Article
pubs.acs.org/JPCBInteraction between Water and Alkali Metal Ions and Its Temperature
Dependence Revealed by Oxygen K‑Edge X‑ray Absorption
Spectroscopy
Masanari Nagasaka,*,†,‡ Hayato Yuzawa,† and Nobuhiro Kosugi†,‡†

Institute for Molecular Science, Myodaiji, Okazaki 444-8585, JapanSOKENDAI (Graduate University for Advanced Studies), Myodaiji, Okazaki 444-8585, Japan‡

ABSTRACT: Interaction between water molecules and alkali
metal ions in aqueous salt solutions has been studied by the oxygen
K-edge soft X-ray absorption spectroscopy (XAS) in transmission
mode. In the measurement of several alkali halide aqueous
solutions with diﬀerent alkali chlorides (Li, Na, and K) anddiﬀerent sodium halides (Cl, Br, and I), the pre-edge component
arising from the hydration water molecules shows a blue shift inpeak energy as strongly depending on cations but not on anions. In
the temperature dependent measurement, the pre-edge component
arising from water molecules beyond the ﬁrst hydration shell shows
the same behavior as that of pure liquid water. On the other hand,the pre-edge component arising from water molecules in the ﬁrst
hydration shell of Li+ ions is not evidently dependent on the temperature, indicating that the hydration water molecules are morestrongly bound with Li+ ions than the other water molecules. These experimental results are supported by the results of radial
distribution functions of the ﬁrst hydration shell and their temperature dependence, evaluated by molecular dynamics
simulations.
spectroscopy.37−39 Smith et al. reported that the bond of anionswith water molecules becomes weaker in the order of F−, Cl−,
Br−, and I− ions.39 On the other hand, the hydration shell of
cations in aqueous salt solutions has not yet been studied indetail. This is because there are few eﬀective methods toinvestigate the interaction of cations with water molecules.
The local structure of liquid water has been extensively
studied by the O K-edge XAS.40−52 The pre-edge peak (535eV) of pure liquid water corresponds to a transition from O 1s
to the 4a1 unoccupied orbital of water molecule, where the 4a1
orbital is mainly distributed on the oxygen atom of watermolecule. Therefore, the pre-edge peak could be aﬀected by theinteraction of hydrating water molecules with cations. Several
groups discussed the hydration structure of cations from theenergy shift of the pre-edge peaks in O K-edge XAS of aqueous
salt solutions by several detection techniques.53−59 Näslund etal. measured O K-edge XAS of NaCl and KCl solutions in
ﬂuorescence yield,53 and discussed that the spectral changes arecaused by the interaction of cations with water molecules. On
the other hand, Cappa et al. studied O K-edge XAS of aqueous
salt solutions from total electron detection of liquid microjet,54,55 and proposed that the spectral changes are caused by
the electronic perturbation of water molecules by anions.
Juurinen et al. measured O K-edge XAS of LiCl solutions at

1. INTRODUCTION
Macroscopic properties of aqueous salt solutions, such as
viscosity, boiling point, and freezing point, are inﬂuenced by theinteraction between solute ions and solvent water molecules.1−3
Recently, hydration structures of ions in aqueous salt solutions
were extensively studied by neutron diﬀraction.4−9 Ohtomo et
al. studied hydration structures of Li+ ions in LiCl solutions.4
Mancinelli et al. studied hydration structures of Na+ and K+
ions in NaCl and KCl solutions.8 Hydration structures of ionsin aqueous salt solutions were also studied by X-ray
scattering10−13 and extended X-ray absorption ﬁne structure.14,15 Megyes et al. studied coordination numbers of Na+ions by X-ray diﬀraction.16 The electronic structures of alkali
halide aqueous solutions were studied by photoelectron
spectroscopy17 and X-ray emission spectroscopy.18−20 Hydration structures of cations were also studied by molecular
dynamic (MD) simulations.21−34 Most of them focused on thecoordination number and the distance of ions with hydratingwater molecules.
In aqueous salt solutions, the cation is weakly bound tooxygen atoms of water molecules, whereas the anion is weakly
bound to hydrogen atoms. Na K-edge X-ray absorption
spectroscopy (XAS) revealed that both Na+ and Cl− ionsshare the same water molecules in highly concentrated NaClsolutions.35 These solvent-shared ion pairs in LiCl solutions
were also conﬁrmed by dielectric relaxation spectroscopy.36
The interaction of anions with water molecules was extensively
studied by using the OH stretch vibration mode in Raman© 2017 American Chemical Society

Received: October 3, 2017
Revised: November 6, 2017
Published: November 13, 2017
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and ﬁnally a photodiode detector. The thickness of liquidsample layer is at most 2000 nm at the helium pressure of
0.1013 MPa in the helium buﬀer region. By increasing thehelium pressure, the liquid sample layer becomes thinner todeform the two elastic membranes of the liquid sample, and theliquid thickness is at minimum 20 nm at the helium pressure of
0.12 MPa. The energy resolution of incident soft X-rays is set to
0.4 eV. XAS spectra are obtained by using Lambert−Beer law,ln(I0/I), where I0 and I are the transmission signal of blank andliquid sample, respectively. The photon energy is calibrated by
the O 1s → π* peak (530.80 eV) of O2 gas mixed a little in thehelium buﬀer gas.74
2.2. MD Simulations. In order to obtain radial distributionfunctions (RDF) of cations with hydrating water molecules, wehave carried out MD simulations by using GROMACS
4.6.1.75,76 The potentials of ions are described by OPLSAA,77,78
and that of water molecule is described by TIP5P.79 The
temperature is controlled by the Nosé−Hoover thermostat
method.80,81 The pressure is adjusted by the Parrinello−
Rahman method.82 The simulation is performed at a time stepof 1 fs with a periodic boundary condition and the partial-mesh
Ewald method.83 The unit cell consists of about 500 watermolecules and several ions considering molar concentrations ofaqueous salt solutions. Randomly distributed structures are
optimized by the simulations, which run during 100 ps at 100 K
in the NVT condition, 100 ps at 200 K and 1 atm in the NPT
condition, and 2 ns at a destination temperature and 1 atm in
the NPT condition. The equilibrium structures are obtained bysampling the structures every 1 ps during a simulation time of
20 ns.

diﬀerent concentrations by using the hard X-ray Raman
scattering, and observed that the pre-edge peak is shifted tothe higher photon energy by increasing the concentration.58
Waluyo et al. measured O K-edge XAS of aqueous salt solutions
in transmission mode by using a droplet of liquid sample
sandwiched between two Si3N4 membranes, and proposed thatthe pre-edge peaks are inﬂuenced by the hydration structures ofcations.57,59 Thus, the previous experimental results anddiscussion are inconsistent with each other. In order to solve
the inconsistency, it is necessary to measure reliable O K-edge
XAS of diﬀerent aqueous salt solutions in transmission mode.60
In addition, the temperature dependence of O K-edge XAS
spectra is also important in investigation of the hydration
structure of cations in aqueous salt solutions. Several groups
measured O K-edge XAS of liquid water at diﬀerent
temperatures.43,44,47,49,51,52 They revealed that the pre-edge
peak in XAS spectra reﬂects a local structural change of thehydrogen bond (HB) network at diﬀerent temperatures. The
temperature eﬀect of liquid water has also been studied byneutron diﬀraction,61 nuclear magnetic resonance,62 X-ray
Compton scattering,63,64 infrared and Raman spectroscopy,65−68 and MD simulations.68−70 Waluyo et al. measured O
K-edge XAS of aqueous salt solutions at diﬀerent temperaturesby using hard X-ray Raman scattering,59 and observed the preedge peak shift dependent on the temperature. However,reliable XAS in transmission mode has not yet been reported
from the viewpoint of temperature dependence of aqueous salt
solutions.
We have been developing several kinds of liquid ﬂow cells for
XAS in transmission mode.48,60,71,72 In the present work, weinvestigate the hydration structure in aqueous alkali halide
solutions by O K-edge XAS in transmission mode. Especially,
we have concentrated on the interaction between alkali metal
ions and hydrating water molecules to select aqueous LiCl,
NaCl, KCl, NaBr, and NaI solutions. From XAS spectra ofdiﬀerent cations and anions, we conﬁrm that the pre-edge peakshift is strongly dependent on cations but not on anions, andthat the temperature dependence of O K-edge XAS spectra of
LiCl solutions arises mainly from bulk water, beyond the ﬁrst
hydration shell of Li+ ions. We perform MD simulations ofsome aqueous salt solutions to support the experimental
investigation.

3. RESULTS AND DISCUSSION
3.1. Dependence of Cation and Anion. Figure 1a shows
O K-edge XAS spectra of liquid water and aqueous salt
solutions with diﬀerent cations (3 M LiCl, 3 M NaCl, and 3 MKCl) at 25 °C. The XAS spectrum of liquid water shows threewell-known features: pre-edge peak (535 eV), main-edge
maximum (537 eV), and post-edge maximum (540 eV). The
intensities of the post-edge maximum of aqueous salt solutions
are decreased as compared to that of pure liquid water,
indicating that the HB network is partially broken by ions andthe multiple scattering in the HB network is weakened. As
shown in the inset of Figure 1a, the pre-edge peaks of aqueous
salt solutions show higher energies (blue shifts) than that ofpure liquid water. Diﬀerent cations show diﬀerent pre-edge
peak energies, and the LiCl solution shows the largest blue shiftof the three alkali chloride solutions. Figure 1b shows O K-edge
XAS of liquid water and aqueous salt solutions with diﬀerent
anions (3 M NaCl, 3 M NaBr, and 3 M NaI) at 25 °C. The preedge peaks of NaCl are slightly higher in energy than those of
NaBr and NaI solutions, but are rather smaller than those of
LiCl. These results indicate that the energy shift of the pre-edge
peaks is predominantly caused by the hydration structures ofalkali metal ions. It is reasonable considering that the 4a1 orbital
related to the pre-edge peak is mainly distributed on the oxygen
atom of water.
Assuming that the water molecules beyond the ﬁrst
hydration shells of cations have almost the same HB networks
as in pure bulk water, the pre-edge features can be decomposed
into the contribution from the water molecules (Ofw) in theﬁrst hydration shells of cations and the contribution from theother water molecules (Obw) beyond the ﬁrst hydration shell. In
Figure 2, the pre-edge peak in O K-edge XAS of 3 M LiCl

2. METHODS
2.1. XAS Measurements. We have carried out O K-edge
XAS measurements at the soft X-ray undulator beamline BL3U
at UVSOR-III Synchrotron.73 Details of a transmission-type
liquid ﬂow cell used in the present work were described
previously.48,60,72 In the liquid ﬂow cell, a liquid layer issandwiched between two 100 nm thick Si3N4 membranes withthe windows size of 2 × 2 mm2 (NTT AT). Teﬂon spacers withthe thickness of 100 μm are set between the support plates of
the Si3N4 membranes. Liquid samples are exchangeable in situ
by using a tubing pump. Temperature of a liquid sample iscontrollable from −5 to 80 °C. The liquid ﬂow cell is in
atmospheric helium condition, which is separated by a 100 nm
thick Si3N4 membrane window from the soft X-ray beamline inultrahigh vacuum condition. This windows size determines thesoft X-ray beam size on the sample. In the present work, wehave chosen 200 × 200 μm2 for use of more photon ﬂuxes
though 30 × 30 μm2 is possible to choose. Soft X-rays undervacuum pass through the membrane window, a liquid layer
sandwiched by the two membranes in the helium buﬀer region,
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solution is ﬁtted by assuming that the pre-edge peak arisingfrom Obw is the same as that of pure liquid water. In this ﬁtting
analysis, the number of Ofw is referred from the neutron
diﬀraction studies:4,8 4 for Li+, 5 for Na+, and 6 for K+ ions. The
intensity ratio of bulk water is evaluated as 78% in 3 M LiClsolution, where the ratio of the number of ions and molecules,
Li+:Ofw:Obw = 1:4:14. There are two major components in the
O K-edge spectra, bulk water beyond the ﬁrst hydration shelland water of the ﬁrst hydration shell, directly interacting withthe cation. The spectral contribution from bulk water (known)
is subtracted in the ﬁtting analysis considering the intensity
ratios of Ofw and Obw, and then the spectral contribution fromhydration water can be obtained, though water of the second
hydration shell may be slightly diﬀerent from bulk water.
Table 1 shows the pre-edge peak energy of Ofw and theenergy shifts ΔE of Ofw in comparison with pure liquid water atTable 1. Pre-Edge Peak Energy of Hydrating Water
Molecules (Ofw) in the First Hydration Shell of Cations at 25°C Determined from the Results Shown in Figure 1 by the
Fitting Shown in Figure 2, and the Energy Diﬀerence ΔEfrom the Pre-Edge Peak Energy of Pure Liquid Water
samplesenergy (eV)ΔE (eV)

pure liquid water
5 M LiCl
3 M LiCl
3 M NaCl
3 M KCl
3 M NaBr
3 M NaI

534.67534.97534.94534.76534.67534.71
534.69

0.30
0.27
0.09
0.00
0.04
0.02

25 °C. In alkali chlorides, the cation eﬀect is remarkable: Li+(0.27 eV), Na+ (0.09 eV), and K+ (0.00 eV). Because theenergy shift of Li+ ion in 5 M LiCl solution (0.30 eV) is close tothat in 3 M, the contribution of Ofw is successfully extracted in
the present ﬁtting procedures. On the other hand, in sodium
halides, the anion eﬀect is rather small: Cl− (0.09 eV), Br−(0.04 eV), and I− (0.02 eV). Note that the energy shifts of preedge peak in diﬀerent anions from pure water include theinteraction of Na+ ions with surrounding water molecules. In
highly concentrated aqueous salt solutions above 1 M, it wasreported that water molecules share both cations and anions
and the electronic structure of water molecules in the ﬁrst
hydration shells of cations is also inﬂuenced by anions.35 Even if
so, the large energy shift of LiCl solutions indicates that thepre-edge peak energy arises predominantly from the interactionof cations with Ofw. Thus, the pre-edge peak shift indicates thatthe hydration of Li+ ions is stronger than that of Na+ ions, andthat of Na+ ions is stronger than that of K+ ions.
In order to conﬁrm the relation between the pre-edge peakshift and the interaction of cations with water molecules in theﬁrst hydration shell, we have evaluated RDF in diﬀerent
aqueous salt solutions by using MD simulations. Figure 3ashows RDF of cations (M+) with oxygen atom of watermolecules (Ow) in diﬀerent aqueous salt solutions with theconcentration of 3 M. The positions of the ﬁrst peaks in RDF
M+-Ow of Li+, Na+, and K+ ions are 1.99, 2.37, and 2.69 Å,
respectively. These results are in good agreement with theneutron diﬀraction results,4,8 showing that the bond lengths of
Li+, Na+, and K+ ions with Ow are 1.90, 2.34, and 2.65 Å,
respectively. The coordination number of Li+ ions obtained by
MD simulation is around 4, and is also consistent with the

Figure 1. (a) O K-edge XAS spectra of pure liquid water and aqueous
salt solutions with diﬀerent cations (3 M LiCl, 3 M NaCl, and 3 MKCl) at 25 °C. (b) O K-edge XAS spectra of pure liquid water andaqueous salt solutions with diﬀerent anions (3 M NaCl, 3 M NaBr, and
3 M NaI) at 25 °C. The insets show the expansions of the pre-edge
regions.

Figure 2. An example of the ﬁtting of the pre-edge peak in O K-edge
XAS of 3 M LiCl solution, assuming that the pre-edge feature consistsof the hydrating water molecules (Ofw) in the ﬁrst hydration shell of
Li+ ions and the other water molecules (Obw). The intensity ratio is thesame as the ratio of numbers of Ofw and Obw. The black line is theﬁtted curve of the main-edge maximum (537 eV).

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Figure 3. (a) RDF of cations (M+) with oxygen atoms of watermolecules (Ow) in diﬀerent aqueous salt solutions with theconcentration of 3 M. (b) RDF of anions (X−) with Ow in diﬀerent
aqueous salt solutions.

Figure 4. O K-edge XAS spectra of liquid water at diﬀerent
temperatures. The dashed lines show the reference spectrum at 25.6
°C. The inset shows the pre-edge region.

neutron diﬀraction results.4 The RDF M+-Ow with diﬀerent
anions (NaCl, NaBr, and NaI) shows that the ﬁrst peak of Na+ions are not changed irrespective of anions. Figure 3b shows
RDF of anions (X−) with Ow in aqueous salt solutions. The
positions of the ﬁrst peaks in RDF X−-Ow of Cl−, Br−, and I−
ions are 3.26, 3.30, and 3.58 Å, respectively, which are rather
longer than those of cations; therefore, the interaction of anionswith water molecules would be too small to show a noticeable
pre-edge energy shift. This result is consistent with theexperimental result.
3.2. Temperature Eﬀects of Hydration Shells. In order
to investigate the temperature dependence of the hydration
shell in aqueous salt solutions, we have measured O K-edge
XAS spectra of liquid water and 5 M LiCl solutions at diﬀerent
temperatures, as shown in Figures 4 and 5, respectively. In O Kedge XAS spectra of liquid water, the intensity of the post-edge
maximum is decreased at the higher temperature, indicatingthat the HB network is partially broken at the higher
temperature and the multiple scattering in the HB network is
weakened. On the other hand, as shown in the inset of Figure 4,
the pre-edge peak energy is lowered by increasing thetemperature. Figure 6a shows the temperature dependence ofthe energy shift of the pre-edge peak from 25.6 °C in liquidwater. The energy shift is −93 meV from 8.3 to 76.7 °C. This
result is almost consistent with previously measured temperature dependence of O K-edge XAS spectra of liquid water by
several detection techniques.43,44,47,49,51,52 The smooth temperature dependence of the pre-edge peak energy arises from thetemperature dependence of the HB distance (namely, volume
expansion), because the longer HB distance reduces theexchange and electrostatic interactions between nearest

Figure 5. O K-edge XAS spectra of 5 M LiCl solutions at diﬀerent
temperatures. The dashed lines show the reference spectrum at 25.6
°C. The inset shows the pre-edge region.
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shifted) at the higher temperature. The pre-edge features in 5
M LiCl solutions can be decomposed into the contributions
from Ofw and Obw by the same ﬁtting procedure shown in
Figure 2, where the bulk water components (Obw) are obtained
from XAS of pure liquid water at diﬀerent temperatures shownin Figure 4. Figure 6b shows the pre-edge peak shift of Ofw in 5
M LiCl solution at diﬀerent temperatures. The energy shift for
Ofw is −23 meV from 8.2 to 76.5 °C, and is much smaller thanthat of bulk water (−93 meV). This result indicates that thehydration water molecules are more strongly bound with Li+
ions than the other water molecules and show much smaller
temperature dependence than the bulk water.
In order to investigate the temperature eﬀect of thehydration shell of Li+ ions theoretically, we have evaluated
RDF in 5 M LiCl solutions at diﬀerent temperatures by using
MD simulations. Figure 7a shows RDF of Li+ ions with oxygen
atoms in water molecules (Ofw) in the ﬁrst hydration shell at
diﬀerent temperatures. The peak position in RDF Li+-Ofw is
2.00 Å at 5 °C, and is slightly shorten by increasing thetemperature (1.98 Å at 80 °C). The hydration shells of Li+ ions
are slightly pressed by the elongation of HB network of water athigher temperatures. This result is consistent with the previous
MD simulations,24 where the ﬁrst peaks in RDF Li+-Ow are notchanged at diﬀerent temperatures. Figure 7b shows RDF of Ofw
with oxygen atoms of water molecules (Obw) beyond the ﬁrst
hydration shell at diﬀerent temperatures in 5 M LiCl solutions.
The ﬁrst peak position in RDF Ofw-Obw becomes longer withincreasing the temperature, in which the distances are 2.71 Å at
5 °C and 2.76 Å at 80 °C. Figure 7c shows RDF Obw-Obw at
diﬀerent temperatures in 5 M LiCl solutions. The ﬁrst peak
position in RDF Obw-Obw in 5 M LiCl aqueous solutions is 2.71
Å at 5 °C and 2.76 Å at 80 °C, the same as RDF Ofw-Obw. As
shown in Figure 7d, RDF Obw-Obw in pure liquid water at
diﬀerent temperatures are also 2.71 Å at 5 °C and 2.76 Å at 80°C, the same as RDF Ofw-Obw and RDF Obw-Obw in 5 M LiClaqueous solutions. The MD results are consistent with thepresent O K-edge XAS results, indicating that the interaction of
Li+ ions with hydrating water molecules (Ofw) is stronger thanthe HB network beyond the ﬁrst hydration shell and is muchless dependent on the temperature. In addition, the MD results
indicate clearly that the HB interaction beyond the ﬁrst
hydration shell is the same as that of pure liquid water.

Figure 6. (a) Temperature dependence of the pre-edge peak energy in
O K-edge XAS spectra of pure liquid water as compared with thereference spectrum at 25.6 °C. The inset shows a typical HB structure
in liquid water. (b) Temperature dependence of the pre-edge peakenergy of the hydrating water (Ofw) of Li+ ions in 5 M LiCl solutions
as compared with the reference spectrum at 25.6 °C. The inset shows atypical hydration structure of Li+ ions.

neighbor water molecules, resulting in red shift of the pre-edge
peak. Here, we do not have to consider the HB network
breaking eﬀect on the pre-edge peak shift.
Figure 5 shows O K-edge XAS spectra of 5 M LiCl solutions
at diﬀerent temperatures. The intensities of the post-edge
maximum are decreased by increasing the temperature,indicating that the HB network is partially broken by ions.
On the other hand, the pre-edge peak energy is lowered (red-Figure 7. (a) RDF of Li+ ions with oxygen atoms of hydrating water molecules (Ofw) in the ﬁrst hydration shell in 5 M LiCl aqueous solutions as afunction of temperature in the order of rainbow color. The inset shows the ﬁrst hydration shell of Li+ ions and water molecules (Obw) beyond theﬁrst hydration shell. (b) RDF of Ofw with Obw in 5 M LiCl solution as a function of temperature. (c) RDF Obw-Obw in 5 M LiCl solution as a functionof temperature. (d) RDF Obw-Obw in pure liquid water as a function of temperature. The inset shows a typical HB structure in pure liquid water.
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4. CONCLUSIONS
We have investigated the hydration eﬀect of alkali metal cations
in aqueous salt solutions on the pre-edge peak of O K-edge
XAS in transmission mode for various aqueous salt solutions
with diﬀerent cations (LiCl, NaCl, and KCl) and anions (NaCl,
NaBr, and NaI). Due to the interaction of cations with watermolecules in the ﬁrst hydration shell, the pre-edge peak isshifted to the higher energy (blue shift) in comparison withpure liquid water, in the order of K+, Na+, and Li+ ions, whichmean that the hydration strength is in the order of Li+, Na+, andK+ ions. We have performed the temperature dependent XAS
measurements of pure liquid water and 5 M LiCl solutions.
Assuming that the number of water molecules in the ﬁrst
hydration shell is 4 and the pre-edge component of the otherwater molecules, beyond the ﬁrst hydration shell, shows thesame temperature dependence as of bulk water, the spectral
decomposition results show that the pre-edge component ofwater molecules in the ﬁrst hydration shell is not evidently
dependent on the temperature. It indicates that the interactionof Li+ ions with hydrating water molecules is strong enough tokeep the ﬁrst hydration shell of Li+ ions irrespective of thetemperature. This experimental result is in good agreementwith the result of MD simulations.

■

AUTHOR INFORMATION

Corresponding Author*E-mail: nagasaka@ims.ac.jp.
ORCID

Masanari Nagasaka: 0000-0002-6249-6553
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work is supported by JSPS Grants-in-Aid for ScientiﬁcResearch (JSPS KAKENHI Grant Nos. 20350014, 23245007,
23685006, 26248010, and 17H03013), Grant for Basic Science
Research Projects from the Sumitomo Foundation, and Morino
Foundation for Molecular Science. The authors acknowledged
Mr. Toshio Horigome for his contribution to the developmentof the liquid ﬂow cell and the staﬀ members of UVSOR-III
Synchrotron for their kind support. The computations were
performed using Research Center for Computational Science,
Okazaki, Japan.

■

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