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TRANSCRIPT
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Effects of Ions on the OH Stretching Band of Wateras Revealed by ATR-IR Spectroscopy
Norio Kitadai • Takashi Sawai • Ryota Tonoue • Satoru Nakashima •
Makoto Katsura • Keisuke Fukushi
Received: 16 December 2013 / Accepted: 12 March 2014 / Published online: 26 June 2014� Springer Science+Business Media New York 2014
Abstract The effects of various cations (Li?, Na?, K?, Rb?, Cs?, Mg2?, Ca2?, Sr2?,
Ba2?, Mn2?, Co2?, and Ni2?) and anions (Cl-, Br-, I-, NO�3 , ClO�4 , HCO
�3 , and CO
2�3 )
on the molar absorptivity of water in the OH stretching band region (2,600–3,800 cm-1)
were ascertained from attenuated total reflection infrared spectra of aqueous electrolyte
solutions (22 in all). The OH stretching band mainly changes linearly with ion concen-
trations up to 2 mol�L-1, but several specific combinations of cations and anions (Cs2SO4,Li2SO4, and MgSO4) present different trends. That deviation is attributed to ion pair
formation and cooperativity in ion hydration, which indicates that the extent of the ion–
water interaction reflected by the OH stretching band of water is beyond the first solvation
shell of water molecules directly surrounding the ion. The obtained dataset was then
N. Kitadai (&)Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-IE-1, Ookayama, Megoro-ku,Tokyo 152-8550, Japane-mail: [email protected]
T. SawaiNakatsugawa Factory Manufacturing, Mitsubishi Electric Corporation, 1213 Matsuoshirota, Iida,Nagano 395-0812, Japane-mail: [email protected]
R. Tonoue � S. Nakashima � M. KatsuraDepartment of Earth and Space Science, Graduate School of Science, Osaka University, 1-1Machikaneyama, Toyonaka, Osaka 560-0043, Japane-mail: [email protected]
S. Nakashimae-mail: [email protected]
M. Katsurae-mail: [email protected]
K. FukushiInstitute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa,Ishikawa 920-1192, Japane-mail: [email protected]
123
J Solution Chem (2014) 43:1055–1077DOI 10.1007/s10953-014-0193-0
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correlated with several quantitative parameters representing structural and dynamic
properties of water molecules around ions: DGHB, the structural entropy (Sstr), the viscosityB-coefficient (Bg), and the ionic B-coefficient of NMR relaxation (BNMR). Results show
that modification of the OH stretching band of water caused by ions has quasi-linear
relations with all of these parameters. Vibrational spectroscopy can be a useful means for
evaluating ion–water interaction in aqueous solutions.
Keywords ATR-IR � Water � OH stretching band � Ion–water interaction
1 Introduction
Ascertaining the physicochemical properties of water in aqueous electrolyte solutions is an
extremely attractive objective in natural sciences because of water’s ubiquitous presence in
daily life and its importance in technical, chemical, and biological processes. Vibrational
spectroscopy, i.e. IR and Raman spectroscopy, provides microscopic information related to
the behaviors of water molecules in aqueous electrolyte solutions because the OH
stretching band of water is sensitive to interactions between ions and hydration water
molecules as well as hydrogen bonds between water molecules. To date, several experi-
mental works have been performed to evaluate structural and dynamic properties of water
molecules around ions from shifts and intensity changes of the OH stretching band of water
caused by the ions [1–12]. The OH stretching band is generally regarded as comprising
several components that are attributed to water molecules embedded in different hydrogen-
bonded environments (Table 1). Components located at the lower frequency region are
attributed to water molecules forming stronger hydrogen bonds, whereas those at the
higher frequency region are attributed to water molecules forming weaker ones. Conse-
quently, ions that shift the OH stretching band toward lower frequencies are typically
interpreted as strengthening the hydrogen bonds between water molecules in the hydration
layer because of tight ion–water interaction, whereas others are interpreted as weakening
the hydrogen bonds. As one example of related research, Masuda et al. [1] performed curve
fitting of the OH stretching bands of NaCl, NaHCO3, and Na2CO3 solutions with four
Gaussian components that were attributed to water molecules having different mean
hydrogen bond distances ranging from 0.273 to 0.295 nm. From the increase of higher- (or
lower-) wavenumber components with increasing NaCl (or Na2CO3) concentration, they
interpreted this to mean that a NaCl solution has longer H-bond distance characteristics,
whereas a Na2CO3 solution has shorter ones.
It is noteworthy that, as shown in Table 1, different researchers use different definitions
for the number, position, and assignment of respective components. Consequently, the
results of their studies are not always consistent. For instance, Li et al. [2] concluded
through Gaussian fitting analyses of the Raman OH stretching band of MgCl2 and CaCl2solutions that Ca2?, Mg2?, and Cl- destroy the tetrahedral water structure through mutual
interaction of water molecules and ions, whereas Chen et al. [5] interpreted that to mean
the structure-making capability of Mg2? is much higher than those of Li? and Na? based
on spectral comparisons among the ATR-IR spectra of Mg(ClO4)2, LiClO4, and NaClO4solutions.
One reason for the lack of a definition of the OH stretching band of water is its strong
dependence on experimental conditions and techniques. For instance, the parallel polarized
1056 J Solution Chem (2014) 43:1055–1077
123
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Raman spectrum of water shows an OH stretching band having its maximum at
3,400 cm-1 with a broad shoulder band around 3,250 cm-1, whereas the perpendicular
polarized Raman spectrum of water shows a symmetrical one centered at 3,430 cm-1 [13–
15]. As a response to dissolving KBr in water, the former band exhibits a shift toward
higher frequency of the maximum, whereas the latter changes its maximum position only
slightly, although the width at half maximum decreases concomitantly with increasing KBr
concentration [14]. Extreme care should also be taken in interpreting the attenuated total
reflection infrared (ATR-IR) spectrum of water because it is distorted strongly by the
anomalous dispersion of aqueous solutions [16].
Another cause arises from the fact that both cations and anions present in aqueous
solutions influence the OH stretching band of water. Their influences generally strongly
overlap and consequently are difficult to separate. Therefore, most reports of experimental
studies have described the overall effect of cations and anions instead of their respective
contributions. That overlap often complicates interpretation of the data and renders con-
clusions ambiguous. Additionally, it should be considered that the OH stretching band of
water has different sensitivity to cations and anions: anions, which interact directly with
Table 1 Reported empirical definitions on the OH stretching band of water
Position(cm-1)
Assignment Technique Reference
3,051 Fully four-hydrogen-bonded water molecules Raman Li et al. [2–4]
3,233
3,393 Partly hydrogen bonded water molecules
3,511
3,628 Free water molecules or free OH
3,014 Single donor–double acceptor (DAA) Raman Sun [11]
3,220 Double donor–double acceptor (DDAA)
3,430 Single donor–single acceptor (DA)
3,572 Double donor–single acceptor (DDA)
3,636 Free OH symmetric stretching vibration
3,080 Largest cluster with the shortest mean H-bond distance ATR-IR Masuda et at. [1]
3,230 Larger cluster with shorter mean H-bond distance
3,400 Smaller cluster with longer mean H-bond distance
3,550 Smallest cluster with the longest mean H-bond distance
3,230 An ice-like component ATR-IR Chen et al. [5], Liuet al. [7],Wei et al. [6],Guo et al. [12]
3,420 An ice-like liquid component
3,540 A liquid-like amorphous phase
3,620 Monomeric water molecules
3,248.9 Strongly hydrogen-bonded components Raman Dong et al. [9]
3,468.4 Weekly hydrogen-bonded components
3,628.8 Slightly hydrogen-bonded components
3,242 Shorter H-bond component ATR-IR Kataoka et al. [10]
3,428 Medium H-bond component
3,562 Longer H-bond component
3,251 Strong hydrogen bond TransmissionIR
Zhao et al. [8]
3,371 Weak hydrogen bond
J Solution Chem (2014) 43:1055–1077 1057
123
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the hydrogen atoms of water molecules, more strongly affect the OH stretching band of
water than cations do. The latter interact with the oxygen atoms of water molecules.
Consequently, a direct interpretation of ion–water interactions based on the total contri-
butions of cations and anions on the OH stretching band can engender misunderstandings.
Raman analysis of Na2SO4 aqueous solutions revealed that SO2�4 influences the OH
stretching band of water only slightly [17]. That fact was confirmed more recently by
Wei et al. [6] through ATR-IR analysis of (NH4)2SO4 solutions. Based on their
observations, Wei et al. [6] regarded SO2�4 as a ‘‘blank anion’’ that has no influence onthe OH stretching band of water. The effects of cations (Na?, Mg2? and Zn2?) were
obtained by subtracting the appropriate amount of pure water spectrum from the spectra
of aqueous sulfate solutions (i.e., Na2SO4, MgSO4, and ZnSO4) [6]. They also obtained
the effect of ClO�4 by calculating the difference spectrum between perchlorate solutionsand sulfate solutions having the same cations at the same concentrations (e.g., NaClO4vs. Na2SO4).
The aim of this study is to re-investigate whether and how the OH stretching band of
water reflects structural and dynamic properties of the ion–water interaction in an aqueous
solution. To this end, we first obtained ATR-IR spectra of many aqueous electrolyte
solutions (22 in total). The electrolytes consist of the sulfate and chloride salts of mono-
and divalent cations plus other electrolytes such as carbonates and nitrates. To avoid
distortions of the ATR-IR spectra attributable to the optical effects, we extracted the molar
absorptivity of water from these ATR-IR spectra using the methodology reported by Bertie
and Eysel [18]. We then determined the effects of individual cations (Li?, Na?, K?, Rb?,
Cs?, Mg2?, Ca2?, Sr2?, Ba2?, Mn2?, Co2?, and Ni2?) and anions (Cl-, Br-, I-, NO�3 ,
ClO�4 , HCO�3 , and CO
2�3 ) on the molar absorptivity of water using the effect of SO
2�4 as a
benchmark (i.e., the effects of all ions are presented as differences from that of SO2�4 ).Finally the obtained dataset was correlated with several quantitative parameters repre-
senting structural and dynamic properties of water molecules around ions: DGHB, structuralentropy (Sstr), the viscosity B-coefficient (Bg), and the ionic B-coefficient of NMR relax-
ation (BNMR). It will be demonstrated herein that modification of the OH stretching band of
water caused by ions has quasi-linear relations with all of these parameters. The OH
stretching band of water can thereby be used as a good indicator to evaluate the ion–water
interactions occurring in an aqueous solution.
2 Experimental
2.1 Materials
The electrolytes used for this study, presented in Table 2, were of analytical reagent
quality, with purities greater than 99.0 % (except for NaClO4, of which the purity was
99 %). They were used without further purification. Stock solutions (1 mol�L-1 for sulfatesolutions and 2 mol�L-1 for the others) were prepared by dissolving weighed amounts ofrespective solids in distilled deionized water. Lower-concentration solutions with con-
centrations of 0.05 and 1.5 mol�L-1 were prepared by diluting the stock solutions withdistilled deionized water in a 5 mL volumetric flask. The resulting total mass was mea-
sured so that the solution densities were obtained (Table 2). The obtained densities are
similar to the values reported in the relevant literature: within 0.3 % [19].
1058 J Solution Chem (2014) 43:1055–1077
123
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2.2 ATR-IR Measurements
ATR-IR measurements were performed using an FTIR spectrometer (FTIR-4200; Jasco
Corp.) equipped with an MCT detector. A 45� ZnSe crystal (six reflections) attached to anATR plate (Benchmark ATR trough top plate; Specac Ltd.) with a horizontal ATR
accessory was used for measurements. All spectra were obtained by collecting 1,024 scans
with a spectral range of 400–7,800 cm-1 with 4 cm-1 resolution. The background spec-
trum was first measured on the ATR plate without a sample. The ATR-IR spectra of sample
solutions were then recorded as absorbance, -log10(I/I0) (where I0 is the background
spectrum intensity), and defined here as pATR. A rubber lid was placed over the ATR plate
during the measurements to prevent evaporation of the solutions (one measurement took
about 11 min). For NaI and MnCl2 solutions, the ATR spectra were measured within 24 h
after dissolving the respective solids in distilled deionized water to prevent the possible
oxidation of I- and Mn2? by oxygen in the air. No change was observed in spectra
measured several days later. Thus the influence of oxidation was negligible. The ATR-IR
measurements were replicated several times to confirm that the obtained spectra were
reproducible in terms of peak position and intensity. The maximum error range in the OH
stretching band region (2,600–3,800 cm-1) of the obtained spectra was estimated as
±0.0015 in pATR units (dimensionless).
All experiments presented above were carried out at 20 �C, which was controlledby an air-conditioner. The accuracy of temperature is supported by the good consis-
tency of solution densities measured in this study (Table 2) with the reported coun-
terparts [19].
2.3 Extraction of the Molar Absorptivity of Water from the ATR-IR Spectra
of Aqueous Electrolyte Solutions
The molar absorptivity of water was extracted from the ATR-IR spectra of aqueous
electrolyte solutions using the procedure reported by Bertie and Eysel [18]. The procedure,
which uses Fresnel equations for reflection, calculates the real (n(m)) and imaginary (k(m))refractive index of the sample.
First, k(m) is calculated from the pATR spectra on the assumption that the penetrationdepth of the evanescent wave at each reflection is calculable from the standard formula for
the distance for the evanescent wave to decrease in magnitude by 1/e in a non-absorbing
sample. This is:
d ¼ k2pn1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sin2h� n2n2r
q ; ð1Þ
where k is the wavelength of radiation in a vacuum, n and nr respectively denote the realrefractive indices of a sample solution and the ZnSe crystal, and h is the angle of incidence,45�. The real refractive index of ZnSe as a function of the wavenumber was taken from theliterature [20]. The real refractive indices of sample solutions were assumed to be constant.
Their values at the sodium D line (589 nm), nD, are used (Table 2).
Based on the assumptions presented above, the path length at each reflection is 2d. If m
is the effective number of reflections, then:
k mð Þ ¼ ln104pmm 2dð Þ pATRðmÞ; ð2Þ
J Solution Chem (2014) 43:1055–1077 1059
123
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where m is not simply the actual number of reflections because it contains contributions
from the approximations in Eq. 1 and experimental factors. We set m = 4.87 here so that
the maximum of the OH stretching band of water becomes the same as that reported by
Bertie and Lan [21] (Fig. 1). The approximate values of k(m) were then replaced by:
k mð Þ ¼ k mð Þ 1þffiffiffiffiffiffiffiffiffi
k mð Þp
� �
ð3Þ
to improve the values calculated from Eq. 2 [18, 22].
Next, n(m) was calculated using the Kramers–Kronig transformation:
n mað Þ ¼ n1 þ2
pP r1
0
mkðmÞm2 � m2a
dm ð4Þ
Therein, m denotes the wavenumber of radiation in a vacuum and n? is the realrefractive index of sample solutions at infinite wavenumber. The values of k(m) outside ofthe experimental spectral region (400–7,800 cm-1) were set to zero. The values of nD were
used as n? because the sodium D line (about 17,000 cm-1) can be regarded as that at near
infinite frequency because it is far from any vibrational or electronic absorption band. The
nD values that were not available in the literature [19, 23, 24] were obtained by extrapo-
lation on the basis that nD is additive for ions in water [25–27]. The nD values of Li2SO4,
Na2SO4, and K2SO4 solutions obtained using this method (Table 2) agree with the liter-
ature values within 0.2 % [28, 29].
In the following step k(m) is refined. The refractivity for light polarized parallel to theinterface between the ZnSe crystal and a sample solution, Rs, is:
Rs ¼cosh�
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nþiknr
� �2
�sin2hr
coshþffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nþiknr
� �2
�sin2hr
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�
2
ð5Þ
and the reflectivity for the perpendicular polarization, Rp, is:
Rp ¼nþik
nr
� �2
cosh�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nþikn1
� �2
�sin2hr
nþiknr
� �2
coshþffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nþiknr
� �2
�sin2hr
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�
2
ð6Þ
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
Wavenumber [cm–1]
8001600240032004000
20
40
60
80
100
0
This studyBertie and Lan (1996)
Fig. 1 Comparison between themolar absorptivity of watercalculated in this study (solidline) and that reported by Bertieand Len [21] (dashed line) in thespectral range of800–4,000 cm-1
1060 J Solution Chem (2014) 43:1055–1077
123
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They were used to calculate the pATR spectrum as presented below:
pATRcalc mð Þ ¼ �log101
2Rmp þ Rms� �
� �
ð7Þ
The calculated pATR spectrum was compared with the experimental one. k(m) wasadjusted as:
k mð Þ ¼ k mð Þ � pATRðmÞpATRcalcðmÞ
� �
ð8Þ
We repeated the calculations from Eqs. 4–8 up to 20 cycles so that the square of the
difference between calculated and observed pATRs,P
j
pATR mj�
� pATRcalc mj� �2
,
became less than 1 9 10-4.
Finally, the molar absorptivity of water, e(m), was calculated using the refined k(m) as
eðmÞ ¼ 4pkðmÞkcln10
; ð9Þ
where c is the concentration (mol�L-1) of water in a sample solution calculated using theexperimentally determined densities of sample solutions. The obtained molar absoptivity
of water in distilled deionized water is presented in Fig. 1 together with that reported by
Bertie and Lan [21]. Their spectra are similar except for the region below 1,000 cm-1. The
difference below 1,000 cm-1 arises because the low-frequency cutoff of the ZnSe crystal
prevents one from obtaining pATR spectra below 700 cm-1. Bertie and Lan [21] overcame
the limitation by adding the known k(m) spectrum of water below 700 cm-1 to that cal-culated from the pATR spectrum. Its influence on the OH stretching band region appears to
be negligible (Fig. 1). Therefore, we use the calculated e(m) spectrum without the furtherrefinements performed by Bertie and Lan [21].
2.4 Extraction of the Effects of Individual Ions on the OH Stretching Band of Water
The effects of individual ions on the OH stretching band of water, which we designate as
‘‘effect’’, were extracted from the e(m) spectra as follows. First, assuming the effect of SO2�4as a benchmark, the effects of Li?, Na?, K?, Rb?, Cs?, and Mg2? were ascertained merely
by calculating the difference spectrum between their sulfate solutions and distilled
deionized water (subtraction factor = 1) (Fig. 2a–e, i).
The effects of anions (Cl-, Br-, I-, NO�3 , ClO�4 , HCO
�3 , and CO
2�3 ) were then
obtained by subtracting the effects of Na? or K? from the spectra of their respective
sodium or potassium salt solutions (e.g., NaCl, KHCO3) after subtracting the spectrum of
distilled deionized water (Fig. 2n–s). The effects of 1 mol�L-1 Na? or 1 mol�L-1 K?were used here because the bands observed in the effects of Na? and K? showed
increasing linear intensity with ion concentration within the range of error (±0.1)
(Fig. 2b, c). Complete subtraction was achieved by multiplying the effects of 1 mol�L-1Na? (or 1 mol�L-1 K?) by a subtraction factor (e.g., the effect of 1.5 mol�L-1 Cl- = thespectrum of 1.5 mol�L-1 NaCl—the spectrum of distilled deionized water—(the effect of1 mol�L-1 Na?) 9 1.5).
Regarding Ca2?, Sr2?, and Ba2?, the solubilities of their sulfates in water are low
(1.5 9 10-2, 6.2 9 10-4, 1.0 9 10-5 mol�L-1 at 20–30 �C, respectively) [30]. Theireffects were therefore obtained from the spectra of their chloride solutions by subtracting
J Solution Chem (2014) 43:1055–1077 1061
123
-
the effect of Cl- (Fig. 2j–l). The effect of 1 mol�L-1 Cl- was used. It was multiplied by asubtraction factor so that the effect of Cl- was subtracted completely (e.g., the effect of
1.5 mol�L-1 Ca2? = the spectrum of 1.5 mol�L-1 CaCl2—the spectrum of distilleddeionized water—(the effect of 1 mol�L-1 Cl-) 9 3). The error in the obtained spectraincreases as the subtraction factor of the effect of Cl- increases. The range of error
21.5
–1.8
–1
–0.2
0.6
1.4
3335
3580
3665
+
Mol
. Abs
. [m
ol–1
·cm
–1]
2900320035003800
–1.2
–0.6
0
0.6
1.2
3335
3560
3665
3120
+
0.53(±0.03)
–0.56(±0.01)
–1.07(±0.04)
3580 cm–13665 cm–1
3335 cm–1
3310 cm–1
3665 cm–13550 cm–1
0.78(±0.06)
–0.42(±0.004)
–1.06(±0.06)
–1.3
–0.6
0.1
0.8
1.5
3310
3550
3665
+
–1.7
–0.8
0.1
1
1.9
3280
3535
3665
+
3535 cm–13665 cm–1
3280 cm–1
0.80(±0.01)
–0.42(±0.01)
–1.01(±0.02)
0 0.5 1
3335 cm–1
3665 cm–13560 cm–1
0.47(±0.01)
–0.50(±0.03)
–0.70(±0.01)
3120 cm–1
–0.9
0.1
1.1
2.1
3.1
3655
3170+
3170 cm–1
3655 cm–1
–0.71(±0.01)
Wavenumber [cm–1] Conc. [mol/L]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
3645
3180
3535
2+
–3
1
5
9
133180 cm–1
3645 cm–1
–2.09(±0.02)
3535 cm–1
–3
–0.5
2
4.5
7
3655
3175
3515
2+3175 cm–1
3515 cm–1
3655 cm–1
3.24(±0.04)
–0.42(±0.01)
–1.84(±0.02)
–2.5
–1
0.5
2
3.53145 cm–1
3555 cm–1
3655 cm–1
1.61(±0.04)
0.68(±0.02)
–1.58(±0.02)
3655
3145
3555
2+
3660
3135
3570
3390
2+ 1.14(±0.01)
0.55(±0.02)
–1.72(±0.02)
–1.60(±0.02)
3135 cm–13390 cm–13570 cm–13660 cm–1
–5
1
7
13
19
3645
3175
3505
2+
9.9(±0.2)
3175 cm–1
3505 cm–1
3645 cm–1
–3.04(±0.03)
–3.38(±0.02)
–4
3
10
17
24
3640
3165
3510
2+3165 cm–1
3510 cm–1
3640 cm–1
10.5(±0.1)
–3.27(±0.02)
–3.60(±0.06)
–6
1
8
15
22
3640
3165
3505
2+3165 cm–1
3505 cm–1
3640 cm–1
10.2(±0.1)
–3.56(±0.01)
–4.57(±0.10)
–2.3
–1.1
0.1
1.3
2.5
–3.52900320035003800 0 0.5 1 21.5
Wavenumber [cm–1] Conc. [mol/L]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
–5.5
–3
–0.5
2
4.5
3215
3475 – 3215 cm–1
3475 cm–1
2.33(±0.3)
–3.18(±0.03)
–7
–3
1
5
9
3255
3500–
3255 cm–1
3500 cm–1
3.30(±0.02)
–4.20(±0.07)
–10
–5
0
5
10 3525
3285
–3285 cm–1
3525 cm–1
–5.89(±0.07)
4.43(±0.02)
–6.5
–3
0.5
4
7.5 3580
3265
3–
3265 cm–1
3580 cm–1
–3.75(±0.09)
3.26(±0.04)
–10
–3
4
11
183315 cm–1
3600 cm–1
–6.13(±0.14)
8.58(±0.09)
3600
3315
4–
0.5
1.5
2.5
3.5
4.53585 cm–1
3075 cm–1
2.09(±0.02)
0.75(±0.02)
0.67(±0.03)
2890 cm–13075
35852890
3460
3045
3–
(d) Rb
(e) Cs
(c) K
(b) Na
(a) Li
(i) Mg
(j) Ca
(k) Sr
(l) Ba
(f) Mn
(g) Co
(h) Ni
(n) Cl
(m) Br
(o) I
(p) NO
(q) ClO
(r) HCO
(s) CO32–3460 cm–1
3045 cm–1
7.64(±0.14)
–5.36(±0.04)
–9
–3
3
9
15
–152900320035003800 0 0.5 1 21.5
Wavenumber [cm–1] Conc. [mol/L]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Mol
. Abs
. [m
ol–1
·cm
–1]
Fig. 2 Effects of univalent cations (a–e), divalent cations (f–l) and various anions (n–s) on the molarabsorptivity of water in the OH stretching band region (2,600–3,800 cm-1). The right sides of the respectivefigures show band intensities observed in effects as a function of the ion concentration. Vertical scales differfor each effect of ions
1062 J Solution Chem (2014) 43:1055–1077
123
-
estimated in the effect of Ca2?, Sr2?, and Ba2? consequently increased from about ±0.1 at
0.1 mol�L-1 up to ±0.4 at 2 mol�L-1.The effects of Mn2?, Co2? and Ni2? were obtained using the same procedure as that
used for Ca2?, Sr2?, and Ba2? (Fig. 2f–h), although the solubilities of their sulfates in
water are sufficiently high to be analyzed because their effects might be distorted by
interactions between these cations and SO2�4 in aqueous solutions (see Sect. 3).
3 Results and Discussion
3.1 Effects of Ions on the OH Stretching Band of Water
Figure 2 shows the effects of univalent cations (Li?, Na?, K?, Rb? and Cs?; Fig. 2a–e),
divalent cations (Mn2?, Co2?, Ni2?, Mg2?, Ca2?, Sr2? and Ba2?; Fig. 2f–l) and various
anions (Cl-, Br-, I-, NO�3 , ClO�4 , HCO
�3 and CO
2�3 ; Fig. 2m–s) on the OH stretching band
of water. The concentration dependences of the band intensities observed in each effect are
also shown at the right side of each figure. The band intensities were measured after
drawing a linear baseline from 2,600 to 3,800 cm-1, except for the effects of HCO�3 and
CO2�3 for which the linear baselines were drawn, respectively, from 2,150 to 3,800 cm-1
and from 2,050 to 3,800 cm-1. In most cases, the band intensities increased linearly with
ion concentration in the investigated concentration range (0.1–2 mol�L-1). This linearrelation shows that the effects of ions are additive. Therefore, the extraction procedures to
obtain effects of individual ions used for this study are valid. However, several band
intensities observed in the effects of Cs?, Li? and Mg2? (Fig. 2a, e, i) departed downward
from the linear relation with concentration. The deviations became greater as their con-
centrations increased. The result suggests that the effects of these three cations are non-
additive. However, because these effects were all derived from sulfate solutions, and
because the effects derived from the other salt solutions (i.e., chloride) showed linear
increases of band intensities, another possibility is that SO2�4 selectively influenced theeffects of Li?, Cs? and Mg2?, and that it does not influence the other cations (i.e., Na?,
K?, and Rb?).
To obtain more evidence of the possible influence of SO2�4 , we extracted the effects ofMg2? and Li? from the spectra of their chloride solutions by subtracting the effect of Cl-.
Subsequently, we compared them with the corresponding ones derived from sulfate
solutions (Fig. 3). The band maxima are located at similar positions irrespective of the
extraction procedure, whereas the effects derived from chloride solutions show linear
increases of the band intensities with concentration (Fig. 3c, g). To evaluate the differences
in band intensity for sulfate and chloride solutions quantitatively, we calculated the
intensity ratios (sulfate/chloride) of the 3,180 and 3,530 cm-1 bands in the effect of Mg2?,
and that for the 3,170 cm-1 band in the effect of Li? at the same Mg2? (or Li?) con-
centration (Fig. 3d, h). The ratios are near one at the lowest concentration (0.1 mol�L-1).They decrease gradually as the cation concentrations increase. Additionally, for the effect
of Li?, a broad shoulder band around 3,500 cm-1 is not significant when the effect was
extracted from the chloride solution (Fig. 3f).
It is noteworthy that these differences are not solely attributable to the overlapping of
the effect of SO2�4 , which was ignored in the extraction procedure of the effects of Mg2?
and Li?. Even if SO2�4 has some influence on the OH stretching band profile of water, itscontribution to the effects of Mg2? (and Li?) obtained using the two extraction procedures
J Solution Chem (2014) 43:1055–1077 1063
123
-
is the same. For instance, the spectrum of 1 mol�L-1 MgSO4 is regarded as including theeffect of 1 mol�L-1 Mg2? and 1 mol�L-1 SO2�4 , if SO2�4 is assumed to have an effect. Thespectral procedure to extract the effect of 1 mol�L-1 Mg2? from the spectrum of 1 mol�L-1MgCl2 yields the following:
1 mol � L�1MgCl2� 1 mol � L�1NaCl � 0:5 mol � L�1Na2SO4�
� 2¼ the effect of 1 mol � L�1Mg2þ þ the effect of 1 mol � L�1SO2�4
ð10Þ
The resultant spectra therefore consist of the same contributions of 1 mol�L-1 Mg2? and1 mol�L-1 SO2�4 . The spectral difference indicates that the effects of Mg
2? and Li? are
distorted in the presence of SO2�4 . In other words, the effects of ions are non-additive forsome specific combinations of cations and anions.
A possible cause of the non-additivity is the formation of ion pairs. Mg2? is known to
form a strong ion pair [contact ion pair (CIP)] with SO2�4 in aqueous solutions. Theconcentration dependence of the fraction of Mg2? present as CIPs in MgSO4 solutions was
determined quantitatively by Hefter and co-authors [31–33]. They revealed that the frac-
tion increases rapidly to about 10 % as the MgSO4 concentration increases from 0 to
0.5 mol�L-1, then it increases moderately to about 13 % at 2.0 mol�L-1. They alsodemonstrated the existence of a triple or more aggregated ion pair (e.g., Mg2SO
2þ4 ) at high
MgSO4 concentration ([1 mol�L-1). However, the CIP between Mg2? and Cl- wasevaluated theoretically as energetically unfavorable and was formed only slightly in
aqueous solutions [34, 35]. It is expected that the penetrations of SO2�4 into the hydrationsphere of Mg2? through the CIP formation partly pushes out the hydration waters of Mg2?
0.9
1
1.1
0.8
0.7
0.8
0.85
0.9
0.95
1
0.75
0.71.510.50 2
0.5
0.6
0.7
0.8
1
0.4
0.3
0.9
Mg2+ Conc. [mol/L]
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
1
6
11
16
21
–4
–9300034003800
Wavenumber [cm–1]
300034003800
(a) Mg2+ (in MgSO4) (b) Mg2+ (in MgCl2)
0 0.5 1 21.5
Mg2+ Conc. [mol/L]
0.5
2
3.5
5
–1
–2.5
(d) Li+ (in Li2SO4) (e) Li + (in LiCl)
Li+ Conc. [mol/L]
0 0.5 1 21.5300034003800
Wavenumber [cm–1]
300034003800
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
1.510.50 2
Li+ Conc. [mol/L]In
tens
ity r
atio
(I M
gSO
4 / I
MgC
l2)
at 3
180
cm-1
Intensity ratio (IMgS
O4 / IM
gCl2 )
at 3535 cm-1
Inte
nsity
rat
io (
I Li2
SO
4 / I
LiC
l)at
317
0 cm
-1
(c)3180 (MgSO4)3180 (MgCl2)
3535 (MgSO4)3535 (MgCl2)
(f)3170 (Li2SO4)3170 (LiCl)
(g)3180 cm-1
3535 cm-1
(h)
Fig. 3 a, b Spectral comparisons of the effects of Mg2? extracted from the spectra of MgSO4 solutions(a) and those of MgCl2 solutions (b). c, g Intensities (c) and the intensity ratios (g) of the band maxima at3,180 cm-1 and at 3,535 cm-1 observed in a and b as a function of the Mg2? concentration. d, e Spectralcomparisons of the effects of Li? extracted from the spectra of Li2SO4 solutions (d) and those of LiClsolutions (e). f, h intensities (f) and the intensity ratios (h) of the band maxima at 3,170 cm-1 observed ind and e as a function of the Li? concentration. Dashed lines in g and h are included only as a visual aid
1064 J Solution Chem (2014) 43:1055–1077
123
-
and consequently induces a decrease of the effect of Mg2?. Decreases of the intensity ratios
(sulfate/chloride) with increasing Mg2? concentration (Fig. 3d) therefore probably reflect a
decrease in the number of hydrated water molecules around Mg2? in MgSO4 solutions,
compared to those in MgCl2 solutions, caused by the increase of the fraction of CIP.
The formation of ion pairs alone, however, cannot fully explain the observed spectral
difference because Li? does not form the CIP with SO2�4 as well as it does with Cl- [36].
Additionally, results show that the fraction of Li? present as weaker ion pairs [i.e., double-
solvent-separated ion pair (2SIP) and solvent-shared ion pair (SIP)] are not significantly
different between those in LiCl solution and those in Li2SO4 solution at the same Li?
concentration [36].
Another possible cause is the cooperativity in ion hydration, which is observed only
when the cations and anions in aqueous solutions are both strongly hydrated [37].
Cooperativity occurs because the cation and anion lock in different degrees of freedom
of water molecules. The local electric field around the cation causes the dipole vector of
water molecules in the solvation shell to point radially away from the cation, whereas for
an anion one OH group of a hydrogen-bonded water molecule points linearly toward the
anion. The nearby presence of the strongly hydrated cation (e.g., Mg2?) and anion (e.g.,
SO2�4 ) can thereby engender a locking in of both directions of the hydrogen bondstructure of several intervening water layers. However, if the counter ion is weakly
hydrated, then the strongly hydrated ion is surrounded by a semi-rigid solvation shell,
where the dynamics of water molecules are restricted only in a certain direction but is
unrestricted in other directions. Such differences in hydration nature probably cause the
different spectral profiles of Li? between a Li2SO4 solution and a LiCl solution. The
extent of ion–water interactions reflected by the OH stretching band of water is therefore
expected to be beyond the first solvation shell of water molecules directly surrounding
the ion.
Tielrooij et al. [37] also observed cooperativity even in the combination of the mod-
erately hydrated cation Na? with the strong anion SO2�4 . The effect of Na? derived from
Na2SO4 (Fig. 2b) might therefore also be modified by SO2�4 . In fact, the negative broad
band around 3,280 cm-1 in the effect of 2 mol�L-1 Na? appears to be somewhat asym-metric compared to that in the effect of 1 mol�L-1 Na?. It is noteworthy, however, that thedifference is near the error level and that its resultant influence on the effects of the other
ions that were extracted using the effect of 1 mol�L-1 Na? (e.g., Cl- and Br-) is expectedto be slight. Evidence supports the expectation that the effect of 1 mol�L-1 Cl- obtainedusing the spectral combination of 1 mol�L-1 KCl and 0.5 mol�L-1 K2SO4 coincides withthat obtained using the spectra of 1 mol�L-1 NaCl and 0.5 mol�L-1 Na2SO4, within therange of error (data not shown). Further detailed experiments might provide useful
information to elucidate the cooperative interaction between Na? and SO2�4 in aqueoussolutions. That discussion is beyond the scope of this study. Therefore, we will not
investigate the slight influence of SO2�4 on the effect of Na? further.
3.2 Correlations Between the OH Stretching Band of Water and Structural
and Dynamic Properties of the Ion–Water Interaction in Aqueous Solution
This section presents an examination of whether and how the OH stretching band of water
reflects structural and dynamic properties of the ion–water interaction in aqueous solution.
Figure 4 presents a comparison of the effects of cations (Fig. 4a) and anions (Fig. 4b) at
the concentration of 1 mol�L-1. We also show the effects of these ions on the OH bending
J Solution Chem (2014) 43:1055–1077 1065
123
-
band of water in the Appendix (Fig. 7). The effects of Mg2? and Li? presented in these
figures were extracted from the spectra of their chloride solutions.
Spectral profiles of the effects of Mn2?, Co2?, Ni2?, and Mg2? in Fig. 4a appear to
comprise a large positive band centered around 3,170 cm-1, a broad shoulder band around
3,310 cm-1, and two negative bands around 3,640 and 3,510 cm-1. Based on previously
reported interpretations of the OH stretching band of water (Table 1), the former two bands
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
3
8
13
18
23
–2
28
33
38
43
48
53
Mg2+
Ca2+
Sr2+
Ba2+
Li+
Na+
K+
Rb+
Cs+
Mn2+
Ni2+
Co2+
25
50
75
100
0
2
12
22
32
42
–2
52
Wavenumber [cm–1]
2600300034003800 3600 3200 2800
25
50
75
100
0
Wavenumber [cm–1]
2600300034003800 3600 3200 2800
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
NO3–
ClO4–
HCO3–
CO32–
Cl–
I–
Br–
(a) (b)
Fig. 4 Effects of 1 mol�L-1 cations (a) and 1 mol�L-1 anions (b) on the molar absorptivity of water in theOH stretching band region (2,600–3,800 cm-1); the OH stretching band of water is shown at the top of eachfigure for comparison
1066 J Solution Chem (2014) 43:1055–1077
123
-
are attributed to water molecules forming stronger hydrogen bonds, whereas the latter are
attributed to water molecules forming weaker ones. Consequently, because of the increase
of the stronger hydrogen bond components and the decrease of the weaker ones, interac-
tions of the four cations with surrounding water molecules are undoubtedly attributable to
strengthening of the water–water hydrogen bonds in their hydration layers. For the other
cations, the lowest frequency band (about 3,170 cm-1) diminishes gradually in the order of
Ca2? [ Sr2? * Li? [ Ba2? [ Cs? and was not observed in the effects of Na?, K?, andRb? (Fig. 4a). The highest frequency band (about 3,640 cm-1), however, was observed in
all effects of cations investigated. The water–Ca2? (or Li?) interaction can still be
attributable to strengthening of the water–water hydrogen bonds, although for the other
cations, particularly Cs? and Ba2?, the interpretation is difficult because these cations
show positive (or negative) bands on both regions of the stronger and weaker hydrogen
bond components.
The effects of anions showed simple spectral profiles compared with those of cations.
The effects of Cl-, Br-, I-, NO�3 , and ClO�4 showed, respectively, a negative band and a
positive band on the region of the stronger and weaker hydrogen bond components
(Fig. 4b). Interactions of the five anions with surrounding water molecules are therefore
interpreted as the result of weakening of the water–water hydrogen bonds in their
hydration layers. The effects of HCO�3 and CO2�3 present spectral features distinct from
those of the other anions: the effect of HCO�3 showed a broad positive band around3,075 cm-1. Its intensity increases strongly as the proton is dissociated
(HCO�3 ! CO2�3 ). Therefore, CO2�3 is expected to cause considerably stronger water–water hydrogen bonds.
The discussion presented above is only qualitative. To correlate the spectral profiles
with structural and dynamic properties of the ion–water interaction in aqueous solution
quantitatively, some clear definition is necessary to quantify the effects of ions. Curve-
fitting analysis might be an appropriate technique to establish the definition. It is note-
worthy, however, that curve fittings with several Gaussians imply that water molecules are
embedded in several distinct hydrogen bond environments. The mixture model of water is
supported by isosbestic points observed in temperature-dependent spectra of water [13].
However, Monte Carlo simulations by Smith et al. [38] show that a continuous distribution
of hydrogen bond geometries and energies in water also generates temperature-indepen-
dent isosbestic points. An ultrafast IR investigation of dilute HDO in D2O solutions also
engendered the conclusion that a continuum, rather than mixture models, is more appro-
priate [39, 40]. Therefore, we did not conduct any curve fitting analyses in the present
study.
As an alternative, we divided the spectral profiles into two regions of 2,600–3,420 and
of 3,420–3,800 cm-1, and then calculated the difference in area between the lower and
higher frequency regions after drawing a linear baseline from 2,600 to 3,800 cm-1
(DMAL–H). For this study, 3,420 cm-1 was chosen because it is near the center of the OH
stretching band of water (Fig. 4) and because the position is near the boundary between the
two regions of the stronger and weaker hydrogen bond components defined qualitatively
above. The value for each ion (at 1 mol�L-1 concentration) is presented in Table 3 withtheir maximum errors estimated from several independent measurements. Based on the
interpretation presented above, ions having positive DMAL–H values are expected tostrengthen the water–water hydrogen bonds in their hydration layers, whereas those ions
having negative DMAL–H weaken them. In fact, Mn2?, Co2?, Ni2?, and Mg2? show large
positive DMAL–H values whereas Cl-, Br-, I-, NO�3 , and ClO
�4 showed negative ones.
J Solution Chem (2014) 43:1055–1077 1067
123
-
Ca2? and Li? show small but definitely positive DMAL–H values, but those of Ba2? and
Cs? are nearly zero.
As quantitative parameters representing structural and dynamic properties of water
molecules around ions, we selected the following four parameters: DGHB, structuralentropy (Sstr), the viscosity B-coefficient (Bg), and the ionic B-coefficient of NMR relax-
ation (BNMR).
DGHB is a dimensionless parameter representing the effect of ions on the averagenumber of hydrogen bonds in which a water molecule participates [41]. DGHB is calculatedusing DtrG (solute, H2O ? D2O), the standard molar Gibbs energy of transfer of the solutefrom light to heavy water:
DGHB ¼ DtrG solute; H2O! D2Oð Þ = ð�929Þ ð11Þ
where -929 J�mol-1 is the molar difference in the hydrogen bonding energies of light andheavy water.
The structural entropy, Sstr, was defined by Marcus [42] to represent the effects of
ions on the structure of water. The value of Sstr is calculated using the standard molar
entropy of ion hydration (DhydS) by subtracting the contribution of the ionic hydrationshell formation:
DSstr ¼ DhydS� DSnt þ DSel1 þ DSel2ð Þ ð12Þ
Therein, DSnt represents the formation of the cavity in water for the accommodation ofthe ion as well as the dispersion interactions of a neutral entity having the same size as the
ion (e.g., a rare gas atom) with surrounding water molecules. DSel1 and DSel2, respectively,represent the electrostatic interactions of the ion with water molecules in the hydration
shell and beyond it. The Sstr values are often used as a quantitative measure to classify ions
into structure makers and breakers. The ions having negative Sstr values are classified as
structure makers and vice versa [41, 42].
The viscosity B-coefficient (Bg) is the slope of the viscosity of an aqueous electrolyte
solution (g) against concentration:
g ¼ gw 1þ Ac1=2 þ Bcþ � � �� �
ð13Þ
where gw stands for the viscosity of water. Also, c represents the molar concentration ofelectrolyte. Coefficient A represents interionic (electrostatic) forces that maintain a space
lattice structure of the electrolyte. The coefficient B is representative of the retardation of
solution flow doe to hydration on ions. In this study, the values of Bg calculated using the
assumption of Marcus [42] that B(Rb?) = B(Br-).
The ionic B-coefficient of NMR relaxation (BNMR) was defined by Engel and Hertz [43]
in order to correlate the NMR longitudinal proton relaxation times in aqueous electrolyte
solution (T1) with those in neat water (T1*), using an expression analogous to Eq. 13:
1=T1ð Þ= 1=T1�ð Þ�1½ � ¼ BNMRc þ � � � ð14ÞThe convention that BNMR(K
?) = BNMR(Cl-) is used to obtain the ionic values. The
rotational correlation times of water molecules (s) are given similarly as:
sion=sW ¼ 1þ 55:51=nionð ÞBNMR ð15ÞSubscripts ion and W, respectively, denote hydration and bulk water. In addition, nion is
the hydration number of the ion. The NMR measurements of longitudinal proton relaxation
1068 J Solution Chem (2014) 43:1055–1077
123
-
times, T1, are confined to diamagnetic ions. Therefore, the BNMR values for transition metal
cations (i.e., Mn2?, Co2?, and Ni2?) were not reported in Engel and Hertz [43].
Figure 5 presents correlations between DMAL–H and the four parameters (DGHB, Sstr,Bg, and BNMR). It is readily apparent that DMAL–H has quasi-linear relations with all ofthese parameters. The values of DMAL–H increase concomitantly with increasing DGHB,Bg, and BNMR (Fig. 5a, c, d), whereas they decrease concomitantly with increasing Sstr(Fig. 5b). In the former correlations, most of the ions occupy the two regions of the upper
right quadrant (DMAL–H [ 0 and DGHB, Bg and BNMR [ 0) and the lower left quadrant(DMAL–H \ 0 and DGHB, Bg and BNMR \ 0) of the diagram, whereas in the DMAL–H -Sstr correlation, the ions mainly occupy the upper left quadrant (DMAL–H \ 0 and Sstr [ 0)and the lower right quadrant (DMAL–H [ 0 and Sstr \ 0). These observations are consistentwith the qualitative interpretation presented above: as the ion–water interaction becomes
stronger, water molecules around the ion are constrained more tightly (Bg and BNMR [ 0)and the strengths and energies of water–water hydrogen bonds increase (DMAL–H andDGHB [ 0). Consequently, the entropy of water decreases (Sstr \ 0). Conversely, as theion–water interactions sufficiently weaken to become comparable to those for water–water
hydrogen bonds, water molecules around the ions become freer to move than bulk water
(Bg or BNMR \ 0). Then the hydrogen bond strengths and energies decrease (DMAL–H \ 0and DGHB \ 0). Consequently, the entropy of water increases (Sstr [ 0). Although furthertheoretical and/or experimental works are necessary to explain the observed correlations
quantitatively, the quasi-linear relations (Fig. 5) are strong evidence that the OH stretching
band of water does reflect the structural and dynamic behaviors of water molecules around
–4000 –2000 0 2000 4000 6000
ΔMAL–H
ΔGH
B
1.2
0.8
0.4
0
–1.2
–0.8
–0.4
Co2+Ni2+
Mg2+
Mn2+
CO32–Ca2+Li
+Sr2+
HCO3–
Ba2+
Na+
SO42–
Cs+Rb+K
+Cl–NO3–
Br–ClO4
–
I–
y = 2.53×10–4x – 0.255R2 = 0.841
(a) vs. ΔGHB (b) vs. Sstr
–4000 –2000 0 2000 4000 6000
ΔMAL–H
Sst
r [J
K–1
mol
–1]
200
100
0
–100
–200
y = –3.11×10–2x – 14.2R2 = 0.840
Co2+
Ni2+
Mn2+
CO32–
Mg2+
Ca2+
Li+
Sr2+
HCO3–
Ba2+
SO42–
Na+
Cs+
Rb+K+Cl–
NO3–
Br–I–
ClO4–
–4000 –2000 0 2000 4000 6000
ΔMAL–H
Vis
cosi
ty B
[L m
ol–1
]
0.5
0.4
0.3
0.2
–0.1
0
0.1
Co2+Ni2+
Mn2+Mg2+
CO32–
Ca2+
Li+
HCO3–
Sr2+
Ba2+SO42–
Rb+ Cs+
Na+
K+
Cl–
NO3–
Br–ClO4
–
I–
(c) vs. Viscosity B
y = 6.82×10–5x – 0.0999R2 = 0.801
–4000 –2000 0 2000 4000 6000
ΔMAL–H
BN
MR
0
0.6
0.4
–0.2
0.2
Mg2+
CO32–
Ca2+
Li+
Sr2+
Ba2+
HCO3–
SO42–
Cs+Rb+
Na+
K+Cl–
NO3–
Br–
I–ClO4
–y = 7.68×10–5x – 0.0849
R2 = 0.724
(d) vs. BNMR
Fig. 5 Correlations between the DMAL–H (difference in area between the lower (2,600–3,420 cm-1) and
the higher (3,420–3,800 cm-1) frequency regions of the bands portrayed in Fig. 4 and: a GHB, b structuralentropy (Sstr), c viscosity B-coefficient (Bg), and d the ionic B-coefficient of NMR relaxation (BNMR)
J Solution Chem (2014) 43:1055–1077 1069
123
-
ions. DMAL–H can thereby be a useful means to evaluate the ion–water interaction in anaqueous solution.
It is noteworthy that although the effects of all ions were calculated using the effect
of SO2�4 as a benchmark, SO2�4 shows non-zero values in the parameters correlated with
DMAL–H (Fig. 5) except for Sstr. In fact, Bg and BNMR show positive values for SO2�4 ,
whereas DGHB shows a negative one. These results suggest that ions located nearer theorigin of each figure (e.g., K?) are more appropriate as a base to calculate the effects of
ions. However, the deviations of the values for SO2�4 from zero are not marked
compared with the scatter of the plots in each figure. Consequently, the switch of SO2�4to another ion (e.g., K?) only slightly influences the general conclusions obtained in
this study.
It is also noteworthy that ions caused similar modifications of the pATR spectra of
water (Fig. 6a, b) and the molar absorptivity spectra (Fig. 4a, b). The similarities are
observed because the modification of the pATR spectrum of water is proportional to
the modification of the k(m) spectrum [25, 27, 44]. Moreover, the difference in areabetween the lower (2,600–3,300 cm-1) and the higher (3,300–3,800 cm-1) frequency
regions in the pATR spectra (DATRL–H; Table 3) show similar linear correlations withthe four parameters (DGHB, Sstr, Bg and BNMR) to those observed in Fig. 5 (only thecorrelation between DATRL–H and DGHB is depicted in Fig. 6c). Ions having positiveDATRL–H values showe positive DGHB values, while ions having negative DATRL–Hshow negative DGHB. ATR-IR spectroscopy is a readily accessible and cost-effectivetechnique to study aqueous solutions. Moreover, it does not require highly skilled
operators. ATR-IR spectroscopy can therefore offer a rapid and simple means to
estimate structural and dynamic properties of the ion–water interaction in aqueous
solution.
Wavenumber [cm–1]2600300034003800
Wavenumber [cm–1]
26003000340038000.5
0.9
1.1
1.5
–0.1
0.7
1.3
0.3
0.1
0.5
0.9
1.1
–0.1
0.7
1.3
0.3
0.1
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
Mg2+
Ca2+
Sr2+
Ba2+
Li+
Na+
K+
Rb+
Cs+
Mn2+
Ni2+
Co2+ NO3–
ClO4–
HCO3–
CO32–
Cl–
I–
Br–
(a) (b)
–25 25 50 100–50 0 75
ΔATRL–H
ΔGH
B
–1.2
–0.8
–0.4
0
0.4
0.8
1.2
Ni2+
Co2+
Mn2+
CO32–
Mg2+
Ca2+Sr2+
Li+
HCO3–
SO42–
Na+ Ba2+
Cs+Rb+K+Cl
–
NO3–
Br–ClO4
–
I–
y = 1.23×10–2x – 0.298R2 = 0.823
(c)
Fig. 6 a, b Effects of cations (a) and anions (b) on the ATR-IR spectrum of water in the spectral range of2,600–3,800 cm-1. c Correlation between the DATRL–H (difference in area between the lower(2,600–3,300 cm-1) and the higher (3,300–3,800 cm-1) frequency regions of the bands observed ina and b and DGHB
1070 J Solution Chem (2014) 43:1055–1077
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4 Conclusion
The effects of various cations (Li?, Na?, K?, Rb?, Cs?, Mg2?, Ca2?, Sr2?, Ba2?, Mn2?,
Co2?, and Ni2?) and anions (Cl-, Br-, I-, NO�3 , ClO�4 , HCO
�3 , and CO
2�3 ) on the molar
absorptivity of water, as a function of ion concentration up to 2 mol�L-1, were determined(1) by measuring the ATR-IR spectra of many electrolyte aqueous solutions (22 in all) (2)
by converting the ATR-IR spectra into the molar absorptivity spectra, and (3) by separating
the effects of individual cations and anions using the effect of SO2�4 as a benchmark.Spectral analyses produced the following conclusions:
(1) Most OH stretching bands in the molar absorptivity spectra show linear intensity
increases and decreases with ion concentration, showing additivity of each effect of
these cations and anions.
(2) For certain specific combinations of cations and anions (Cs2SO4, Li2SO4, and
MgSO4), the band intensities depart downward from the linear trend at higher
concentrations. That type of deviation was attributed to the formation of ion pairs and
to cooperativity in ion hydration, which indicates that the extent of the ion–water
interaction as reflected by the OH stretching band of water is beyond the first
solvation shell of water molecules directly surrounding the ion.
(3) The difference in areas (DMAL–H) between the lower (2,600–3,420 cm-1) and the
higher (3,420–3,800 cm-1) frequency regions showed quasi-linear relations with
several quantitative parameters representing structural and dynamic properties of
water molecules around ions: DGHB, Sstr, Bg and BNMR. This observation indicatesthat the modification of the OH stretching band of water caused by ions is useful as an
indicator to evaluate ion–water interactions in an aqueous solution.
Acknowledgments We greatly appreciate Dr. Tadashi Yokoyama and Mr. Naoki Nishiyama of OsakaUniversity for their help with sample preparations. We also thank two anonymous referees and associatededitor Luigi Paduano for their careful reviews of this manuscript. This research was financially supported bya JSPS Research Fellowship for Young Scientists to Norio Kitadai.
Appendix
Although it is not the main subject of this study, we show the effects of 1 mol�L-1 cationsand 1 mol�L-1 anions on the molar absorptivity of water in the OH bending band region(1,200–2,000 cm-1) in Fig. 7. This presentation of the influences of various ions on the
OH bending band of water is the first ever reported. We hope that these results can
stimulate future theoretical and/or experimental studies in this area.
We also present in Tables 2 and 3 a summary of the aqueous solutions measured in this
study, and the numerical results on the areas of DMAL–H and DATRL–H for each ion (at1 mol�L-1 concentration), respectively.
J Solution Chem (2014) 43:1055–1077 1071
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Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
Mol
ar a
bsor
ptiv
ity [m
ol–1
·cm
–1]
Mg2+
Ca2+
Sr2+
Ba2+
Li+
Na+
K+
Rb+
Cs+
Mn2+
Ni2+
Co2+
Wavenumber [cm–1]
120016002000 1800 1400
3
7
11
15
19
–1
10
15
20
25
5
1
3
5
7
9
–1
11 CO32–
Cl–
Br–
I–
ClO4–
NO3–
10
15
20
25
5
Wavenumber [cm–1]
120016002000 1800 1400
(a) (b)
Fig. 7 Effects of 1 mol�L-1 cations (a) and 1 mol�L-1 anions (b) on the molar absorptivity of water in theOH bending band region (1,200–2,000 cm-1). The effect of HCO�3 is not shown in this figure because of thestrong overlap with a HCO�3 band [1]. The OH bending band of water is shown at the top of each figure forcomparison
Table 2 Summary of the aqueous solutions measured in this study
Electrolyte Solid Purity Conc.(mol�L-1)
Density(g�mL-1)
Refractiveindex (nD)
Manufacturer
LiCl LiCl�H2O [99.9 0.1 1.000 1.334 Wakoa
0.25 1.004 1.335
0.5 1.008 1.338
0.75 1.016 1.340
1 1.021 1.342
1.5 1.033 1.346
2 1.044 1.350
Li2SO4 Li2SO4�H2O [99.0 0.05 1.002 1.334 Wakoa
0.125 1.009 1.335
0.25 1.019 1.338
0.375 1.030 1.340
0.5 1.041 1.342
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Table 2 continued
Electrolyte Solid Purity Conc.(mol�L-1)
Density(g�mL-1)
Refractiveindex (nD)
Manufacturer
0.75 1.064 1.346
1 1.087 1.350
NaCl NaCl [99.5 0.1 1.001 1.334 Wakoa
0.5 1.017 1.338
1 1.037 1.343
1.5 1.057 1.348
2 1.077 1.352
NaBr NaBr [99.9 0.1 1.004 1.335 Wakoa
0.5 1.037 1.340
1 1.075 1.347
1.5 1.115 1.354
2 1.152 1.360
NaI NaI [99.5 0.1 1.009 1.335 Wakoa
0.5 1.054 1.344
1 1.113 1.354
1.5 1.171 1.365
2 1.226 1.375
NaNO3 NaNO3 [99.0 0.1 1.002 1.334 Wakoa
0.5 1.026 1.338
1 1.052 1.342
1.5 1.079 1.346
2 1.107 1.350
NaClO4 NaClO4�H2O 99 0.1 1.005 1.333 Stremb
0.5 1.037 1.336
1 1.076 1.340
1.5 1.113 1.344
2 1.154 1.347
Na2SO4 Na2SO4 [99.0 0.05 1.004 1.334 Wakoa
0.25 1.028 1.338
0.5 1.059 1.343
0.75 1.088 1.348
1 1.119 1.352
KCl KCl [99.5 0.1 1.002 1.334 Wakoa
0.5 1.021 1.338
1 1.044 1.343
1.5 1.067 1.347
2 1.090 1.352
KHCO3 KHCO3 [99.7 0.1 1.004 1.334 Aldrichc
0.5 1.030 1.338
1 1.060 1.343
1.5 1.090 1.348
2 1.118 1.353
K2CO3 K2CO3 [99.5 0.1 1.010 1.335 Wakoa
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Table 2 continued
Electrolyte Solid Purity Conc.(mol�L-1)
Density(g�mL-1)
Refractiveindex (nD)
Manufacturer
0.5 1.056 1.344
1 1.112 1.354
1.5 1.165 1.363
2 1.217 1.372
K2SO4 K2SO4 [99.0 0.05 1.004 1.334 Wakoa
0.25 1.031 1.338
0.5 1.065 1.343
Rb2SO4 Rb2SO4 [99.0 0.05 1.011 1.333 Wakoa
0.25 1.052 1.339
0.5 1.105 1.345
0.75 1.159 1.351
1 1.210 1.357
Cs2SO4 Cs2SO4 [99.9 0.05 1.012 1.334 Wakoa
0.25 1.071 1.340
0.5 1.147 1.346
0.75 1.221 1.352
1 1.296 1.358
MgCl2 MgCl2�6H2O [99.0 0.1 1.005 1.335 Aldrichc
0.25 1.017 1.339
0.5 1.034 1.345
0.75 1.055 1.350
1 1.072 1.356
1.5 1.107 1.366
2 1.142 1.377
MgSO4 MgSO4�7H2O [99.5 0.1 1.010 1.335 Wakoa
0.25 1.027 1.339
0.5 1.057 1.344
0.75 1.083 1.350
1 1.111 1.355
1.5 1.165 1.364
2 1.221 1.373
CaCl2 CaCl2�2H2O [99.0 0.1 1.006 1.336 Wakoa
0.5 1.042 1.346
1 1.086 1.358
1.5 1.129 1.370
2 1.171 1.381
SrCl2 SrCl2�6H2O [99.0 0.1 1.010 1.336 Wakoa
0.5 1.064 1.347
1 1.131 1.360
1.5 1.197 1.372
2 1.260 1.384
BaCl2 BaCl2�2H2O [99.0 0.1 1.015 1.336 Wakoa
0.5 1.088 1.348
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Table 2 continued
Electrolyte Solid Purity Conc.(mol�L-1)
Density(g�mL-1)
Refractiveindex (nD)
Manufacturer
1 1.178 1.362
1.5 1.265 1.376
MnCl2 MnCl2�4H2O [99.0 0.1 1.007 1.336 Wakoa
0.5 1.050 1.346
1 1.103 1.358
1.5 1.150 1.371
2 1.199 1.383
CoCl2 CoCl2�2H2O [99.0 0.1 1.008 1.336 Wakoa
0.5 1.054 1.347
1 1.111 1.361
1.5 1.166 1.373
2 1.220 1.386
NiCl2 NiCl2�6H2O [99.9 0.1 1.009 1.336 Wakoa
0.5 1.057 1.348
1 1.116 1.363
1.5 1.171 1.379
2 1.223 1.395
a Wako Pure Chemical Industries Ltd.b Strem Chemicals Inc.c Sigma–Aldrich Chemie GmbH
Table 3 Left side: areas in the lower (2,600–3,420 cm-1) and the higher (3,420–3,800 cm-1) frequencyregions of the bands portrayed in Fig. 4 (MAL and MAH, respectively) and their difference (DMAL–H)calculated after drawing a linear baseline from 2,600 to 3,800 cm-1
MAL MAH DMAL-H ATRL ATRH DATRL-H
Li? 635 5 630 15.0 2.4 12.6
Na? -232 92 -324 1.6 5.2 -3.7
K? -233 102 -335 -2.1 3.1 -5.1
Rb? -225 30 -255 -2.3 1.9 -4.2
Cs? 6 22 -15 0.1 0.9 -0.9
Mg2? 2,976 -545 3,522 72.3 2.0 70.3
Ca2? 1,077 -164 1,241 39.8 11.0 28.8
Sr2? 347 -30 377 27.7 14.3 13.4
Ba2? 66 -108 174 23.8 14.0 9.8
Mn2? 3,440 -742 4,183 85.2 -1.8 87.0
Co2? 3,596 -797 4,392 93.9 0.1 93.8
Ni2? 3,398 -914 4,312 94.7 1.1 93.6
Cl- -871 353 -1,224 -19.6 5.4 -25.0
Br- -1,289 515 -1,804 -24.4 8.2 -32.7
I- -1,787 618 -2,405 -28.3 11.2 -39.6
NO�3 -1,330 440 -1,770 -30.1 1.3 -31.4
J Solution Chem (2014) 43:1055–1077 1075
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Table 3 continued
MAL MAH DMAL-H ATRL ATRH DATRL-H
ClO�4 -2,092 492 -2,584 -46.4 -3.1 -43.3
HCO�3 884 111 773 12.5 -4.4 16.9
CO2�3 2,914 -914 3,828 74.6 -16.2 90.8
SO2�4 0 0 0 0 0 0
The errors in the DMAL–H for monovalent ions are within ±30 and those for divalent ions are within±60 cm-1, which values were estimated from several independent measurements. Right side: areas in thelower (2,600–3,300 cm-1) and the higher (3,300–3,800 cm-1) frequency regions of the bands portrayed inFig. 6a, b (ATRL and ATRH, respectively) and their difference (DATRL–H) calculated after drawing a linearbaseline from 2,600 to 3,800 cm-1. The maximum error ranges in the DATRL–H values for monovalent ionsare ±0.45 and that for divalent ions are ±0.9 cm-1. All values for SO2�4 were set to zero
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Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR SpectroscopyAbstractIntroductionExperimentalMaterialsATR-IR MeasurementsExtraction of the Molar Absorptivity of Water from the ATR-IR Spectra of Aqueous Electrolyte SolutionsExtraction of the Effects of Individual Ions on the OH Stretching Band of Water
Results and DiscussionEffects of Ions on the OH Stretching Band of WaterCorrelations Between the OH Stretching Band of Water and Structural and Dynamic Properties of the Ion--Water Interaction in Aqueous Solution
ConclusionAcknowledgmentsAppendixReferences