Are All Polar Molecules Hydrophilic? Hydration Numbers of Ketones

Article pubs.acs.org/JPCB

Are All Polar Molecules Hydrophilic? Hydration Numbers of Ketones and Esters in Aqueous Solution 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 numbers of typical polar compounds like ketones and esters in aqueous solution were precisely determined using high-frequency dielectric relaxation techniques up to a frequency of 50 GHz at 25 °C. Because the hydration number is one of the most quantitative parameters to demonstrate how much are molecules hydrophilic, it is a critical parameter to determine the hydrophilicity of compounds. Hydration numbers of some ketones bearing carbonyl groups were determined to be ca. 0 irrespective of the species of molecules. Moreover, hydration numbers of some esters were also evaluated to be ca. 0 as well as the ketones. These findings suggested that there is no hydrogen bond formation between the ester group and water molecules, nor is there the hydrogen bond formation between the carbonyl group and water molecules. Consequently, esters and ketones bearing typical polar groups are not classified into hydrophilic compounds, but into “hydroneutral” compounds positioned between hydrophilic and hydrophobic ones. Molecular motions of the examined polar molecules in aqueous solution were well described with single Debye-type rotational relaxation modes without strong interaction between solute and water molecules, and also between solute molecules because of the obtained Kirkwood factor close to unity. This independent rotational mode for the polar compounds results from the hydroneutral characteristics caused by the relationship nH = 0.

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°C, since the finite nH values imply the formation of hydrogen bonds between the polar groups and solute water molecules. On the other hand, although low molar mass esters, such as propyl acetate and methyl propionate, are typically polar compounds bearing finite permanent dipole moments and highly soluble in water, slightly bigger esters possess low water solubilities such as 0.058 M for butyl acetate.9 Polymeric compounds like poly(methyl acrylate) and poly(vinyl acetate) bearing ester groups as side chains and poly(glicolide) and poly(lactide) bearing ester groups as backbones do not dissolve into water at all. Then, a question arises in one’s mind due to the low solubilities, “Is the typical polar ester group, −C(O)O−, really phydrophilic?” Moreover, although low molar mass ketones like acetone, 3-pentanone, and cyclohexanone are also typical polar liquid compounds and highly soluble in water, solubilities of slightly bigger ketones are quite low, e.g., 0.125 M for 3-heptanone.10 Polymeric compounds, such as poly(vinyl methyl ketone), bearing carbonyl groups as side chains and poly(ketone)s bearing carbonyl groups as backbones have also quite low solubilities in water. Then, the same question about the hydrophilicity of the carbonyl group, −C(O)−, arises again in one’s mind as given to the ester group.

INTRODUCTION 1−4

According to the description in many chemistry textbooks and also the definition given by IUPAC,5 all the polar compounds and groups are classified into hydrophilic species and nonpolar ones into hydrophobic ones. This clear-cut classification is comprehensive and effective for some polar compounds such as alcohols, ethers, and also amides. In the case of alcohols bearing a polar hydroxy group, −OH, low molar mass alcohols dissolve well into water. Moreover, poly(vinyl alcohol)s bearing −OH group as side chains also dissolve into water. Situations of compounds bearing an ether group, −CH2OCH2−, and amide group, −C(O)NH2, are similar to that of the alcohol. Hydration numbers per functional group (nH) have been evaluated experimentally for some polar groups. For example, when the hydroxy group, −OH, is isolated from other −OH groups without intramolecular hydrogen bond formation, the relationship nH ∼ 5 at room temperature of 25 °C has been reported, and the nH value decreases with increasing temperature.6 In the case of the −CH2OCH2− group, it has been reported that the nH value is ca. 4 at room temperature and it decreases with increasing temperature.7 Furthermore, the nH has been evaluated to be ca. 6 for the −C(O)NH− group of low-mass compounds irrespective of temperature.8 Such finite positive nH values strongly confirm that the three polar groups are really hydrophilic at least in a temperature region lower than 25 © XXXX American Chemical Society

Received: March 26, 2013 Revised: May 26, 2013

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dx.doi.org/10.1021/jp4029968 | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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To make clear answers to the questions, the determination of hydration number, nH, for the two polar group is necessary. Numerous experimental techniques to determine the hydration number have been proposed, such as nuclear magnetic resonance (NMR) techniques,11 neutron scattering experiments,12 ultrasound interferometry,13,14 Raman scattering techniques with multivariate curve resolution,15 and dielectric spectroscopic measurements.6−8,16 However, different methods have provided rather different values of hydration numbers for the same substance preserving the order in molecular series.14 The reason for the discrepancy observed in the hydration number evaluated for the same compound is that the physical meaning of hydration numbers slightly differs depending on employed techniques.16,17 As described above, the hydration number, nH, is one of the most reliable numerical parameters to demonstrate quantitatively how much hydrophilic compounds or functional groups are in aqueous solution. Furthermore, it has been well-known that the temperature dependence of hydration numbers usually well corresponds to that of solubilities of compounds.6−8 Very recently, the essence of hydrophobic hydration of alkyl chains in aqueous solution observed at relatively high temperatures was elucidated using Raman scattering techniques.15 Our group recently 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 20 or 50 GHz.6−8,18−21 Because relaxation strength of free water molecules in aqueous solutions is precisely evaluable in such a high frequency range, the amount of water molecules hydrated to solute molecules can be exactly determined. In this study, the dielectric measurements over a frequency range from 50 MHz to 50 GHz were carried out at 25 °C for aqueous solutions of propyl acetate (PrAc), methyl propionate (MePr), 3-pentanone (3P), and cyclohexanone (cH) to determine hydration numbers, nH, of two typical polar groups, −C(O)− and −C(O)O−. Then, we made a decision on the issue whether the two typically polar groups are really hydrophilic or not, based on the obtained nH values. Molecular dynamics of PrAc, MePr, 3P, and cH in aqueous solution and interaction between these polar compounds and solvent, water, molecules are also discussed.

frequency range from 50 MHz to 50 GHz (3.14 × 108 to 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 Hex, 3P, and water as the standard materials was performed prior to all the dielectric measurements. Details for the three-point calibrating procedure used in this studies have been described elsewhere.22,23 Dielectric measurements were performed at the temperature of T = 25 °C (accuracy of ±0.1 °C) using a temperature-controlling unit made of a Peltier device. Density measurements for all the aqueous sample solutions were carried out using a digital density meter DMA4500 (Anton Paar, Graz) to determine the partial molar volumes of solute molecules at the same temperature as dielectric relaxation measurements.

EXPERIMENTAL SECTION Materials. 3-Pentanone, 3P (>99%), was purchased from Sigma-Aldrich (St. Louis, MO). Cyclohexanone, cH (>99%), propyl acetate, PrAc (>97%), methyl propionate, MePr (>98%), and n-hexane (Hex, > 96%) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and used without any further purification. Highly deionized water with specific resistance higher than 15 MΩ cm obtained by an ElixUV3 system (Millipore-Japan, Tokyo) was used as a solvent for aqueous sample solution preparation. The concentration, c, of aqueous solutions of PrAc ranged from 0.5 to 1.2 M. In the case of aqueous solutions of 3P and cH, the values of c were altered in a range from 0.1 to 0.4 M. On the other hand, the c values ranged from 0.05 to 0.15 M and 0.1 to 0.5 M for hexane solutions of PrAc and MePr, respectively. Methods. A dielectric probe kit 8507E equipped with a network analyzer N5230C, ECal module N4693A, and performance probe 05 (Agilent Technologies, Santa Clara, CA) was used for dielectric relaxation measurements over a

examples of the obtained spectra for aqueous solutions of 3P. The dielectric spectra seen in Figure 1 are well decomposed into two Debye-type relaxation modes as given by eq 1 below

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RESULTS AND DISCUSSION Dielectric Behavior. Dielectric spectra (frequency, ω, dependencies of ε′ and ε″) for an aqueous solution of 3P at c = 0.37 M and T = 25 °C are shown in Figure 1 as typical

Figure 1. Frequency, ω, dependence of real and imaginary parts of electric permittivity, ε′ (open circles) and ε″ (open squares), for aqueous solution of 3P at the concentration of c = 0.37 M and 25 °C. Broken lines mean constituent Debye-type relaxation functions to describe the experimental ε′ and ε″ as solid lines via eq 1. Closed circles and squares represent data of ε′ − ε1′−ε∞ and ε″ − ε1″, respectively.

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2

ε′ = ε′1 + ε′2 + ε∞ =

∑ j=1

2

ε″ = ε″1 + ε″2 =

∑ j=1

εj 1 + ω 2τj 2

+ ε∞ ,

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). Closed circular and square symbols in this figure represent the values of ε′ − ε1′ − ε∞ and ε″ − ε1″, respectively. Agreement between ε′ − ε1′ − ε∞ and ε2′, and also ε″ − ε1″ and ε2″, manifests the presence of the second mode j = 2 and the accuracy of eq 1. Such a decomposition procedure into two kinds of relaxation modes held well in the dielectric spectra obtained for other aqueous 3P at different c values and for all the solutions examined in this study. B



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The dependencies of εj (j = 1 and 2) values on the concentration, c, for 3P and cH solutions are shown in Figure 2.

Figure 3. Concentration, c, dependencies of relaxation strength, εj (j = 1, 2, and ∞), of constituent Debye-type modes for aqueous solutions of PrAc (open symbols) and MePr (closed symbols) at 25 °C. Figure 2. Concentration, c, dependencies of relaxation strength, εj (j = 1, 2, and ∞), of constituent Debye-type modes for aqueous solutions of 3P (open symbols) and cH (closed symbols) at 25 °C.

Debye relationship25 provides the dielectric relaxation time of the mode j = 2 as τ2 ∝ Vsηw at the same temperature, where ηw means the viscosity of the medium, water. This equation suggests that the τ2 value is simply proportional to Vs, whereas one should not compare quantitatively the τ2 values between PrAc and MePr. The physical meaning of ηw is not clear at the molecular level so far. Hydration Numbers and Dynamics. The observed depression in the ε1 value as seen in Figures 2 and 3 is explained by two factors: a volumetric effect of solute molecules and a hydration effect, as described in detail elsewhere.18−21 How much the ε1 value is depressed by the presence of solute molecules is quantitatively described by eq 218−21 given below

The relaxation times weakly depended on c. The magnitude of relaxation strength of the mode j = 1, ε1, decreased in proportion to c. However, the magnitude of the relaxation strength of the mode j = 2, ε2, increased, preserving the proportionality to c in the examined c range as seen in Figure 2. 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 state 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 value in proportion to c observed in Figure 2 is directly related to the evaluation of hydration number, nH, as described later. The relaxation mode j = 2 possessing the relaxation times of τ2 ∼ 20 and ∼25 ps irrespective of c values was assigned to the rotational process of solute molecules, 3P and cH, because of the presence of dipole moments possessing a relatively large value, ca. 3.0 D,24 for carbonyl groups. A small difference in the τ2 values between 3P and cH reflects a slight difference in the effective sizes of these molecules in water. The partial molar volume of cH, Vs = 98.7 cm3 mol−1, is slightly greater than that of 3P, Vs = 97.0 cm3 mol−1. If the solute molecules possessed certain finite (nonzero) hydration numbers, nH, the mode j = 2 was possibly assigned to an exchange process of hydrated water molecules by free ones. However, since the nH values for both 3P and cH will be determined to be zero in the next section, one does not have to pay attention to the exchange process of hydrated water molecules. Figure 3 shows the concentration, c, dependencies of εj (j = 1 and 2) for aqueous solutions of esters, PrAc and MePr. Because essentially the same c dependencies of τj and εj as observed in aqueous 3P and cH solutions (cf. Figure 2) were obatained, assignment for each mode should be the same as in the 3P and cH solutions. However, the clearly smaller ε2 value of the esters than that of ketones suggests that the dipole moment of the ester group is smaller than the carbonyl group. Furthermore, the partial molar volumes of PrAc and MePr in aqueous solution were determined to be Vs = 104.9 and 88.08 cm3 mol−1 at 25 °C, respectively. Such a distinct difference in the effective sizes of PrAc and MePr in aqueous solution is clearly recognized as a difference between τ2 values, ∼40 and ∼25 ps, respectively. If one assumes that the two esters are spherical molecules possessing volumes proportional to Vs, Stokes−Einstein−

1 − 10−3Vsc ε1 = − 10−3VwcnH εw 1 + 10−3Vsc /2

(2)

where εw means the dielectric relaxation strength of pure water and Vw is the partial molar volume of water ones. The first term of eq 2 represents the contribution of the volumetric effect of the solute and the second one the hydration effect. Figure 4, a and b, shows the concentration, c, dependencies of depression ratios, ε1εW−1, for aqueous solutions of 3P and cH at T = 25 °C, respectively. The c dependencies of ε1εW−1 for aqueous solutions of PrAc and MePr are shown in Figure 5, a and b. If one assumes that there is no hydration effect, the ε1εW−1 data should stay on the line calculated via eq 2 assuming nH = 0. The obtained data well followed the line of nH = 0 irrespective of c for both the systems. Then, the hydration number, nH, is zero at the temperature of T = 25 °C for both the ketones, 3P and cH, and the esters, PrAc and MePr. From these considerations, one may conclude that the carbonyl, −C(O)−, and ester, −C(O)O−, groups should not be classified into hydrophilic groups, but into newly defined “hydroneutral” groups. Here, the hydroneutral groups are defined as functional groups positioned between hydrophilic and hydrophobic groups due to nH = 0 and high solubilities in water observed in low molar mass compounds bearing the groups. The absence of hydrated water molecules, nH = 0, makes consideration of molecular dynamics of dissolved molecules in the aqueous solutions much simpler. Because one does not have to take account of an exchange process of hydrated water molecules to the ketones by free water molecules, the second relaxation mode j = 2 is attributed to the rotational relaxation mode of the solutes, 3P, cH, PrAc, and MePr, in water. If the interaction between solvent, water, and solute molecules, 3P (or cH, PrAc, and MePr), is not strong and that between solute C



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Figure 4. Dependence of the ratio, ε1εW−1, on c for aqueous solutions of 3P (a) and cH (b) at 25 °C.

Figure 5. Dependence of the ratio, ε1εW−1, on c for aqueous solutions of PrAc (a) and MePr (b) at 25 °C.

in this study corresponds to that of the isolated state found in aqueous acetone solutions. If the −C(O)− group of 3P and cH is hydrated by water molecules strongly at a certain hydration number, nH, spectroscopic data such as infrared adsorption spectra should show clear alternation at a composition in relation to the nH value. However, such clear changes in spectroscopic data were not reported due to their low solubilities. In the case of the ester group, we determined the value of intrinsic dipole moment, |μ0|, in this study. Dielectric spectra for n-hexane, Hex, solutions of PrAc and MePr at 25 °C were well described by a Debye-type relaxation mode with a single set of relaxation time, τ1, and strength, ε1. Because Hex is a common nonpolar solvent, the observed single dielectric relaxation mode was naturally assigned to the rotational modes of PrAc and MePr molecules. By use of the Kirkwood26 and Fröhlich27 relationship, in which ε2 of eq 3 is replaced by ε1, the |μap| values were calculated from the data of ε1 as a function of c. The extrapolated values of |μap| at c = 0 M was evaluated to be 1.8 ± 0.1 D as the |μ0| for both PrAc and MePr. This value perfectly agreed with the value reported for many low mass esters in the literature.30 Using this value of |μ0| = 1.8 D, the dependencies of |μap| and gK on c were calculated from data shown in Figure 3 via eq 3. Both PrAc and MePr showed the relationship |μap| ∼ 1.8 D and gK ∼ 1 irrespective of c. These observations strongly manifested that molecular dynamics of solute, PrAc and MePr, molecules in the systems are rather independent from that of the solvent, water, as well as observed in the 3P and cH solutions. Consequently, all the ketones and esters examined in this study rotate almost freely in aqueous solution. Such a simple dynamic behavior of ketones and esters found in this study results from the characteristics of “hydroneutral” compounds, which dissolve but are not hydrated at all in water. Controlling Factor of Hydrophilicity. Acetonitrile (AcCN) is known as a typical dipolar compound and is highly

molecules can be simply described with a Kirkwood factor (gK), the square of the apparent dipole moment of the solute molecule (μap2) is described by an equation given below assuming Kirkwood26 and Fröhlich27 relationship. μap2 =

9ε2(2ε2 + 3ε∞)εv kBT (ε2 + ε∞)(ε∞ + 2)2 ϕNA

,

μap2 = gK μ0 2

(3)

In this equation, ϕ, NA, ε∞,εv, kBT, and μ02 represent the molar concentration of the solute in units of mol cm−3, Avogadro’s number, the high-frequency limiting electric permittivity, the electric permittivity in a vacuum, the product of a Boltzmann constant and absolute temperature, and the square of the intrinsic dipole moment of the solute, respectively. The |μ0| value of the carbonyl group, −C(O)−, was previously determined to be ca. 3.0.24 Then, the concentration, c, dependencies of |μap| and gK were obtained from Figure 2 via eq 3. Both 3P and cH showed the relationship |μap| ∼ 3.0 D and gK ∼ 1 almost irrespective of c. The obtained c independent gK value of ∼1 reveals that the interaction between water and 3P (or cH) molecules and the interaction between 3P (or cH) molecules are rather weak as we supposed first in the procedure of |μap| and gK analysis. Many people believe that low molar mass ketones like acetone are highly hydrated in aqueous solution over an entire composition range due to hydrogen bond formation to water molecules. However, experimental evidence has elucidated the presence of isolated acetone molecules in a dilute regime at concentrations lower than ca. 2.4 M (0.05 in molar fraction) which is identical to the composition of the type II clathrate hydrate.28 It has been reported that interaction between acetone and water molecules in the isolated state observed in the dilute regime is rather repulsive and hydrophobic hydration due to the presence of hydrophobic methyl groups is more important than hydrophilic hydration caused by the hydrogen bond formation between the −C(O)− groups and water molecules.28,29 The concentration range of 3P and cH covered D



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Table 1. Values of nH and pKa for Some Polar Compounds Bearing Various Kinds of Polar Groups at the Room Temperature of 25 °C compound

nitro alkyl34

acetonitrile35

acetone34

methyl acetate36

diethyl ether37

ethanol38

acetamide39

polar group nH pKa

−NO2 (0) −12

−CN (0) −11.8

−C(O)− 0 −7.5

−C(O)O− 0 −6.0

−CH2OCH2− 4 −3.6

−OH 5 −2.4

−C(O)NH2 6 −0.6

hydrated (cf. Table 1). It is well-known that nitrogen lone-pair electrons are delocalized to the carbonyl oxygen, and the amide group actually has a tendency to be protonated at the oxygen atom. This tendency would be an essential reason for the high hydration number of the amide group. A project to determine the nH values of nitriles and nitro compounds directly by using high-frequency dielectric relaxation techniques is now in progress.

soluble in water. A cyano group (−CN) neither has hydrogen nor oxygen atoms which make hydrogen bonds between water molecules. However, a nitrogen atom of the −CN group possibly has the ability to be a water hydrogen (proton) acceptor for the hydrogen bond formation to water molecules because of the presence of its lone-pair electron orbital. Valeronitrile (1-cyanobutane, BuCN), which is a slightly larger nitrile compound than AcCN, has a quite low solubility in water of 0.01−0.05 M.31 Moreover, poly(acrylonitrile) is insoluble in water. These low solubilities of BuCN and poly(acrylonitrile) possibly suggest that the −CN group has the hydration number of nH = 0, and AcCN is not assisted by the hydration behavior to get its high solubility in water. In such the case, the cyano group, −CN, should not be classified into the hydrophilic groups. Similar consideration is possible for another polar compound, nitromethane (MeNO2), bearing a highly polar nitro group (−NO2). MeNO2 is completely soluble in water. However, 1-nitropropane (PrNO2), which is a slightly larger nitro compound than MeNO2, has a low solubility of 0.17 M.32 The solubility of PrNO2 in water is comparable to that of PrAc, and the −NO2 group also has the same hydration number of nH = 0 as the −CN group. Then, the nitro group, −NO2, should not be classified into the hydrophilic groups. Although the two typical polar groups, −CN and −NO2, possess large dipole moments such as 3.9−4.123 and 3.3−4.0 D,33 respectively, both the groups do have the hydration number of nH = 0. Consequently, the nH values are not controlled by the magnitudes of dipole moments of polar groups. Because the considered hydroneutral polar groups, −C(O)−, −C(O)O−, −CN, and −NO2, are all aprotic, the groups have to behave as water proton acceptors to be hydrated in aqueous solution. Basically, protonated form pKa values of the groups quantitatively demonstrate how easily the groups are positively protonated as seen in eq 4 as an example for the group, −C(O)−. pK a

⟩CO⊕H HooI ⟩CO + H⊕

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CONCLUSIONS The hydration numbers of water-soluble ketones, 3-pentanone and cyclohexanone, bearing a carbonyl group and esters, propyl acetate and acetyl propionate, bearing an ester group were determined to be ca. zero at 25 °C using extremely high frequency dielectric relaxation measurements up to 50 GHz. Then, these typical polar compounds should not be classified into hydrophilic compounds but into newly defined “hydroneutral” compounds. Such the hydroneutral compounds exhibit rather simple dynamic behavior recognized as almost free rotation in aqueous solution due to very weak interaction between the compounds and water molecules resulting from the unique characteristics of zero hydration number. Whether the polar compounds or groups have finite nonzero hydration numbers or not is not controlled by magnitudes of dipole moments of the compounds, but by the ability to be water proton acceptors roughly quantifiable via the pKa values.

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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.

(4)

Nevertheless, it is expected that the pKa value is also a rough measure of the ability for a group to be a water proton acceptor. Table 1 summarizes the reported pKa values for the hydroneutral polar groups together with those for hydrophilic polar groups, −OH, −CH2OCH2−, and −C(O)NH2.34−39 The magnitudes of pKa values for the hydroneutral polar groups are obviously larger than those for hydrophilic polar groups. This fact reveals that the hydrophilic polar groups are more easily protonated than the hydroneutral polar ones. Then, one might conclude that the hydrophilic polar groups are hydrated to have finite positive nH values more easily than the hydroneutral polar groups. It seems that a certain threshold pKa value between −6 and −4 divides polar groups into hydrophilic and hydroneutral polar groups. It is noteworthy to describe the reason why an amide group consisting of a carbonyl group and an amino group is highly hydrated at nH = 6, but an isolated carbonyl group is not

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