Interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids with glucose in water: A study of volumetric properties, viscosity, conductivity and NMR

Article abstract of DOI:10.1039/c1cp20948e

Extensive applications of ionic liquids (ILs) may result in their accumulation in the ecological environment and organisms. Although ILs are popularly called "green solvents", their toxicity, in fact, has been exhibited. Therefore the interaction of ILs with biomolecules is a cutting-edge research subject. Herein, the interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids ([C₄mim][HCOO], [C₄mim][CH ₃COO] and [C₄mim][CH₃CH₂COO]) with glucose in water were studied for their volumetric properties, viscosity, conductivity and NMR spectra. Limiting apparent molar volumes (V0Φ,IL), viscosity B-coefficients, limiting molar conductivities (Λ₀) and Walden products (Λ₀η₀) were evaluated for the ILs in glucose + water solutions. Volumetric interaction parameters were also obtained from the transfer volumes of the ionic liquids. The contributions of the solvent properties (B₁) and the ionic liquid-solvent interactions (B₂) to the B-coefficient were extracted, together with molar activation energies (Δμ0≠IL) of the ionic liquids for viscous flow of the aqueous glucose + IL solution. In addition, the ¹³C and ¹H NMR spectra of methyl β-d-glucopyranoside and ILs in β-d-glucopyranoside + IL + D₂O were studied. The NMR results show that no special and strong interactions were observed between glucopyranoside and the ILs. However, it was confirmed that the H2 on the imidazolium ring has more activity (acidity) than atoms H4 and H5. The macro-properties and their changes were also discussed in terms of the size, structure and solvation of the ILs and glucose. the Owner Societies 2011.

Full text of DOI:10.1039/c1cp20948e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
PCCP
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 14542–14549
www.rsc.org/pccp
PAPER
Interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids
with glucose in water: a study of volumetric properties, viscosity,
conductivity and NMRw
Kelei Zhuo,* Yujuan Chen, Jing Chen, Guangyue Bai and Jianji Wang
Received 28th March 2011, Accepted 24th May 2011
DOI: 10.1039/c1cp20948e
Extensive applications of ionic liquids (ILs) may result in their accumulation in the ecological
environment and organisms. Although ILs are popularly called ‘‘green solvents’’, their toxicity, in
fact, has been exhibited. Therefore the interaction of ILs with biomolecules is a cutting-edge
research subject. Herein, the interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids
([C4mim][HCOO], [C4mim][CH3COO] and [C4mim][CH3CH2COO]) with glucose in water were
studied for their volumetric properties, viscosity, conductivity and NMR spectra. Limiting
apparent molar volumes (V0F,IL), viscosity B-coefficients, limiting molar conductivities (L0) and
Walden products (L0Z0) were evaluated for the ILs in glucose + water solutions. Volumetric
interaction parameters were also obtained from the transfer volumes of the ionic liquids. The
contributions of the solvent properties (B1) and the ionic liquid–solvent interactions (B2) to the
0a
IL
B-coefficient were extracted, together with molar activation energies (Dm ) of the ionic liquids
1
for viscous flow of the aqueous glucose + IL solution. In addition, the 13C and H NMR spectra
of methyl b-D-glucopyranoside and ILs in b-D-glucopyranoside + IL + D2O were studied.
The NMR results show that no special and strong interactions were observed between
glucopyranoside and the ILs. However, it was confirmed that the H2 on the imidazolium ring has
more activity (acidity) than atoms H4 and H5. The macro-properties and their changes were also
discussed in terms of the size, structure and solvation of the ILs and glucose.
nonflammability.6 Despite the ‘‘green’’ aspects of ILs, it is
1. Introduction
irresponsible to ignore the potential toxicity of the ILs released
into the environment.7 Due to the high stability of ILs in
Glucose (Glc), as an important kind of saccharide (S), is a
basic necessity for living organisms and a ubiquitous fuel for
water, they may accumulate in the ecological environment
biological processes. An unusual glucose level in a body fluid is
and organisms, and therefore may be a health hazard. The
a warning signal of a medical condition: for example, an
potential toxicity of ILs may originate from the interaction
irregular concentration of glucose in human blood or urine
between ILs and biomolecules. Thus it is a cutting-edge
implies a biological dysfunction. Glucose detection is, there-
research subject to explore the interactions between ILs and
fore, of biomedical importance.
biomolecules, including saccharides.
Ionic liquids (ILs) are organic salts having melting points
In recent years, there have been a number of research
less than 100 1C. They are called ‘‘designer solvents’’ because
publications about the dissolution of cellulose in ionic
liquids.8,9 Cellulose is a linear polymer of cellobiose that
their physical properties can be modified according to the
nature of the desired reactions by the modification of their
consists of two glucose units linked by glucosidic linkages
cations and anions.1 The ability to control their physical
(C–O–C) at the C1 and C4 positions. In order to understand
properties has led to a vast number of papers concerning the
the dissolution of cellulose in ionic liquids, it is also necessary
use of ionic liquids as solvents.2–4 Ionic liquids are often touted
to study the interactions between ILs and its monomer,
as being ‘‘green’’ solvents, due to their low vapor pressure5 and
glucose.
In the present work, the basic physico-chemical properties –
densities r, viscosities Z and conductivities k of aqueous ionic liquid
School of Chemistry and Environmental Science, Key Laboratory of
Green Chemical Media and Reactions, Ministry of Education,
Henan Normal University, Xinxiang, Henan 453007,
People’s Republic of China. E-mail: klzhuo@263.net
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cp20948e
(1-butyl-3-methylimidazolium carboxylates: [C4mim][HCOO],
[C4mim][CH3COO] and [C4mim][CH3CH2COO], shown in
Scheme 1) solutions were measured in the presence of glucose
and in its absence at 298.15 K. Some macro-parameters
c
14542 Phys. Chem. Chem. Phys., 2011, 13, 14542–14549
This journal is the Owner Societies 2011

View Article Online
Scheme 1 Chemical structures and numbering of [C4mim]+, HCOOꢁ, CH3COOꢁ, CH3CH2COOꢁ and methyl b-D-glucopyranoside.
obtained from the above data can not only explain well the
interaction behavior of glucose, as a model biological molecule,
with1-butyl-3-methylimidazolium carboxylate ionic liquids,
but also give a deep insight to other IL + biomolecule mixed
systems, especially providing theoretical foundations and
important data for studies on the potential toxicity of ILs.
On the other hand, the 13C and 1H NMR spectra can
provide micro-information on the atomic/molecular level
about the S–IL, S–W, and IL–W (W stands for water) inter-
actions in solution. However, glucose exists as an equilibrium
mixture of its various isomers in aqueous solution, so it is
difficult to accurately recognize the NMR spectra peaks. There-
fore, 13C and 1H NMR spectra of methyl b-D-glucopyranoside,
as a replacement for glucose, and the ionic liquids were studied
at different concentrations in D2O, and used to investigate the
interactions of the ILs with glucose.
performed in ethyl acetate at 343 K for 48 h. [C4mim]Cl was
washed with ethyl acetate. The residual solvents were removed
by rotary evaporation, and the resulting products were dried
under vacuum for 24 h.
An aqueous solution of [C4mim]Cl was allowed to pass
through a column filled with anion exchange resin to obtain
[C4mim][OH].10 During this process, the progress of the
reaction was monitored using AgNO3. Upon the complete
substitution of chloride for hydroxide, no precipitation of
AgCl can be found. The aqueous [C4mim][OH] solution was
then neutralized with equal moles of acetic acid. After
removing water by evaporation under reduced pressure, the
viscous liquid [C4mim][CH3COO] was thoroughly washed
with diethyl ether, and finally dried under vacuum at 70 1C
for 72 h. The other two ionic liquids ([C4mim][HCOO] and
[C4mim][CH3CH2COO]) were prepared by similar processes.
2.3 Measurements of density and viscosity
2. Experimental section
Solution densities were measured using a vibrating-tube digital
densimeter (model DMA 60/602 Anton Paar Austria), which
has been described elsewhere.11–13 The temperature around the
densimeter cell was controlled by circulating water from a
constant-temperature bath (Schott, Germany). A CT-1450
temperature controller and a CK-100 ultracryostat were
employed to maintain the bath temperature at (298.15 ꢂ
0.005) K. The densimeter was calibrated with pure water
(the value of density was taken to be 0.997046 g cmꢁ3 at
298.15 K14,15) and dry air. The relative uncertainty of
molalities of glucose and the ionic liquids was evaluated to
be less than ꢂ0.03%. The uncertainty in density was estimated
2.1 Chemicals
D-Glucose (499.5%, Sigma) was dried under vacuum at
333 K to constant weight. Methyl b-D-glucopyranoside
(499.0%, Sigma) was dried under vacuum at room temperature
to constant weight. These dried reagents were stored over P2O5
in desiccators. 1-Methylimidazole (99%, Alfa), 1-chlorobutane
(99%, Alfa), deuterium oxide (499.9%, Sigma) and sodium
4,4-dimethyl-4-silapentan sulfonate (DSS, Beijing Chem.
Factory) were used without further purification. Anion
exchange resin (Ambersep 900 OH) was purchased from Alfa
Aesar; formic acid (498%), acetic acid (499.5%), and
propionic acid (499.5%) were purchased from Aladdin
Chem. Reagent Co., Ltd. The deionized water was doubly
distilled over KMnO4. Water with a conductivity of (0.8–1.0) ꢀ
10ꢁ6 S cmꢁ1 was used throughout the experiments.
to be ꢂ3 ꢀ 10ꢁ6 g cmꢁ3
.
Solution viscosities were measured using a suspended level
Ubbelohde viscometer, which was placed in a water thermo-
stat (Schott, Germany), with a flow time of about 200 s for
water at 298.15 K. The temperature of the thermostat was still
controlled at 298.15 ꢂ 0.005 K. The viscometer was calibrated
at 298.15 and 308.15 K with water. Viscosities for water at
different temperatures were taken from the literature.16
Flow time measurements were performed using a Schott
AVS310 photoelectric time unit with a resolution of 0.01 s.
2.2 Synthesis of the ionic liquids
[C4mim]Cl was prepared and purified using the procedure
described previously in the literature.9 Briefly, reactions of
1-methylimidazole with an excess of 1-chlorobutane were
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 14542–14549 14543

View Article Online
calculated from the equation19
ð1000þmSMSÞðr ꢁ r1Þ
MIL
r
VF;IL
¼
ꢁ
ð2Þ
mILrr1
where MS and MIL are the molar masses of glucose and ionic
liquids, mS and mIL are the molalities of glucose and ionic
liquids (moles of glucose and ILs per kilogram of water), and r
and r1 are the densities of the ternary solution (glucose +
ionic liquid + water) and mixed solvent (glucose + water),
respectively. The results are also included in Table S1.w Limit-
ing apparent molar volumes V0F,IL, which are equal in value to
the standard partial molar volumes V0IL, were obtained by
least-squares fitting experimental data to the following
equation
1/2
IL
VF,IL = V0F,IL + SVm + bVmIL
(3)
Fig. 1 Limiting apparent molar volumes for the ionic liquids in
glucose + water solutions as a function of the molality of glucose
mS at 298.15 K.
where SV is the Debye–Huckel limiting law coefficient and b
¨
V
is an adjustable parameter related to the pair interactions and
equivalent to the second virial coefficient, which indicates the
deviation from the limiting law due to the non-electrostatic
solute–solute interactions.20 The VF0 ,IL values obtained
by least-squares analysis are included in Table S2.w Plots of
V0F,IL versus mS are represented in Fig. 1, indicating that the
values of V0F,IL for the three ionic liquids studied are positive
and increase with increasing mS.
A linear relation is also observed between V0F,IL and the
number of methylene groups in the alkyl chain of the anions
(see Fig. 2):
The estimated relative uncertainty of experimental viscosity is
less than ꢂ0.2%. Solution viscosity Z is given by the following
equation
Z/r = Ct ꢁ K/t (1)
where C and K are the cell constants and t is the flow time. The
details of the experimental procedure have been given elsewhere.17
2.4 Measurements of conductivity
VF0;IL ¼ VF0;½C mimꢃ½HCOOꢃ þ VF0;CH nCH
ð4Þ
Solution conductivities were measured on a Wayne-Kerr
6430B Autobalance Bridge fitted with a Shanghai DJS-1
electrode. The solution in the conductance cell was stirred
magnetically. The conductance cell was equipped with a water
circulating jacket, and the temperature was controlled at
298.15 ꢂ 0.01 K with a temperature thermostat HAAKE
V26 (Thermo Electron, Germany). The cell was calibrated
by adding potassium chloride solution successively.18 All data
were corrected at 298.15 K with the conductivities of the
corresponding solvents. At the beginning of every measuring
cycle, the cell was filled with a weighed amount of the solvent.
After measurements of the solvent conductivity, the stepwise
concentration was carried out by successive additions, using a
gas-tight syringe, of weighed amounts of the IL (with or
without monosaccharide). The relative uncertainty of conduc-
tivity is about ꢂ 0.02%. For the uncertainties of density,
viscosity and conductivity, the coverage factor and the
confidence interval are 2 and 95%, respectively.
2
4
2
where VF0;CH represents the contribution per methylene group
(CH2) in the alkyl chain of the anion to the standard partial
2
molar volume of the ILs at given concentrations of mS. The
VF0;CH values obtained are included in Table 1. It can be
2
seen that VF0;CH values in glucose + water solutions are larger
2
than that in water, and are almost constant within error in the
concentration range (mS = 0.2–1.6 mol kgꢁ1).
McMillan and Mayer21 proposed a theory of solution that
permits the formal separation of the interactions between two
1
2.5 Measurements of 13C and H NMR spectra
1
The 13C and H NMR spectra were recorded using a Bruker
AV-400 MHz NMR spectrometer equipped with a 5 mm tube
at 298.15 ꢂ 0.1 K at 100.62 MHz. DSS was used as the
external reference. D2O was used for the deuterium lock.
3. Results and discussion
3.1 Apparent molar volume
Fig. 2 Limiting apparent molar volumes for the ionic liquids in water
Densities of solutions measured are listed in Table S1 in the
with the number nCH of methylene groups in the alkyl chain of the
2
ESI.w Apparent molar volumes of ionic liquids, VF,IL, were
anion.
c
14544 Phys. Chem. Chem. Phys., 2011, 13, 14542–14549
This journal is the Owner Societies 2011

View Article Online
Table 1 Limiting apparent molar volumes for the methylene group in water and in glucose + water solutions at 298.15 K, derived from eqn (4)
mS/mol kgꢁ1
0
0.2
0.4
0.8
1.2
1.6
0
14.40 ꢂ 0.11
14.57 ꢂ 0.10
14.64 ꢂ 0.09
14.65 ꢂ 0.14
14.64 ꢂ 0.19
14.71 ꢂ 0.18
V
F;CH2
or more solute molecules. According to this theory, the
transfer apparent molar volume of an ionic liquid from water
to glucose + water mixtures (DtVF,IL) can be expressed as22
Values of A, B and D were obtained by the fit of experimental
data to the equation and are given in Table 3, together with
correlation coefficients.
The A-coefficient represents the contribution from interionic
electrostatic forces. The B-coefficient represents the order or
disorder of the solvent molecules in the solvation shell of the
ions besides the size and shape of the ions. In other words, the
B-coefficient is connected with the ion–solvent interactions.24
The B-coefficients for [C4mim][HCOO], [C4mim][CH3COO]
and [C4mim][CH3CH2COO] exhibit positive values and
increase with increasing molalities of glucose. At a given
molality of glucose, the B-coefficients are in the order:
[C4mim][CH3CH2COO] 4 [C4mim][CH3COO] 4 [C4mim]-
[HCOO]. The mobility for mixed solutions reduces with
increasing sizes of groups attached to the –COOꢁ of carboxy-
late ions. Thus the B-coefficients are in the above order.
DtVF,IL = VF,IL(mIL,mS) ꢁ VF,IL(mIL
= 2uvILSmS + 3u2vILILSmILmS + 3uvILSSmS2 +. . .
(5)
)
where v is the number of ions into which the ionic liquid
dissociates, and vILS, vILILS and vILSS are pair and triplet
interaction parameters, respectively. When mIL - 0, eqn (5)
can be rearranged as follows
DtV0F,IL/mS = 2uvILS + 3uvILSSmS +. . .
(6)
These interaction parameters were obtained using a least-
squares fit, and are included in Table 2.
Table 2 shows that the vILS values are positive and in
the order: [C4mim][CH3CH2COO] 4 [C4mim][CH3COO] 4
[C4mim][HCOO]. Positive vILS values mean that the IL–Glc
interactions contribute a positive value to volume. Based
on the group additivity principle of Savage–Wood,23 the
difference in vILS values among these three ILs would be
mainly attributed to the Glc–R interactions, where R stands
for the alkyl group (CH3, CH3CH2). The contribution of the
Glc–R interactions to vILS comes from the dehydration of
hydroxyl groups of glucose molecules, which gives a positive
volume and depends on the size of R. This works out as a
reasonable interpretation of the positive vILS values and the
above-mentioned order.
3.3 Activation parameters for the viscous flow
The viscosity data can also be analyzed on the basis of
the transition-state theory of relative viscosity suggested by
Feakins et al.25,26 The transition-state is the activation state for
viscous flow.25,26 According to the theory, the B-coefficients can
be expressed as
0a
B = (V1 ꢁ V0F,IL)/1000 + (V1/1000)(Dm ꢁ Dma1 )/(RT) (9)
IL
where V1 is the molar volume of the solvent. When a binary
solvent is used, V1 is the molar volume of the mixed solvent,
which is equal in value to the mean molar mass of the mixed
solvent divided by its density. V0F,IL is the limiting apparent
3.2 Viscosity B-coefficient
The experimental viscosities for the studied systems are
collected in Table S3.w The relative viscosity, Zr, can be
analyzed using the Jones–Dole equation24
Table 3 Viscosity A-, B- and D-coefficients of the ionic liquids in
water and in glucose + water solutions at 298.15 K
mS
mol kgꢁ1
A
B
D
dm3/2 molꢁ1/2
dm3 molꢁ1
dm6 molꢁ2
Zr = Z/Z1 = 1 + Ac1/2 + Bc + Dc2
(7)
[C4mim][HCOO]
0
where Z and Z1 are, respectively, the viscosities of the ionic
liquid solutions and the solvent (water or glucose + water
mixed solvent), c (mol dmꢁ3) is the molarity of the ionic liquid,
and A, B and D are empirical constants known as the viscosity
A-, B- and D-coefficients. Eqn (7) can be rearranged as
0.0110 ꢂ 0.0117
0.496 ꢂ 0.045
0.510 ꢂ 0.019
0.538 ꢂ 0.026
0.542 ꢂ 0.018
0.548 ꢂ 0.020
0.553 ꢂ 0.023
0.161 ꢂ 0.094
0.126 ꢂ 0.055
0.094 ꢂ 0.056
0.074 ꢂ 0.042
ꢁ0.018 ꢂ 0.065
ꢁ0.004 ꢂ 0.059
0.2000
0.4000
0.8000
1.2000
1.6000
0.0057 ꢂ 0.0041
ꢁ0.0057 ꢂ 0.0066
ꢁ0.0066 ꢂ 0.0046
ꢁ0.0078 ꢂ 0.0041
ꢁ0.0077 ꢂ 0.0052
[C4mim][CH3COO]
(Zr ꢁ 1)/c1/2 = A + Bc1/2 + Dc3/2
(8)
0
0.0119 ꢂ 0.0065
0.608 ꢂ 0.029
0.621 ꢂ 0.014
0.653 ꢂ 0.078
0.672 ꢂ 0.008
0.682 ꢂ 0.024
0.708 ꢂ 0.017
0.332 ꢂ 0.080
0.324 ꢂ 0.043
0.221 ꢂ 0.023
0.218 ꢂ 0.021
0.166 ꢂ 0.070
0.090 ꢂ 0.058
0.2000
0.4000
0.8000
1.2000
1.6000
0.0129 ꢂ 0.0030
0.0014 ꢂ 0.0017
ꢁ0.0036 ꢂ 0.0018
ꢁ0.0034 ꢂ 0.0053
ꢁ0.0050 ꢂ 0.0034
Table 2 Volumetric interaction parameters for the ionic liquid +
glucose + water solutions at 298.15 K
2uvILS
3uvILSS
sa
[C4mim][CH3CH2COO]
cm3 molꢁ2 kg cm3 molꢁ3 kg2 cm3 molꢁ1
Ionic liquid
0
0.0207 ꢂ 0.0172
0.682 ꢂ 0.068
0.728 ꢂ 0.030
0.734 ꢂ 0.026
0.749 ꢂ 0.010
0.755 ꢂ 0.069
0.761 ꢂ 0.023
0.435 ꢂ 0.152
0.363 ꢂ 0.085
0.359 ꢂ 0.064
0.344 ꢂ 0.028
0.356 ꢂ 0.169
0.346 ꢂ 0.073
0.2000
0.4000
0.8000
1.2000
1.6000
0.0098 ꢂ 0.0064
0.0068 ꢂ 0.0061
0.0064 ꢂ 0.0022
0.0056 ꢂ 0.0166
0.0076 ꢂ 0.0048
[C4mim][HCOO]
[C4mim][CH3COO]
0.698 ꢂ 0.208 0.427 ꢂ 0.211 0.24
1.649 ꢂ 0.263 ꢁ0.228 ꢂ 0.267 0.31
[C4mim][CH3CH2COO] 2.268 ꢂ 0.204 ꢁ0.383 ꢂ 0.207 0.24
a
s is the standard deviation of the fit.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 14542–14549 14545

View Article Online
0a
Table 4 Activation free energies of binary glucose + water solvents Dma1 and of the ionic liquids Dm in binary glucose + water solvents at
infinite-dilution, and B1, B2 values at 298.15 K
IL
0a
IL
kJ molꢁ1
0a
IL
kJ molꢁ1
0a
mS
mol kgꢁ1
Dma1
B1
dm3 molꢁ1
m
B2
dm3 molꢁ1
Dm
B2
dm3 molꢁ1
Dm
IL
B2
dm3 molꢁ1
kJ molꢁ1
kJ molꢁ1
Glucose + Water
[C4mim][HCOO]
[C4mim][CH3COO]
[C4mim][CH3CH2COO]
0
9.2
9.4
9.7
10.2
10.8
11.3
ꢁ0.049
98.14
98.64
101.01
98.87
97.25
95.62
0.545
115.44
115.51
118.10
117.30
115.48
115.78
0.657
127.63
131.94
130.83
129.08
126.39
124.02
0.731
0.780
0.789
0.810
0.822
0.835
0.2000
0.4000
0.8000
1.2000
1.6000
ꢁ0.052
ꢁ0.055
ꢁ0.061
ꢁ0.067
ꢁ0.074
0.562
0.593
0.603
0.615
0.627
0.673
0.708
0.733
0.749
0.782
molar volume of the ionic liquid. Dma1 is the free energy of
activation per mole of solvent, and is given by
Dma1 = RTln(Z1V1/hNA)
(10)
where h is the Planck constant, NA is the Avogadro constant,
0a
IL
Z1 is the viscosity of the solvent, and Dm is the contribution
per mole of solute (ionic liquid) to the free energy of activation
for viscous flow of the solution at infinite dilution. Hence,
eqn (10) can be rearranged as follows
0a
Dm = Dma1 + (RT/V1)[1000B ꢁ (V1 ꢁ V0F,IL)] (11)
IL
0a
IL
Using eqn (11), values of Dm
in water and in glucose +
water mixed solvents were calculated and are given in Table 4.
0a
IL
The positive Dm
values suggest that the formation of the
transition state is less favored in the presence of ions and
accompanied by the breaking of attractive interactions
between ions and solvents. The addition of glucose strengthens
Fig. 3 B1 and B2 values for the ionic liquid + glucose + water
systems as a function of the molality mS of glucose at 298.15 K.
the interactions of ions with the mixed solvent. It is also found
0a
IL
that the Dm values are in the order: [C4mim][CH3CH2COO] 4
[C4mim][CH3COO]
4
[C4mim][HCOO], meaning that
3.4 Limiting molar conductivity
it is more difficult to form the transition-state for
[C4mim][CH3CH2COO] than for [C4mim][CH3COO] and
[C4mim][HCOO]. This can be ascribed to the stronger
solvation effect of [C4mim][CH3CH2COO].
The molar conductivities of the ionic liquids in the water +
glucose mixtures measured are collected in Table S4.w The
experimental data were analyzed using the Lee-Wheaton
conductivity equation29,30 in the form suggested by Pethybridge
Rearrangement of eqn (9) yields27,28
and Taba,31 and the following set of equations was used:
!
ꢀ
ꢁ
VF0;IL
V1
Dma1 V1
1000 1000RT
V1Dm0ILa
ꢂ
B ¼
B1 ¼
B2 ¼
ꢁ
þ
ꢁ
1000RT 1000
ð12aÞ
ð12bÞ
ð12cÞ
2
3
L ¼ g L0½1 þ C1ðbkÞ þ C2ðbkÞ þ C3ðbkÞ ꢃ
ð13Þ
ꢃ
ꢄꢅ
ꢀ
ꢁ
Dma1 V1
1000 1000RT
rk
ð1 þ kRÞ
kR
12
V1
2
ꢁ
1 þ C4ðbkÞ þ C5ðbkÞ þ
ꢁ
!
1 ꢁ g
cg2fꢂ2
VF0;IL
V1Dm0ILa
1000RT 1000
KA
¼
ð14Þ
ꢁ
f2 = exp[ꢁ(kb)/(1 + kR)]
(15)
(16)
ꢂ
where B1 and B2 are the contributions of the solvent properties
(V1 and Dma1 ) and the ionic liquid–solvent interactions to
the B value (listed in Table 4), respectively. Fig. 3 shows that
the B1 values are relatively small and negative and they get
more negative with increasing mS. The B2 values are much
larger than B1 and positive, and increase with increasing mS.
Obviously, the contributions of ionic liquid–solvent inter-
actions are dominant. The B2 values are also in the order:
[C4mim][CH3CH2COO] 4 [C4mim][CH3COO] 4 [C4mim]-
[HCOO], confirming that the interactions between the ionic
liquids and the solvent (water + glucose) are in the
above order.
b = z2e2/(DkT)
!
1=2
2
8pNe2jzj gc
k ¼
ð17Þ
1000DkT
r = |z|Fe/(299.79 ꢀ 3pZ)
where C1–C5 are complex functions of (KA and R), L0 is the
limiting molar conductivity, KA is the association constant,
(18)
R is the ion distance parameter, f is the mean ionic activity
ꢂ
coefficient, and k is the Debye parameter. The other symbols
c
14546 Phys. Chem. Chem. Phys., 2011, 13, 14542–14549
This journal is the Owner Societies 2011

View Article Online
have their usual meanings. Three-parameter fits were used to
obtain the limiting molar conductivity L0, the association
constant KA, and the ion distance parameter R by non-linear
least squares iterations.
of the nature of the solvent, provided that the radius of the ion
remains unchanged. The Walden products increase as mS
increases, indicating that these ions do not have the same
effective radius in different solvent compositions, and conse-
quently providing evidence for the solvation of ions in
solution. Similar phenomena were observed in other mixtures
of water with organic co-solvents.37,38
The lower limit a of the association integral is the distance of
closest approach of cation and anion (contact distance)
a = a+ + a
(19)
ꢁ
1
3.5 13C and H NMR spectra
where a+ and a are the radii of the cation and anion,
ꢁ
respectively. Their values were taken from the literature:
a+([C4mim]+) = 330 pm,32 a ([HCOO]ꢁ) = 169 pm,33
In order to deeply understand the difference in microscopic
information for the studied systems, we performed NMR
measurements for two cases (i) the addition of methyl
ꢁ
a ([CH3COO]ꢁ) = 162 pm.33 The densities of [C4mim][CH3COO]
ꢁ
and [C4mim][CH3CH2COO] were measured to be 1.05223 and
1.03491 g cmꢁ3 at 298.15 K, respectively. Using these density
b-D-glucopyranoside to ionic liquid + D2O solutions
([C4mim][HCOO], [C4mim][CH3COO], [C4mim][CH3CH2COO];
mIL = 0.2 mol kgꢁ1) and (ii) the addition of ionic liquids
data, values of a for [CH3COO]ꢁ and [CH3CH2COO]ꢁ can
ꢁ
be calculated using the following equation:
to methyl b-D-glucopyranoside + D2O mixtures (mS
=
Vm = (2a+)3N+ + (2a )3N
(20)
0.4 mol kgꢁ1). The 13C and H NMR chemical shifts for the
above different cases are given in Tables S5–S8w according
to the numbering marked in Scheme 1. For convenience of
comparison, we also calculated the Dd values: Dd = dobs ꢁ d0,
where dobs and d0 are the measured chemical shifts of the
solute in the mixed solvent and in pure solvent (D2O). The
values are also collected in Tables S9–S12,w respectively.
For the cation [C4mim]+ of the ILs, all of the Dd values of
C and H are relatively small, meaning that the interactions
between the cation and glucopyranoside are very weak. It is
worth noting that the ratios (Dd/d) of C are nearly invariable
with increasing mS, whereas the ratios (Dd/d) of H are slightly
larger than those of C. This is ascribed to the fact that the H
atoms are on the outside of the imidazolium ring, which makes
it easier for them to interact with other molecules compared
with internal C atoms.
1
ꢁ
ꢁ
where Vm is the molar volume, calculated from the density and
molar mass of IL, N+ and N are the number of cation and
ꢁ
anion per mole of IL, respectively. The resulting values
are a ([CH3COO]ꢁ) = 147 pm and a ([CH3CH2COO]ꢁ) =
ꢁ
ꢁ
188 pm. The a ([CH3COO]ꢁ) value is in good agreement with
ꢁ
in the literature value.33
From extended investigations on ionic liquid solutions in
amphiprotic hydroxylic solvents (water, alcohols), the upper
limit of association can be given by an expression of the type34
R = a + ns
(21)
where s is the length of an oriented solvent molecule, n is an
integer, n = 0, 1, 2,. . .. In this work, s is the length of an
OH-group (dOH) and s = dOH = 280 pm.34 The limiting molar
conductivity L0 and the association constant KA were
obtained by fitting at R = a + s, and summarized in Table 5.
The L0 values for [C4mim][HCOO], [C4mim][CH3COO] and
[C4mim][CH3CH2COO] decrease with the increase of mS
(Fig. 4). This can be ascribed to the facts that (i) the associa-
tion of the ionic liquids and glucose strengthens; and (ii) the
microscopic viscosity of the mixed solutions increases with
increasing mS, and then reduces the ionic mobility.
For the chemical shifts of the protons in the imidazolium
ring, it was observed that the chemical shift of H2 is more
sensitive to the addition of glucopyranoside, compared with
those of H4 and H5. As we know, H2 in the imidazolium ring
has greater acidity, and can form hydrogen bonds with water
molecules. The addition of glucopyranoside into the solution
also leads to the formation of hydrogen bonds between
glucopyranoside and water molecules, and meanwhile
weakens the hydrogen bonds between H2 in the imidazolium
ring and water molecules. Therefore, the chemical shifts of C2
and H2 shift highfield more evidently than those of C4, C5,
H4, and H5. In other words, the interactions between the ILs
and glucose would be mainly determined by their dehydra-
tions/hydrations in the processes.
For a given molality of glucose, the limiting molar
conductivities are in the order: [C4mim][HCOO]
4
[C4mim][CH3COO] 4 [C4mim][CH3CH2COO]. These three
carboxylate ions have the same –COOꢁ group that can form
strong hydrogen bonds with water/glucose molecules; their
differences in L0 values are chiefly attributed to their atom/
groups attached to the –COOꢁ group. Since CH3CH2
4
For the anions of the ILs, the chemical shifts d of C11 for
CH3 4 H, the mobility of these three ILs should have
the above-mentioned order: [C4mim][HCOO] 4 [C4mim]-
[CH3COO] 4 [C4mim][CH3CH2COO].
the carboxylate ions are in the order: [CH3CH2COO]ꢁ
4
[CH3COO]ꢁ 4 [HCOO]ꢁ. After the addition of glucopyrano-
side to the solution, the absolute values Dd of C11 are the
largest of all of the C atoms (including cation and anions) and
the order of d remains the same. This should be attributed to
the decrease of the hydrogen bonds between the carboxylate
ions and water molecules with the addition of glucopyrano-
side. Youngs et al.39 and Remsing et al.40 studied the inter-
action of carbohydrates in pure IL. The results showed that
the solvation process of carbohydrates is governed by
the interactions between the IL anion and the carbohydrate.
To eliminate the influence of macroscopic viscosity on the
ionic mobility, the viscosities of the mixtures were determined
using the method reported in the literature.35 The values of the
Walden product (L0Z0) were calculated and are also given in
Table 5. An increase in viscosity leads to a decrease in
conductivity. This effect was formulated quantitatively by
the Walden rule,36 which states that the product (L0Z0) should
be approximately constant for a given ionic liquid, irrespective
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 14542–14549 14547

View Article Online
Table 5 Values of L0, KA and the Walden product (L0Z0) for the ionic liquids in water and in binary water + glucose mixtures at 298.15 K
mS mol kgꢁ1
L0 S cm2 molꢁ1
KA
L0Z0 S cm2 molꢁ1 mPa s
[C4mim][HCOO]
0
0.2000
0.4000
0.6000
0.8000
1.0000
1.2000
[C4mim][CH3COO]
0
0.2000
0.4000
0.6000
0.8000
1.0000
1.2000
[C4mim][CH3CH2COO]
0
0.2000
0.4000
0.6000
86.78 ꢂ 0.02
79.99 ꢂ 0.02
74.28 ꢂ 0.02
68.43 ꢂ 0.03
66.43 ꢂ 0.04
58.99 ꢂ 0.01
55.10 ꢂ 0.02
2.25 ꢂ 0.18
77.27
77.91
79.18
79.79
84.57
81.70
82.71
0.47 ꢂ 0.21
1.54 ꢂ 0.21
ꢁ0.10 ꢂ 0.31
3.72 ꢂ 0.42
0.30 ꢂ 0.13
1.39 ꢂ 0.30
72.25 ꢂ 0.02
66.69 ꢂ 0.02
61.71 ꢂ 0.01
56.99 ꢂ 0.03
54.68 ꢂ 0.03
48.86 ꢂ 0.03
45.34 ꢂ 0.02
0.37 ꢂ 0.20
0.43 ꢂ 0.19
0.88 ꢂ 0.12
ꢁ0.57 ꢂ 0.31
0.75 ꢂ 0.31
ꢁ0.87 ꢂ 0.35
ꢁ0.26 ꢂ 0.22
64.33
64.96
65.78
66.45
69.61
67.67
68.06
66.89 ꢂ 0.04
61.84 ꢂ 0.03
57.17 ꢂ 0.02
53.59 ꢂ 0.02
50.49 ꢂ 0.03
45.21 ꢂ 0.03
41.93 ꢂ 0.03
ꢁ0.24 ꢂ 0.37
0.04 ꢂ 0.32
59.56
60.23
60.94
62.49
64.27
62.62
62.94
ꢁ0.58 ꢂ 0.20
3.63 ꢂ 0.18
0.8000
1.0000
1.2000
ꢁ0.65 ꢂ 0.38
ꢁ0.74 ꢂ 0.28
ꢁ0.90 ꢂ 0.34
the H2 of the imidazolium ring has more activity (acidity) than
the H4 and H5 atoms.
4. Conclusion
In this work, we have focused on the interaction characteristics
of glucose, as a model biological molecule, with 1-butyl-3-
methylimidazolium carboxylate ionic liquids. The macro-
parameters calculated from the basic physico-chemical
properties, along with the micro-information obtained from
the 13C and 1H NMR spectra, give us a more complete
explanation for the interactions of the ILs with glucose.
For a given molality of glucose, the differences in the macro-
properties of these ILs depend mainly on the sizes of groups
attached to the –COOꢁ of the carboxylate ions. Analyses
of NMR results revealed that no specific and stronger inter-
actions were observed between glucopyranoside and the ILs,
however the results confirmed that the H2 on the imidazolium
ring has more activity (acidity) than the H4 and H5 atoms.
Furthermore, the interaction of the IL anion with glucose is
stronger than that of the IL cation. The contributions of the
interactions between the ILs and glucose to macro-properties
should result mainly from their hydrations/dehydrations in
the interaction processes. This study gives a deep insight to
IL + biomolecule mixed systems, especially providing theore-
tical foundations and important data for studies on the
potential toxicity of ILs. Further studies on this kind of mixed
systems are in progress.
Fig. 4 The variation in the limiting molar conductivities L0 of the
ionic liquids with the molalities mS of glucose.
For our systems, the interaction between the IL anion and
glucose is stronger than that between the IL cation and
glucose, which is consistent with the results reported in the
literature.39,40
Similar to the ILs, all of the Dd values of C and H for
glucopyranoside are relatively small, especially the d values of
C, which are nearly invariable as the IL molality increases.
This indicates that the interactions take place mainly between
the OH groups of glucopyranoside and the ILs or water,
whereas the interactions of other atoms or groups on gluco-
pyranoside molecules with the ILs or water should be
quite weak.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (No. 20973055) and the Innovation Foundation
of Colleges and Universities of Henan Province are gratefully
acknowledged.
Summarily, no specific and strong interactions were
observed between glucopyranoside and the ILs from the
chemical shifts recorded. But the results also confirmed that
c
14548 Phys. Chem. Chem. Phys., 2011, 13, 14542–14549
This journal is the Owner Societies 2011

View Article Online
´
20 M. Gutierrez-Pichel, P. Taboada, L. M. Varela, D. Attwood and
V. Mosquera, Langmuir, 2002, 18, 3650.
References
1 J. Dupont, R. F. De Souza and P. A. Z. Suarez, Chem. Rev., 2002,
102, 3667.
2 M. U. Araos and G. G. Warr, Langmuir, 2008, 24, 9354.
3 Y. He, Z. Li, P. Simone and T. P. Lodge, J. Am. Chem. Soc., 2006,
128, 2745.
4 J. Eastoe, S. Gold, S. E. Rogers, A. Paul, T. Welton, R. K. Heenan
and I. Grillo, J. Am. Chem. Soc., 2005, 127, 7302.
5 F. Endres and S. Z. EI Abedin, Phys. Chem. Chem. Phys., 2006,
8, 2101.
21 W. G. McMillan and J. E. Mayer, J. Chem. Phys., 1945, 13, 276.
22 G. Perron, D. Joly and J. E. Desnoyers, et al., Can. J. Chem., 1978,
56, 552.
23 J. J. Savage and R. H. Wood, J. Solution Chem., 1976, 5, 733.
24 G. Jones and M. Dole, J. Am. Chem. Soc., 1929, 51, 2950.
25 D. Feakins, W. E. Waghorne and K. G. Lawrence, J. Chem. Soc.,
Faraday Trans. 1, 1986, 82, 563.
26 D. Feakins, D. J. Freemantle and K. G. Lawrence, J. Chem. Soc.,
Faraday Trans. 1, 1974, 70, 795.
6 (a) M. J. Rosen, F. Li, S. W. Morrall and D. J. Versteg, Environ.
Sci. Technol., 2001, 35, 954; (b) A. S. Wells and V. T. Coombe,
Org. Process Res. Dev., 2006, 10, 794.
7 P. Luis, I. Ortiz, R. Aldaco and A. Irabien, Ecotoxicol. Environ.
Saf., 2007, 67, 423–429.
8 (a) A. Pinkert, K. N. Marsh, S. s. Pang and M. P. Staiger, Chem.
Rev., 2009, 109, 6712; (b) H. Zhao, G. A. Baker, Z. Y. Song,
O. Olubajo, T. Crittle and D. Peters, Green Chem., 2008, 10, 696;
(c) J. M. Zhang, H. Zhang, J. Wu, J. Zhang, J. S. He and
J. F. Xiang, Phys. Chem. Chem. Phys., 2010, 12, 1941.
9 A. R. Xu, J. J. Wang and H. Y. Wang, Green Chem., 2010, 12, 268.
10 Y. Fukaya, A. Sugimoto and H. Ohno, Biomacromolecules, 2006,
7, 3295.
27 K. L. Zhuo, H. X. Liu, X. K. Zhang, Y. H. Liu and J. J. Wang,
Carbohydr. Res., 2008, 343, 2428.
28 K. L. Zhuo, H. X. Liu, J. M. Tang, Y. J. Chen and J. J. Wang,
J. Phys. Chem. B, 2009, 113, 13638.
29 W. H. Lee and R. J. Wheaton, J. Chem. Soc., Faraday Trans. 2,
1978, 74, 743.
30 W. H. Lee and R. J. Wheaton, J. Chem. Soc., Faraday Trans. 2,
1978, 74, 1456.
31 A. D. Pethybridge and S. S. Taba, J. Chem. Soc., Faraday Trans. 1,
1980, 76, 368.
32 H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
M. Watanabe, J. Phys. Chem. B, 2004, 108, 16593.
33 Y. Marcus, J. Chem. Soc., Faraday Trans., 1991, 87(18), 2995.
34 M. Bester Rogac, V. Babic, T. M. Perger, R. Neueder and
J. Barthel, J. Mol. Liq., 2005, 118, 111.
11 K. L. Zhuo, J. J. Wang, Y. K. Yue and H. Q. Wang, Carbohydr.
Res., 2000, 328, 383.
12 K. L. Zhuo, J. J. Wang, H. Q. Wang and Y. K. Yue, Z. Phys.
Chem., 2001, 215, 561.
35 C. W. Zhao, P. S. Ma and J. D. Li, J. Chem. Thermodyn., 2005,
37, 37.
13 K. L. Zhuo, J. J. Wang, H. H. Zheng, X. P. Xuan and Y. Zhao,
J. Solution Chem., 2005, 34, 155.
36 R. H. Robinson and R. H. Stokes, Electrolyte Solutions,
Butterworth, London, 1955, p. 125.
14 G. S. Kell, J. Chem. Eng. Data, 1975, 20, 97.
15 R. D. Weir, J. Chem. Thermodyn., 1996, 28, 261.
16 C. J. James, D. E. Mulcahy and B. J. Steel, J. Phys. D: Appl. Phys.,
1984, 17, 225.
37 J. Gregorowicz, A. Bald, A. Szejgis and A. Chmielewska, J. Mol.
Liq., 2000, 84, 149.
38 J. Gregorowicz, A. Bald, A. Szejgis and M. Zurada, J. Mol. Liq.,
1999, 79, 167.
17 Z. N. Yan, J. J. Wang and J. S. Lu, Biophys. Chem., 2002, 99, 199.
18 J. Barthel, F. Feuerlein, R. Neueder and R. Wachter, J. Solution
Chem., 1980, 9, 209.
19 C. De. Visser, G. Perron and J. E. Desnoyers, J. Am. Chem. Soc.,
1977, 99, 5894.
39 T. G. A. Youngs, C. Hardacre and J. D. Holrey, J. Phys. Chem. B,
2007, 111, 13765.
40 R. C. Remsing, G. Hernandez, R. P. Swatloski, W. W. Massefski,
R. D. Rogers and G. Moyna, J. Phys. Chem. B, 2008, 112,
11071.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 14542–14549 14549