Hydration of Short-Chain Poly(oxyethylene) in Carbon Tetrachloride

A. Andres Leal , Joseph M. Deitzel , Steven H. McKnight , John W. Gillespie. Journal of Polymer Science Part B: Polymer Physics 2009 47 (18), 1809-182...
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19704

J. Phys. Chem. B 2005, 109, 19704-19710

Hydration of Short-Chain Poly(oxyethylene) in Carbon Tetrachloride: An Infrared Spectroscopic Study Md. Ruhul Matin, Yukiteru Katsumoto, Hiroatsu Matsuura, and Keiichi Ohno* Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima 739-8526, Japan ReceiVed: May 25, 2005; In Final Form: August 18, 2005

Hydration of short-chain poly(oxyethylene)s, CH3(OCH2CH2)mOCH3 (abbreviated as C1EmC1) (m ) 1-3), in carbon tetrachloride has been studied by infrared spectroscopy. The O-H stretching vibrations of water in ternary solutions with H2O:C1EmC1:CCl4 mole ratios of 0.000418:0.005:0.995 to 0.000403:0.04:0.96 were analyzed. Two types of hydrogen bonds are formed in the interaction between water and C1EmC1 in carbon tetrachloride; one is a monodentate hydrogen bond, in which only one of the O-H bonds of a water molecule participates in hydrogen bonding, and the other is a bidentate hydrogen bond, in which both of the O-H bonds of a water molecule participate in hydrogen bonding by bridging oxygen atoms separated by two or more monomer units on the polymer chain. An important finding is that the bidentate hydrogen-bond bridge is not formed between the nearest-neighbor oxygen atoms. This experimental observation supports the results of previous molecular dynamics simulations. The shortest oligomer of poly(oxyethylene), i.e., CH3OCH2CH2OCH3 (1,2-dimethoxyethane) with a single monomer unit, is suggested not to be an adequate model for this polymer with respect to hydrogen bonding to water. The hydrogen bonding in a 1:1 C1EmC1water adduct in carbon tetrachloride represents primitive incipient hydration of poly(oxyethylene). The present results indicate that both monodentate and bidentate hydrogen bonds are important and the latter is destabilized more rapidly than the former with increasing temperature. This dehydration process can be a potential mechanism of the poly(oxyethylene)-water phase separation.

Introduction A polymer with its chemical formula (-OCH2CH2-)m is called poly(oxyethylene) (POE), poly(ethylene oxide), or poly(ethylene glycol) and is used in diverse fields of science and technology, in particular, in biological and biomedical applications.1-4 The relevant functions of the polymer include the ability to improve the biocompatibility of foreign materials in both in vivo and ex vivo applications and the ability to be a stabilizing surface coating in biological environments. POE is also used widely as a polymer matrix for drug delivery. These biological and biomedical functions are associated for the most part with a peculiar property of this polymer, namely, a dual nature of hydrophilicity and hydrophobicity.5 Actually, POE has a practically unlimited solubility in water at ambient temperatures, but at elevated temperatures the aqueous solution separates into two phases, one water-rich and the other water-depleted.6-10 At ambient temperatures, hydrophilicity and hydrophobicity are balanced in such a way that POE is effectively hydrophilic. According to the experimental and theoretical results,11-13 the hydration of the POE chain is one of the important factors of the high solubility of this polymer in water, and the direct hydrogen bonding between the oxyethylene group and water, in particular, is a determining factor of the hydrophilicity/ hydrophobicity of POE. Previous studies14,15 have yielded some results on the hydration of short-chain POE in aqueous solution in a wide rage of concentrations. Water molecules in the concentrated solutions exist in two stable configurations, either participating in water clusters enclosed in cavities of POE * To whom correspondence should be addressed. E-mail: kohno@ sci.hiroshima-u.ac.jp.

molecules or as monomers bridging ether oxygen atoms of POE molecules.15 Investigations of the hydration in the immediate vicinity of the POE chain are critically important in understanding the dual nature of POE in water. In a situation where a hydrogen bond is formed exclusively between a water molecule and a single POE chain, there is a possibility of observing an incipient primitive hydration structure of POE. This situation is substantiated if we study hydrogen bonding of water dissolved in a large excess of carbon tetrachloride that contains an appropriate amount of POE. In line with this scheme, we have undertaken an infrared spectroscopic study, focusing on the O-H stretching vibrations of water, on ternary solutions consisting of water, short-chain POE, and carbon tetrachloride. The essential idea in our study lies in the fact that only a small amount of water is soluble in carbon tetrachloride with a solubility of 8.7 × 10-3 mol dm-3 at 25 °C,16 which is equivalent to a H2O:CCl4 mole ratio of 0.00084:1. The short-chain POE compounds studied in this work are R-methyl-ω-methoxymono-, -bis-, and -tris(oxyethylene)s (or mono-, di-, and triethylene glycol dimethyl ethers), i.e., CH3(OCH2CH2)mOCH3 with m ) 1-3 (abbreviated as C1EmC1; C1E1C1 is also called 1,2-dimethoxyethane, usually abbreviated as DME). Diethyl ether was also studied as a reference that contains only one ether oxygen atom in the molecule. Experimental Section Reagents used in this work are C1E1C1 (supplied by Nacalai Tesque), C1E2C1 (Tokyo Kasei Kogyo), C1E3C1 (Tokyo Kasei Kogyo), diethyl ether (Wako Pure Chemical Industries), and carbon tetrachloride (Hayashi Pure Chemical Industries). The

10.1021/jp0527554 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/04/2005

Hydration of Poly(oxyethylene)

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TABLE 1: Compositions of Ternary Solutions Studied in This Work, Consisting of Water, C1EmC1, and Carbon Tetrachloride mole fraction H2O

C1EmC1

CCl4

mole ratio H2O:C1EmC1

0.000 42 0.000 418 0.000 417 0.000 416 0.000 412 0.000 403

0.0 0.005 0.008 0.010 0.02 0.04

1.0 0.995 0.992 0.990 0.98 0.96

1:0 1:12 1:19 1:24 1:49 1:99

purity of C1E1C1, C1E2C1, and C1E3C1 is higher than 99.0% and that of diethyl ether and carbon tetrachloride is higher than 99.5%. These reagents were dried with molecular sieves before use. Water was carefully purified by distillation. The infrared spectra were recorded on a Bruker IFS66V vacuum spectrometer equipped with a deuterated triglycine sulfate detector by coaddition of 100 scans at a resolution of 2 cm-1. The liquid samples for infrared measurements were contained in an airtight quartz cell of 2-cm path length (Tosoh Quartz, type T-31-IR-20). The cell with sample was enclosed by a homemade holder which was temperature-regulated by a Peltier device. The temperature was monitored by a thermistor attached directly to the cell wall. The spectra were measured at five temperatures: 5, 15, 25, 35, and 45 °C. The ternary solutions consisting of water, C1EmC1 (m ) 1-3), and carbon tetrachloride with compositions given in Table 1 were prepared as follows. First, water-saturated carbon tetrachloride was prepared at 25 °C with use of a glass separatory funnel, in which carbon tetrachloride and excess water were mixed, and a carbon tetrachloride-rich layer was separated therefrom after the liquid mixture was equilibrated. A binary solution of water and carbon tetrachloride with a concentration of water of half the concentration for the full water saturation was prepared by diluting the water-saturated carbon tetrachloride with the same amount of dry carbon tetrachloride. The concentration of water in this solution is 1/2(8.7 × 10-3) mol dm-3 at 25 °C,16 which is equivalent to a H2O:CCl4 mole ratio of 0.00042:1. The H2O-C1EmC1-CCl4 ternary solutions for infrared measurements were prepared by adding an appropriate amount of C1EmC1 to the 0.00042:1 H2O-CCl4 binary solution. The C1EmC1 mole fraction in the solution was varied from 0.005 to 0.04, with a H2O:CCl4 mole ratio being held constant at 0.00042:1 (Table 1). The H2O-diethyl ether-CCl4 ternary solutions were also prepared in the same way. Preparation and handling of the samples were performed in a sealed box under a dry nitrogen atmosphere. The infrared spectra of water in the H2O-C1EmC1 (or diethyl ether)-CCl4 solutions were studied in a wavenumber region between 3300 and 3800 cm-1. The absorptions of C1EmC1 or diethyl ether in this region associated with the combinations and/or the overtones were eliminated from the spectra of the H2O-C1EmC1 (or diethyl ether)-CCl4 solutions by subtracting the spectra of water-free C1EmC1 (or diethyl ether)-CCl4 solutions at the same concentrations of C1EmC1. The overlapping O-H stretching bands in the 3300-3800 cm-1 region were decomposed by curve fitting using a nonlinear least-squares method based on the quasi-Marquardt algorithm.17,18 Each of the bands was fitted with a Lorentzian function or a mixed Lorentzian-Gaussian function. Results and Discussion Interpretation of Infrared Spectra. Because the H2O:CCl4 mole ratio for all ternary solutions studied is 0.00042:1, most

Figure 1. Structures and O-H stretching vibrations of water in H2OC1EmC1-CCl4 solutions: free water (a), monodentate hydrogen-bonded water (b), and bidentate hydrogen-bonded water (c).

of the water molecules are anticipated to exist as monomers. A previous study19 in fact showed that water molecules in watersaturated carbon tetrachloride (H2O:CCl4 mole ratio of 0.00084: 1) exist predominantly as monomers, with 3-4% dimers. Accordingly, the existence of water dimers in the ternary solutions does not affect the substantial results from our spectroscopic measurements. In the solutions where a minor amount of water is dissolved in a large excess of carbon tetrachloride with added C1EmC1, we expect three types of water molecules: (1) a free water molecule without hydrogen bonding, (2) a water molecule bound to an ether oxygen atom of C1EmC1 by a single hydrogen bond (monodentate hydrogen-bonded water), and (3) a water molecule bound to two ether oxygen atoms of C1EmC1 by bridging hydrogen bonds (bidentate hydrogen-bonded water). The structures of the three types of water molecules are depicted in Figure 1. We studied the O-H stretching vibrations of water20 to gain accurate information of the state of hydrogen bonding in the H2O-C1EmC1-CCl4 solutions. The vibrations of water relevant to the present study are summarized as follows. A free water molecule with C2V local symmetry has two O-H stretching vibrations, symmetric stretching (νfree(sym)) and antisymmetric stretching (νfree(asym)). For a monodentate hydrogen-bonded water molecule with Cs local symmetry, the two O-H bonds are nonequivalent, one participating in hydrogen bonding and the other being unbonded, giving a stretching vibration of the bonded O-H bond (νmono(bond)) and a stretching vibration of the unbonded O-H bond (νmono(unbond)). A bidentate water molecule with C2V local symmetry gives a symmetric O-H stretching vibration (νbi(sym)) and an antisymmetric O-H stretching vibration (νbi(asym)). A bidentate water molecule, nonetheless, may have a symmetry lowered from C2V because of possible nonequivalent configurations of the two hydrogen bonds. The modes of the O-H stretching vibrations of water are shown in Figure 1. The infrared spectra of water in the H2O-C1EmC1-CCl4 ternary solutions (m ) 1-3) observed at 5 °C are shown in Figure 2, where the spectra of water in the H2O-diethyl etherCCl4 solution are also shown. The spectra on the left side in the figure are those for a H2O:C1EmC1:CCl4 mole ratio of 0.000418:0.005:0.995 (to be denoted as a C1EmC1 mole fraction 0.005 for the sake of simplicity) and the spectra on the right side are those for a mole ratio of 0.000403:0.04:0.96 (C1EmC1 mole fraction 0.04). A comparison of the spectra of the ternary solutions containing C1E1C1, C1E2C1, and C1E3C1 shows considerable differences in their appearance, depending on the

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Figure 2. Infrared spectra of water at 5 °C in ternary solutions of H2O-diethyl ether-CCl4 (a, b), H2O-C1E1C1-CCl4 (c, d), H2O-C1E2C1-CCl4 (e, f), and H2O-C1E3C1-CCl4 (g, h). The H2O:C1EmC1 (diethyl ether):CCl4 mole ratios are 0.000418:0.005:0.995 (a, c, e, g) and 0.000403:0.04: 0.96 (b, d, f, h). The decomposed bands, A-F, are shown by thinner lines.

length of the molecular chain. At a C1EmC1 mole fraction 0.005, the spectrum for C1E1C1 shows four distinct bands, while the spectra for C1E2C1 and C1E3C1 show six resolved bands. The spectrum for C1E1C1 closely resembles that for diethyl ether at the same composition of the solution. Assignment of the observed bands of water in the ternary solutions will be discussed below. The spectra of the H2OC1E1C1-CCl4 solution (Figure 2c,d) agree substantially with the spectra of the H2O-diethyl ether-CCl4 solution (Figure 2a,b). This implies that the state of water molecules in the two ternary solutions is essentially the same. Accordingly, we start with an examination of the spectra of the H2O-diethyl etherCCl4 solution, since we can interpret the observed bands for this solution more definitively. At a mole fraction of diethyl ether of 0.005, water molecules exist as free monomers or participating in 1:1 diethyl ether-water adducts. Water in carbon tetrachloride with no added substance gives infrared bands at

3707 and 3616 cm-1, which are associated, respectively, with the antisymmetric O-H stretching vibration (νfree(asym)) and the symmetric O-H stretching vibration (νfree(sym)) of free water.19,21-23 These bands coincide with the bands for the H2Odiethyl ether-CCl4 solution at 3707 cm-1 (band A) and 3616 cm-1 (band C). Accordingly, bands A and C are assigned to νfree(asym) and νfree(sym), respectively. The decrease in intensities of these bands with increasing diethyl ether fraction (Figure 2b) confirms this assignment. The spectra of the H2O-diethyl ether-CCl4 solution exhibit two other bands at 3686 cm-1 (band B) and 3495 cm-1 (band F). These bands are assigned to the O-H stretching vibrations of water in the 1:1 diethyl ether-water adduct. Since diethyl ether possesses only one oxygen atom in the molecule, the hydrogen bond formed in the adduct is unambiguously of the monodentate form. Accordingly, band B is assigned to the stretching vibration of the unbonded O-H bond of monodentate

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TABLE 2: O-H Stretching Vibrations of Water in Ternary Solutions of Water, C1EmC1, and Carbon Tetrachloride band

wavenumber/cm-1

designation

assignment

A B C D E F

3707 3686 3616 3590-3595 3525 3490-3510

νfree(asym) νmono(unbond) νfree(sym) νbi(asym) νbi(sym) νmono(bond)

antisymmetric O-H stretching of free water unbonded O-H stretching of monodentate H-bonded water symmetric O-H stretching of free water antisymmetric O-H stretching of bidentate H-bonded water symmetric O-H stretching of bidentate H-bonded water bonded O-H stretching of monodentate H-bonded water

hydrogen-bonded water (νmono(unbond)), in conformity with its sharp feature and its wavenumber close to that of band A due to free water. Band F at 3495 cm-1, on the other hand, is significantly broader and is assigned to the stretching vibration of the bonded O-H bond (νmono(bond)). The present assignment of the O-H stretching vibrations of monodentate hydrogenbonded water is consistent with the previous results for the H2O-diethyl ether-CCl4 solution.21,22,24 Having established the assignment of the bands for the H2Odiethyl ether-CCl4 solution, we can interpret the spectra of the H2O-C1E1C1-CCl4 solution (Figure 2c,d) in a straightforward way; bands A and C are assigned to νfree(asym) and νfree(sym), respectively, of free water, and bands B and F are assigned to νmono(unbond) and νmono(bond), respectively, of monodentate hydrogen-bonded water. In the spectra of the H2O-C1E2C1-CCl4 and H2O-C1E3C1CCl4 solutions at a C1EmC1 mole fraction of 0.005 (Figure 2e,g), we observe six O-H stretching bands. Of these, bands A and C are assigned to free water and bands B and F are assigned to monodentate hydrogen-bonded water, in accordance with the assignment for the H2O-diethyl ether-CCl4 and H2O-C1E1C1CCl4 solutions. The spectra of the H2O-C1EmC1-CCl4 solutions (m ) 2 and 3) show two additional bands with fairly strong intensities, one at 3590-3595 cm-1 (band D) and the other at 3525 cm-1 (band E). They are assigned to the O-H stretching vibrations of water bound to C1EmC1 in a bidentate form. The most appropriate way to establish this assignment is to examine the infrared spectra of water in a binary solution of water and diethyl ether, where the majority of water molecules are engaged in hydrogen bonding that bridges oxygen atoms belonging to different diethyl ether molecules. A previous study21 has shown that a solution of 0.1-0.2 mol dm-3 of water in diethyl ether exhibits strong infrared bands at 3600 and 3525 cm-1, which are assigned to the O-H stretching vibrations of bidentate hydrogen-bonded water. These bands agree with bands D and E observed for the H2O-C1E2C1-CCl4 and H2O-C1E3C1CCl4 solutions. Accordingly, band D at 3590-3595 cm-1 is assigned to the antisymmetric O-H stretching vibration (νbi(asym)) and band E at 3525 cm-1 is assigned to the symmetric O-H stretching vibration (νbi(sym)) of bidentate hydrogen-bonded water. The assignment of the O-H stretching bands of water in the ternary solutions studied is summarized in Table 2. Monodentate versus Bidentate Hydrogen Bonding. The interpretation of the spectra presented in the preceding section has shown that water molecules in the H2O-C1E1C1-CCl4 solution exist as being free without hydrogen bonding or monodentate hydrogen-bonded, while water molecules in the H2O-C1E2C1-CCl4 and H2O-C1E3C1-CCl4 solutions are free, monodentate hydrogen-bonded, or bidentate hydrogen-bonded. Thus, there is a striking contrast between C1E1C1 and the longer homologues, C1E2C1 and C1E3C1, in that a water molecule is bound to C1E1C1 only with monodentate coordination, while it is bound to C1EmC1 (m g 2) with monodentate or bidentate coordination.25 We can state alternatively that the bidentate hydrogen-bond bridge is formed between ether oxygen atoms

separated by at least two monomer units, i.e., between Oa and Oc in the OaCH2CH2ObCH2CH2Oc segment or between Oa and a remoter oxygen atom, but is not formed between the nearestneighbor oxygen atoms Oa and Ob or between Ob and Oc. However, we should leave open the possibility that there may exist a slight amount of bidentate hydrogen bond that bridges the nearest-neighbor oxygen atoms, as the O-H stretching bands associated with this hydrogen bond might be obscured by the overwhelming band due to monodentate hydrogen bond in the spectra of the H2O-C1E1C1-CCl4 solution. The structures of the hydrogen bonds between water and C1E1C1 and between water and C1EmC1 (m g 2) are schematically shown in Figure 3.

Figure 3. Structures of hydrogen bonds between water and C1E1C1 (a) and between water and C1EmC1 (m g 2) (b, c). a and b are monodentate hydrogen bonds, and c is a bidentate hydrogen bond.

It is anticipated that a water molecule has more chances to form a bidentate hydrogen bond to a longer chain POE than to a shorter one, because a longer POE molecule has more oxygen atoms to which hydrogen atoms of water are accessible. Figure 2 in fact shows that the intensities of bands D and E (bidentate hydrogen bonding) relative to bands B and F (monodentate hydrogen bonding) are stronger for C1E3C1 than for C1E2C1 at the same composition of the solution (compare Figure 2g with Figure 2e, and Figure 2h with Figure 2f). This spectral observation demonstrates that water molecules interacting with longer chain POE have stronger possibilities of forming a bidentate hydrogen bond. A previous quantum chemical study of a 1:1 C1E1C1-water adduct26 has indicated that all of the three lowest energy configurations are stabilized by monodentate coordination of water, not bidentate, being consistent with the experimental results of the present work. The unattainable bidentate hydrogenbond bridge between the nearest-neighbor oxygen atoms can be explained by unfavorable dipole-dipole interactions between C1E1C1 and water with this configuration. The favorable dipoledipole interaction accords more likely with the monodentate coordination of water.27 The present spectroscopic observation also supports the results of molecular dynamics simulations.28 The simulations showed that water molecules form hydrogen bonds to ether oxygen atoms of C1E1C1 regardless of the ether

19708 J. Phys. Chem. B, Vol. 109, No. 42, 2005 conformation. Possible conformers of C1E1C1 that may allow, simply from a geometrical point of view, the bidentate coordination of water are those with the gauche conformation around the central C-C bond (e.g., trans-gauche(-trans (TG(T), trans-gauche(-gauche( (TG(G(), and ( ( ( ( ( ( gauche -gauche -gauche (G G G )). Accordingly, if the bidentate hydrogen bond were formed between C1E1C1 and water, the extent of hydrogen bonding would depend significantly on the conformation of the C1E1C1 molecule. The formation of a monodentate hydrogen bond, on the other hand, is possible for most of the conformers, being independent of the molecular conformation. The formation of only the monodentate hydrogen bond with C1E1C1 is supported by the results of a previous infrared spectroscopic study.13 It was shown that the conformational behavior of C1E1C1 in water is different from that of C1EmC1 (m ) 2-4); namely, the rate of increase in the gauche population for the C-C bond of C1E1C1 with increasing water fraction is considerably smaller than that of the longer homologues. This implies that the hydrogen bond between a water molecule and a single oxyethylene unit (C1E1C1) does not stabilize efficiently the gauche conformation around the C-C bond, in contrast to the high stabilization of the gauche conformation in consecutive oxyethylene units (C1EmC1, m g 2). The bidentate hydrogen bond by a water molecule that bridges ether oxygen atoms on the same molecular chain has often been realized in molecular dynamics simulations of oligo(oxyethylene) and 18-crown-6 (cyclic hexakis(oxyethylene)) interacting with water.12,29-31 Favorable conformations for the formation of a bidentate hydrogen-bond bridge between Oa and Oc in the OaCH2CH2ObCH2CH2Oc segment are gauche( for the OaCH2-CH2Ob bond and gauche- for the ObCH2-CH2Oc bond, namely, the conformations that have a pair of gauche( and gauche- for adjoining OCH2-CH2O bonds.12,29 One of the most favorable conformations is trans-gauche(-trans-transgauche--trans (TG(TTG-T) for the Oa-CH2-CH2-ObCH2-CH2-Oc segment. A theoretical calculation study31 has shown that, among possible forms of a 1:1 adduct of 18-crown-6 and water, the one with a crown of D3d symmetry and a bidentate hydrogen-bonded water is the most stable. The conformation of an 18-crown-6 molecule with this symmetry is TG(TTG-TTG(TTG-T-TG(TTG-T, where all of the adjoining OCH2CH2O bonds on the crown ring have a pair of gauche( and gauche- conformations. The D3d form of 18-crown-6 has in fact been shown by Raman spectroscopy to be the most stable in water.32 A previous infrared spectroscopic study23 has demonstrated that water in a H2O-18-crown-6-CCl4 ternary solution exhibits prominent bands at about 3600 and 3535 cm-1 (bands D and E), indicating the dominant existence of a bidentate hydrogen bond between water and 18-crown-6. The results described above for the 18-crown-6-water system are consistent with the present interpretation of the hydrogen bonding between water and short-chain POE. The discussions given above indicate that C1E1C1, the shortest oligomer of POE with a single monomer unit, is not an adequate model for longer chain POE with respect to hydrogen bonding to water. Composition Dependence of Hydrogen Bonding. The extent of hydrogen bonding in the H2O-C1EmC1-CCl4 ternary solutions depends on the composition of the solution. This is clearly seen in Figure 2 if the spectra of the solutions observed at different compositions are compared. The intensities of bands A and C due to free water are reduced remarkably when the

Matin et al.

Figure 4. Integrated intensities of bands A-F at 25 °C plotted against the composition of the solution: H2O-C1E1C1-CCl4 (a), H2OC1E2C1-CCl4 (b), and H2O-C1E3C1-CCl4 (c). Symbols: 4, band A; 9, band B; 2, band C; O, band D; b, band E; and 0, band F. Integrated intensities are given in arbitrary units on a scale common to this figure and Figure 5. The estimated errors in the integrated intensities are ((1-2).

fraction of C1EmC1 is increased or the fraction of water is decreased. At a C1EmC1 mole fraction of 0.04 (Figure 2d,f,h), which corresponds to a H2O:C1EmC1 mole ratio of ca. 1:100, the majority of water molecules are engaged in hydrogen bonding to C1EmC1 molecules in a monodentate or bidentate form. Free water molecules at this composition are severely diminished. We should note that at large C1EmC1 (or diethyl ether) mole fractions (e.g., at 0.04) the spectra of the H2O-C1E1C1-CCl4 and H2O-diethyl ether-CCl4 solutions exhibit a broad illdefined band centered around 3580-3610 cm-1 (Figure 2b,d). This band, to be called band Z, can be ascribed to the O-H stretching vibration of water that bridges ether oxygen atoms belonging to different C1EmC1 (diethyl ether) molecules. In the spectral analysis of the H2O-C1E2C1-CCl4 and H2O-C1E3C1CCl4 solutions, band Z was not considered, because band D appears at a wavenumber in close proximity to band Z, and this situation makes the separation of band D and band Z, if any, difficult in the analysis. Accordingly, the observed band D for C1E2C1 and C1E3C1 at large mole fractions may include the unresolved band Z. To examine closely the composition dependence of hydrogen bonding in the H2O-C1EmC1-CCl4 solutions, we have plotted in Figure 4 the integrated intensities of bands A-F at 25 °C against the composition. The composition dependence of hydrogen bonding at other temperatures closely resembles the

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Figure 5. Integrated intensities of bands A-F plotted against temperature: H2O-C1E1C1-CCl4 (a, b), H2O-C1E2C1-CCl4 (c, d), and H2OC1E3C1-CCl4 (e, f). The H2O:C1EmC1:CCl4 mole ratios are 0.000418:0.005:0.995 (a, c, e) and 0.000403:0.04:0.96 (b, d, f). Symbols: 4, band A; 9, band B; 2, band C; O, band D; b, band E; and 0, band F.

one at 25 °C. In the H2O-C1E1C1-CCl4 solution, where free water and monodentate hydrogen-bonded water exist in equilibrium, the former is replaced by the latter when the fraction of C1E1C1 is increased. In the H2O-C1E2C1-CCl4 and H2OC1E3C1-CCl4 solutions, where three types of water (free, monodentate hydrogen-bonded, and bidentate hydrogen-bonded) coexist, the population of free water decreases, while the population of hydrogen-bonded water increases, with increasing fraction of C1EmC1. The plots for these solutions show that the rate of intensity increase of bands B and F due to monodentate hydrogen-bonded water and that of bands D and E due to bidentate hydrogen-bonded water are rather similar. This observation implies that on addition of C1EmC1 the populations of monodentate hydrogen bond and bidentate hydrogen bond increase in parallel at similar rates. Temperature Dependence of Hydrogen Bonding. The infrared spectra of water in the H2O-C1EmC1-CCl4 solutions show significant dependence on temperature. In Figure 5, the integrated intensities of bands A-F are plotted against temperature. The plots on the left side in the figure are those for a H2O:C1EmC1:CCl4 mole ratio of 0.000418:0.005:0.995, and the plots on the right side are those for a mole ratio of 0.000403:0.04:0.96. For the H2O-C1E1C1-CCl4 solution, bands B and F due to monodentate hydrogen-bonded water decrease in intensity when the temperature is raised, while bands A and C due to free water increase. For the H2O-C1E2C1-CCl4 and H2O-C1E3C1-CCl4 solutions, the intensities of bands D and E due to bidentate hydrogen-bonded water decrease with

increasing temperature, and those of bands B and F also decrease, but to a lesser extent, or are almost constant at larger fractions of C1EmC1. The intensities of bands A and C increase as observed for C1E1C1. The spectral observations described above demonstrate that hydrogen-bonded water in the H2O-C1EmC1-CCl4 solutions is replaced by free water when the temperature is raised. Namely, increasing temperature brings about the destabilization of the hydrogen bonds between water and C1EmC1. The rapider decrease in intensities of bands D and E than bands B and F with increasing temperature gives us a suggestion for a possible process of the hydrogen-bond breakup; the dominant bidentate hydrogen bonds are converted into monodentate hydrogen bonds as a result of the breaking of one of the two hydrogen bonds, and the monodentate hydrogen bonds are subsequently broken at higher temperatures. The higher stability of the monodentate hydrogen bond than the bidentate hydrogen bond is assured by a spectral observation that the wavenumber (ca. 3500 cm-1) of the stretching vibration of the monodentate hydrogen-bonded O-H bond (νmono(bond)) is lower than the averaged wavenumber (ca. 3560 cm-1) of the antisymmetric and symmetric vibrations of the bidentate hydrogen-bonded O-H bonds (νbi(asym), νbi(sym)). Previous molecular dynamics simulations27,33 have shown that an increase of temperature leads to a great reduction in favorable water-POE hydrogen bonding. In this work, we have provided direct evidence of the breakup of the hydrogen bonding between water and short-chain POE by spectral observations of the O-H

19710 J. Phys. Chem. B, Vol. 109, No. 42, 2005 stretching vibrations of monodentate and bidentate hydrogenbonded water. The dehydration process we observed is a potential mechanism, among others, of the POE-water phase separation.34 Conclusions In this work we have found two types of hydrogen bonds in the interaction between water and short-chain POE (C1EmC1) in carbon tetrachloride; one is a monodentate hydrogen bond, in which only one of the O-H bonds of a water molecule participates in hydrogen bonding, and the other is a bidentate hydrogen bond, in which both of the O-H bonds of a water molecule participate in hydrogen bonding by bridging oxygen atoms separated by two or more monomer units on the POE chain. An important finding is that the bidentate hydrogen-bond bridge is not formed between the nearest neighbor oxygen atoms. The shortest oligomer of POE, i.e., CH3OCH2CH2OCH3 (1,2-dimethoxyethane) with a single monomer unit, is thus suggested not to be an adequate model for longer chain POE with respect to hydrogen bonding to water. The bidentate hydrogen bond is formed most favorably with a POE segment involving a pair of gauche( and gaucheconformations for adjoining OCH2-CH2O bonds. The previous finding35 that the population of the gauche conformation around the OCH2-CH2O bond decreases with increasing temperature is correlated well with the present result of the temperature dependence of hydrogen bonding. Thus, the hydrogen bonding and the chain conformation of POE are closely interrelated. The hydrogen bonding in a 1:1 C1EmC1-water adduct in carbon tetrachloride represents primitive incipient hydration of POE. The present results indicate that both monodentate and bidentate hydrogen bonds are important and the latter is destabilized more rapidly than the former with increasing temperature. This dehydration process can be a potential mechanism of the POE-water phase separation. Acknowledgment. The authors are grateful to Drs. Shaheda A. Wahab and Akimitsu Tonegawa for their help in the experimental work. Md.R.M. thanks the Japanese Government for a Monbukagakusho Scholarship for International Students (Research Students Program). References and Notes (1) Poly(ethylene glycol) Chemistry: Biotechnical and Biomedical Applications; Harris, J. M., Ed.; Plenum: New York, 1992. (2) Biomedical Functions and Biotechnology of Natural and Artificial Polymers: Self-assemblies, Hybrid Complexes and Biological Conjugates of Glycans, Liposomes, Polyethylene Glycols, Polyisopropylacrylamides, and Polypeptides; Yalpani, M., Ed.; ATL Press: Shrewsbury, MA, 1996. (3) Poly(ethylene glycol): Chemistry and Biological Applications; Harris, J. M., Zalipsky, S., Eds.; American Chemical Society: Washington, DC, 1997. (4) Elbert, D. L.; Hubbell, J. A. Annu. ReV. Mater. Sci. 1996, 26, 365394.

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