Article
pubs.acs.org/JPCB

Local Structures of Methanol−Water Binary Solutions Studied bySoft X‑ray Absorption Spectroscopy
Masanari Nagasaka,†,‡ Kenji Mochizuki,‡ Valentin Leloup,† and Nobuhiro Kosugi*,†,‡†The Institute for Molecular Science, Myodaiji, Okazaki 444-8585, JapanThe Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan‡

ABSTRACT: Liquid methanol shows one- and two-dimensional (1D/2D) hydrogen bond (HB) networks, and liquidwater shows three-dimensional (3D) HB networks. We haveclearly found three diﬀerent local structures around the methylgroup of methanol−water binary solutions(CH3OH)X(H2O)1−X at diﬀerent concentrations in C K-edge
soft X-ray absorption spectroscopy (XAS). With the help ofmolecular dynamics simulations, we have discussed theconcentration dependence of the hydrophobic interaction at
the methyl group in the C K-edge XAS spectra. In the methanol-rich region I (1.0 > X > 0.7), a small amount of water molecules
exists separately around dominant 1D/2D HB networks of methanol clusters. In the region II (0.7 > X > 0.3), the hydrophobic
interaction of the methyl group is dominant due to the increase of mixed methanol−water 3D network structures. In the waterrich region III (0.3 > X > 0.05), methanol molecules are separately embedded in dominant 3D HB networks of water. On theother hand, the pre-edge feature in the O K-edge XAS shows almost linear concentration dependence. It means the HBinteraction between methanol and water is almost the same as that of water−water and of methanol−methanol.
dielectric relaxation methods.16 Takamuku et al. measured thenumber of water molecules per 6 methanol molecules as afunction of the methanol molar fraction by the mass
spectrometry of methanol−water liquid microjets14 and foundthree diﬀerent dependences with the borders at X = 0.7 and X =0.3. They proposed that the chain structures of methanol
clusters are dominant at X > 0.7, the tetrahedral-like water
clusters gradually evolve at 0.7 > X > 0.3, and the water cluster
is a main species at 0.3 > X > 0.0.
The interaction in the binary solution was also investigatedby vibrational spectroscopies: infrared spectroscopy17−24 andRaman spectroscopy.22,25−28 Dixit et al. found nonlinear
proﬁles of the C−O stretching vibration in the Raman
spectroscopy when decreasing the methanol molar fraction,27
in which the behavior of the energy shifts changes at X = 0.70
and X = 0.25. They proposed diﬀerent local structures: in theregion X > 0.7, water molecules connect the terminal of themethanol chain and the chain structure is preserved; in theregion 0.7 > X > 0.25, the methanol chain is broken by adding
water molecules; in the region 0.25 > X > 0.05, the hydration
structure of methanol molecules is formed.
The structure of liquid methanol and methanol−water binary
solutions has been investigated theoretically by using molecular
dynamics (MD)29−43 and Monte Carlo simulations.44−49 In themethanol-rich condition, the 1D/2D HB network structure ofmethanol clusters is not strongly inﬂuenced by water

1. INTRODUCTION
It is known that methanol−water binary solutions show smaller
entropy than expected in an ideal solution of randomly mixed
molecules1 and show a nonlinear proﬁle in viscosity as changing
the mixing ratio.2 These characteristics have been discussed byusing clathrate-like structure models of methanol moleculeswith surrounding water molecules and with hydrophobic
interactions between methyl groups.1 However, a consistentpicture of the microscopic structure of methanol−water binary
solutions is not yet established.
The oxygen atom in a water molecule has two hydrogendonating (“donor”) sites and two hydrogen-accepting (“acceptor”) sites, and liquid water forms tetrahedrally coordinatedthree-dimensional (3D) hydrogen bond (HB) networks.3 On
the other hand, a methanol molecule has one donor and one or
two acceptor sites due to the replacement of one donor site bya hydrophobic methyl group, and liquid methanol forms oneand two-dimensional (1D/2D) HB networks, such as chain andring structures.4−9 In the neutron diﬀraction experiments ofmethanol−water binary solutions,10−12 it is found that 3D HB
networks of methanol−water mixtures are formed by hydrophilic and hydrophobic interactions between water andmethanol molecules. Dixit et al. measured neutron diﬀraction
at X = 0.7 in the methanol−water binary solutions(CH3OH)X(H2O)1−X11 and revealed that the distance between
methyl groups of methanol molecules becomes closer byadding water molecules.
The interaction between methanol and water molecules inthe binary solution was studied by nuclear magnetic
resonance,13 mass spectrometry,14 Rayleigh scattering,15 and© 2014 American Chemical Society

Received: September 13, 2013
Revised: March 4, 2014
Published: April 3, 2014
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Articlemolecules.32 As the mixing ratio of water increases, the HB
networks of both water and methanol molecules grow to bemixed with each other.38−40,46,48 When the mixing ratio ofwater is high, the 3D hydration shell is formed aroundmethanol molecules.41,43,45
Although methanol−water binary solutions have beenstudied experimentally and theoretically as described above,
microscopic structures of methanol−water mixtures, such asnearest-neighbor interactions, have not yet been known indetail. Soft X-ray absorption spectroscopy (XAS) is an elementselective method to investigate local structures of liquid andaqueous solutions. The structure of liquid water was extensively
studied by the O K-edge XAS.50−52 Because the X-ray
absorption process occurs within a few femtoseconds, XAS
enables us to investigate the HB interaction of liquid water
before the geometrical rearrangement. The hydration structureof cations in aqueous salt solutions was also investigated by the
O K-edge XAS.53,54 Wilson et al. studied the O and C K-edge
XAS of liquid methanol in the total electron yield of liquid
microjet.55 Tamenori et al. measured the O and C K-edge XASof free methanol clusters.56 Guo et al. investigated liquidmethanol and methanol−water binary solutions at X = 0.5 byusing the O K-edge XAS and X-ray emission spectroscopy.57,58
Guo et al. suggested that liquid methanol shows chains andrings of 6−8 methanol molecules and proposed that the
number of pure methanol chains decreases and the number of
mixed methanol−water networks increases when adding watermolecules. However, the O K-edge XAS shows contributions ofoxygen atoms in both methanol and water molecules and isdiﬃcult to analyze the local structure of methanol−water
mixtures. It is necessary to measure the C K-edge XAS to
analyze the local structure of the methyl group of methanol
molecules in the binary solution.
In the present work, we investigate the local structure ofmethanol−water binary solutions at diﬀerent concentrations by
the O and C K-edge XAS. The XAS measurement is based on atransmission mode by using a recently developed liquid cellthat enables to optimize the absorbance by changing the
thickness of liquid layer.59 The pre-edge feature in the O Kedge XAS is found to show almost linear dependence of theconcentration. On the other hand, in the C K-edge XAS, thesecond peak that is related to the methyl group is found tochange its behavior at X = 0.7 and X = 0.3. With the help of the
MD simulation, we have revealed diﬀerent local structures ofmethanol−water mixtures at the three concentration regions.

present liquid cell, the thickness of the liquid layer can becontrolled from 2000 to 20 nm by increasing the heliumpressure in the regions II and IV. The thickness is set to 300
nm in the present O K-edge XAS. In the C K-edge XAS, on theother hand, the thickness of liquid methanol is set to 550 nm,
and the thickness is set larger in more dilute methanol aqueous
solutions. The energy resolutions of incident soft X-rays at the
O and C K-edges are set to 0.40 and 0.19 eV, respectively. The
XAS spectra are based on the Beer−Lambert law, ln(I0/I),
where I0 and I are the detection current through the cellwithout and with samples, respectively. The liquid ﬂow is
stopped during the XAS measurement because the sample
liquid has no radiation damage from the long (say, more than 1h) exposure of soft X-rays in the present photon ﬂux. The
photon energy in the O K-edge is calibrated by the O 1s π*
peak (530.80 eV)63 for free O2 molecules and that in the C Kedge is calibrated by the ﬁrst peak (287.96 eV)56 of freemethanol molecules, which are mixed with helium gas in theregions II and IV.

3. RESULTS AND DISCUSSION
3.1. Oxygen K-Edge XAS. Figure 1 shows O K-edge XAS
spectra for methanol−water binary solutions at diﬀerent

Figure 1. O K-edge XAS spectra of methanol−water binary solutions
at diﬀerent concentrations at 25 °C. The mixing ratio of methanol inthe solution decreases with molar fraction steps of 0.1 along indicatedarrows. The inset shows isosbestic points (dashed lines) in the preedge region.

concentrations at 25 °C. The absorbance in the O K-edge
XAS spectra was normalized by the sample thickness and theconcentration of the binary solution considering the soft X-ray
absorption coeﬃcients of water and methanol in the O Kedge.62 After this normalization, we subtracted a constantbackground and show the resultant absorption coeﬃcients in
Figure 1. In the previous work,57 a small peak was observed
around 532 eV in methanol−water mixtures; on the other
hand, it is not observed for any concentration in the present
measurement. It should not be regarded as an intrinsic peak.
The pre-edge peak of liquid water (534.7 eV) corresponds to
the O 1s transition to an unoccupied 4a1* orbital of a watermolecule (533.9 eV), which is mainly distributed at the oxygen
atom in water molecule and is blue-shifted and broadened by
the HB interaction.51 On the other hand, the pre-edge featureof liquid methanol (534.9 eV) is embedded in the main peakbut is similarly blue-shifted from the gas-phase peak (534.0
eV).55Figure 1 shows that the intensity of the pre-edge region
around 535.2 eV decreases as the molar fraction of methanol

2. EXPERIMENTS
The experiments were performed at an in-vacuum soft X-ray
undulator beamline BL3U at UVSOR-II.60 Details of the liquidcell were described previously.59,61 The liquid cell consists offour regions, which are separated by 100 nm thick Si3N4
membranes (NTT AT Co., Ltd.). Soft X-rays under vacuum(region I) pass through the buﬀer region ﬁlled with helium gas(region II) and the liquid thin layer (region III) and ﬁnallyreach a photodiode detector ﬁlled with helium gas (region IV).
The regions II and IV are connected and can be mixed withother gas molecules for the precise gas−liquid energy shiftmeasurement and photon energy calibration. A liquid sample(region III) is sandwiched between two Si3N4 membranes withpressed Teﬂon spacers and can be substituted by other samples
in combination with a tubing pump system.
The thickness of liquid layer should be optimized in order totransmit soft X-rays with an appropriate absorbance.62 In the4389

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Articlean O 1s hole. There are two possibilities. One is a negligible HB
interaction between water and methanol, Om*HOw and
Ow*HOm, indicating that water aggregates are segregatedfrom methanol ones in solution. The other is almost the same
HB interaction in Ow*HOm as in Ow*HOw and that in
Om*HOw as in Om*HOm. The pre-edge peak of liquid water issensitive to the HB interaction of liquid water and is dependent
on the temperature.52 The concentration dependence in the
pre-edge region of the methanol−water binary solution issmaller than the temperature dependence of liquid water.
Therefore, nearly the same HB interaction between methanol
and water could be possible.
3.2. Carbon K-Edge XAS. Figure 4 shows C K-edge XAS
spectra of molecular (gas) and liquid methanol at 25 °C by(X) decreases in the binary solution (CH3OH)X(H2O)1−X. It is
known that the pre-edge peak in liquid water reﬂects the HBinteraction, and the intensity of methanol is diﬀerent from thatof water. The pre-edge region shows isosbestic points at 534.8
and 535.9 eV. It indicates that the pre-edge region in the XAS
spectrum contains only two contributions, which is the HBinteraction of liquid methanol and that of liquid water. In orderto obtain the change in the HB interaction at the diﬀerent
concentrations, the pre-edge region between 534 and 536 eV in
the O K-edge XAS spectra at the diﬀerent concentrations is
ﬁtted by the superposition of pure liquid methanol (X = 1.0)
and pure liquid water (X = 0.0), as shown in Figure 2.

Figure 2. Fitting results of the pre-edge region in the O K-edge XAS
spectra at the molar fractions of X = 0.8, X = 0.5, and X = 0.2 in themethanol−water binary solutions (CH3OH)X(H2O)1−X by superposition of the reference spectra of pure liquid methanol (X = 1.0) andpure liquid water (X = 0.0).

Figure 3 shows the fraction of the pure methanol
contribution in the pre-edge region as a molar fraction step
Figure 4. C K-edge XAS spectra of (a) methanol gas and (b) liquidmethanol at 25 °C.

using the same sample cell. The XAS spectrum of methanol gas
was measured by mixing methanol vapor into helium buﬀer gas
and is in agreement with published spectra.55,56,64,65 As shownin Figure 4a, the ﬁrst peak (287.96 eV) in a gas-phase spectrum
arises from a transition of the C 1s electron to the lowest
unoccupied orbital (8a′) of C−O antibonding and O−Hbonding characters. The second peak around 289.44 eV arises
from a transition of the C 1s electron to the second lowest
unoccupied orbital (9a′) of pseudo CH3-π* character with avery small OH component. The broad peak around 293 eVarises from a transition of the C 1s electron to the highest
unoccupied orbital (11a′) within a minimal basis picture, which
is of both C−O antibonding and O−H antibonding character.
Figure 4b shows the present C K-edge XAS spectrum ofliquid methanol with a simple structure of three main
contributions around 288.4, 289.55, and 293 eV. The publishedC K-edge XAS spectrum of free methanol clusters56 is almost
the same as the present liquid spectrum. On the other hand, thepublished spectrum of liquid microjet methanol55 is diﬀerent
from the present one and is rather similar to the gas-phase
spectrum. It could be diﬃcult to completely remove thecontribution from the molecular methanol in the liquid
microjet experiment.
In the liquid spectrum, the ﬁrst peak (8a′-related), which isan excited state with both CH3 and OH components, shows a
0.44 eV blue-shift from that in the gas spectrum. On the other
hand, the second peak (9a′-related), which is an excited state

Figure 3. Fraction of the pure methanol contribution in the pre-edge
region of the O K-edge XAS spectra as a function of the methanol
molar fraction (X) in the methanol−water binary solutions(CH3OH)X(H2O)1−X, which is obtained by ﬁtting from superpositionof pure liquid methanol (X = 1.0) and pure liquid water (X = 0.0).
Each fraction includes an error bar.

of 0.05, which corresponds to the HB interaction. The intensity
decreases almost linearly as the molar fraction of methanol
decreases. There may be some information about diﬀerent local
structures on the oxygen atom behind small deviations from thelinear dependence, but it is consistent with the result ofvibrational spectroscopy,22,24 which explains that the ratio of
HB interaction of methanol−methanol to that of methanol−
water is linearly dependent on the molar fraction of methanol.
Observation of the isosbestic points suggests that the binary
solution has two major HB components, though there are four
diﬀerent HB interactions: Om*HOm, Om*HOw, Ow*HOm, andOw*HOw, where Om and Ow denote oxygen atoms of methanol
and water, respectively, and the asterisk denotes the atom with
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ArticleBecause the second peak related to the methyl group shows aquasi-isosbestic point around 290 eV, two contributions would
be contained in the second peak: One is the interaction ofsurrounding methanol with the methyl group, which is obtainedby the C K-edge XAS spectrum of liquid methanol (X = 1.0).
The other is the hydrophobic interaction of surrounding waterwith the methyl group, which is obtained by the XAS spectrumof the dilute methanol solutions (X = 0.05). In order to obtainthe change in the hydrophobic interaction of methyl group at
the diﬀerent concentrations, the second peak between 288.5
and 290.5 eV in the C K-edge XAS spectra at the diﬀerent
concentrations are ﬁtted by superposition of the reference
spectra of liquid methanol (X = 1.0) and the dilute methanol
solution (X = 0.05), as shown in Figure 6.

with a large CH3 component, shows a 0.11 eV blue-shift fromthat in the gas spectrum. Considering atomic components inthe corresponding molecular orbitals of a methanol molecule,the second excited state has mainly a hydrophobic interaction,
but the ﬁrst excited state has not only a hydrophobic
interaction but also an HB interaction. The unoccupied orbital
level could be destabilized by both the hydrophobic andhydrophilic interactions in liquid, similarly to the case of theblue-shifted O 1s pre-edge peak in liquid water as observed in
Figure 1.
Figure 5 shows C K-edge XAS spectra of methanol−water
binary solutions of diﬀerent concentrations at 25 °C. The C K-

Figure 5. C K-edge XAS spectra of methanol−water binary solutions
at diﬀerent concentrations at 25 °C. The mixing ratio of methanol inthe solution decreases with molar fraction steps of 0.1 along indicatedarrows.

Figure 6. Fitting results of the second peak in the C K-edge XAS
spectra at the molar fractions of X = 0.9, X = 0.6, and X = 0.2 in themethanol−water binary solutions (CH3OH)X(H2O)1−X by superposition of the reference spectra of pure liquid methanol (X = 1.0) andthe dilute methanol solution (X = 0.05).

edge XAS spectrum is more appropriate than the O K-edge
XAS as regards the analysis of the intermolecular interaction of
methanol because the carbon atom is contained only inmethanol. The absorbance in the C K-edge XAS spectra wasnormalized by the sample thickness and the concentration of
the binary solution considering the soft X-ray absorption
coeﬃcients of methanol in the C K-edge.62 After thisnormalization, we subtracted the absorbance of water in the
C K-edge by considering the sample thickness, the concentration of the binary solution, and the soft X-ray absorption
coeﬃcient of water in the C K-edge.62 Figure 5 shows resultant
absorption coeﬃcients. The ﬁrst peak that is related to the HBinteraction of the OH and CH3 groups does not change its
energy position so much at diﬀerent concentrations. This isreasonable if the HB interaction of methanol with water is not
so diﬀerent from that with methanol and is consistent with theresults of O K-edge XAS shown in Figure 3.
On the other hand, the second peak, which corresponds tothe excited state with a large CH3 component, increases theblue-shift as the mixing ratio of water increases. It is reasonable,considering that the blue-shift arises from the interaction of
methyl group in methanol molecule. Liquid methanol forms
1D/2D network structures, and the methyl groups are apartfrom each other due to its hydrophobic interaction. Whenwater molecules join the 1D/2D HB network of methanol, the3D HB network might be formed. Then, the interaction of
methyl groups can be enhanced in binary solutions and causes
the blue-shift of the second peak in the C K-edge XAS spectra.
The general behavior observed in Figure 5 is consistent withthat of the neutron diﬀraction,11 where mixed methanol−water
networks are formed and methyl groups become closer to eachother in the methanol−water binary solution.

Figure 7 shows the fraction of the pure liquid methanol
contribution in the second peak at diﬀerent molar fractions of

Figure 7. Fraction of the pure liquid methanol contribution in thesecond peak in the C K-edge XAS spectra as a function of themethanol molar fraction (X) in the methanol−water binary solutions(CH3OH)X(H2O)1−X, which is obtained by ﬁtting from superpositionof pure liquid methanol (X = 1.0) and the dilute methanol solution (X= 0.05). Each fraction includes an error bar. Three characteristic
regions are found with the borders of X = 0.7 and X = 0.3.

methanol (X) in the binary solution (CH3OH)X(H2O)1−X,
which corresponds to the hydrophobic interaction of themethyl group. The fraction of the pure liquid methanol
contribution is changed nonlinearly and shows three diﬀerent
behaviors with the borders of X = 0.7 and X = 0.3. In themethanol-rich region I (X > 0.7), the intensity is not so muchchanged as compared to the intensity of liquid methanol (X =1.0). The phase transition-like behavior of the intensity change
is found at X = 0.7. The slow decrease in intensity (indicating
blue-shift of the second pre-edge peak) continues when
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Articleincreasing the mixing ratio of water in the region II (0.7 > X >0.3). The decrease in intensity becomes faster in the water-richregion III (0.30 > X > 0.05). These results suggest diﬀerent
local interactions of the methyl group at the diﬀerent
concentration regions.
3.3. MD Simulations. Without any spectral calculation
based on time-consuming density functional theory or ab initio
approaches, it would be simply understood that the C K-edge
region is sensitive to the hydrophobic interaction around the
methyl group of methanol. In order to get such informationfrom the radial distribution function (RDF) of intermolecularinteraction in the solutions, we have carried out the MD
simulation by using GROMACS 4.5.5.66 The potential ofmethanol molecule is described by OPLSAA,67,68 and that ofwater molecule is TIP5P.69 The temperature is controlled by
the Nosé−Hoover thermostat method.70 The pressure isadjusted by the Parrinello−Rahman method.71 The simulation
was performed at a time step of 1 fs with a periodic boundary
condition and the particle-mesh Ewald method.72 The unit cellconsists of 500 molecules, and the molar fraction of methanol(X) is changed from X = 0.0 to X = 1.0. Randomly distributed
structures were optimized by the simulations, which run during
50 ps at 100 K in the NVT condition, 50 ps at 200 K and 1 atmin the NPT condition, and 400 ps at 298.15 K and 1 atm in the
NPT condition. The equilibrium structures were obtained bysampling the structures every 1 ps during a simulation time of 2
ns.
First, we calculated RDF of four diﬀerent HB: Om−HOm,
Om−HOw, Ow−HOm, and Ow−HOw. The distances of both theﬁrst peak and the ﬁrst minimum point in RDF are not changed
even at diﬀerent molar fractions. It means that the HBinteraction of water is nearly the same as that of methanol as
already discussed in the O K-edge XAS.
The number of HB was counted at diﬀerent molar fractions,based on the criterion of the distance between HO and O
within the ﬁrst minimum point (2.5 Å) in RDF.73 By increasing
the molar fraction of water, the total number of HB increases
linearly. The average number of HB around water molecules is
between 3.2 and 3.8, and that around methanol is between 1.8
and 2.5. This is consistent, considering the water molecule has
two donor and two acceptor sites and the methanol molecule
has one donor and one or two acceptor sites. The total average
number of HB is increasing at the higher mixing ratio of watermolecules.
Figure 8 shows RDF of C in the methyl group of methanol
with surrounding atoms at diﬀerent molar fractions of methanol(X) in the binary solution (CH3OH)X(H2O)1−X. Figure 8ashows RDF of C with C and hydrogen H (HC) atoms in theCH3 group of neighboring methanol molecules. The RDF
distances of both C−C and C−HC are slightly reduced byincreasing the mixing ratio of water. This result is consistent
with the results of neutron diﬀraction.11 Figure 8b shows RDF
of C with Om and HOm in neighboring methanol molecules.
The intensities of both HOm and Om in the ﬁrst coordination
peak decrease as increasing the mixing ratio of water. On theother hand, as shown in Figure 8c, the intensities of both HOw
and Ow in the ﬁrst coordination peak increase as increasing themixing ratio of water.
Figure 9 shows the number of the coordination by nearest
neighbors HOm and HOw to the C atom in the methyl group of
methanol at diﬀerent binary solutions. The coordination isdeﬁned within the RDF distance of 3.2 Å, which is the ﬁrst
minimum point of HOm and HOw as shown in Figure 8. The

Figure 8. Results of the MD simulation for the RDF of the C atomwith surrounding atoms: (a) C and HC in methanol, (b) Om and HOm
in methanol, and (c) Ow and HOw in water. The mixing ratio of
methanol in the solution decreases from X = 1.0 to X = 0.1 with molar
fraction steps of 0.1 along indicated arrows. Note that RDF in (c) ischanged from X = 0.9 to X = 0.1.

Figure 9. Results of the MD simulation for the coordination numbersof HOm (square) and HOw (circle) to the C atom of the methyl groupof methanol at diﬀerent molar fractions of methanol (X) in the binary
solutions (CH3OH)X(H2O)1−X. Below X = 0.73 (region II), thecoordination number of HOw becomes larger than that of HOm. In theregion III (0.3 > X), the linear dependence of HOm and HOw is not
valid as shown by dashed lines.

methyl group in liquid methanol (X = 1.0) is surrounded byHOm (square). By increasing the molar fraction of water, thenumber of HOm coordination decreases and instead that ofHOw (circle) increases. When the methanol molar fraction is
below X = 0.7, the number of HOw coordination becomeslarger than the HOm coordination. It is reasonable consideringthat the molar ratio of methanol and water is 2:1 at X = 0.67
and the ratio of the H donating site is 1:1. However, the rate ofincrease in the number of HOw coordination is larger than therate of decrease in the number of HOm coordination. The rateof increase in the number of HOw coordination is accelerated in
the region III (0.3 > X). On the other hand, the number of
HOm coordination is nearly zero in the region III. Note that X= 0.7 and X = 0.3 are almost the same borders in the spectralchange of the C K-edge XAS as shown in Figure 7.
Next, we investigate the mesoscopic scale HB network in thebinary solution. The average size of methanol clusters in pureliquid methanol (X = 1.0) is 40 in the present MD simulation,
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Articlewhich contains 500 molecules. The hydrophobic interaction of
the methyl group prevents a large HB network formation andpermits only formation of small methanol clusters with anaverage size of 40. This size is larger than the previously
predicted size, 6−8 molecules.8
On the other hand, water molecules like to meet (bond)
together to form a large HB network in solution. Figure 10

Figure 10. Results of the MD simulation for the average HB network
size (square) and the average size of water-only clusters (circle)
embedded in total HB networks in a unit cell (500 molecules) of thebinary solution (CH3OH)X(H2O)1−X as a function of molar fraction of
methanol (X). The dashed line is a total number of water molecules inthe solution.

shows results of the MD simulation for the average HB network
size and the average size of water-only clusters embedded in
total HB networks at diﬀerent molar fractions of methanol (X)in the binary solutions (CH3OH)X(H2O)1−X. A unit cell in thepresent MD simulation contains totally 500 molecules.
In the methanol-rich region I (X > 0.7), the average size ofwater-only clusters is rather small considering that a watermolecule is diﬃcult to meet another water molecule in thisregion. Water clusters start to grow at X = 0.7. As the molar
fraction of methanol is down to X = 0.3 in the region II (0.7 >X > 0.3), the rate of growing in size of water clusters is
accelerated, and ﬁnally the average size of water clusters is equal
to the total number of water molecules in the region III. On theother hand, at X = 0.7, all the methanol and water molecules
join a large HB network, though a large water-only cluster is
not yet formed in the network. The ratio of the total number of
H donating (accepting) sites is 1:1 for water and methanol at X
= 0.67; therefore, all the water and methanol molecules canmeet (bond) together to form a large HB network at around X= 0.7.
3.4. Structures of Methanol−Water Mixtures. From theresult of the C K-edge XAS shown in Figure 7, the interaction
around the methyl group of methanol molecule shows
characteristic changes at the three concentration regions. The
MD simulations also show similar three concentration regions
from the coordination number around the methyl group shownin Figure 9 and the average cluster size shown in Figure 10.
Figure 11a shows a typical structure in the binary solution at
X = 0.9 in the methanol-rich region I. As shown in Figure 10,
the average size of water-only clusters is much smaller than the
total number of water molecules. Water molecules form the HB
network with methanol clusters and stabilize the total energy ofthe binary solution. However, the interaction around the
methyl group of methanol is not so much inﬂuenced by watermolecules because of a small amount of isolated watermolecules. It is consistent with the previous work,16,27 wherewater molecules are coordinated to the terminal of methanol
chains in the methanol-rich region.

Figure 11. Typical structures of methanol−water binary solutions at
diﬀerent concentrations: (a) X = 0.9, (b) X = 0.5, and (c) X = 0.1.
Methanol and water molecules are marked as green and white,
respectively. HB between methanol and water is marked as an orange
line.

Figure 7 shows a phase-transition-like intensity change at X =0.7 in the C K-edge XAS. Figure 9 shows that the number of
HOw coordination to the methyl group becomes larger thanthat of HOm coordination below X = 0.7. In addition, Figure 10
shows that the HB network of water clusters grows rapidly
below X = 0.7. Figure 11b shows a typical structure in thebinary solution at X = 0.5 in the region II. Water molecules
form a large cluster and have the 3D HB network withmethanol molecules, resulting in the increase of the interactionof the methyl group in methanol with water molecules. The
phase-transition-like behavior at X = 0.7 in the C K-edge XAS(Figure 7) indicates that the 3D HB network involving water
clusters is dominant over the 1D/2D HB network of methanol
in the binary solutions. This result is consistent with theprevious MD simulation, in which the 1D chain structure ofmethanol molecules is changed to 3D mixed clusters by adding
water molecules.40
Figure 11c shows a typical structure in the binary solution at
X = 0.1 in the water-rich region III. The HB networks between
methanol molecules are mostly diminished, and methanol
molecules are isolated in the 3D HB network of water. The
hydration structures of methanol molecules are dominated by
the 3D HB network of water, and the numbers of water
coordination to the methyl group increase. As a result, thehydrophobic interaction around the methyl group is enhancedin this region, increasing a blue-shift in the C K-edge XAS. The
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Articlemolecules. These features are successfully revealed by the ﬁtting
analysis of the second peak in the C K-edge XAS, which issensitive to the hydrophobic interaction of the methyl group,and the MD simulation.

previous theoretical studies suggested that hydration structuresof methanol molecules are formed in this concentration
region,41,43,45 consistent with the present result.
It is known that the thermodynamic parameter such as
entropy and viscosity shows an extreme value at the molar
fraction of X = 0.30.1,2 Dougan et al. studied neutron diﬀraction
experiments and MD simulations and suggested that bothmethanol and water molecules are percolated in this region, andthe thermodynamic parameters show extreme values at themolar fraction of X = 0.27.12 It means that the structure andabundance of large mixed methanol−water HB networks in thebinary solution, as clearly shown in Figures 9 and 10, aﬀect
macroscopic thermodynamic properties.

■

AUTHOR INFORMATION

Corresponding Author*E-mail: kosugi@ims.ac.jp (N.K.).
Present Address

V.L.: École Nationale Supérieure de Chimie de Paris (Chimie
ParisTech), Paris, 75231 Cedex 05, France.
Notes

The authors declare no competing ﬁnancial interest.

■

4. CONCLUSIONSThe local structure of methanol−water binary solutions wasstudied by the O and C K-edge XAS. The pre-edge peak in the
O K-edge XAS reﬂects the HB interaction of oxygen atoms andshows almost linear concentration dependence. It indicates thatthe HB interaction of methanol with surrounding watermolecules is nearly the same as in pure liquid methanol andthe HB interaction of water with surrounding methanol
molecules is nearly the same as in pure liquid water.
The C K-edge XAS enables us exclusively to investigate local
structures around the methyl group of methanol molecules inthe binary solution. The peak around 290 eV in the C K-edge
XAS corresponds to a transition of the C 1s electron to theunoccupied orbital around the methyl group and shows thehigher photon energy (blue-shift) as the mixing ratio of water
increases. It predicts enhancement in the interaction between
the hydrophobic methyl groups by large water clusters in mixed
methanol−water networks. The intensity change shows anonlinear proﬁle with three characteristic concentration regionsin the binary solution (CH3OH)X(H2O)1−X.
The three regions are consistently interpreted with the helpof the MD simulation. Liquid methanol is known to have the
1D/2D HB network structure.8 In the methanol-rich region I(X > 0.7), a small amount of water molecules form the HB
network with methanol clusters and stabilize the total energy ofthe binary solution. However, the interaction around the
methyl group of methanol is not so much inﬂuenced by isolated
water molecules. The phase-transition-like decrease in theintensity occurs at the molar fraction of X = 0.7. The 3D HB
network of water start to grow rapidly, and the HOw
coordination to the methyl group becomes dominant overthe HOm coordination when the molar fraction is below X =0.7. In the region II (0.7 > X > 0.3), methanol molecules form alarge HB network with water molecules. As a result, thehydrophobic interaction of the methyl group is enhanced in thisregion. This behavior is reasonable considering that the molar
ratio of methanol and water is 2:1 at X = 0.67 and the ratio of
the H donating site is 1:1. The thermodynamic parameters suchas entropy and viscosity are closer to extreme values as thenumber of mixed HB networks increases at X = 0.3. In thewater-rich region III (0.3 > X > 0.05), the interaction ofsurrounding water molecules with the methyl group isincreased rapidly, indicating methanol molecules are separately
embedded in the 3D HB network of water molecules.
The methyl group in the methanol−water binary solution
shows three characteristic local structures: methanol-dominant
1D/2D HB network structure with isolated water molecules,
methanol−water mixed 3D HB network structure, and waterdominant 3D HB network structure with isolated methanol

ACKNOWLEDGMENTS
This work is supported by JSPS Grants-in-Aid for ScientiﬁcResearch (Nos. 20350014, 23245007, and 23685006). The
authors acknowledge Dr. Takaki Hatsui and Mr. Toshio
Horigome for the contributions of the liquid cell and the staﬀ
members of the UVSOR-II facility for their kind support.
Theoretical calculations were performed at Research Center for
Computational Science, Okazaki, Japan.

■

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