin-Aid (Nos. 12640500 and 15550016 to T.T., No. 15076211 to T.Y.) and HITEKU (2000-2004) to T.Y. from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References and Notes (1) Lee, H.; Hong, W.-H.; Kim, H. J. Chem. Eng. Data 1990, 35, 371. (2) Sakurai, M. J. Chem. Eng. Data 1991, 36, 424. (3) Tsierkezos, N. G.; Molinou, I. E. J. Chem. Eng. Data 1998, 43, 989. (4) Villaman˜a´n, M. A.; Gonzalez, C.; Van Ness, H. C. J. Chem. Eng. Data 1984, 29, 427. (5) Kracht, C.; Ulbig, P.; Schulz, S. J. Chem. Thermodyn. 1999, 31, 1113. (6) Batov, D. V.; Zaichikov, A. M.; Slyusar, V. P.; Korolev, V. P. Russ. J. Gen. Chem. 2001, 71, 1208. (7) Cramer, C. J.; Truhlar, D. G. J. Am. Chem. Soc. 1994, 116, 3892. (8) Bultinck, P.; Goeminne, A.; Van de Vondel, D. J. Mol. Struct. (THEOCHEM) 1995, 357, 19. (9) Csonka, G. I.; Csizmadia, I. G. Chem. Phys. Lett. 1995, 243, 419. (10) Chang, Y.-P.; Su, T.-M.; T.-W. Li; Chao, I. J. Phys. Chem. A 1997, 101, 6107. (11) Bako´, I.; Grosz, T.; Palinkas, G.; Bellissent-Funel, M. C. J. Chem. Phys. 2003, 118, 3215. (12) Hayashi, H.; Tanaka, H.; Nakanishi, K. Fluid Phase Equilib. 1995, 104, 421.
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12379 (13) Padro´, J. A.; Saiz, L.; Gua`rdia, E. J. Mol. Struct. 1997, 416, 243. (14) Saiz, L.; Padro´, J. A.; Gua`rdia, E. J. Chem. Phys. 2001, 114, 3187. (15) Gubskaya, A. V.; Kusalik, P. G. J. Phys. Chem. A 2004, 108, 7151. (16) Ambrosone, L.; D’Errico, G.; Sartorio, R.; Costantino, L. J. Chem. Soc., Faraday Trans. 1997, 93, 3961. (17) Ishihara, Y.; Okouchi, S.; Uedaira, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3337. (18) Hayashi, H.; Tanaka, H., Nakanishi, K. J. Chem. Soc., Faraday Trans. 1995, 91, 31. (19) Gubskaya, A. V.; Kusalik, P. G. J. Phys. Chem. A 2004, 108, 7165. (20) Vital de Oliveira, O.; Freitas, L. C. G. J. Mol. Struct. (THEOCHEM) 2005, 728, 179. (21) Yamanaka, K.; Yamaguchi, T.; Wakita, H. J. Chem. Phys. 1994, 101, 9830. (22) Ihara, M.; Yamaguchi, T.; Wakita, H.; Matsumoto, T. AdV. X-ray Anal. Jpn. 1994, 25, 49. Yamaguchi, T.; Wakita, H.; Yamanaka, K. Fukuoka UniV. Sci. Rep. 1999, 29, 127. (23) Furukawa, K. Rep. Prog. Phys. 1962, 25, 395. (24) Krogh-Moe, J. Acta Crystallogr. 1956, 2, 951. (25) Norman, N. Acta Crystallogr. 1957, 10, 370. (26) Takamuku, T.; Tabata, M.; Yamaguchi, A.; Nishimoto, J.; Kumamoto, M.; Wakita, H.; Yamaguchi, T. J. Phys. Chem. B 1998, 102, 8880. (27) Johanson, G.; Sandstro¨m, M. Chem. Scr. 1973, 4, 195. (28) Yamaguchi, T. Doctoral Thesis, Tokyo Institute of Technology, 1978. (29) Otomo, T.; Furusaka, M.; Satoh, S.; Itoh, S.; Adachi, T.; Shimizu, S.; Takeda, M. J. Phys. Chem. Solids 1999, 60, 1579. (30) Sears, V. F. Thermal-Neutron Scattering Lengths and Cross Sections for Condensed-Matter Research; Chalk River Lab.: Ontario, 1984. (31) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Techniques of Chemistry, Vol. II, Organic SolVents, Fourth Edition; Wiley: New York, 1986. (32) Cordray, D. R.; Kaplan, L. R., Woyciesjes, P. M.; Kozak, T. F. Fluid Phase Equilib. 1996, 117, 146. (33) Takamuku, T.; Yamaguchi, T.; Asato, M.; Matsumoto, M.; Nishi, N. Z. Naturforsch. 2000, 55a, 513. (34) Nishi, N.; Takahashi, S.; Matsumoto, M.; Tanaka, A.; Muraya, K.; Takamuku, T.; Yamaguchi, T. J. Phys. Chem. 1995, 99, 462. (35) Matsumoto, M.; Nishi, N.; Furusawa, T.; Saita, M.; Takamuku, T.; Yamagami, M.; Yamaguchi, T. Bull. Chem. Soc. Jpn. 1995, 68, 1775. (36) (a) Nishi, N.; Matsumoto, M.; Takahashi, S.; Takamuku, T.; Yamagami, M.; Yamaguchi, T. Structures and Dynamics of Clusters; Kondow, T., Kaya, K., Terasaki, A., Eds.; Universal Academic Press, Inc., and Yamada Science Foundation: 1996; pp 113-120. (b) Matsumoto, M. Master Thesis, Kyushu University, 1996. (37) Ichikawa, K.; Kameda, Y.; Yamaguchi, T.; Wakita, H.; Misawa, M. Mol. Phys. 1991, 73, 79. (38) Takamuku, T.; Maruyama, H.; Watanabe, K.; Yamaguchi, T. J. Solution Chem. 2004, 33, 537. (39) Takamuku, T.; Saisho, K.; Aoki, S.; Yamaguchi, T. Z. Naturforsch. 2002, 57a, 982. (40) Takaizumi, K.; Wakabayashi, T. J. Solution Chem. 1997, 26, 927. (41) Murthy, S. S. N. J. Phys. Chem. A 1999, 103, 7927. (42) Takamuku, T.; Saisho, K.; Nozawa, S.; Yamaguchi, T. J. Mol. Liquids 2005, 119, 133. (43) Guinier, A.; Fournet, G. Small-Angle Scattering of X-rays; Wiley: New York, 1955. (44) Boese, R.; Weiss, H.-C. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1998, C54, IUC9800024. (45) Powder Diffraction File Sets 16 to 18, Inorganic Volume No. PD1S18iRB; Berry, L. G., Ed.; Joint Committee on Powder Diffraction Standards: Newtown Square, PA, 1974. (46) Kratly, O.; Glatter, O. Small-angle X-ray Scattering; Academic Press: San Diego, 1982.
