Thermal Properties and Mixing State of DiolâWater Mixtures Studied

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J. Phys. Chem. B 2008, 112, 13300–13309

Thermal Properties and Mixing State of Diol-Water Mixtures Studied by Calorimetry, Large-Angle X-Ray Scattering, and NMR Relaxation Toshiyuki Takamuku,*,† Youichi Tsutsumi,† Masaru Matsugami,‡ and Toshio Yamaguchi§ Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga 840-8502, Japan, Department of General Education, Kumamoto National College of Technology, 2659-2 Suya, Koshi, Kumamoto 861-1102, Japan, and AdVanced Materials Institute and Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: August 21, 2008

Differential scanning calorimetry (DSC) has been performed on aqueous mixtures of three diols, which involve a linear carbon chain, HO-(CH2)n-OH (n ) 3, 4, and 5), over the whole mole fraction range of diols. The DSC results have shown the alkyl chain parity for the freezing process of the aqueous mixtures: aqueous mixtures of 1,3-propanediol (PrD) and 1,5-pentanediol (PeD) are kept in the supercooled state or vitrified over a wide mole fraction range, while those of 1,4-butanediol (BuD) are easily crystallized. The structure of PrD-water mixtures has been elucidated by using the large-angle X-ray scattering (LAXS) technique. It has been suggested that the structural change of PrD-water mixtures occurs at PrD mole fractions of xPrD ) 0.4 and 0.8: in the range of xPrD e 0.4 where the tetrahedral-like structure of water predominates, in the range of 0.4 < xPrD < 0.8 where both PrD and water structures coexist, and in the range of xPrD g 0.8 where the inherent structure of PrD is mainly formed. 17O and 1H NMR relaxation measurements have been made on aqueous mixtures of ethylene glycol (EG, n ) 2), PrD, and BuD to clarify the dynamics of H217O and diol molecules. The 17O NMR relaxation rates have suggested that the rotational motion of water molecules is gradually retarded in the diol-water mixtures with increasing diol content and that the restriction of the motion is more remarkable in the order of EG < PrD < BuD. On the basis of all the results, together with comparison with those of methanol-water, ethanol-water, and 1-propanol-water mixtures previously reported, the mixing state of diol-water mixtures has been discussed at the molecular level. Introduction At ambient conditions, linear diols, HO-(CH2)n-OH with n ) 2 to 5, are miscible with water at any ratio and have several interesting properties.1 For example, their melting points do not monotonously increase with increasing length of the alkyl chain: the melting points for 1,3-propanediol (PrD) and 1,5-pentanediol (PeD) (246.5 and 257.6 K, respectively) are lower than those for ethylene glycol (EG) and 1,4-butanediol (BuD) (260.6 and 292.8 K, respectively). However, those for monohydroxyl alcohols, such as methanol and ethanol, simply depend on the length of the alkyl chain. Moreover, the viscosities of the diols do not monotonously increase with increasing length of the chain.2 Thus, the properties show the alkyl chain parity. It is probable that the difference in the intermolecular interactions among diol molecules, which cannot be simply understood in terms of the van der Waals force between the alkyl chains, contributes to their freezing processes and viscosities. Many researchers have made numerous thermodynamic experiments on diol-water mixtures. The heats of mixing for diol-water mixtures have been measured over the entire range of diol mole fraction. A comparison of the thermodynamics properties between diol and monohydroxyl alcohol is of interest. For instance, the mole fraction dependence of the heat of * Author to whom all correspondence should be addressed. E-mail: [email protected]. † Saga University. ‡ Kumamoto National College of Technology. § Fukuoka University.

mixings for aqueous mixtures of EG and PrD at 298 K bears resemblance to that for aqueous mixtures of methanol: a minimum appears for all the mixtures at mole fraction of ∼0.3 with ca. -850, -720, and -800 J/mol for methanol-water,3 EG-water, and PrD-water mixtures, respectively.4,5 Nevertheless, PrD-water mixtures in the wide mole fraction range from xPrD ) 0.3 to 0.7 are kept in the supercooled state and vitrified below ∼153 K,6 whereas aqueous mixtures of methanol, ethanol, and propanol are easily crystallized by cooling.7-11 In contrast to the investigations at a macroscopic level, there have been a smaller number of investigations on structure and dynamics for diol-water mixtures at a microscopic level. An ab initio study has been made on the PrD molecule to clarify its conformation. It has been suggested that the intramolecular hydrogen bond plays an important role in the conformation of the PrD molecule.12 The structure of linear diols with the numbers of hydrocarbons from 9 to 12 adsorbed on a graphite surface has been investigated by using an X-ray diffraction technique. All the diol molecules are arranged in a herringbone structure in the metastable monolayer on a graphite surface, independent of the number of hydrocarbons.13 However, the alkyl chain parity was clearly observed in the stable monolayer of the diols on a graphite surface: diols with the odd numbers of hydrocarbons (n ) 9 and 11) take a parallel arrangement, where the alkyl chains of diol molecules are set in parallel and hydrogen bonded chains are formed among diol molecules, while those with the even numbers of hydrocarbons (n ) 10 and 12) take a herringbone one, where both ends of the hydroxyl groups form hydrogen bonds with those of other molecules.13

10.1021/jp804495n CCC: $40.75  2008 American Chemical Society Published on Web 09/30/2008

Mixing State of Diol and Water Molecules 17O

NMR relaxation measurements for water oxygen atoms in aqueous solutions of linear diols with the number of hydrocarbons from n ) 2 to 6 have been made to clarify the dynamics of water molecules.14 A linear correlation of the partial molar volumes and heat capacities with the dynamic hydration number of diol molecules estimated from the 17O relaxation times T1 has been found for the aqueous diol solutions in the range of the molality from 0 to 1.2 mol/kg, which corresponds to the diol mole fraction range from 0 to 0.02. However, the dynamics of water molecules in aqueous diol mixtures at higher mole fractions has not yet been investigated by the 17O NMR relaxation technique. The cluster formation of diols in their aqueous mixtures has been investigated by using mass spectrometry, where mass numbers of hydrated diol clusters isolated from liquid droplets by adiabatic expansion in a vacuum were measured.15 The stabilities of hydrated clusters estimated suggest that the stabilities of hydrated clusters depend on the distance between the two hydroxyl groups within a diol molecule. In the previous investigation, we have elucidated the thermal properties and mixing state of EG-water mixtures over the entire range of EG mole fraction by using DSC, LAXS, and small-angle neutron scattering (SANS) techniques.16 The results have shown that the thermal properties of the mixtures strongly relate to the mixing states. EG-water mixtures in the EG mole fraction range of 0.3 e xEG e 0.8 are in kept in the supercooling state until ∼100 K or vitrified, while those in the range of 0 e xEG e 0.2 and 0.9 e xEG e 1 are easily crystallized by cooling. The SANS experiments revealed that EG and water molecules are homogeneously mixed with each other in the first range; i.e., crystallization of EG and water molecules is inhibited by interaction with each other. On the other hand, the heterogeneity of the mixtures in the second and third ranges increases with decreasing temperature. It results in formation of the peritectic crystal or EG crystal from the mixtures. Despite these efforts, the relation between thermal properties for aqueous mixtures of diols involving a longer alkyl chain than EG and their structure and dynamics has not yet been elucidated. In the present investigation, thermal properties of aqueous mixtures of linear diols, HO-(CH2)n-OH (n ) 3, 4, and 5), have been clarified by means of DSC. The DSC results, accompanied by the previous ones on EG-water mixtures,16 and the chain length dependence of the thermal properties of the diol-water mixtures are discussed. LAXS experiments have been made on PrD-water mixtures over the entire diol mole fraction range to elucidate the mixing state of them at the molecular level. The rotational motion of water molecules in the diol-water mixtures, where H217O is enriched at the ratios for 17O atoms of water to those of diol molecules of 35:1-50: 1, has been observed by 17O NMR relaxation experiments. Furthermore, to clarify the dynamics of diol molecules, 1H NMR relaxation measurements have been made on the diol-water mixtures. Those have also been carried out on methanol-water, ethanol-water, and 1-propanol mixtures for comparison. On the basis of all the results, the effects of hydrocarbon chain of diol molecules on the thermal properties and structure of the diol-water mixtures are discussed at the molecular level. Experimental Section Sample Solutions. EG (Wako Pure Chemicals, grade for organic synthesis), PrD (Aldrich Chemicals, 98%), BuD (Aldrich Chemicals, g 99%), and PeD (Aldrich Chemicals, 96%) were used without further purification. Sample solutions of diol-water mixtures for DSC, LAXS, and 1H NMR relaxation measurements were prepared by weighing diol and doubly distilled water

J. Phys. Chem. B, Vol. 112, No. 42, 2008 13301 to required diol mole fractions. H217O (CDN, 17O atom content of 27%) was used without further purification. To observe the motion of water molecules alone, sample solutions for 17O NMR relaxation experiments were prepared by mixing diol, H217O, and doubly distilled water so that 17O concentrations of water are 35-50 times higher than those naturally contained in diol. Methanol, ethanol, and 1-propanol (Wako Pure Chemicals, grade for high performance liquid chromatography) were used for preparation of sample solutions for 1H NMR relaxation measurements. DSC Measurements. DSC measurements were made on aqueous mixtures of PrD, BuD, and PeD over the entire diol mole fraction range with a differential scanning calorimeter (SEIKO Instruments Inc., DSC220CU). Cooling DSC curves for the sample solutions were measured in the temperature range from 298 to 120 K, and then heating curves were recorded up to 298 K. Both cooling and heating rates were controlled at 2 K/min by a heater and cold nitrogen stream from liquid nitrogen. LAXS Experiments. LAXS measurements at 298 K have been made on pure PrD and PrD-water mixtures at PrD mole fractions of xPrD ) 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 (BURKER AXS, DIP 301) with an imaging plate (IP) (Fuji Film Co.) as a twodimensional detector was used in the present measurements. Details of the diffractometer have been described in the literature.17,18 X-rays were generated at a rotary Mo anode (Rigaku, RU-300) operated at 50 kV and 200 mA and monochromatized by a flat graphite crystal to obtain Mo KR radiation, whose wavelength is λ ) 0.7107 Å. X-ray scattering intensities for a sample solution, which was sealed into a glass capillary of 2 mm inner diameter with wall thickness of 0.01 mm, were accumulated on the IP for 1 h. Those for an empty capillary were also measured as background. The observed range of the scattering angle (2θ) was 0.2° to 109°, corresponding to the scattering vector s () 4πλ-1sinθ) of 0.03-14.4 Å-1. Densities of the sample solutions were measured at 298 K by using a densimeter (ANTON Paar K.G., DMA60 and DMA602) for analysis of LAXS data. Two-dimensional X-ray data measured on the IP were corrected for polarization and then integrated to obtain onedimensional data by conventional procedures.17 The intensities for the samples and empty capillary were corrected for absorption.18 The contribution of the sample solution alone was obtained by subtracting the intensities for the empty capillary from those for the sample. The corrected intensities were normalized to absolute units by conventional methods.19-21 The structure function, i(s), for the sample solution was obtained as previously reported22 and then Fourier transformed into the radial distribution function, D(r), in a usual manner.22 To make quantitative analysis on the X-ray data, a comparison between experimental structure function and theoretical one, which was calculated on a structure model for the short-range interactions, was made by a least-squares refinement procedure by using the first term of eq 5 in ref 22. In the data treatment, the stoichiometric volume V was chosen to contain one O atom from both diol and water molecules in the solutions. These data treatments were made by using programs KURVLR23 and NLPLSQ.24 NMR Relaxation. 17O spin-lattice relaxation times, T1, for H217O in aqueous mixtures of EG, PrD, and BuD, where the concentration of H217O is enriched at 35-50 times higher than 17O concentration naturally involved in diol molecules, were measured in the temperature range from 278 to 318 K on an FT-NMR spectrometer (JEOL, JNM-AL300). T1 measurements

13302 J. Phys. Chem. B, Vol. 112, No. 42, 2008

Takamuku et al.

Figure 1. DSC curves of (a) PrD-water, (b) BuD-water, and (c) PeD-water mixtures at various diol mole fractions. The values in the parentheses give the factor to expand or reduce the intensity of the curve.

Figure 2. States of diol-water mixtures at low temperatures determined by DSC measurements against diol mole fraction xdiol.

at 293 K were also made on the alkyl 1H atoms of diol and monohydroxyl alcohol molecules in the aqueous mixtures of diols, methanol, ethanol, and 1-propanol using the same spectrometer. The resonant frequencies of 17O and 1H nuclei were 40.741 and 300.40 MHz, respectively. Before the measurements, the sample solutions were degassed by five cycles of freeze-pump-thaw. The sample solution was kept in a 5 mm sample tube (Shigemi, PS-003), and its temperature required was controlled within ( 0.1 K by mixtures of hot air and a dry nitrogen stream generated from liquid nitrogen. T1 was measured by the inversion-recovery method with a pulse sequence (π-τ-π/2)n, where the number n of the delay times τ in the series of the sequence was 13. The longest delay time exceeded 5T1. The T1 data for each sample solution were measured three times at least and averaged to give a final value. Results and Discussion Thermal Properties. DSC curves for PrD-water, BuD-water, and PeD-water mixtures at various diol mole fractions are depicted in Figure 1a, b, and c, respectively. According to the features of both cooling and heating DSC curves observed, the states of the diol-water mixtures at low temperatures are classified into four regimes as shown in Figure 2: glass, supercooled state, formation of peritectic or eutectic crystal, and crystallization of diol and/or water. For the glassy state for pure PrD and the PrD-water mixture at xPrD ) 0.9, a small kink appears at 131 and 127 K,

respectively, due to vitrificaton in the cooling curve (see the inserted intensity-expanded one), as observed in the previous investigation on EG-water mixtures,16 while a pair of endothermic and exothermic peaks at ∼210 and ∼250 K due to crystallization of glass and its rapid melting, respectively, appears in the heating curve. For the supercooled state of the diol-water mixtures, no significant peaks appear in both cooling and heating curves over the entire temperature range measured, e.g., in the case of the PrD-water mixtures in the xPrD range from 0.3 to 0.8. When the eutectic crystal of the diol-water mixtures is formed due to crystallization of hydrated diol, a broad exthothermic peak is observed in the cooling curve, whereas two endothermic peaks are observed in the heating curve because the eutectic crystal may melt with two processes: e.g., the DSC curve for the BuD-water mixture at xBuD ) 0.8 in Figure 1b. In the case of crystallization of diol and/or water for the diol-water mixtures, a very sharp and large exothermic peak appears in the cooling curve, and one or two endothermic peaks assigned to melting of the crystals are observed in the heating curve: e.g., the curves for the PrD-water at xPrD ) 0.1. In the cooling curve for pure PeD (Figure 1c), neither a small kink nor a sharp exothermic peak is observed; however, the broad hump appears in the range of 200-230 K, which is below the melting point of 257.6 K. This feature suggests that the supercooled liquid is slowly frozen with decreasing temperature as an irregular crystal or an amorphous solid. Thus, this state can be regarded as a glass. For the PeD-water mixtures at xPeD ) 0.2, 0.8, and 0.9, no significant peaks appear in the cooling curve though one or two exothermic humps and one endothermic peak are observed in the heating curve. It is thus suggested that the mixtures are kept in the supercooled state during the cooling process, while the unstable supercooled mixtures freeze once by heating and then melt. The mixtures are classified into a supercooled state. In Figure 2, the states of the diol-water mixtures are illustrated against the diol mole fraction xdiol. For comparison, those of EG-water mixtures previously reported16 are also depicted in the figure. The PrD-water and PeD-water mixtures are kept in the supercooled or glassy state in the wider mole fraction range than the BuD-water mixtures. The PrD-water and PeD-water mixtures are crystallized only in the low xdiol

Mixing State of Diol and Water Molecules

Figure 3. Structure functions i(s) weighted by s for PrD, water,25 and their mixtures at various PrD mole fractions xPrD. The dotted and solid lines represent experimental and theoretical ones. The latter is omitted in a range of s e 1.5 Å-1 for clarity because the corresponding interactions were not taken into account in the present analysis.

range (at the high water contents), whereas the BuD-water mixtures are frozen over the entire range. On the other hand, the EG-water mixtures are crystallized in both low and high xdiol ranges and kept in the supercooled or glassy state in the mole fraction range of 0.2 < xEG < 0.9. Thus, the thermal properties of the diol-water mixtures clearly show the alkyl chain parity: the aqueous mixtures of the diols involving odd numbers of the hydrocarbons are readily kept in the supercooled state or vitrified by cooling, whereas those of the diols containing even numbers of the hydrocarbons are crystallized in the wide xdiol range. This finding may arise from the difficulty of crystallization for pure PrD and PeD, as seen in the lower melting points of PrD and PeD than those of EG and BuD. It is probable that the diols involving the odd numbers of the hydrocarbons do not easily form the long-range ordering by hydrogen bonds due to a structural hindrance of the hydrocarbon chain.13 LAXS Experiments. The structure functions factored by s for the PrD-water mixtures at various mole fractions are depicted in Figure 3. The corresponding radial distribution functions (RDFs) in the form of D(r) - 4πr2F0 are shown in Figure 4. The structure function and RDF for pure water previously measured25 are also depicted in the respective figures for comparison. In the RDF for pure PrD (xPrD ) 1), peaks below ∼4 Å are mainly attributed to intramolecular interactions within a PrD molecule. Two large and broad peaks centered at ∼5 and ∼9 Å arise from intermolecular interactions among PrD molecules, showing the long-range ordering of PrD molecules as previously seen in monohydroxyl alcohols25-29 and EG.16 Various conformations are possible for a PrD molecule due mainly to two rotational C-C bonds. Hence, to search the most stable conformation, DFT calculations with the B3LYP/631+G* level by using the Gaussian98 program package were performed on possible 20 conformations of the PrD molecule, where four dihedral angles with respect to two HO-CC and two OC-CC bonds were considered. The results suggested that tGG’g and gGG’g conformers (Figure 5) are the most stable among the 20 conformers because of the intramolecular hydrogen bond between the terminal hydroxyl groups. However, the conformers depending on the dihedral angles of the HO-CC bonds are not distinguishable in the RDF obtained because positions of hydrogen atoms cannot be determined by the LAXS experiments. On the basis of the conformations defined only

J. Phys. Chem. B, Vol. 112, No. 42, 2008 13303

Figure 4. Radial distribution functions in the D(r) - 4πr2F0 form for PrD, water,25 and their mixtures at various xPrD. The solid lines represent experimental values (original RDFs), and the dashed ones are intermolecular RDFs (IRDFs) obtained by subtraction of intramolecular interactions within PrD and water molecules from the original RDFs. The dotted lines give theoretical values calculated by using optimized parameter values (Tables 1 and 2), and the dot-dashed ones are residual curves after subtraction of theoretical values from the original RDFs.

by the dihedral angles for the OC-CC bonds, thus the plausible conformers can be classified into xGGx, xGG’x, xTGx, and xTTx, where “x” means positions of the hydrogen atom unknown by the LAXS experiments. In Figure 5, the structure models of the conformers are illustrated. The present DFT calculations revealed that the conformers are more stable in the order of xGG’x > xGGx > xTGx > xTTx. The four conformers were examined to explain the RDF for pure PrD. Figure 6a,b shows the results of the model fits based on the four conformers on both s- and r-spaces, respectively. The structure parameters for each conformer used for the fits are listed in Table 1. In addition, the structure parameters for O · · · O hydrogen bonds between PrD molecules were fixed as the distance r, 2.79 Å, the temperature factor b, 13.0 × 10-3 Å2, and the number n, 1.0. In the s-space, the theoretical values (solid line) for the xGGx and xTTx conformers satisfactorily reproduce the observed ones (dotted line) in the range of s > ∼4 Å-1, to which intramolecular interactions of the PrD molecule mainly contribute. On the other hand, the theoretical values for xGG’x and xTGx cannot explain the observed ones, particularly in the range of ∼4 e s/Å-1 e 7. For the observed values (solid line) in the r-space, intramolecular interactions appear mainly in the range of 0 < r/Å e ∼4. The first sharp peak at 1.5 Å is attributed to C-C and C-O bonds, and the second one at 2.6 Å is assigned to the nonbonding interactions, such as C1 · · · C3 and C2 · · · O (the notation of atoms is indicated in Figure 5), whose distances do not change among the conformers. However, the distances for the longer nonbonding interactions, C1 · · · O2, C3 · · · O1, and O1 · · · O2, vary in the range from 2.8 to 5 Å depending on the conformation; i.e., a shoulder at 3 Å and a peak at 3.8 Å arise from such nonbonding interactions. The differences in the theoretical values in the r-space among the conformers are clearer than those in the s-space. The theoretical RDFs calculated by using the structure parameters listed in Table 1 are depicted by the dotted lines, and the residual curves, which were obtained by subtracting the theoretical values from the observed ones, are shown by the dot-dashed lines. The residual curve for the xGGx conformer over the range from 0 to ∼4.5 Å is mostly smooth among those for the conformers examined; i.e., the structure parameters for the conformer well explain the observed RDF in the range. In


Takamuku et al.

TABLE 1: Intramolecular Interactions for the PrD Molecule in Four Plausible Conformations and Water Moleculea interaction

r

C-C C-H C-O O-H C1 · · · C3 C2 · · · O C1 · · · O 2 C3 · · · O 1 O1 · · · O 2 O · · · H1,3 O · · · H2 O · · · H2 O2 · · · H 1 O2 · · · H 1 O1 · · · H 3 O1 · · · H 3 C· · ·H C1,3 · · · H3,1 C1 · · · H 3 C3 · · · H 1

1.514 1.111 1.421 0.942 2.550 2.500 2.980 3.000 3.770 2.100 2.750 3.350 2.550 3.700 2.550 3.700 2.160 2.650 3.300 3.300

xGGx

xGG′x

103

3

xTGx

b

n

r

10 b

1 1 1 1 4.5 4 9 9 4 4.5 3 4 4 4 4 4 4.5 3 4 4

2 6 2 2 1 2 1 1 1 4 2 2 1 1 1 1 8 2 1 1

1.514 1.111 1.421 0.942 2.550 2.500 2.980 2.980 2.800 2.100 2.750 3.350 3.300 4.000 3.300 4.000 2.160 2.650 3.300 3.300

1 1 1 1 4.5 4 9 9 4 4.5 4 4 4 6 4 6 4.5 3 4 4

xTTx

3

n

r

10 b

n

r

103b

n

2 6 2 2 1 2 1 1 1 4 2 2 1 1 1 1 8 2 1 1

1.514 1.111 1.421 0.942 2.550 2.500 2.980 3.770 4.270 2.100 2.550 2.550 2.750 3.350 4.000 4.500 2.160 2.650 2.650 3.300

1 1 1 1 4.5 4 9 9 4 4.5 3 4 4 4 6 6 4.5 3 3 4

2 6 2 2 1 2 1 1 1 4 2 2 1 1 1 1 8 2 1 1

1.514 1.111 1.421 0.942 2.550 2.500 3.800 3.800 4.950 2.100 2.550 2.550 4.100 4.100 4.100 4.100 2.160 2.650 2.650 2.650

1 1 1 1 4.5 4 9 9 9 4.5 4 4 6 6 6 6 4.5 4 4 4

2 6 2 2 1 2 1 1 1 4 2 2 1 1 1 1 8 2 1 1

PrD

Waterb O-H H· · ·H a

0.970 1.555

2 10

2 1

The distance r (Å), the temperature factor b (Å2), and the number of interactions n. b Ref 30.

TABLE 2: All Optimized Parameter Values of the Interactions in Water, PrD, and Their Mixtures Obtained by Least-Squares Fitsa xPrD interaction parameter

0b

0.05

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1

O· · ·O

r 103b n

Linear hydrogen bond of water-water, PrD-water, and PrD-PrD 2.826(2) 2.834(2) 2.833(2) 2.824(3) 2.820(4) 2.812(4) 2.805(6) 2.805(9) 2.800(11) 2.798(13) 2.799(15) 2.791(14) 17 17 17 17 17 16 16 16 15 14 14 13 3.43(6) 3.28(4) 3.03(4) 2.64(4) 2.35(5) 2.13(6) 1.80(6) 1.52(11) 1.25(10) 1.01(10) 0.98(12) 1.02(11)

O· · ·O

r 103b n

3.35 15 1.0

3.35 15 1.0

3.35 15 0.98

3.35 15 0.94

O· · ·O

r 103b n

4.00 90 3.0

4.00 90 3.0

Second 4.00 90 2.8

neighbor of water-water, PrD-water, and PrD-PrD 4.00 4.00 4.00 4.00 4.00 90 90 90 90 90 2.6 2.4 2.2 2.0 1.8

4.00 90 1.4

O· · ·O

r 103b n r 103b n r 103b n

4.50

4.50 90 0.6 3.57 16.5 0.5 4.00 18 0.5

4.50 90 0.6 3.57 16.5 1.1 4.00 18 1.1

First neighbor of PrD-PrD and PrD-water 4.50 4.50 4.50 4.50 4.50 90 90 90 90 90 0.5 0.5 0.4 0.4 0.3 3.57 3.57 3.57 3.57 3.57 16.5 16.5 16.5 16.5 16.5 1.1 1.2 1.2 1.3 1.6 4.00 4.00 4.00 4.00 4.00 18 18 18 18 18 1.1 1.2 1.2 1.3 1.6

3.57 16.5 1.6 4.00 18 1.6

O· · ·C C· · ·C

0.6

Interstitial water molecules 3.35 3.35 3.35 15 15 15 0.90 0.80 0.70

3.35 15 0.60

3.35 15 0.50

3.57 16.5 1.6 4.00 18 1.6

3.57 16.5 1.6 4.00 18 1.6

3.57 16.5 1.6 4.00 18 1.6

a The interatomic distance r (Å), the temperature factor b (Å2), and the number of interactions n per oxygen atom. 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 Ref 25.

the residual curve for the xGG’x conformer, a valley at 2.8 Å reveals the excess contribution of the theoretical values due probably to an intramolecular O · · · O hydrogen bond. The LAXS results thus suggest that the PrD molecule hardly forms an intramolecular hydrogen bond in the liquid though the DFT calculations suggest the most stable xGG’x conformer. This is because the DFT calculations were applied to a PrD molecule in the gas phase. On the other hand, a hump at ∼3.2 Å and a peak at ∼3.9 Å in the residual curve for the xTGx conformer show that the theoretical values cannot reproduce the observed

ones. Although the theoretical values for the xTTx conformer satisfactorily explain the observed ones in the s-space, two small peaks at ∼3.1 and ∼3.8 Å remain in the residual curve in the r-space. In consequence, the present fits reveal that a PrD molecule prefers the xGGx conformation and scarcely forms an intramolecular hydrogen bond. It is likely that the xGGx conformer can more easily form intermolecular hydrogen bonds to make the long-range ordering than the xGG’x one. In Figure 4, intermolecular RDF (IRDF) for PrD, which was obtained by subtraction of the intramolecular interactions of the



Figure 5. Structure models of four conformations of the PrD molecule with total energies (hartrees) for tGGt, tGG′g, g′TGt, and gTTg conformers estimated by DFT calculations with the B3LYP/6-31+G* level.

Figure 6. Results of model fitting by xGGx, xGG′x, xTGx, and xTTx conformations of the PrD molecule in (a) s- and (b) r-spaces. In the s-space, the dotted and solid lines give experimental and theoretical ones, respectively. The latter is omitted in a range of s e 1.5 Å-1 for clarity. In the r-space, the solid and dotted lines represent experimental and theoretical ones, and the dot-dashed ones represent residual values obtained by subtracting the theoretical ones from the experimental ones.

xGGx conformer of the PrD molecule from the observed RDF (original RDF), is depicted by the dashed line. A small peak at 2.8 Å is observed in the IRDF, suggesting hydrogen bonds between PrD molecules. A hump at 3.8 Å in the IRDF is ascribed to intermolecular interaction between the carbon atom and the hydroxyl oxygen atom of a neighbor molecule hydrogenbonded. To elucidate the structure parameters related to the hydrogen bonds between PrD molecules, a least-squares refinement procedure was performed on the structure function of PrD in the range of 4.5 e s/Å-1 e 14.4 by using structure parameters of the O · · · O hydrogen bond and C1 · · · O and C3 · · · O interactions. The intramolecular interactions of the xGGx conformer listed in Table 1 were fixed during the refinement procedure. The optimized structure parameters are summarized in Table 2. As shown in Figure 3, the theoretical structure function calculated with the structure parameters in Table 2 well reproduces the observed one. In Figure 4, the residual curve obtained from subtracting the theoretical RDF (dotted line) from the observed one is smooth in the range of 0 e r/Å e 4, suggesting that there are no other significant interactions left. The distance (2.791 ( 0.014 Å) of the O · · · O hydrogen bond between PrD molecules is slightly longer than that (2.717 ( 0.010 Å) between EG molecules previously estimated.16 On the other hand, the number (1.02 ( 0.11) of O · · · O hydrogen bonds

for PrD is comparable with that (1.03 ( 0.06) for EG.16 The longer O · · · O distance suggests a slightly weaker hydrogen bond between PrD molecules than that between EG ones. In the RDFs for the PrD-water mixtures (Figure 4), the broad peaks centered at ∼5 and ∼9 Å assigned to the intermolecular interactions in the hydrogen-bonded structure of PrD are gradually weakened when the xPrD decreases from 1 to 0.2. In particular, the longer-range interactions centered at ∼9 Å significantly diminish and almost disappear at xPrD ) 0.2. The RDFs for the PrD-water mixtures at xPrD ) 0.05 and 0.1 are comparable with that for pure water (xPrD ) 0): the first, second, and third neighbor interactions in the tetrahedral-like structure of water are observed at 2.8, 4.5, and 7 Å, though the 4.5 Å peak may also include interactions in the hydrogen-bonded structure of PrD molecules.25 To observe intermolecular interactions in the PrD-water mixtures, the IRDFs (dashed line) for the mixtures were obtained by subtracting the intramolecular interactions of PrD and water molecules from the observed RDFs for the mixtures. Here, the intramolecular interactions for the xGGx conformer of the PrD molecule obtained from the RDF for pure PrD (Table 1) were utilized. Those for water molecules were fixed to the values previously determined by a large-angle neutron scattering technique.30 In the IRDFs for the mixtures, a peak at 2.8 Å gradually grows with decreasing xPrD (increasing water content); on the contrary, a hump at 3.8 Å arising from the intermolecular interactions of PrD-PrD and PrD-water by hydrogen bonding, such as C1 · · · O and C3 · · · O, is weakened when the xPrD decreases. These features suggest that hydrogen bonds among water molecules are gradually evolved in the mixtures with increasing water content, while those of PrD-PrD and PrD-water decrease. The structure parameters related to O · · · O hydrogen bonds, which are an average of those for PrD-PrD, PrD-water, and water-water, were estimated to clarify the structural change in the mixtures with xPrD. A least-squares refinement procedure was performed on the structure function of the PrD-water mixtures in the range of 4.5 e s/Å-1 e 14.4 by using the model parameters of the O · · · O hydrogen bond for PrD-PrD and PrD-water and the intermolecular interactions of C1 · · · O and C3 · · · O for PrD molecules hydrogen-bonded with either PrD or the water molecule, accompanied by O · · · O interactions for the tetrahedral-like structure of water and the non-hydrogen bonding interstitial water molecules.25 During the refinement procedure, the intramolecular interactions for both PrD and water were not allowed to vary. The optimized parameter values are


Takamuku et al.

Figure 8. Diol mole fraction dependence of 17O NMR relaxation rates of H217O in (a) EG-water, (b) PrD-water, and (c) BuD-water mixtures at various temperatures.

Figure 7. Numbers of hydrogen bonds per molecule of PrD-water (filled circles) and EG-water16 (opened circles) mixtures as a function of diol mole fraction. The standard deviations σ were indicated as error bars.

listed in Table 2, together with those for pure water previously determined25 for comparison. Figure 3 shows that the theoretical structure functions calculated by using intra- and intermolecular structure parameters listed in Tables 1 and 2 well reproduce the observed ones over the range analyzed. In the RDFs for the PrD-water mixtures (Figure 4), the residual curve (dot-dashed line) obtained by subtracting the theoretical values (dotted line) from the observed one (solid line) is smooth. It is thus concluded that the structure parameters of PrD-PrD and PrD-water by hydrogen bonding and the water structure can explain the observed RDFs for the mixtures. The present analysis also suggests that a PrD molecule prefers the xGGx conformer in the mixtures over the entire mole fraction range. Table 2 shows that the distance of the O · · · O hydrogen bond is gradually elongated when the xPrD decreases. The elongation of the O · · · O hydrogen bond, which has often been observed in various alcohol-water and EG-water mixtures, suggests the evolution of the hydrogen-bonded structure in the PrD-water mixtures because hydrogen bonds among water molecules are longer than those between alcohol molecules.16,25 The number of O · · · O hydrogen bonds per oxygen atom of both PrD and water molecules increases with decreasing xPrD, revealing the evolution of the hydrogen-bonded structure in the mixtures again. In Figure 7, the numbers of O · · · O hydrogen bonds for PrD, water, and the PrD-water mixtures, which are converted to the value per molecule because of two hydroxyl oxygen atoms in a PrD molecule, are plotted as a function of xPrD. For comparison, those for EG-water mixtures previously estimated from the LAXS experiments16 are also depicted in the figure. The number of O · · · O hydrogen bonds for pure PrD is comparable with that for pure EG. However, the number for the PrD-water mixtures seems to slightly dip at xPrD ) 0.8, while that for EG-water mixtures monotonously increases with decreasing xEG. This may be ascribed to the uncertainties of the numbers for the PrD-water mixtures because several intramolecular interactions of the PrD molecule overlap to that of the O · · · O hydrogen bonds. Hence, the numbers of O · · · O hydrogen bonds for the PrD-water mixtures scarcely change with decreasing xPrD from 1 to 0.8 and are smaller than those for EG-water mixtures in the range. This suggests that PrD molecules less easily form hydrogen bonds with other PrD and water molecules than EG molecules. The numbers for the PrD-water mixtures significantly increase with decreasing mole fraction from xPrD ) 0.7, and the numbers below xPrD ) 0.4 are

comparable with those for EG-water mixtures. The change in the numbers of O · · · O hydrogen bonds for the PrD-water mixtures shows two break points at ∼0.4 and ∼0.8. In accordance with both changes in the RDFs and the numbers of O · · · O hydrogen bonds for the PrD-water mixtures with mole fraction, the inherent structure of PrD predominates in the range of 0.8 e xPrD < 1, while the water structure is considerably formed in the range of xPrD e 0.4. In the middle range of 0.5 e xPrD < 0.8, both PrD and water structures coexist in the mixtures. 17O NMR Relaxation. Figure 8 shows the 17O relaxation rates R1 () 1/T1) of H217O molecules in the EG-water, PrD-water, and BuD-water mixtures in the temperature ranges of 278-318, 288-318, and 293-318 K, respectively, as a function of xdiol. The 17O relaxation occurs mainly via the quadrupole process because of the spin number I ) 5/2 for 17O nuclei. Thus, the rotational correlation time of an H217O molecule is related to the relaxation rate R1 under an extreme narrowing condition by the following equation31

R1 )

(

)( )

12π2 2I + 3 η2 e2Qq 2 τ2R 1 + 40 I2(2I - 1) 3 h

(1)

Here, e2Qq/h is the quadrupole coupling constant (QCC); η represents the asymmetric parameter; and τ2R is the rotational correlation time. The R1 values for all the mixtures monotonously increase with increasing diol mole fraction. It is suggested that the rotational motion of water molecules is gradually retarded in the diol-water mixtures with increasing xdiol. This is caused by the fact that water molecules are gradually accommodated in the structure of diol evolved in the mixtures with the increase in diol content. In addition, the R1 value at each mole fraction increases when the temperature is lowered, and the increase in the R1 values with decreasing temperature is more significant at high mole fractions. This implies that the restriction of the rotational motion of water molecules in the mixtures is caused by not only cooling but also the structure of the diol enhanced in the mixtures at low temperatures. In contrast to the diol-water mixtures, the R1 values for H217O molecules in aqueous mixtures of methanol, ethanol, and 1-propanol increase with increasing alcohol mole fraction, and then, the increase in the R1 values becomes more moderate with a further increase of mole fraction.32 It thus leads to a break point for the mixtures at the alcohol mole fractions of ∼0.3, ∼0.2, and ∼0.15, respectively, where the structural change in main clusters formed in the mixtures occurs: the tetrahedrallike structure of water predominates in the mixtures below the break point, whereas the chain structure of alcohol molecules via hydrogen bonds is mainly formed above the break point. The rotational motion of water molecules is retarded by hydrogen bonding with alcohol molecules when the mole fraction increases toward the break point, leading to the increase


Figure 9. Average activation energy of the rotational motion of H217O molecules in (a) diol-water and (b) monohydroxyl alcohol-water32 mixtures estimated from 17O NMR relaxation rates as a function of mole fraction.

in the R1 values. On the other hand, water monomers may gradually increase in the mixtures with increasing alcohol mole fraction from the maximum because the chainlike structure of alcohol via hydrogen bonds is predominantly formed in the mixtures. Hence, the restriction of the rotational motion of water molecules moderately occurs with increasing mole fraction. However, no significant break point appears in the change in the R1 values for all the diol-water mixtures with increasing diol mole fraction. The rotational correlation time τ2R of H217O molecules was estimated from the T1 values measured for EG-water, PrD-water, and BuD-water mixtures through eq 1. The τ2R values for all the mixtures examined are listed in Tables S1-S3 in the Supporting Information. The QCC (8.9 kHz) and η (0.72) for 17O in pure water were utilized in the present estimation.33 However, the two parameters might be influenced by diol content of the mixtures and temperature because the charge distribution around the 17O nucleus is varied by the degree of hydrogen bonding depending on diol content and temperature. Indeed, the value of (1 + η3)(e2Qq/h)2 for the 17O nucleus of H217O molecules ranges from 98 MHz at 275 K to 105 MHz at 370 K.34 For the diol-water mixtures at each mole fraction, the R1 value at the lowest temperature studied is twice as large at least than that at the highest one. Moreover, at each temperature, the value at the highest diol content is five times larger than those for pure water at least. In the present analysis, thus, it can be regarded that the change in the R1 values with both diol content and temperature is mainly governed by the rotational dynamics of H217O molecules in the mixtures. The constant QCC and η were thus adopted in the present analysis. From the Arrhenius plots of the τ2R values, the activation energies Ea of the rotational motion of water molecules in the diol-water mixtures are estimated. The average Ea was obtained in the temperature ranges of 288-318 K for the EG-water, 288-313 K for the PrD-water, and 293-318 K for the BuD-water mixtures, respectively. In Figure 9a, the Ea values for the diol-water mixtures are depicted as a function of diol mole fraction xdiol. For comparison, those for aqueous mixtures of methanol (MeOH), ethanol (EtOH), and 1-propanol (1-PrOH) previously reported are also plotted as a function of alcohol mole fraction xA in Figure 9b.32 As seen in Figure 9a, the Ea values for the EG-water mixtures monotonously increase with increasing EG mole fraction. Those for the PrD-water and BuD-water mixtures also gradually increase when the diol mole fraction increases to 0.6 and 0.7, respectively, but reach almost a plateau or slightly decrease with further increasing mole fraction. These features for the diol-water mixtures appreciably differ from those for the monohydroxyl


Figure 10. 1H NMR relaxation rates of the methylene and methyl groups in (a) diol-water, (b) methanol-water and ethanol-water, and (c) 1-propanol-water mixtures at 293 K as a function of mole fraction.

alcohol-water mixtures and, in particular, the methanol-water and ethanol-water ones. The Ea values for methanol-water and ethanol-water mixtures similarly change with increasing mole fraction to the R1 values of them: a maximum appears at xA ) 0.2-0.3. The rotational motion of water molecules in methanol-water and ethanol-water mixtures is gradually retarded when the xA increases from 0 to 0.2-0.3 due to hydrogen bonds between alcohol and water molecules in the predominant water structure, whereas the restriction of the rotational motion diminishes with the further increase in the xA because alcohol clusters mainly formed gradually release water molecules as monomers. For 1-propanol-water mixtures, although the weak break point in the Ea values is observed at xA ) ∼0.2, the values still increase with further increasing xA. This may arise from the more defined water structure in 1propanol-water mixtures than methanol-water and ethanolwater mixtures due to the larger hydrophobic group, as discussed in the previous investigation.32 Thus, the motion of water molecules in the water structure is gradually retarded in the hydrophobic environment evolved in the mixtures when the xA increases. The features of the Ea values for the diol-water mixtures should also be related to the structural change of the diol-water mixtures with increasing xdiol. On the basis of the changes in the R1 and Ea values with mole fraction, the structural change of the diol-water mixtures with increasing xdiol may not drastically occur at a specific mole fraction. Thus, the rotational motion of water molecules is gradually restricted in the diol structure monotonously evolved with increasing xdiol. However, in the PrD-water and BuD-water mixtures in the range of xdiol > 0.6-0.7, the rotational motion of water molecules becomes freer again rather than further restricted. Thus, water monomers are released from the inherent structure of diol, which is barely evolved in the mixtures at the higher mole fractions of xdiol ) 0.6-0.7 than the alcohol-water mixtures. The reason for no plateau in the change in the Ea values for the EG-water mixtures may be that water molecules are more easily embedded into the inherent structure of EG by hydrogen bonding than the structure of PrD and BuD due to the smaller hydrophobicity of EG.16 1H NMR Relaxation. To clarify the change in the structure of the diol-water mixtures with increasing xdiol from the dynamics of diol molecules, 1H NMR relaxation measurements at 293 K were made on the methylene (CH2) groups within diol molecules in the EG-water and PrD-water mixtures. Additionally, those were performed on the methyl and methylene groups of monohydroxyl alcohol molecules in methanol-water, ethanol-water, and 1-propanol-water mixtures for comparison. Figure 10 shows the relaxation rates R1 for 1H of the methylene

13308 J. Phys. Chem. B, Vol. 112, No. 42, 2008 and methyl groups within diol and monohydroxyl alcohol molecules as a function of mole fraction. The 1H relaxation for both groups is governed mainly with intramolecular and intermolecular 1H-1H dipole interactions of diol and alcohol molecules. Furthermore, the intermolecular 1H-1H dipole interaction between diol or alcohol and water molecules contributes to the 1H relaxation. Thus, the relaxation rate of the 1H nucleus gives complex information on both translational and rotational motions of diol and alcohol molecules. As shown in Figure 10, however, the changes in the R1 values for 1H of the methylene and methyl groups in the aqueous mixtures of diols, methanol, and ethanol are comparable with those for 17O of water molecules,32 respectively. The R1 values for the methylene groups of EG molecules monotonously increase with increasing xEG. This suggests that the motion of EG molecules is gradually restricted in the mixtures with the increase in the EG content due to the evolution of EG structure. Those for the center (C2) and both sides (C1 and C3) of methylene groups within PrD molecule overlap each other within the experimental errors. The R1 values for the methylene groups of PrD molecules in the PrD-water mixtures also gradually rise when the xPrD increases up to 0.6 but are almost constant at xPrD g ∼0.7. This feature is also comparable with the LAXS results: the numbers of hydrogen bonds for the PrD-water mixtures estimated from the RDFs are almost constant at xPrD g 0.8 (Figure 7). Thus, the inherent PrD structure is predominantly evolved in the mixtures above xPrD ) 0.8. On the other hand, the R1 values for the methyl and methylene groups of methanol and ethanol molecules similarly change with increasing alcohol content: the values increase when the xA increases and then decrease or reach almost constant with the further increase in the xA, leading to a maximum. The changes in the R1 values suggest that the motion of methanol and ethanol molecules is gradually retarded with increasing xA to the maximum due to hydrogen bonding between alcohol and water molecules, while the motion does not significantly change with further increasing xA from the maximum because alcohol molecules are stabilized by forming the chainlike structure via hydrogen bonds in the mixtures. The changes in the R1 values for the methyl and methylene groups of the 1-propanol molecule with increasing xA are comparable with those of methanol and ethanol molecules, although the change in the Ea values of the rotational motion of water molecules in 1-propanol-water mixtures differs from those in methanol-water and ethanol-water mixtures, as shown in Figure 9. This may be because the motions of 1-propanol and water molecules given by the R1 and Ea values, respectively, are not strongly related with each other due to the well-defined structures of 1-propanol and water. Thus, the R1 values for the methyl and methylene groups of the 1-propanol molecule mainly give the information on the change in the structure of 1-propanol in the mixtures with increasing xA. In the xA range below the break point or the maximum at xA ) ∼0.2, 1-propanol molecules form hydrogen bonds with water molecules, while in the range of xA g ∼0.2, the chainlike structure of 1-propanol by hydrogen bonding is formed in the mixtures, i.e., the motion of the 1-propanol molecule is stabilized in the structure. Here, the R1 values for the primary methylene (C1) group within the 1-propanol molecule in the xA range of xA g ∼0.2 are almost constant; on the contrary, those for the tail methyl and the secondary methylene (C2) groups dip with increasing xA and reach a plateau with the further increase in xA. This shows that the motion of the primary methylene (C1) group is immediately restricted in

Takamuku et al. the chainlike structure of 1-propanol with increasing xA because of the vicinal hydroxyl group, whereas those of the methyl and methlyene (C2) groups are still freer in the xA range. The different features of the R1 values between the diols and monohydroxyl alcohols suggest that the structural change of the diol-water mixtures moderately takes place with increasing diol mole fraction, while the structure of the main clusters formed in the methanol-water, ethanol-water, and 1-propanolwater mixtures drastically changes at xA ≈ 0.3, 0.2, and 0.15, respectively, as concluded in the previous investigations.25-29,32 Figure 10 indicates that the R1 values for the EG-water and PrD-water mixtures are larger by a factor of ∼10 than those for the monohydroxyl alcohol-water mixtures. It is shown that the structure of diols is more rigid than that of monohydroxyl alcohols. Mixing States of Diol-Water. The present results from the LAXS and NMR experiments show that the structure of the diol-water mixtures moderately changes from the hydrogenbonded network of water to the inherent structure of diol with increasing xdiol. The number of hydrogen bonds and the activation energy Ea of the rotational motion of water molecules for the diol-water mixtures smoothly change with increasing xdiol. On the contrary, as the break point remarkably appears in those for the monohydroxyl alcohol-water mixtures, the structural change in the main clusters formed in the alcohol-water mixtures drastically occurs with increasing alcohol mole fraction.11,25-29 This difference arises from the molecular structure of diol and monohydroxyl alcohol. For the change in the structure of the diol-water mixtures with increasing xdiol, water molecules in the hydrogen-bonded network of water may be gradually and easily replaced with diol molecules. The hydrophobic effect of the hydrocarbon chain within the diol molecules will not be remarkable rather than the two terminal hydroxyl groups. On the other hand, the alkyl group of the monohydroxyl alcohol molecule may play a role of a terminal in the hydrogen-bonded structure among alcohol and water molecules. This leads to the less homogeneous mixing of the alcohol-water binary solutions; thus, the change in the structure of the mixtures from water clusters to alcohol clusters with increasing alcohol content drastically takes place. As discussed in the LAXS section, the PrD molecule prefers the compact conformation of xGGx to reduce its hydrophobicity. However, the PrD molecule scarcely forms an intramolecular hydrogen bond with the xGG’x conformation, while an intramolecular hydrogen bond is formed in an EG molecule.16 This may be because the xGGx conformation of the PrD molecule is suitable to form intermolecular hydrogen bonds with other PrD and water molecules. The effect of the hydrocarbon chain length of diol molecules on the structure of diol-water mixtures is clearly found at high diol contents. The numbers of hydrogen bonds for the PrD-water mixtures estimated from the LAXS experiments suggest that the inherent structure of PrD predominates in the range of xPrD g 0.8. However, those for the EG-water mixtures do not show a specific mole fraction range, where EG structure is dominant. In addition, the Ea values of the rotational motion of water molecules determined by the 17O NMR relaxation indicate that the rotational motion of water molecules is gradually restricted in the PrD-water and BuD-water mixtures with increasing xdiol to ∼0.6. When the xdiol further increases, however, the rotational motion becomes freer again. The relaxation rates R1 for 1H NMR of the methylene groups of diol molecules in the diol-water mixtures suggest that the restriction for the motion of PrD molecules with increasing xPrD is moderate in the range of xPrD

Mixing State of Diol and Water Molecules g ∼0.7. On the other hand, the motions of water and EG molecules in the EG-water mixtures are monotonously retarded with increasing EG content. These findings imply that the structure of PrD and BuD is rigidly formed in the diol-water mixtures at xdiol g ∼0.7. It results in the formation of water monomers, whose rotational motion is freer than that at the low diol contents. The hydrophobicity of EG molecules is lower than those of PrD and BuD due to the shortest hydrocarbon chain. Thus, the interactions between EG and water molecules are still significant in the EG-water mixtures even at the high EG mole fractions. The Ea values of the rotational motion of water molecules suggest that the increase of water monomers in the mixtures at the high diol contents is more significant in the sequence of BrD > PrD > EG; i.e., the inherent structure of the diols is more rigid in the same sequence. Finally, the parity of the hydrocarbons on the thermal properties of the diol-water mixtures is considered as follows. In the liquid state, the longer the hydrocarbon chain, the more rigid the inherent structure of diol. This may be caused mainly by van der Waals forces between the chains. Probably, the strength of hydrogen bonds between diol molecules is not significantly different among the diols studied. However, the ordering of the diol molecules by hydrogen bonding is different among them. As seen in the melting points of the diols1 and the adsorption structure of diols on graphite surface,13 the twoor three-dimensional ordering of the diol molecules involving the odd number of the hydrocarbon chain is less easy than those with the even number of the hydrocarbon chain because of the discontinuity of hydrogen bonding. Thus, the aqueous mixtures of the former are readily kept in the supercooled and glassy states in the wider mole fraction range due to the difficult crystallization of diol. On the contrary, those of the latter are easily crystallized in the wider mole fraction range. Conclusions The mixing states of the aqueous mixtures of PrD, BuD, and PeD have been investigated by using DSC, LAXS, and NMR relaxation techniques and compared with those of aqueous mixtures of EG, methanol, ethanol, and 1-propanol previously reported. Although the structural change in aqueous mixtures of methanol, ethanol, and 1-propanol with increasing alcohol mole fraction drastically takes place at the specific mole fraction, the change in the aqueous mixtures of PrD and BuD with mole fraction moderately occurs as well as the shortest diol of EG. It was concluded that the different mixing states between the aqueous mixtures of diols and monohydroxyl alcohols are caused mainly by the two hydrophilic hydroxyl groups of the former and the terminal effect of the hydrophobic alkyl groups of the latter in the hydrogen-bonded structure. On the other hand, the parity of the thermal properties of the diol-water mixtures observed by the DSC measurements arises from the discontinuity of hydrogen bonding of diol molecules depending on their odd or even number of the hydrocarbon chain. Acknowledgment. The authors are grateful to MS. Eiji Goto for his technical assistance in the 1H NMR relaxation measurements for 1-propanol-water mixtures. This work was supported partly by Grants-in-Aid (No. 15550016 and 19550022) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. The density measurements and NMR ex-

J. Phys. Chem. B, Vol. 112, No. 42, 2008 13309 periments for the sample solutions were carried out at Analytical Research Center for Experimental Sciences of Saga University. Supporting Information Available: Tables S1-S3 for the τ2R values. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic SolVents: Physical Properties and Methods of Purification, 4th ed.; Wiley: New York, 1986. (2) Lech, T.; Czechowski, G.; Jadz˙yn, J. J. Chem. Eng. Data 2001, 46, 725. (3) Franks, F.; Ives, D. J. G. Q. ReV. Chem. Soc. 1966, 20, 1. (4) Matsumoto, Y.; Touhara, H.; Nakanishi, K.; Watanabe, N. Nestu Sokutei 1977, 4, 3. (5) Matsumoto, Y.; Touhara, H.; Nakanishi, K.; Watanabe, N. J. Chem. Thermodyn. 1977, 9, 801. (6) Jabrane, S.; Le´toffe´, J. M.; Claudy, P. Thermochim. Acta 1998, 311, 121. (7) Ott, J. B.; Goates, J. R.; Waite, B. A. J. Chem. Thermodyn. 1979, 11, 739. (8) Takaizumi, K.; Wakabayashi, T. J. Solution Chem. 1997, 26, 927. (9) Takaizumi, K. J. Solution Chem. 2000, 29, 377. (10) Murthy, S. S. N. J. Phys. Chem. A 1999, 103, 7927. (11) Takamuku, T.; Saisho, K.; Nozawa, S.; Yamaguchi, T. J. Mol. Liq. 2005, 119, 133. (12) Bultinck, P.; Goeminne, A.; Van de Vondel, D. J. Mol. Struct. (Theochem) 1995, 357, 19. (13) Morishige, K.; Takeuchi, A.; Kato, T. J. Phys. Chem. B 1998, 102, 5495. (14) Ishihara, Y.; Okouchi, S.; Uedaira, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3337. (15) Takahashi, S.; Nishi, N. Bull. Chem. Soc. Jpn. 1995, 68, 539. (16) Matsugami, M.; Takamuku, T.; Otomo, T.; Yamaguchi, T. J. Phys. Chem. B 2006, 110, 12372. (17) Yamanaka, K.; Yamaguchi, T.; Wakita, H. J. Chem. Phys. 1994, 101, 9830. (18) 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. (19) Furukawa, K. Rep. Prog. Phys. 1962, 25, 395. (20) Krogh-Moe, J. Acta Crystallogr. 1956, 2, 951. (21) Norman, N. Acta Crystallogr. 1957, 10, 370. (22) Takamuku, T.; Tabata, M.; Yamaguchi, A.; Nishimoto, J.; Kumamoto, M.; Wakita, H.; Yamaguchi, T. J. Phys. Chem. B 1998, 102, 8880. (23) Johanson, G.; Sandstro¨m, M. Chem. Scr. 1973, 4, 195. (24) Yamaguchi, T. Doctoral Thesis, Tokyo Institute of Technology, 1978. (25) Takamuku, T.; Yamaguchi, T.; Asato, M.; Matsumoto, M.; Nishi, N. Z. Naturforsch. 2000, 55a, 513. (26) Nishi, N.; Takahashi, S.; Matsumoto, M.; Tanaka, A.; Muraya, K.; Takamuku, T.; Yamaguchi, T. J. Phys. Chem. 1995, 99, 462. (27) Matsumoto, M.; Nishi, N.; Furusawa, T.; Saita, M.; Takamuku, T.; Yamagami, M.; Yamaguchi, T. Bull. Chem. Soc. Jpn. 1995, 68, 1775. (28) (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 Academy Press, Inc. and Yamada Science Foundation: 1996; pp 113-120. (b) Matsumoto, M. Master Thesis, Kyushu University, 1996. (29) Takamuku, T.; Maruyama, H.; Watanabe, K.; Yamaguchi, T. J. Solution Chem. 2004, 33, 641. (30) Ichikawa, K.; Kameda, Y.; Yamaguchi, T.; Wakita, H.; Misawa, M. Mol. Phys. 1991, 73, 79. (31) Abragam, A. The Principles of Nuclear Magnetism: The International Series of Monographs on Physics; Oxford University Press: Oxford, 1961. (32) Yoshida, K.; Kitajo, A.; Yamaguchi, T. J. Mol. Liq. 2006, 125, 158. (33) Eggenberger, R.; Gerber, S.; Huber, H.; Searles, D.; Welker, M. Mol. Phys. 1993, 80, 1177. (34) Ropp, J.; Lawrence, C.; Farrar, T. C.; Skinner, J. L. J. Am. Chem. Soc. 2001, 123, 8047.

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