Does the ethanolammonium acetate ionic liquid mix homogeneously with molecular solvents?

Article abstract of DOI:10.1002/mrc.4363

In order to study micro-structural heterogeneities in the solutions of ionic liquids (ILs), ethanolammonium acetate (EOAA) IL has been synthesized and ¹H NMR spectra of its binary solutions, namely EOAA/DMSO and EOAA/n-butanol, have been measured at 298.15 K. Then the concentration-dependent chemical shifts δ (-CH₃, EOAA), δ (-CH₃, DMSO) and δ (-CH₃, n-butanol) have been correlated separately using the local composition (LC) model. The results indicated that self-association of EOAA played the leading role within EOAA-rich region in mixtures. However with the molecular solvents increasing, the interactions between EOAA and solvents gradually predominated. Moreover the comparisons between DMSO and n-butanol have demonstrated that DMSO could break the network of IL more easily, indicating that the influence of DMSO on the properties of EOAA should be more obvious, which was consistent with experimental results of viscosity and conductivity. Furthermore the maximum difference between the local and the bulk appeared at x(EOAA) ≈ 0.5 for both systems, which reflected remarkable nonideality of mixtures at this concentration.

Full text of DOI:10.1002/mrc.4363

Research article
Received: 7 June 2015
Revised: 15 August 2015
Accepted: 3 September 2015
Published online in Wiley Online Library: 19 October 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4363
Does the ethanolammonium acetate ionic liquid
mix homogeneously with molecular solvents?
Xiao Zhu,a* Huan Zhanga and Yingjie Xub
In order to study micro-structural heterogeneities in the solutions of ionic liquids (ILs), ethanolammonium acetate (EOAA) IL has
been synthesized and 1H NMR spectra of its binary solutions, namely EOAA/DMSO and EOAA/n-butanol, have been measured at
298.15K. Then the concentration-dependent chemical shifts δ (―CH3, EOAA), δ (―CH3, DMSO) and δ (―CH3, n-butanol) have been
correlated separately using the local composition (LC) model. The results indicated that self-association of EOAA played the lead-
ing role within EOAA-rich region in mixtures. However with the molecular solvents increasing, the interactions between EOAA and
solvents gradually predominated. Moreover the comparisons between DMSO and n-butanol have demonstrated that DMSO could
break the network of IL more easily, indicating that the influence of DMSO on the properties of EOAA should be more obvious,
which was consistent with experimental results of viscosity and conductivity. Furthermore the maximum difference between
the local and the bulk appeared at x(EOAA) ≈ 0.5 for both systems, which reflected remarkable nonideality of mixtures at this con-
centration. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: ammonium-based ionic liquids; local composition model; 1H NMR chemical shifts; binary solutions
Recently NMR has been frequently utilized to study the in-
Introduction
homogeneous behavior in the mixtures of ILs because of its accu-
racy and sensitivity.[13–18] Undoubtedly 1H NMR chemical shifts
As an extremely hot topic, ionic liquids (ILs) have attracted world-
wide attentions recently because of their many outstanding prop-
erties, such as negligible vapor pressure, high thermal stability,
wide liquid range and properties-adjustable according to different
combinations of cations and anions. Numerous investigations have
reported applications of ILs in many fields, such as catalysts,[1]
organic synthesize,[2] self-assembly,[3] electrochemistry,[4] dissolv-
ing cellulose[5] and so on. Apart from pure ILs, there has been a
growing interest in the study of binary mixtures of ILs with other
molecular solvents,[6] because the ILs usually have to coexist with
other solvents in real process. Many properties of IL/solvents solu-
tions, such as viscosity, density and conductivity, have been mea-
sured and interactions between ILs and solvents have also been
discussed.[7] It is common knowledge that the physicochemical
properties of ILs could be considerably affected by the molecular
solvents, indicating that the micro-structure of solutions could be
characteristically changed. Actually it is worth noting that solutions
of ILs may not be homogeneous in the local areas. Theoretical and
experimental methods have clearly revealed that nano-
inhomogeneity could be generated in aqueous solutions of
ILs.[8,9] Moreover the study[10] on the DMSO/either trimethyl(butyl)
ammonium bis((trifluoromethyl)sulfonyl)imide ([N4111] [Tf2N])
and DMSO/1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfo-
nyl)imide ([bmim] [Tf2N]) have suggested that DMSO in mixtures
with ILs could tend to produce clusters, instead of being completely
mixed. In addition, the formation of localized, self-segregated,
hydrogen-bonded nanostructures within the 1-butyl-3-methylimi
dazolium hexafluorophosphate ([bmim][PF6])/tetraethylene glycol
(TEG) binary mixture has also been shown.[11] Accordingly it has
been mentioned that[12] considerable efforts are expected to be
applied to the investigation of structural heterogeneities in mix-
tures of ILs, in particular with molecular solvents.
can help us to determine the numbers and types of protons in pure
substance. Furthermore 1H NMR chemical shifts could also offer us
much information about microscopic structures within mixtures
conveniently, because chemical environments of protons could
be changed because of the intermolecular interactions. In addition,
1H NMR chemical shifts could be analyzed quantitatively by certain
theoretical models. In 2003, a simple semi-empirical physical local
composition (LC) model was proposed by our group[19] to study
1
the H NMR chemical shifts of binary mixtures with satisfying re-
sults. Afterwards, the relations between thermodynamic and spec-
troscopic properties for mixtures have been established
successfully with such LC model.[20–22] And our previous work[23]
has reported the local structures of aqueous solutions of ILs using
1
the H NMR spectra coupled with LC model. So far as we knew,
the ILs involved in most of literatures are imidazolium-based ILs.
Other kinds of ILs have seldom been mentioned, which is obviously
unfavorable for the comprehensive understanding of ILs family. Ac-
tually ammonium-based ILs are showing enormous potential as
catalysts and solvents with many advantages compared to
imidazolium-based ILs, such as low-cost, easy to prepare and
smaller toxicity. Moreover investigations have revealed their excel-
lent performances in many fields like carbon dioxide absorption,[24]
wastewater treatment,[25] catalytic reaction[26] and electrochemistry.[27]
*
Correspondence to: Xiao Zhu, School of Chemistry and Chemical Engineering,
Qufu Normal University Qufu 273165, China. E-mail: xiaoxiao1982820@163.com
a School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu
273165, China
b Department of Chemistry, Shaoxing University, Shaoxing 312000, China
Magn. Reson. Chem. 2016, 54, 205–212
Copyright © 2015 John Wiley & Sons, Ltd.

X. Zhu, H. Zhang and Y. Xu
Some understandings about the structures and properties of pure
ammonium-based ILs have already been obtained.[28,29] Neverthe-
less, their special behaviors in solutions because of the influence of
molecular solvents have seldom been discussed.
48h before use. The water content in EOAA determined by Karl
Fischer titration (ZSD-1, ShangHai Anting Scientific Instrument
Factory) is lower than 200ppm. The structure of EOAA was iden-
tified by Fourier transform infrared (FT-IR, Nicolet FT-IR/
Nexus470) and 1H NMR spectroscopy (Bruker, 400 MHz, CDCl3).
IR vibrational frequencies (cmꢀ1) were as follows: 2872 (+N―H
and O―H stretching vibration), 1552 (C¼O stretching vibration
and +N―H bending vibration), 1337(C―N stretching vibration)
and 1073 (C―O stretching vibration), which was consistent with
In the present paper, the ethanolammonium acetate (EOAA) IL,
as a kind of ammonium-based ILs, has been synthesized. In fact
their aggregations in pure state have been studied before.[30]
However the understanding of local structure in mixtures of EOAA
is extremely scarce at present. The selective systems involved here
are EOAA/DMSO and EOAA/n-butanol. DMSO is famous for its
special dissolving capacity and high polarity, which is, often in
company with ILs, used to promote chemical reactions[31,32] and
biological processes.[33] n-butanol, as another common organic
solvent, is considered to be a very competitive biofuel.[34] And
some kinds of ILs are reported to be suitable for the extraction
of n-butanol.[35] Herein we reported the microscopic local struc-
tures of EOAA/DMSO and EOAA/n-butanol solutions using 1H
NMR coupled with LC model, which is interesting and necessary
for their applications.
1
the structure of EOAA. The H NMR chemical shifts δ (ppm) were
as follows: 8.1 (4H, N―H and O―H), 3.8(2H, ―CH2―N), 3.1
(2H, ―CH2―O), 2.0 (3H, ―CH3), which was corresponding well
to the protons of EOAA. All of solutions of ILs were prepared
by weight using an analytical balance (Shanghai Optical
Instrument Factory, FA2004) with a precision of 0.0001 g over
the whole concentration range.
Measurement of 1H NMR chemical shifts of solutions at differ-
ent concentrations
The internal reference method[36] was adopted to obtain the
concentration-dependent chemical shifts of ―CH3 in EOAA
(because the δ(―CH3) changed more obviously compared to other
protons) and ―CH3 in DMSO as well as ―CH3 in n-butanol. To
avoid the influence of deuterated reagents, a 2-mm capillary tube,
in which the deuterated chloroform (CHCl3-d) was sealed, was
placed at the center of a 5-mm sample tube filled with the chem-
ical shift reference of tetramethylsilane (TMS) and the sample
solutions. It is demanded that the inner capillary tube should be
Experimental section
Synthesis of EOAA IL
EOAA was prepared from ethanolamine and acetate acid
according to the procedure reported previously by our labora-
tory (see Scheme 1). Acetate acid (CAS NO. 64-19-7), ethanol-
amine (CAS No.141-43-5), DMSO (CAS NO.67-68-5) and
n-butanol (CASNO.71-36-3) were of analytical grade. The above-
mentioned materials were purchase from Tianjin Damao Chemi-
cal Reagent Factory (Tianjin, China). Specific synthesis processes
were as follows: into a 100-ml three-necked flask under vigorous
stirring, 0.55 mol of ethanolamine was placed, and then 0.50 mol
of acetate acid was dropped in slowly. The temperature was
kept at 25 °C. After the neutralization reaction for 4 h, extremely
viscous and colorless EOAA was obtained, and after rotary evap-
oration the sample were dried under vacuum at 70 °C for at least
1
kept parallel with the exterior sample tube. The H NMR spectra
were measured using a Bruker AV 400 spectrometer operating
at 400 MHz at different concentrations at 298.15 K, and all of the
experimental δ(―CH3) have been included in Table 1.
Viscosity measurements
The viscosities of the samples were measured with a digital dis-
play viscometer produced by Shanghai Sunny Hengping Scien-
tific Instrument Co., Ltd (NDJ-5S). Each sample was placed in
an external water bath which was controlled to 25 0.1°C for
30min. Then the measurements of viscosity were performed
quickly. Each viscosity value of the samples was reported by
averaging three consecutive runs (Table 2).
Scheme 1. Molecular structure and synthesis of EOAA ionic liquid.
Table 1. Concentration-dependent 1H NMR chemical shifts of two systems at 298.15 K
EOAA(1)/DMSO(2)
EOAA(1)/n-butanol(2)
x1
δ(―CH3,DMSO)/ppm
δ(―CH3,EOAA)/ppm
x1
δ(―CH3,n-butanol)/ppm
δ(―CH3,EOAA)/ppm
0
3.09
3.08
3.07
3.03
3.01
2.94
2.91
2.89
2.85
/
/
0
1.34
1.31
1.28
1.25
1.15
1.11
1.08
1.06
1.01
/
/
0.05
0.10
0.19
0.39
0.50
0.59
0.70
0.89
1
2.26
2.23
2.19
2.17
2.11
2.08
2.06
2.03
1.99
0.05
0.10
0.20
0.40
0.50
0.60
0.69
0.80
1
2.27
2.24
2.2
2.13
2.09
2.06
2.03
2.00
1.99
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Magn. Reson. Chem. 2016, 54, 205–212

local composition behavior of EOAA ionic liquid in solutions
Table 2. Viscosity η of the mixtures at 298.15 K
Theory
EOAA (1)+DMSO(2)
EOAA(1)+n-butanol(2)
In 1964, Wilson[37] proposed the local composition concept to de-
scribe the microscopic solution structure, which has been exten-
sively applied to investigate some inhomogeneous behavior
within associated mixtures.[38] Based on the Wilson’s assumption,
x2
η/mPa·s
x2
η/mPa·s
0.000
0.069
0.143
0.223
0.311
0.404
0.502
0.612
0.732
0.852
1.000
2300.01
1634.33
1064.21
420.05
375.13
139.51
66.42
0.000
0.088
0.179
0.272
0.369
0.476
0.574
0.666
0.776
0.881
1.000
2300.01
1548.30
734.15
460.34
325.31
190.28
74.63
1
Deng[19] assumed that the observed H NMR chemical shifts at a
certain composition in a binary system could be simply expressed
as follows:
δ1;cal ¼ δ01φ11 þ δ1∞φ21
(1)
29.50
30.51
where φ11 and φ21 are local volume fractions of molecules 1 and 2
around the central molecule 1, respectively. δ01 and δ1∞ are pure sub-
stance and infinitely dilute chemical shifts of a certain proton of
molecule 1, respectively. The latter is usually obtained by extrapo-
lating the dilute chemical shift to zero concentration. Then the local
molar volume fractions can be defined as following:
12.73
19.04
6.71
14.02
5.20
5.51
Conductivity measurements
x1
The electric conductivities were measured with an electric conduc-
tivity meter produced by Shanghai Precision & Scientific Instrument
Co., Ltd. (DDS-11D). The cell constant was determined by
0.1 mol·Lꢀ1 aqueous solution of KCl at 298.15 K (Table 3). Each sam-
ple was placed in water bath which was controlled to 25 0.1°C for
30 min before measurement. Each conductivity value of the sam-
ples was reported by averaging three consecutive runs. The viscos-
ity and electric conductivity data of binary solutions have been
illustrated separately in Figs. 11-12.
φ11
¼
¼
(2)
(3)
(4)
x1 þ Λ21x2
Λ21x2
x1 þ Λ21x2
φ21
V2
Λ21
¼
exp½ꢀðg21 ꢀ g11Þ=RTꢁ
V1
where g21 and g11 are proportional to the 1–2 and 1–1 interaction
energies, respectively, where V1 and V2 are the mole volumes of
molecule 1 and 2 which are calculated by division of molecular
weight and density (the measured density of EOAA, DMSO and n-
Table 3. Conductivity κ of the mixtures at 298.15 K
EOAA (1)+ DMSO(2)
EOAA(1)+ n-butanol(2)
,
butanol at 298.15 K are 1.1565 g·cmꢀ3 1.1088 g·cmꢀ3 and
x1
κ/103μS·cmꢀ1
x1
κ/103μS·cmꢀ1
0.8059 g·cmꢀ3 respectively using the pycnometer method). x1 and
x2 denote bulk mole fractions of molecules 1 and 2, respectively.
(g21–g11) is the mutual interaction energy parameter connected
with the 1–2 and 1–1 pairs of molecules. The following objective
function is optimized using the Powell[39] method to get the inter-
molecular interaction parameter (g21–g11),
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.090
0.100
0.197
0.393
0.492
0.604
0.701
0.799
0.900
0.910
0.920
0.930
0.940
0.949
0.959
0.970
0.979
0.989
1
0.75
1.02
1.21
1.26
1.49
1.60
1.68
1.73
1.77
2.15
1.45
1.38
1.02
0.68
0.61
0.56
0.52
0.48
0.45
0.42
0.41
0.37
0.36
0.33
0.29
0.28
0.010
0.024
0.031
0.040
0.050
0.060
0.070
0.080
0.090
0.100
0.193
0.284
0.400
0.500
0.601
0.700
0.798
0.900
0.910
0.920
0.930
0.9399
0.950
0.960
0.970
0.980
0.990
1
0.06
0.11
0.16
0.20
0.26
0.31
0.34
0.41
0.44
0.45
0.93
1.15
1.15
0.98
0.80
0.69
0.65
0.64
0.60
0.58
0.56
0.55
0.52
0.50
0.45
0.41
0.36
0.28
1
"
#
=2
m
∑
ꢀ
ꢁ
1
2
O:F ¼
δ1;cal;k ꢀ δ1; exp;k
(5)
m ꢀ 1 k¼1
where m denotes the size of the experimental data set. δ1,cal,k de-
notes k-th calculated chemical shifts according to Eqn (1). δ1,expl,k
denotes the k-th experimental value, and (g21–g11) is the only opti-
mized parameter. According to Eqns (1–4), the correlation of ―CH3
chemical shifts in binary mixtures can be achieved using one set of
energy parameter as shown in Fig. 1–4. The root-mean square devi-
ations are defined as follows,
1
2
"
#
ꢀ
ꢂ
ꢁ
ꢃ
2
m
δ1;cal;k ꢀ δ1; exp;k
1
Δδ% ¼
∑
ꢂ100%
(6)
(7)
m k¼1
δ1; exp;k
1
ꢄ
ꢅ
=2:
2
m
ꢀ
ꢁ
1
Δδ ¼
∑ δ1;cal;k ꢀ δ1; exp;k
m k¼1
The (g21–g11) and correlated deviations of all systems were listed
in Table 4. Actually according to Wilson’s local composition defini-
tion in a binary mixture, the ratios of the probability of finding the
molecule 2 around the central molecule 1 can be defined in terms
Magn. Reson. Chem. 2016, 54, 205–212
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X. Zhu, H. Zhang and Y. Xu
Figure 4. Experimental and correlated 1H NMR chemical shifts of ―CH3 of
n-butanol in EOAA/n-butanol system at 298.15 K: (●: experimental values; —:
correlated values).
Figure 1. Experimental and correlated 1H NMR chemical shifts of ―CH3 of
EOAA in EOAA/DMSO system at 298.15 K: (●: experimental values; —:
correlated values).
Table 4. Energy parameter (g21–g11) and deviations of chemical shifts
correlation
(g21–g11)/ Δδ/ppm Δδ %
J·molꢀ1
EOAA(1)/DMSO(2)
(1) as molecule 1 ꢀ756.15
503.30
0.012
0.017
0.017
0.017
0.55%
0.57%
0.81%
1.48%
(2) as molecule 1
EOAA(1)/n-butanol(2) (1) as molecule 1 781.84
(2) as molecule 1 ꢀ563.22
of bulk mole fractions and interaction energies between 1–2 pair
g21 and 1–1 pair g11 shown below,
x21 x2 expðꢀg21=RTÞ x2
Figure 2. Experimental and correlated 1H NMR chemical shifts of ―CH3 of
DMSO in EOAA/DMSO system at 298.15 K: (●: experimental values; —:
correlated values).
¼
¼
exp½ꢀðg21 ꢀ g11Þ=RTꢁ
(8)
x11 x1 expðꢀg11=RTÞ x1
where x11 and x21 denote the local mole fraction of molecule 1 and
2, respectively, around the central molecule 1. Apparently the local
mole fraction of every molecule in the vicinity of the central mole-
cule 1 should be unity. Thus,
x11 þ x21 ¼ 1:
(9)
Subsequently the values of x11 and x21 can be calculated at corre-
sponding compositions after correlation of chemical shifts.
Results
Correlation of 1H NMR chemical shifts
Taking EOAA/DMSO for example, if EOAA is considered as the cen-
tral molecule 1, then molecular 2 is assigned to DMSO. Thus, ac-
cording to Eqns (1–5) the chemical shifts of ―CH3 in EOAA can
be correlated with the only optimized parameter (g21–g11). Similarly
δ (―CH3, DMSO) and δ (―CH3, n-butanol) in solutions can also be
correlated. Clearly as shown in Fig. 1–4, δ (―CH3) in EOAA, DMSO
and n-butanol could be well correlated by this semi-empirical LC
model.
Figure 3. Experimental and correlated 1H NMR chemical shifts of ―CH3 of
EOAA in EOAA/n-butanol system at 298.15 K: (●: experimental values; —:
correlated values).
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local composition behavior of EOAA ionic liquid in solutions
The local structure of EOAA/DMSO system
According to the idea of local composition, the binary solutions of
ILs should not absolutely be homogeneous in the local areas be-
cause of distinction in interactions of different molecule pairs IL–
IL, IL–solvent and solvent–solvent. Therefore in the vicinity of any
component, the local mole fractions of ILs and molecular solvents
should be not always the same as in the bulk, which is the so-called
local composition behavior. In EOAA/DMSO system, when EOAA is
considered as the central molecule 1, x11 denotes the local mole
fractions of EOAA around the EOAA. And x21 denotes the local mole
fractions of DMSO around the EOAA because DMSO is assigned to
the molecular 2. With the help of the LC model, the local mole frac-
tions x11 and x21 at different compositions could be calculated
using Eqns (8, 9) because the (g21–g11) can be obtained by the cor-
relation of δ(―CH3) in EOAA. In order to illustrate the local compo-
sition behavior more clearly, the concentration-dependent (x11–x21)
and (x1–x11) of EOAA/DMSO system have been shown graphically
in Fig. 5–7.
Figure 6. Concentration-dependent (x11–x21) when DMSO is the central
molecule 1 for EOAA/DMSO system.
As illustrated in Fig. 5, the (x11–x21), which denotes the differ-
ences between the local mole fractions of EOAA and DMSO, be-
haves decreasing from 1 to ꢀ1 with x2 (the bulk mole fractions of
DMSO) over the whole concentration range. Obviously in the pure
IL there are only EOAA, so x11 = 1 and x21 = 0. With DMSO increasing,
(x11–x21) keeps gradually reducing, indicating more and more
DMSO molecules locate around EOAA. In other words, DMSO could
destroy the network of EOAA because of the intermolecular interac-
tions between DMSO and EOAA. In addition, it is clear that when x
(DMSO) < 0.4, the value of (x11–x21) keeps positive, indicating that
there are more EOAA but fewer DMSO distributed around EOAA.
However the values of (x11–x21) exhibit negative within the
DMSO-rich region (when x(DMSO) > 0.5), indicating that there are
more DMSO than EOAA located around EOAA IL. Subsequently it
could be deduced that self-associations among EOAA IL should
preferably happen within the EOAA-rich region, maybe x(EOAA)
= 0.6–1. While in the DMSO-rich region, the vicinity of EOAA could
be mainly occupied by DMSO molecules, showing the predomina-
tion of interactions between EOAA and DMSO instead of
self-association of EOAA. It has been reported that DMSO could
not completely mixed with IL in DMSO/trimethyl(butyl)ammo-
nium bis((trifluoromethyl)sulfonyl)imide [N(4)111][Tf2N] mixtures.[9]
P. Venkatesu and co-workers[40,41] have studied the mixtures of
trimethylammonium-based ILs/DMSO and detected that DMSO
molecules could reduce hydrogen bonding between the cation
Figure 7. Concentration-dependent (x1–x11) of EOAA/DMSO system: -●-:
EOAA as the central molecule 1; -★-: DMSO as the central molecule 1.
and anion because of ion–dipole interactions between DMSO and
ions of ILs. However at higher ILs concentration, the interactions be-
tween DMSO and IL became weaker, and extensive self-association
in the IL has been detected. Obviously our findings that DMSO
could gradually break in the self-association structure of EOAA are
in agreement with the former conclusions.
On the other hand, when DMSO is regarded as central molecule
1, the local composition analysis has been illustrated in Fig. 6. With
EOAA increasing, the (x11–x21) becomes decreasing, which can
demonstrate that more and more EOAA distribute around DMSO.
And when x(EOAA) > 0.5,(x11–x21) < 0, indicating that interactions
between DMSO and EOAA play the leading role rather than self-
association of DMSO molecules. That is to say, DMSO could mainly
interact with EOAA instead of DMSO itself in EOAA-rich region.
The difference between the local and bulk composition of
EOAA/DMSO system
Furthermore the analysis of difference between the local mole frac-
tion x11 and bulk mole fraction x1 in EOAA/DMSO has also been per-
formed, which have been illustrated in Fig. 7. It is clear that no
matter which one is considered as central molecule 1, the values
of (x1–x11) reach the maximum or minimum at x(EOAA) ≈ 0.5. Such
interesting phenomenon should be attributed to the remarkable
changes in structure of solutions, which could be expressed in
some excess physicochemical properties. Actually the experimental
Figure 5. Concentration-dependent (x11–x21) when EOAA is the central
molecule 1 for EOAA/DMSO system.
Magn. Reson. Chem. 2016, 54, 205–212
Copyright © 2015 John Wiley & Sons, Ltd.
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X. Zhu, H. Zhang and Y. Xu
excess volumes values,[42] index of non-ideality of solutions, in
triethylammonium acetate (TEAA)+ DMSO system have maximum
values at mole fractions of this IL of about x(IL) ≈ 0.5. And the excess
molar volumes have a minimum at the mole fraction of x(IL)≈ 0.46
for trimethylammonium hydrogen sulfate (TMAS)+ DMSO
system.[43] Consequently our observations combined with other ex-
perimental data could offer us a clear insight that the structure of
region, EOAA could prefer self-associations to form so-called ‘clus-
ter’. Instead in the n-butanol-rich region, the interactions between
EOAA and n-butanol gradually dominate which lead to the break-
down the ‘cluster’ of EOAA.
Also n-butanol could be regarded as molecule 1, and the local
composition behaviors of n-butanol have been illustrated in Fig. 9.
It is interesting to notice that (x11–x21) < 0 when x(EOAA) > 0.5,
which indicates that the number of n-butanol is fewer than EOAA
distributed around n-butanol. Consequently it could be concluded
that the interactions between EOAA and n-butanol, instead of the
self-association of n-butanol molecules, predominate within
EOAA-rich region.
EOAA/DMSO mixtures behaves characteristic change at
x
(EOAA)≈ 0.5.
The local structure of EOAA/n-butanol system
Similar to the approach of the EOAA/DMSO system, the local mole
fractions x11 and x21 in the EOAA/n-butanol solutions have also
been calculated through correlation of δ (―CH3, EOAA) and
(―CH3, n-butanol). As illustrated in Fig. 8 when EOAA as central
molecule 1, (x11–x21) behaves decreasing from 1 to ꢀ1 with the x2
(mole fraction of n-butanol) increasing, which is similar to
EOAA/DMSO system. It is interesting to focus on the point x(n-buta-
nol) ≈ 0.6 (when(x11–x21) ≈ 0). The values of (x11–x21) exhibit positive
when x(n-butanol) < 0.6, suggesting that there are more EOAA than
n-butanol locating around the EOAA in such region. However the
values of (x11–x21) keep negative when x(n-butanol) > 0.6, indicat-
ing that there are more n-butanol but fewer EOAA distributed
around EOAA. Such results could reveal that within EOAA-rich
The difference between the local and bulk composition of
EOAA/n-butanol system
The difference between the local and bulk mole fractions,
namely (x1–x11), for EOAA/n-butanol solutions has been illus-
trated in Fig. 10. It can be detected that the extremum of (x1–x11)
appears at x(EOAA) ≈ 0.5. Such phenomenon is similar to that of
EOAA/DMSO system. Because the peak position of (x1–x11) in
EOAA/DMSO system is consistent with their excess properties, it
could be hypothesized reasonably that remarkable changes on
structure would happen when x(EOAA) ≈ 0.5 for EOAA/n-butanol
solutions and excess functions, such as excess molar volumes, will
be detected in our future studies.
The comparison of DMSO and n-butanol
Although (x11–x21) decreases with solvent increasing for
EOAA/DMSO and EOAA/n-butanol systems when EOAA is assigned
to molecule 1, the difference between these two mixtures should
not be neglected. As shown in Fig. 5 and Fig. 8, (x11–x21) ≈ 0 when
x(DMSO) is about 0.4 in EOAA/DMSO system. However (x11–
x21) ≈ 0 when x(n-butanol) is close to 0.6 in EOAA/n-butanol system.
In other words, if x(DMSO) is more than 0.4, there could be more
DMSO than EOAA locating around the EOAA. Whereas the interac-
tions between EOAA and n-butanol could not dominate until x(n-
butanol) is more than 0.6. Thus it could be deduced that compared
to n-butanol, DMSO could breakdown the network of EOAA more
easily. It has been reported that the influences of the organic sol-
vents on the network of ILs depended on the solvent dielectric
constant.[44] And the solvents with high dielectric constant show
better miscibility with ILs while nonpolar solvents demonstrate
Figure 8. Concentration-dependent (x11–x21) when EOAA is the central
molecule 1 for EOAA/n-butanol system.
Figure 9. Concentration-dependent (x11–x21) when n-butanol is the central
molecule 1 for EOAA/n-butanol system.
Figure 10. Concentration-dependent (x1–x11) of EOAA/n-butanol system: -
●-: EOAA as the central molecule 1; -★-: n-butanol as the central molecule 1.
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Magn. Reson. Chem. 2016, 54, 205–212

local composition behavior of EOAA ionic liquid in solutions
in EOAA/DMSO solutions because of its higher dielectric constant,
which is indeed consistent with structures variations of
EOAA/DMSO deduced from the LC model.
Discussion and conclusions
1
With the help of LC model, the correlations of H NMR chemical
shifts in EOAA/DMSO and EOAA/n-butanol solutions have been
performed. The local mole fractions have revealed that solvents
could not mix homogeneously with ILs in the local areas. Instead
the structure of systems could change with the concentration of so-
lutions. Specifically within EOAA-rich region, EOAA mainly self-
associated with each other, and solvent molecules prefer to interact
with EOAA rather than self-associate. Within solvent-rich region, sol-
vent molecules prefer to self-associate with each other, and EOAA
tends to interact with solvents instead of EOAA self-association. In
addition, compared to n-butanol, the DMSO molecules could be
more ready to destroy the network of EOAA because of its higher
dielectric constant. And the studies on the viscosity and conductiv-
ity of EOAA/n-butanol and EOAA/DMSO solutions have indicated
that DMSO could be more effective to reduce the viscosity and to
increase the conductivity of EOAA, which is consistent with remark-
able effects of DMSO on the micro-structure of EOAA. Furthermore,
the differences between the bulk and the local compositions in
EOAA/DMSO are consistent with changes of the excess molar vol-
umes VE, which further indicated that structure of solutions could
undergo special variations when x(EOAA) ≈ 0.5. Consequently ac-
cording to the extreme value of (x1–x11) in EOAA/n-butanol mixture
when x(EOAA) ≈ 0.5, it is reasonable to predict that excess proper-
ties of EOAA/n-butanol, like VE, may possibly perform turnover at
x(EOAA) ≈ 0.5.
Figure 11. Viscosity η versus volume fraction of solvent for binary solutions
at 298.15 K:
: EOAA/DMSO system;
: EOAA/n-butanol system.
Evidently it is considerable interesting and practical to study the
local heterogeneity of ILs in solutions which could dramatically af-
fect the properties of ILs/solvent systems. Our work can offer us
clear insight into the microscopic structures of solutions containing
the EOAA IL using an extremely convenient method. In order to ex-
amine the above findings, the further researches on the excess
properties of such systems will be carried out, and relevant findings
will be reported shortly.
Figure 12. Conductivity κ versus mole fraction of EOAA for binary solutions
at 298.15 K: : EOAA/DMSO system; : EOAA/n-butanol system.
poorer miscibility, indicating that polar solvents have stronger mo-
lecular interaction with ILs.[40,41] Moreover other studies[42,45] have
pointed out that molecular solvents with high dielectric constant
are more impactful in disruption of the electrostatic attraction be-
tween the ions of the ILs. In order to confirm the distinction be-
tween DMSO and n-butanol, the concentration-dependent
viscosities η and conductivities κ of EOAA/DMSO and EOAA/n-
butanol systems have been measured. As shown in Fig. 11,
although the viscosity gradually becomes smaller with volumes
fraction of solvents Фs increasing for the two systems, the slope
of curves of EOAA/DMSO appears more negative, indicating that
the ability of DMSO to decrease the viscosity of EOAA should be
stronger than that of n-butanol. In the case of conductivities κ
shown in Fig. 12, with mole fraction of EOAA increasing, κ sharply
increases in solvent-rich region and then progressively becomes
smaller in EOAA-rich region. And the κ reaches maximum when
the mole fraction of EOAA is about 0.2 for the two systems. How-
ever the maximal value of κ of EOAA/DMSO appears larger than
that of EOAA/n-butanol system, indicating the DMSO should be
more effective to promote the dissociation of EOAA. Such proper-
ties of solutions have proved that DMSO are more capable of reduc-
ing the electrostatic attraction between the ions of EOAA and
consequently could induce more remarkable changes of structure
Acknowledgements
This work was supported by the National Natural Science Founda-
tion of China (No. 21206085 and 21406140) and Shandong Provin-
cial Doctoral Foundation (No. BS2011CL023).
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