J. Phys. Chem. 1995, 99, 462-468
Hydrogen Bonding Cluster Formation and Hydrophobic Solute Association in Aqueous Solution of Ethanol Nobuyuki Nishi,* Satoru Takahashi, Masaki Matsumoto, Aki Tanaka, and Keisuke Muraya Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1,Fukuoka 812, Japan
Toshiyuki Takamuku Research & Development Div. TOTO Ltd. 1-1,Nakashimu 2-chome, Kokurakita-ku, Kitakyushu 802, Japan
Toshio Yamaguchi Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Fukuoka 814-01, Japan Received: August 15, 1994; In Final Form: October 17, I994@
Hydrogen bonding (HB) cluster formation of ethanol and water molecules in diluted aqueous solutions (XA I 0.03) is investigated on the bases of infrared (IR) absorption spectroscopy, mass spectrometric analyses of the clusters, and X-ray diffraction measurement. With increasing alcohol molar fraction (XA) up to 0.03, IR spectra showed the intensity decrease on the high-frequency side of the 0-H stretching band of water. The si@) curve at XA = 0.02 showed striking resemblance to the reported pure water curve at 1 kbar. The X-ray radial distribution function exhibited the decrease of linear hydrogen bonds (LHB) at 2.85 8, and the new peaks at 3.3 and 3.8 A. The new peaks correspond to the 0-0 distance of an angular hydrogen bond and the distance from an ethyl carbon to a water oxygen of the hydrogen-bonding cage, respectively. Mass spectra 0.001) at 35 “C. The of the clusters revealed the dimerization of ethanol in a very diluted solution (XA clusters isolated from an XA = 0.02 solution were composed of water and ethanol molecules with an average molecular number ratio (water / ethanol) of 2 ( j z 1). This indicates that the solute-solute association is highly preferable even at such a diluted concentration. A hydrophobic core structure composed of coherent ethyl groups with a strong water hydrogen-bonding cage is presented to explain the observed results.
Introduction Addition of small amount of ethanol into water is known to cause the contraction in volume that might reflect the strength of intercomponent attraction. Mitchell and Wynne-Jones found a remarkable decrease of the partial molar volume of ethanol (V A) with the minimum at an ethanol molar fraction (XA) of 0.08.2 Franks and Johnson made more precise measurement of VA of ethanol in the region 0 < XA < 0.1 with changing solution temperature from 10 to 30 0C.3 The most remarkable decrease of VA was seen in the low ethanol concentrationregion of 0 < XA < 0.03 independent of the temperature. At XA = 0.03, the decrease of VA exhibited an inflection that is accentuated by temperature rise to the extent that the 30 “C isotherm showed just a shallow maximum at XA = 0.06. However, the isotherm again showed the smallest VA at XA = 0.08 followed by a marked increase with increasing ethanol content. Franks and Ives inferred that the “solute contractions” have nothing to do with intercomponent attraction but have a common origin in “hydrophobic hydration”. On the basis of the partial molar volumes of ethanol and water in the mixture, Ben-Naim evaluated the three quantities of GAA, GAW,and GWWthat convey sorts of averages of the pair correlation functions of alcohol-alcohol, alcohol-water, and water-water pairs, re~pectively.~He stressed that a very complicated behavior of intermolecular interaction could be found in the region 0 < XA < 0.05. The most salient feature of GAAis its very steep increase on the successive addition of a little ethanol into pure water. The initial behavior at XA < 0.05 was thought to be undoubtedly a reflection of the enhancement @
Abstract published in Advance ACS Abstracts, December 1, 1994.
OO22-3654/95/2099-0462$09.OO/O
of the strength of the “hydrophobic interaction” between two ethanol molecules. His finding showed that ethanol-ethanol pair interaction is coming into play in a very diluted aqueous solution of ethanol. Nishikawa and Iijima studied fluctuations in ethanol-water mixtures by small-angle X-ray scattering experiments and obtained GAAvalues close to the Ben-Naim’s result, although they did not pay much attention to the drastic increase of the GAAvalue in the low ethanol concentration r e g i ~ n .We ~ studied the molecular association or cluster formation equilibria in ethanol-water mixtures at 60 “C by the method of isolating strongly bound clusters through adiabatic expansion of liquid droplets in vacu~m.63~ One of the findings related to the present study is that even in a solution with XA = 0.01 ethanol polymers such as dimer, trimer, and tetramer were observed in the form of their hydrate clusters. With decreasing temperature, the intensities of the hydrate signals of the ethanol polymers increased more eminently relative to the signals of the pure water clusters or the monomer-hydrate species. In this study, solutesolute association in such dilute solutions at lower temperatures (20-35 “C) is examined by IR absorption, the mass spectrometric analysis of the clusters isolated from the solutions, and X-ray diffraction measurement of the intact solutions.
Experiments Infrared absorption spectra were measured with a Shimadzu FIIR-8600 infrared spectrometer. A drop of the sample solution was put between a couple of water-free fused quartz plates in a cell holder. The thickness of the solution layer was of the order of 5 p m but it varied delicately depending on the pressure added on the two plates, so that it was adjusted by monitoring the C-H stretching band intensity of ethanol at 2977 cm-’. 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 1, 1995 463
Cluster Formation in Aqueous Solution of Ethanol The method of adiabatic expansion of liquid droplets in A droplet flow vacuum has been reported in previous was generated by hydrodynamic conversion of liquid stream from a notched small nozzle with a diameter of -40 pm. These droplets were introduced into a high vacuum chamber through two conical skimmers. "Superheated" droplets in the vacuum suffered from immediate expansion due to the pressure change. Droplets with diameters larger than 1 pm were found not to show explosion in a flight distance of 10 cm when the deposition patterns of aqueous dye solution was observed through a microscope. Only submicrometer particles showed instantaneous fragmentation for a flight of a few centimeters. The clusters produced through the adiabatic expansion were ionized by electron impact at 40 eV and mass-analyzed by a quadrupole mass spectrometer (Extrel 7-162-8 311-12H) with an axial ionizer (Extrel 041-3). The ionizer was located at 7 cm downstream from the nozzle. Signals from a ceramic electron multiplier (Murata EMS-608 1B) were accumulated using a Nicolet 1170 signal averager. Average temperature of the liquid droplets was estimated by the mass spectral change of an aqueous solution of propionic acid with XA = 0.005. Spectral pattern of this solution is highly dependent on the temperature. Intensities of the higher polymer hydrates became dominant with decreasing temperature. The temperature dependence of the spectrum of this solution was carefully examined for a reference aqueous solution of 2-butoxyethanol with X A = 0.05 that shows anomalous spectral change due to phase separation in the region between the lower critical temperature (49 "C) and the upper critical temperature (129 0C).8,9Because the adiabatic expansion of liquid droplets requires sufficient internal energy, the lower limit of the average droplet temperature was found to be 30 "C with our nozzle condition. A rapid liquid X-ray diffractometer was used coupled with an imaging plate that simultaneously detects two-dimensional pattern of scattered X-rays in a wide range of scattering angle within a short time of measurement.1° Mo K a radiation (A = 0.7107 A) monochromatized by a graphite crystal was scattered in a transmission mode from a sample solution sealed in a glass capillary of 1 mm diameter. The observed range of the scattering angle (20) is 2"-140°, correspondingto the scattering vectors (= 4 n sin 8/A) of 0.3-16 A-l. It takes approximately 3 h for the measurement of one sample. The useful dynamic range of the detector is higher than lo5 and its resolution is 150 pm, while the pixel resolution of the imaging plate reader is 125 x 125pm2. The structure function i(s) is given by the following equation:
w
1
1
1
1
(
1
,
,
I
I
1
+
where K represents a normalization factor for the corrected total intensity Z(s) to absolute units, nj is the atomic fraction of atom j in a stoichiometIic volume V, andfi(s) is the atomic scattering factor of atomj corrected for the anomalous dispersion. A radial distribution function D(r) is obtained by Fourier transform of eq 1 according to D(r) = 4 n 2 e 0
+ (2 r/n) hsmm si@)M(s) sin(r s) ds (2)
where eo (= >jf/2(0)) stands for the average scattering density of molecules in the sample solution and smaxis the maximum s value attained in the measurements (sa = 14.5 A-1>. The modification function M(s) has the form
(3)
4000
3500
3000 WAVENUMBER
Figure 1. Infrared absorption spectra of pure water (a), and aqueous solutions of ethanol with XA = 0.02 (b), and XA = 0.03(c).
with the damping factor k being chosen as 0.01 A2 in the present case.
Results
IR Absorption Band of OH Stretching Vibrations of Water. Spectral change of IR absorption with increasing ethanol concentration was observed in the OH stretching vibration region of 2750-3800 cm-' as shown in Figure 1. The spectral shape of the pure water OH absorption is very similar to that reported by Mar6chal.l' Pure liquid ethanol showed the 0-H stretching band at 3250 cm-' with approximately the same peak intensity as that of the C-H stretching band at 2977 cm-'. Thus the most of absorption intensity can be attributed to the water 0-H stretching band. The most interesting spectral change is seen on the high-frequency side; the half-width at half-maximum (hwhm) in the high-frequency region of spectrum c for the 3% solution (XA = 0.03) was narrower by 6 cm-' than that of pure water (spectrum a), while the hwhm in the lowfrequency region was kept as it was. to see the shape changes more clearly, the intensity of each absorption spectrum was normalized at 3410 cm-' (the peak position). The difference spectra obtained by subtracting the intensity normalized pure water spectrum are shown in Figure 2. The intensity decrease in the high-frequency region shows the deepest dip at 3600 cm-', while the intensity in the low-frequency region becomes slightly stronger with increasing alcohol concentration. The alcoholic 0-H band at 3250 cm-' must be responsible to this slight increase around 3250 cm-' in the difference spectra because it is as strong as the C-H stretching band at 2977 cm-' as stated above. Since the peak intensities at 3250 cm-' (in Figure 2) are much weaker than those at 2977 cm-', the net change of the water absorption at 3250 cm-' becomes negative after the subtraction of the increased alcohol 0-H absorption. Thus we have to say that the relative intensity decrease at 3600 cm-' is accompanied by a slight decrease in intensity at 3250 cm-' making the width of the water band much narrower. Markhal showed that the peak intensity as well as the intensities in the lower frequency region drastically decreased with raising the temperature from 27 to 75 "C. The temperature change of the pure water spectrum also showed that the relative intensity at 3600 cm-I increased with increasing temperature in contrast to the opposite behavior of the shoulder at 3250 cm-'. This observation is in accord with the temperature change of the Raman spectra of pure water observed by several group^.'^-'^ The changes observed on the addition of small amount of
Nishi et al.
464 J. Phys. Chem., Vol. 99, No. I, 1995
n
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WAVENUMBER Figure 2. Difference spectra (absorbance of solution - absorbance of pure water) of aqueous solutions of ethanol with XA = 0.01 (a), XA = 0.02 (b), and XA = 0.03 (c). Both the original solution spectra and the reference water spectrum are intensity normalized at 3410 cm-I. The sharp peaks at 2977 and 2905 cm-* are the C-H stretching vibrational bands of the ethyl group of the solute.
ethanol are partly similar to the effects for the increasing temperature of the pure water. However, the decrease in the intensity at 3600 cm-’ indicates that the addition of ethanol reduces “free” water molecules with “gaslike” 0-H stretching vibrational frequencies. Such simultaneous decrease of “free waters” and “icy hydrogen bonds” is expected for highly pressurized waters.
Mass Spectrometry of the Clusters Isolated from Liquid Droplets. We have reported that alkyl alcohols and alkylcarboxylic acids in aqueous solution provided the cluster mass spectra highly sensitive to temperature and solute concentration.6-1° Figure 3 shows a mass spectrum of an aqueous solution of ethanol with a solute-to-water molar ratio of 1:2000 (XA = 0.0005). The temperature of the droplets just before the expansion is estimated to be 35 “C based on the calibration stated The main sequence of the mass peaks is composed of the protonated pure water clusters ((HzO),H+) characteristic of diluted aqueous solutions. The electron impact ionization evaporates some of free water molecules and an OH radical from the original clusters: (H20),
+ e- - (H20),Hf + OH + ( m - n - l)HzO+ 2e- (4)
where the evaporation number (m - n) is estimated to be approximately in the range of 4-8 for the present ionization condition as shown in a later section. The spectral pattern does not change for the ionization electron energies of 20-40 eV. The lower limit of the energies required for the evaporation of 4-8 water molecules is 1.7-3.5 eV. Thus one should notice that the observed ions originate from the parent clusters with 4-8 more water molecules on the average. The observed smooth intensity decrease of the clusters with increasing cluster
size (n) may originate from the smooth size distribution of the clusters in the original solution. The most important point of the mass spectrum is enhanced intensities of ethanol monomerhydrates (CZH~OH(H~O)~~H+). The ionization potential of ethanol (10.65 eV) is not so different from those of water clusters (11.5-9.87 eV for n = 4-8;15 11.1 eV for ice16), and the volume of total water molecules (n ’ I30) in the clusters is large enough to be ionized dominantly. Thus the signal intensities of ethanol monomer-hydrates could be compared with those of pure water clusters with nearly the same number of water molecules. This point has been carefully investigated by comparing the intensity ratios of phenol monomer-hydrates to pure water clusters as a function of decreasing electron energy. Intensity ratios of nonprotonated ions of phenol hydrates to the protonated ones increased drastically with decreasing electron energy, while those of protonated phenolhydrates to the pure water clusters (with identical water numbers) showed constant ratios over the wide electron energies from 11 to 70 eV. This observation indicates the protonated signals are produced by the ionization of a water molecule followed by the evaporation of neighboring water molecules similar to the ionization of pure water clusters. These conditions may be seen for sufficiently large hydrate clusters where the volume of solute species is small enough compared with that of the water cluster part. Another important point is that the evaporation of an ethanol molecule from such an hydrate cluster hardly occurs except for ethanol dominant binary clusters.” Thus the intensities of the ethanol monomer or the dimer-hydrate signals in the high mass region may base quantitative estimates for the abundance of the monomer- and dimer-hydrates relative to pure water clusters with identical molecular numbers. Figure 3 demonstrates that even at XA = 0.0005 the dimerhydrates and the trimer-hydrates also appear to some extent. (Here one should notice that the ethanol polymer-hydrate signals are contaminated with the l8O signals of strong hydrate species with two less solute molecules.) It should be also noted that the intensities of monomer-hydrate signals are stronger by more than 30% of the pure water signals with similar mass numbers at 550-850. The relative intensities of the monomerhydrate signals increase with increasing mass numbers. In order to evaluate the relative stability of solute hydrates to pure water clusters, we proposed to use the stability constant ~i for solute i-mer-hydrates. The constant ~i is independent of hydration numbers and solute concentrations but dependent on the solution temperatures and the size of hydrophobic gr0ups.7~~The stability constant of monomer-hydrates should be measured at sufficiently diluted solution where the dimer-hydrates are negligibly small. This condition is seen for the solution with XA values lower than 0.0004 at 35 “C and 0.001 at 60 “C. At the lower concentrations the intensities of the monomer-hydrates change in proportion to XA. Figure 4 shows the plots of the intensity ratios [(CzHsOH)(HzO),-~H+]/[(H*O),H+]obtained from the mass spectra of the solution with XA = 0.0002 at 35 “C as a function of the observed hydration numbers. The plots show a linear dependence of the ratios against the hydration number and provide a slope value of 0.0034 (20 = f0.0002). This value gives a stability constant ~1 of 16.9 (fl.O) for the monomer-hydrates. The line in Figure 4 is the least squares fitting of the plots. The zero-point of the line originates from n -5. This shift of the origin from n = 0 to n PX -5 originated from the loss of water molecules by the ionization both from the monomer-hydrates and pure water clusters. Judging from the possible errors for the plotting procedure, we estimated the real loss of the water molecules is in the range of 4-8.
J. Phys. Chem., Vol. 99, No. I, 1995 465
Cluster Formation in Aqueous Solution of Ethanol I
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at 35°C
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Figure 3. Mass spectrum of the clusters isolated from liquid droplets with an ethanol to water molar ratio of 1:2000 (xA= 0.0005). Average temperature of the droplets is 35 "C (see text). (m.n) stands for (C&ISOH),(H~O),H+. I
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CzH50H : HzO = 1 : 100
Hydration Number (n)
Figure 4. Plots of the intensity ratios [(C~H~OH)(HZO),-IH+] / [(HzO),H+] as a function of hydration number n. The solid line is the least-squares fitting of the plots providing the slope of 0.0034 (*0.0002) and the origin number no % -5 (f1.5). ~1 is independent of the solute concentration as long as the nature of aqueous solvent condition is preserved, while it is dependent on temperature. With increasing inverse temperature (UT) it increased exponentially. This condition allows us to take the van't Hoff plots for the thermochemical analyses of the formation-dissociation equilibria of the hydrate clusters. Detailed analyses will be reported soon with systematic studies for various alcohols. From the stochastic background for the definition of KI,* one can say that the ethanol hydrates have an enhanced stability 16.9 times higher than those of the pure water clusters with the same sizes. (Strictly speaking, this is valid for the clusters with n > 10.) This enhanced hydration around the ethyl group is so called "hydrophobic hydration" of ethanol.' This hydration takes an important role in the solution with a little bit higher solute concentration. The compositional change of the main clusters on the increase of solute concentration is shown in Figure 5 for the solutions with ethanol molar fractions (XA) of 0.002, 0.01, and 0.02. As
CzHtiOH : HzO = 1 : 500
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MASS NUMBER (m/Z)
Figure 5. Mass spectral change of the clusters isolated from three solutions with ethanol to water molar ratios of 150 (xA= 0.02, top), 1:100 (XA = 0.01,middle), and 1500 (XA = 0.002, bottom). Average temperature of the droplets is 35 "C (see text). (m,n) stands for (C2H5OH)m(H2O)nH+.
466 J. Phys.
Nishi et al.
Vol. 99, No. 1, 1995 I
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C2HsOH : H2O = 1 : 50 at 35 "C
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MASS NUMBER (m/Z 1 Figure 6. Low-mass part of the mass spectrum of the ethanol-water binary clusters isolated from the aqueous solution with an --.mol molar ratio XA = 0.02, the higher mass part of which is shown at the top of Figure 5. Average temperature of the droplets is 35 "C (see text). (m,n) stands for (C2HO-MH20),H+.
expected from the strong intensities of the monomer-hydrates in the mass spectrum of the diluted solution with XA = 0.0005 (Figure 3), the mass spectrum of the 0.2% solution (XA = 0.002) is composed of the main sequence of ethanol monomerhydrates in the mass region of 500-750. The spectrum of the solution with XA = 0.01 is already complicated enough and its main peaks in the same mass region originate from the trimer-, tetramer-, and pentamer-hydrates. At XA = 0.02, the main clusters with mass numbers 500-750 are hexamer-, heptamer-, and octamer-hydrates. Figure 6 shows the low mass region (mh, = 315-500) of the mass spectrum at XA = 0.02. Important is the relative molecular composition of the solvent to the solute. Both in the low mass and in the high mass regions the solvent-to-solute molecular ratios are in the range of 2 f 1. An average of twice of water molecules is associated to ethanol molecules. This kind of proportionality in the molecular composition of the binary clusters is usually seen in the mass spectra of "aqueous" solutions, indicating the high-mass clusters are just the adducts of low-mass clusters. X-ray Diffraction Measurement. Structural information of a solution system can be obtained from the X-ray diffra~tion.'~J~ The observed structure functions si(s) of the 2% solution is shown in Figure 7 as well as that of the pure water obtained with the same experimental system. It should be particularly emphasized that the si ( s ) curves of pure water and 2% solution in the figure bear striking resemblance to the pure water curves at 1 atm and 1 kbar (987 atm), respectively.20 The radial distribution functions obtained from the Fourier transform of these functions are shown in Figure 8 for the 2% solution (solid line) and the pure water (broken line). In spite of the low molar fraction of ethanol, one can see clear differences in the two functions. The addition of 2 mol % ethanol induced the decrease of the hydrogen-bond peak at 2.85 A but it caused the appearance of a new 0-0 pair at 3.3 A. Intensity increment around 3.8 A is also seen in the figure. The appearance of this peak is quite important. According to a recent neutron diffraction measurement on a 1:9 molar ratio methanol-water mixture,21a distance of -3.7 A corresponds to that from the methanol carbon atom to the water oxygen atom of a disordered
-80 b 0
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s I A-' Figure 7. Structure functions i(s) multiplied by s for pure water (top) and aqueous solution of ethanol with X A = 0.02 (bottom).
cage. The analysis of the data by Soper and Finney confirmed the existence of a definite hydration shell of water molecules.21 The observation of the peak at 3.8 A in the 2% solution is consistent with the neutron diffraction data on the methanolwater mixture. The decrease of the 2.85 A peak intensity is partly compensated by the increase of the interstitial 0-0pairs in the 3-4 A
Cluster Formation in Aqueous Solution of Ethanol 2 0.4
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J. Phys. Chem., Vol. 99, No. 1, 1995 467
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rlA Figure 8. Radial distribution functions in the form of D(r) -4n for pure water (broken line) and aqueous solution of ethanol with XA = 0.02 (solid line).
region. Since it is characteristic of the 0-0 distance in the ice structure, the peak at 2.85 A can be attributed to linear hydrogen bonds (LHB) that produce a hexagonal or a pentagonal ring as an energetically most stable unit.
Discussion The most striking feature of the X-ray diffraction measurement is that the s*i(s)curve of 2% solution (lower part of Figure 7) resembles to the pure water curve at 1 kbar.20 The depletion of the infrared absorption at 3600 cm-’, corresponding to the decrease of free water molecules, is well understood for such highly pressurized water. These results may suggest that the addition of a little ethanol in water creates water structure similar to that of highly pressurized water. The radial distribution
function provided clear information on this structural change. The most important point is the decrease of the ice-like linear hydrogen bonds at 2.8 A. That is partly compensated by the increase of the interstitial water molecules with the 0-0 distance at 3.3 A. This change is not so curious when we see the mass spectrum of the clusters isolated from the same solution (top of Figures 5 and 6). The XA = 0.02 solution showed the binary clusters with average of 2 times more water molecules. This molecular ratio corresponds to nearly equal volume of total water molecules to that of ethyl groups of the ethanols in the binary clusters. This 1:l volume balance indicates that the a hydrogen-bonding cage containing water molecules can be formed with the core of the stacked ethyl groups. Since it has only one hydrogen atom attached to the oxygen, an ethanol molecule can participate in a linear chain hydrogen-bond (HB) or a cyclic cluster like a pentamer or a hexamer. Therefore, random insertion of ethanol molecules into the water network results in the termination of a three-dimensional HB structure. The replacement of 30% water molecules by ethanol molecules causes drastic destruction of the network structure. Cohesion of hydrophobic groups, however, makes the formation of outside water HB networks possible. The core of this type of cluster is made of stacked ethyl groups. They are closely located to each other due to the tension from the strong network of the outer water molecules coupled with the OH groups of ethanols. A schematic model of this type of heterogeneous cluster is shown in Figure 9. On the basis of the molecular composition of the isolated strongly bound clusters, a mantle-type HB network is likely to exist at the interface and it may be composed of one (or two) water layer(s). This layer must be surrounded by bulk waters with nonlinear hydrogen bonds (NLHB) as well as linear hydrogen bonds (LHB). The decrease of the partial will molar volume of ethanol in this concentration be elucidated by introducing the stacked hydrophobic core and the water networks. Ben-Naim found that the GAAvalue very
Figure 9. A model of ethanol polymer-hydrates with a stacked hydrophobic core of ethyl groups and an interfacing HB water layer coupled with the OH groups of ethanols. The outer “bulk” water area has a rather glassy structure composed of many pairs with nonlinear hydrogen bonds (NLHB) as well as linear hydrogen bonds (LHB). This outer area may act as “solvent area” for strongly bound hydrate clusters and the adiabatic expansion of this liquid may leave only the strongly bound clusters of the core ethanols with the interface HB water layers.
468 J. Phys. Chem., Vol. 99, No. 1, I995
steeply increases on the addition of small amounts of alcohol to pure water. The initial behavior at XA < 0.05 was thought to be a reflection of the enhancement of the strength of the “hydrophobic interaction” between two ethanol molecules brought about by the addition of alcohol. The results from the X-ray diffraction and mass spectrometric measurements gave us an impression that the bulk water region composed of rather weakly bonded pairs increases on the addition of ethanol. Gigutre discussed the presence of bifurcated hydrogen bonds (BHB) based on the results of the temperature changes of the 0-H stretching bands in the Raman spectra and the neutron diffraction of heavy water22 and pure methan01.~~ He estimated the 0-0 distances between the outer two water molecules connected with the central water through BHB to be 4.2 8, by keeping the nearest-neighbor 0-0 distance at 2.8 8, (ice site). The angle between the two BHB is 97” in this case. The nearest-neighbor 0-0 distance will be elongated when this apex angle becomes smaller. M6 et al. studied the structure and binding energies of water trimers by using ab initio molecular orbital c a l c u l a t i ~ n .As ~ ~a stable trimer they presented another kind of bifurcated hydrogen bond in which each of the two nonbonding orbitals (or the two hydrogen atoms) of one water molecule is coordinated with a hydrogen atom (or a nonbonding orbital) of each of the other water molecules. TQe calculated 0-0 distances are in the range of 3.123-3.142 A. The appearance of other 0-0 bonds with somewhat weaker interactions at 3-4 8, (Figure 8) indicates the presence of NLHB. BHB and trifurcated HB23,25are possible candidates for such nonlinear bindings. The water networks surrounding the cohesive ethyl group core are expected to form strong hydrogen bonds. The outside of these strong water networks is thought to be surrounded with many of NLHB water molecules. With increasing number of ethanol molecules in a cluster, the hydrogen-bonding networks may suffer from higher motional fluctuation of the hydrophobic groups in a massive core of stacked ethanols. This motional fluctuation of a massive core may make the hydration shell structure more flexible leaving the strongly bound networks only at the monolayer interface region. The strong and thin layer of LHB network may work as an interface mantle of a hydrophobic core. Because of its thinness, the interface mantle layer can follow the motional fluctuation, but the outer three dimensional networks surrounding the strong interface layer are highly prone to be damaged producing many of NLHB water pairs. Because of their largeness, the motion of these binary clusters may affect the structure of bulk water. This damaged structure of bulk
Nishi et al. water may produce an X-ray diffraction pattern similar to that of highly pressurized pure water. The structural model composed of three heterogeneous regions in the solution must be exposed to new theoretical and experimental studies as well as molecular dynamics simulation with appropriate potentials.
Acknowledgment. This work was supported by Grant-inAids for New Program “Intelligent Molecular Systems with Controlled Functionality” (06NPO301)and for General Research Program (04403002) from the Ministry of Education, Science, and Culture of Japan. We thank to Messrs. M. Ihara, M. Yamagami, and H. Ohzono for the technical assistance in the X-ray diffraction measurement. References and Notes (1) Franks, F.; Ives, D. J. G. Q. Rev. Chem. SOC. 1966, 20,l. (2) Mitchell, A. G.; Wynne-Jones, W. F. K. Discuss. Faraday SOC. 1953, 15, 161. (3) Franks, F.; Johnson, H. H. Trans Faraday SOC., 1962, 58, 656. (4) Ben-Naim, A. J. Chem. Phys., 1977, 67, 4884. ( 5 ) Nishikawa, K.; Iijima, T. J. Phys. Chem., 1993, 97, 10824. (6) Nishi, N.; Koga, K. ; Ohshima, C. ; Yamamoto, K.; Nagashima, U. ; Nagami, K. J. Am. Chem. SOC.1988, 110, 5246. (7) Nishi, N. Z. Phys. D-Atoms, Mol., Clusters 1990, 15, 239. (8) Nishi, N.; Yamamoto, K. J. Am. Chem. SOC.1987, 109, 7353. (9) Yamamoto, K.; Nishi, N. J. Am. Chem. SOC.1990, 112, 549. (10) Ihara, M.; Yamaguchi, T.; Wakita, H.; Matumoto, T. Adv. X-ray Anal. Jpn. 1994, 25, 49. (11) Markchal, Y. J. Chem. Phys. 1991, 95, 5565. (12) Walrafen, G. E.; Hokmabadi, M. S.; Yang, W.-H. J. Chem. Phys. 1986, 85, 6964. (13) D’Anigo, G. D.; Maisano, G.; Mallamace, F.; Migliardo, P.; Wanderlingh, F. J. Chem. Phys. 1981, 75, 4264. (14) Hare, D. E.; Sorensen, C. M. J. Chem. Phys. 1991, 95, 5565. (15) Tomoda, S.; Kimura, K. Ions and Molecules in Solution; Tanaka, N. et al., Eds.; Elsevier: Amsterdam, 1983; p 13. (16) Yu, K. Y.; McMenamin, J. C.; Spicer, W. E. Sur$ Sci. 1975, 50, 149. (17) Stace, A. J.; Shukla, A. K. J. Am. Chem. SOC.1982, 104, 5314. (18) Morgan, J. ; Warren, B. E. J. Chem. Phys. 1938, 6, 666. (19) Narten, A. H.; Danford, M. D.; Levy, H. A. Discuss. Faraday SOC. 1967, 43, 97. (20) Okuhulkov, A. V.; Demianets, Yu, N.; Gorbaty, Yu. E. J. Chem Phys. 1994, 100, 1578. (21) Soper, A. K.; Finney, J. L. Phys. Rev. Letters, 1993, 71, 4346. (22) GiguBre, P. A. J. Chem. Phys. 1987, 87, 48. (23) GiguBre, P. A,; Pigeon-Gosselin, M. J. Solution Chem. 1988, 17, 1007. (24) M6, 0.;Yfifiez, M. J. Chem. Phys. 1992, 97, 6628. (25) Mezei, M.; Dannenberg, J. J. J. Phys. Chem. 1988, 92, 5860. JP9421646
