Dehydration Behavior of Polyalcoholic Compounds

Feb 14, 2013 - ... to 70 °C. The temperature dependencies of hydration numbers per hydroxy group (mOH) in the polyalcoholic compounds were determined...
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Hydration/Dehydration Behavior of Polyalcoholic Compounds Governed by Development of Intramolecular Hydrogen Bonds Toshiyuki Shikata*,† and Misumi Okuzono‡ †

Division of Natural Resources and Echo-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan ‡ Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan ABSTRACT: Hydration/dehydration behavior of polyalcoholic compounds with a rigid cyclohexane-type aliphatic molecular frame, such as 1,4-cyclohexanediol (ch(OH)2), 1α,3α,5α-cyclohexanetriol (cis-phloroglucitol, ch(OH)3), and cis-1,2,3,5-trans-4,6-cyclohexanehexaol (myo-inositol, ch(OH)6) was investigated using extremely high-frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The temperature dependencies of hydration numbers per hydroxy group (mOH) in the polyalcoholic compounds were determined. Although the obtained mOH value was ca. 5 in a temperature range lower than 30 °C, it decreased and approached to ca. 3 with increasing temperature for ch(OH)2 and ch(OH)3, in which hydroxy groups hardly form intramolecular hydrogen bonds due to separations between them. This temperature-dependent hydration/ dehydration behavior is characteristic of isolated hydroxy groups without intramolecular hydrogen bond formation. On the other hand, a constant mOH value of ca. 1 was observed irrespective of temperature for ch(OH)6. Because all the separations between adjacent hydroxy groups in ch(OH)6 are proper for (circular type) intramolecular hydrogen bond formation, the obtained temperature-independent small mOH value resulted from an inherent feature of intramolecularly hydrogen bonded hydroxy groups.

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INTRODUCTION Most of the nonelectrolyte water-soluble substances contain at least one of the typically hydrophilic groups in spite of the total molecular size, such as hydroxy (−OH ), ether (−CH2OCH2−), amino (−NH2), and amido (−NHCO−) groups. Interaction called hydration between solvent, water, molecules, and the hydrophilic groups is caused by the formation of hydrogen bonds. In the typical hydrophilic groups, since the hydroxy group possesses chemical structure partially the same as that of water molecules, affinity or favorability between hydroxy groups and water molecules would be distinguishably higher than those in other combinations. However, it has been known that the presence of hydroxy groups is insufficient to improve the favorability between solutes and water molecules.1 The hydration number per hydrophilic group, for example, hydroxy group (mOH), should be a key parameter to govern the water solubility of substances.2−5 Numerous experimental techniques to determine the hydration number have been proposed, such as nuclear magnetic resonance (NMR) techniques,6 neutron scattering experiments,7 ultrasound interferometry,8,9 and dielectric spectroscopic measurements.10 However, different methods have provided rather different values of hydration numbers for the same substance preserving the order in molecular series.9 The reason for the discrepancy observed in the hydration © 2013 American Chemical Society

number evaluated for the same compound is that the physical meaning of hydration numbers differs depending on the employed techniques.10,11 Three-dimensional hydration structure and the lifetime of hydration for each hydrophilic group should also be important parameters responsible for the water solubility dependent on temperature. Unfortunately, no methods proposed so far are able to determine such the important parameters relevant to the hydration behavior simultaneously. Recently, our group developed a technique to determine hydration numbers of solute molecules dissolved in water using dielectric relaxation measurements performed in an extremely high-frequency range up to some tens of gigahertz.12−15 Because the relaxation strength of free water molecules in solutions is precisely evaluated in such a high-frequency range, the amount of water molecules hydrated to solute molecules can be exactly determined. The value of mOH has been evaluated to be ca. 2 at room temperature, 25 °C, for poly(vinyl alcohol)s (PVAs) bearing different average molar masses.14 Because PVA has only hydroxy groups as hydrophilic ones, the relationship mOH ∼ 2 was possibly recognized as the inherent feature of the hydroxy group. On the other hand, the fact that Received: January 9, 2013 Revised: February 8, 2013 Published: February 14, 2013 2782

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the values of mOH for 1,4-cyclohexanediol (ch(OH)2) and 1,4cyclohexanedimethanol (ch(MeOH)2) were determined to be ca. 5 at room temperature revealed that the relationship mOH ∼ 2 is not unique. If one pays attention to the molecular structure of ch(OH)2 and ch(MeOH)2, the separations between the two hydroxy groups in these compounds are too long to form intramolecular hydrogen bonds. However, in the case of PVA the separations between adjacent hydroxy groups are in the range adequate for the formation of intramolecular hydrogen bonds. Consequently, the reason for the discrepancy between the values of mOH in PVA and ch(OH)2 (or ch(MeOH)2) has been attributed to the presence/absence of intramolecular hydrogen bond formation.14 In this study, the dielectric spectroscopic technique to determine the hydration number for solute molecules was extended over a wide temperature range from 10 to 70 °C. Then, the temperature dependence of the mOH was investigated for some model polyalcoholic compounds with a cyclohexanetype molecular frame such as ch(OH)2, 1α,3α,5α-cyclohexanetriol (cis-phloroglucitol, ch(OH)3), and cis-1,2,3,5-trans4,6-cyclohexanehexaol (myo-inositol, ch(OH)6). In the most stable chair conformation of ch(OH)6, five hydroxy groups are in an equatorial conformation, and the remaining one is in an axial conformation. As such, the conformation of ch(OH)6 provides proper distances between adjacent hydroxy groups to generate a cyclic intramolecular hydrogen bond network in a ch(OH)6 molecule in a dilute condition.16 The mOH values determined in this study were compared with that resulting from other techniques, and the reliability of our method was also discussed. Moreover, the origin for a difference in the temperature dependence of the mOH values due to the presence of intramolecular hydrogen bonds was discussed.

Another dielectric measuring system consisting of an RF LCR meter 4287A (Agilent Technologies) and a handmade electrode cell possessing the vacant electric capacitance (C0) of ca. 0.23 pF was operated in a lower-frequency range from 1 MHz to 3 GHz (6.28 × 106 ∼ 1.88 × 1010 s−1 in ω). In this system, ε′ and ε″ were calculated from electric capacitance (C) and conductivity (G) measured by the RF LCR meter via the relationship ε′ = CC0−1 and ε″ = (G − Gdc)(ωC0)−1, where Gdc represents direct conductivity due to ionic impurities. Dielectric measurements were performed in the temperature range from T = 10 to 70 °C (accuracy of ±0.1 °C) using a temperaturecontrolling unit made of a Peltier device. Density measurements for all the sample solutions were carried out using a digital density meter DMA4500 (Anton Paar, Graz) to determine the partial molar volumes of solute molecules at each temperature where dielectric relaxation measurements were performed.

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RESULTS AND DISCUSSION Dielectric Behavior. Dielectric spectra (frequency, ω, dependencies of ε′ and ε″) for an aqueous solution of ch(OH)2 at c = 1.0 M and T = 25 °C are shown in Figure 1 as

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EXPERIMENTAL SECTION Materials. 1,4-Cyclohexanediol (>99%, a mixture of cis and trans form), ch(OH)2, was purchased from Aldrich (St. Louis). cis-Phloroglucitol (>98%), ch(OH)3, was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo). myo-Inositol (>99%), ch(OH)6, was purchased from Wako Pure Chemical Industries, Itd. (Osaka). All the compounds were used without any further purification as model water-soluble polyalcohols. Highly deionized water with the specific resistance higher than 15 MΩ cm obtained by an Elix-UV3 system (Millipore-Japan, Tokyo) was used as a solvent for sample solution preparation. The concentration, c, of sample solutions was ranged from 0.5 to 1.2 M for ch(OH)2 and ch(OH)6. In the case of ch(OH)3, the value of c was varied in a range from 0.05 to 0.15 M. Methods. A dielectric probe kit 8507E equipped with a network analyzer N5230C, ECal module N4693A, and performance probe 05 (Agilent Technologies, Santa Clara) was used for dielectric relaxation measurements over a frequency range from 50 MHz to 50 GHz (3.14 × 108 ∼ 3.14 × 1011 s−1 in angular frequency (ω)). Real and imaginary parts (ε′ and ε″) of electric permittivity were automatically calculated from reflection coefficients measured by the network analyzer via a program supplied by Agilent Technologies. A three-point calibration procedure using hexane, 3-pentanone, and water as the standard materials was performed prior to dielectric measurements at each measuring temperature. Details for the three-point calibrating procedure used in this study has been described elsewhere.17,18

Figure 1. Frequency, ω, dependencies of real and imaginary parts of electric permittivity, ε′ (open circles) and ε″ (open squares), for aqueous solution of ch(OH)2 at the concentration of c = 1.0 M and 25 °C. Broken lines in this figure mean constituent Debye-type relaxation functions to describe the experimental ε′ and ε″ as solid lines via eq 1.

typical examples of the obtained spectra for aqueous solutions of sample compounds. The dielectric spectra seen in Figure 1 are perfectly decomposed into two dielectric relaxation modes described by Debye-type relaxation modes as given by eq 1 below 2

ε′ =

∑ j=1

εj 1+

ω 2τj 2

2

+ ε∞ ,ε″ =

∑ j=1

εjωτj 1 + ω 2τj 2

(1)

where τj and εj mean a dielectric relaxation time and strength of mode j (= 1 and 2 from the shortest relaxation time). Such the decomposition procedure into two kinds of relaxation modes held well also in the dielectric spectra obtained for ch(OH)3, whereas three relaxation modes were necessary for ch(OH)6. The dependencies of τj and εj on the concentration, c, for ch(OH)2 at T = 25 °C are shown in Figures 2(a) and (b), respectively. The relaxation times seem to be weakly dependent on c, whereas magnitudes of relaxation strength of each mode were clearly proportional to c. These behaviors reveal that the 2783

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not far from that per free water molecule looks natural, as described later in detail. The third relaxation mode observed only in ch(OH)6 and possessing a relaxation time of τ3 ∼ 90 ps at 25 °C, which will be shown later (in Figure 6(b)), was attributed to the rotation of hydroxy groups after the lifetime of intramolecular hydrogen bonds between adjacent hydroxy groups. In the cases of ch(OH)2 and ch(OH)3, intramolecular hydrogen bond formation is impossible due to long distances between adjacent hydroxy groups. Then, the rotational motions for hydroxy groups should be rather free and occur on the time scale of the relaxation model j = 2 for exchange of hydrated water molecules. Overall molecular rotation of ch(OH)6 molecules might be the origin for the relaxation mode j = 3, if the molecules possess permanent dipoles because of the fixed orientation of hydroxy groups. However, it is hardly conceivable for hydroxy groups in ch(OH)6 to keep special conformations generating finite dipole moments. The rotation of hydroxy groups is able to make dielectric relaxation independently of the overall rotation of ch(OH)6 molecules. Hydration Numbers and Dynamics. The depression observed in ε1 with increasing c as seen in Figure 2(b) is explained by two factors: a volumetric effect of solute molecules and a hydration effect, as described in detail elsewhere.12−15 How much the ε1 value is depressed by the presence of solute molecules is quantitatively described by eq 212−15 given below 1 − 10−3Vpc ε1 = − 10−3Vwcm 10−3Vpc εw 1+ 2

Figure 2. Concentration, c, dependencies of relaxation times, τj (a), and strength, εj (b), of constituent Debye-type modes for aqueous solutions of ch(OH)2 at 25 °C.

(2)

where εw means the dielectric relaxation strength of pure water; Vp and Vw are the partial molar volume of the solute molecules and that of water ones; and m is the hydration number per solute molecule, respectively. The first term of eq 2 represents the contribution of the volumetric effect of the solute molecule and the second one the hydration effect. Figure 3 shows the concentration, c, dependence of a depression ratio, ε1εW−1, for aqueous ch(OH)2 solutions at T =

contribution of contact between ch(OH)2 molecules, which might be observed in a high concentration region, was less considerable. Because the value of the shortest relaxation time, τ1, was essentially identical to the dielectric relaxation time, τw (= 8.3 ps), of water molecules in the pure liquid sate at the same temperature as 25 °C, the mode j = 1 is assigned to the rotational mode of free water molecules in solution. The depression in the ε1 in proportion to c observed in Figure 2(b) is related to the evaluation of a hydration number as described in the next section. The second relaxation mode j = 2 showing the relaxation time of τ2 ∼ 20 ps has been assigned to an exchange process of hydrated water molecules to ch(OH)2 molecules by free water molecules.14 If this assignment is correct, the relaxation strength would be proportional to c. The reason for this consideration is that the number of hydrated water molecules responsible for the relaxation strength should be proportional to c as observed in Figure 2(b). Moreover, many water-soluble substances in spite of molecular sizes show relaxation modes bearing relaxation times close to 20−30 ps at T = 25 °C in aqueous solution. Then, the modes have been attributed to the exchange process of water molecules hydrated to solutes.13−15 Furthermore, some recent computer simulation studies have revealed that the residence time of water molecules hydrated to solute molecules is on the order of 10 ps, which is close to the τ2 value observed in Figure 2(a).19−21 Additionally, supposing the second relaxation mode j = 2 is the exchange process of hydrated water molecules, the fact that dielectric relaxation strength per water molecule hydrated to a solute molecule is

Figure 3. Concentration, c, dependence of a depression ratio of relaxation strength of the mode j = 1 relative to that of pure water, ε1εW−1, for aqueous ch(OH)2 solutions at T = 25 °C.

25 °C as a typical example. If one assumes that there is no hydration effect, the data should stay on the broken line calculated via eq 2 assuming m = 0. However, the obtained data well followed a solid line calculated assuming m = 10 irrespective of c. Then, we may conclude the hydration 2784

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number, m, is ca. 10 at the temperature. According to the same procedure, the temperature dependence of m was obtained for other model compounds. The reduced value of m by the number of hydroxy groups, mOH, which means the hydration number per hydroxy group, is shown in Figure 4 as functions of temperature, T.

Figure 5. Optimized hydration structures calculated by MOPAC201222 via the method of PM3 for an isolated hydroxy group at the hydration number of mOH = 5 in a temperature region lower than 30 °C (a) and for hydroxy groups forming an intramolecularly sequential hydrogen bonded network in ch(OH)6 at the hydration number of mOH = 1 (b).

neutral condition by using nuclear magnetic resonance (NMR) techniques and quantum chemical calculations. They also concluded that the chair conformation is stabilized by the cyclic intramolecular hydrogen bonds. This pioneering study strongly suggests that ch(OH)6 forms a cyclic intramolecular hydrogen bond network as well as myo-inositol-2-phosphate. Consequently, a difference between hydroxy groups in ch(OH)2 (or ch(OH)3) and ch(OH)6 is the presence/absence of intramolecular hydrogen bond formation. Then, we may conclude that the development of intramolecular hydrogen bond formation determines the hydration number, mOH, and its temperature dependence. For the complete sequential intramolecular hydration formation in linear poly(vinyl alcohol), PVA, we have speculated hydration structure, in which one water molecule directly hydrates to the oxygen atom in a hydroxy group.14 Semiempirical quantum chemical calculations using the MOPAC201222 provided one of the optimized structures for hydrated ch(OH)6 at mOH = 1 via the method of PM3 as seen in Figure 5(b). In the hydration structure of ch(OH)6, one of the lone-pair-electron orbitals of oxygen atoms is occupied (or hydrated) by the hydrogen atom of a neighboring hydroxy group, and the other lone-pair-electron orbital is hydrated by a water molecule directly as seen in the figure. In the case of ch(OH)6, such side-by-side hydrogen bonding develops sequentially to make a cyclic hydrogen bond network. PVA does not form the complete sequential intramolecular hydration in aqueous solution in nature, whereas PVA possesses a mixture of hydroxy groups forming the sequential intramolecular hydration and isolated hydroxy groups with mEO = 5 as depicted in Figure 5(a).14 Temperature dependencies of relaxation times, τj, provide information about differences in dynamics for each relaxation mode j. Figures 6(a) and (b) show so-called Arrhenius plots of relaxation times, with a logarithmic value of τj vs T−1, for aqueous sample solutions. Slopes evaluated from the plots provide the apparent activation energies for each τj. The activation energy, E1*, for the relaxation mode j = 1 was evaluated to be 19 kJ mol−1, which is identical to that of rotational relaxation time of pure water molecules, Ew*. Then, there is no doubt for the assignment of the mode j = 1. In the case of τ2, the plot for ch(OH)2 and ch(OH)3 seen in Figure 6(a) does not seem to be a straight line to provide a single activation energy in the T range examined but has a break point at T−1 ∼ 0.0033 K−1. This observation reveals that

Figure 4. Dependence of hydration number per hydroxy group, mOH, on temperature, T, for aqueous solutions of ch(OH)3, ch(OH)3, and ch(OH)6. Solid curves are a guide for the eyes.

The mOH value seemed to be a constant value of 5 in a temperature range lower than 30 °C for both ch(OH)2 and ch(OH)3. However, the mOH value decreased gradually with increasing temperature and reached a constant value of ca. 3 in a higher temperature region close to 70 °C as seen in Figure 4. This observation suggests that isolated hydroxy groups without forming intramolecular hydrogen bonds in both ch(OH)2 and ch(OH)3 show dehydration behavior around T = 40 °C. Nevertheless, three water molecules still keep a hydrated state even above 60 °C. In the previous study,14 we considered hydration structure for an isolated hydroxy group. Then, we speculated the hydration structure consisting of five hydrated water molecules. In the hydration structure, three primary hydrated water molecules form a first hydration shell and two second hydrated water molecules a second hydration shell. In this study, we carried out semiempirical quantum chemical calculations using a widely accepted semiempirical quantum chemistry program, molecular orbital package (MOPAC2012),22 and obtained one of the optimized hydration structures for the isolated hydroxy group at mOH = 5 as seen in Figure 5(a) via a method of PM3. Three primary hydrated water molecules and two secondary ones are obviously recognized. It is likely that the secondary hydrated two water molecules are more easily dehydrated above 40 °C than the directly hydrated ones. On the other hand, since the temperature dependence of mEO for ch(OH)6 molecules was very weak and the relationship mOH = 1 was recognized, hydroxy groups in ch(OH)6 hardly showed dehydration behavior in the temperature range examined. It is likely that the most stable conformation of ch(OH)6 in aqueous solution is the chair-type one possessing one axial and five equatorial hydroxy groups. Yang et al.16 clearly elucidated that myo-inositol-2-phosphatea phosphate derivative of ch(OH)6 at the axial hydroxy group attached to the second carbon atompossesses a cyclic intramolecular hydrogen bond network due to the formation of hydrogen bonds between adjacent hydroxy groups at an electrically 2785

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Figure 7. Relationship between the relative dielectric strength, ε2(cEOmEOε1VW)−1, and T for aqueous solutions of ch(OH)2, ch(OH)3, and ch(OH)6.

molecules in spite of their restricted situation in the hydration sites. Rotational motions of isolated hydroxy groups bearing a finite dipole moment in aqueous solutions of ch(OH)2 and ch(OH)3 is possibly the reason why the ε2(cmε1VW)−1 values were slightly greater than unity irrespective of temperature as seen in Figure 7. However, in the case of ch(OH)6, the ε2(cmε1VW)−1 values greater than unity are not attributed to the rotations of hydroxy groups. The rotations of hydroxy groups have been already assigned to the mode j = 3. Comparison of Hydration Numbers. It has been wellknown that adiabatic compressibility determination of aqueous solutions using ultrasonic velocity measurements also permit one to determine hydration numbers of solute molecules on the basis of assumption that the compressibility of hydrated solute molecules dissolved in the tested solution is zero.23 Recently, Boscaino et al.24 investigated the hydration numbers of cyclohexanol (ch(OH)), ch(OH)2, and ch(OH)6 at 27 °C via ultrasonic velocity measurements. The converted mOH values from their experiments are 1−2 (ch(OH)), 1.7−2 (ch(OH)2), and 0.7−0.8 (ch(OH)6), respectively. The value for ch(OH)6 fairly agrees with the mOH value of unity obtained in this study. However, the values for ch(OH)2 with isolated hydroxy groups seem to be less than half of the mOH value of ca. 5 determined in this study at the temperature. The fact that the mOH value for ch(OH)2 evaluated by dielectric techniques is more than twice that obtained by ultrasonic velocity measurements suggests that the physical meanings of hydration (number) are slightly different between the two methods. In our previous study,14 a speculated hydration structure for an isolated hydroxy group was proposed as shown in Figure 5(a). Additional two water molecules secondary hydrate to the directly hydrated three water molecules. According to this speculated hydration structure, it is likely that the ultrasonic velocity measurement technique is less sensitive to the presence of the secondary hydrated water molecules. Burakowski et al.25,26 have investigated hydration behavior for aqueous solutions of many kinds of substances using adiabatic compressibility determination using ultrasonic velocity measurements. In the cases of small monoalcohols such as methanol and ethanol, hydration numbers, mOH, are reported to be 0.7 and 2.2 at 25 °C, respectively.25,26 Parke et al.27 also reported that mOH = 2.3 and 1.4 at 20 and 37 °C, respectively, for ethanol. However, with increasing the size of alkyl chains the mOH values increase up to ca. 5, for example, mOH = 4 (for n-butanol), 4.9 (tert-butanol), and 5.2 (n-pentanol), respec-

Figure 6. Dependencies of logarithmic τj on the reciprocal temperature, T−1, for aqueous solutions of ch(OH)2 and ch(OH)3 (a) and ch(OH)6 (b).

the relaxation mechanism of the mode j = 2 alters from a mechanism possessing E2* (∼E1*) to another with lower activation energies with increasing T. These observations strongly suggest there is a difference between the exchanging mechanism of hydrated water molecules in a low-temperature range possessing mOH = 5 and that in a higher-temperature range with mOH = 3 for ch(OH)2 and ch(OH)3. On the other hand, the relationship τ2 and T−1 for ch(OH)6 shown in Figure 6(b) seems to be a straight line to provide a single activation energy. This single activation energy for the mode j = 2 is related to a constant hydration number of mOH = 1 contrary to the temperature-dependent hydration number for the ch(OH)2 and ch(OH)3 systems. The relaxation mode j = 3 observed only in ch(OH)6 showed an activation energy, E3*, slightly higher than the value of E1* and EW* as seen in Figure 6(b). This difference between the E3* and E1* values reveals that the mode j = 3 is not related to overall rotational motion of ch(OH)6 molecules in the medium, water. Dielectric relaxation strength per hydrated water molecule reduced at 1 M is calculated to be ε2(cm)−1 and that per free water molecule reduced at 1 M to be ε1VW. The ratio of ε2(cm)−1 to ε1VW, ε2(cmε1VW)−1, implies relative dielectric strength between hydrated and free water molecules. The relationship between the relative dielectric strength, ε2(cmε1VW)−1, and T for aqueous solutions of ch(OH)2, ch(OH)3, and ch(OH)6 is shown in Figure 7. Weakly temperature-dependent ε2(cmε1VW)−1 values slightly greater than unity were obtained for all the systems. This observation suggests that hydrated water molecules still keep dielectric relaxation strength not so different from that of free water 2786

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tively, at the same temperature.25,26 Burakowski et al.25 claimed that increasing the mOH values with increasing the size of monoalcohols resulted from the contribution of hydrophobic hydration induced by the presence of alkyl groups. However, the mOH value possibly approaches to an inherent value for isolated hydroxy groups connected to relatively large alkyl groups with increasing the size of monoalcohols. It is likely that the reason why the mOH values are smaller for methanol and ethanol is shorter lifetime of hydration due to faster molecular motions of the small alcohols in water. Very recently, Davis et al.28 investigated the hydration structure of water molecules in aqueous solutions of some linear monoalcoholic substances from n-butanol to n-heptanol using Raman scattering techniques with multivariate curve resolution. They clearly confirmed the presence of strongly hydrated water molecules to the solute alcohols. They also found that the minimum number of perturbed water molecules by the presence of the alcohols, which might correspond to the value of mOH, is ca. 5 at 20 °C and decreases with increasing temperature (down to ca. 3 at 60 °C) irrespective of the sizes of the examined alcohols as well as observed in this study for ch(OH)2 and ch(OH)3. Nevertheless, in a temperature range higher than 80 °C, the mOH values show a steep increase with increasing temperature highly depending on the size of the alcohols.28 These temperature and alcohol size dependencies of mOH observed in the high-temperature range are obviously related to the hydrophobic hydration effect.

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CONCLUSIONS Hydration/dehydration behaviors of 1,4-cyclohexanediol (ch(OH)2), 1α,3α,5α-cyclohexanetriol (cis-phloroglucitol, ch(OH)3), and cis-1,2,3,5-trans-4,6-cyclohexanehexaol (myo-inositol, ch(OH)6) were investigated using dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The hydration number per hydroxy group was ca. 5 in a temperature range lower than 30 °C, and it decreased and approached to ca. 3 with increasing temperature for ch(OH)2 and ch(OH)3. On the other hand, a constant hydration number per hydroxy group of ca. 1 was observed irrespective of temperature for ch(OH)6. Although hydroxy groups in ch(OH)2 and ch(OH)3 hardly form intramolecular hydrogen bonds due to long separations between them, separations between adjacent hydroxy groups in ch(OH)6 are adequate for intramolecular hydrogen bond formation. Consequently, the development of intramolecular hydrogen formation in polyalcohol molecules determines the hydration number and its temperature dependence.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.S. is indebted to DIC Co. (Tokyo) for their financial support of this study.

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REFERENCES

(1) Hine, J.; Moorkerjee, P. K. Structural Effects on Rates and Equilibriums. XIX. Intrinsic Hydrophilic Character of Organic 2787

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The Journal of Physical Chemistry B

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dx.doi.org/10.1021/jp400290b | J. Phys. Chem. B 2013, 117, 2782−2788