Temperature dependence measurements and structural characterization of trimethyl ammonium ionic liquids with a highly polar solvent

Article abstract of DOI:10.1021/jp2059084

We report the synthesis and characterization of a series of an ammonium ionic liquids (ILs) containing acetate, dihydrogen phosphate, and hydrogen sulfate anions with a common cation. To characterize the thermophysical properties of these newly synthesized ILs with the highly polar solvent N,N-dimethylformamide (DMF), precise measurements such as densities (ρ) and ultrasonic sound velocities (u) over the whole composition range have been performed at atmospheric pressure and over wide temperature ranges (25 - 50 °C). The excess molar volume (V^E) and the deviation in isentropic compressibilities (ΔK_s) were predicted using these temperature dependence properties as a function of the concentration of ILs. The Redlich - Kister polynomial was used to correlate the results. The ILs investigated in the present study included trimethylammonium acetate [(CH₃) ₃NH][CH₃COO] (TMAA), trimethylammonium dihydrogen phosphate [(CH₃)₃NH][H₂PO₄] (TMAP), and trimethylammonium hydrogen sulfate [(CH₃)₃NH][HSO ₄] (TMAS). The intermolecular interactions and structural effects were analyzed on the basis of the measured and the derived properties. In addition, the hydrogen bonding between ILs and DMF has been demonstrated using semiempirical calculations with help of Hyperchem 7. A qualitative analysis of the results is discussed in terms of the ion - dipole, ion-pair interactions, and hydrogen bonding between ILs and DMF molecules and their structural factors. The influence of the anion of the protic IL, namely, acetate (CH ₃COO), dihydrogen phosphate (H₂PO₄), and hydrogen sulfate (HSO₄), on the thermophysical properties is also provided.

Full text of DOI:10.1021/jp2059084

ARTICLE
pubs.acs.org/JPCB
Temperature Dependence Measurements and Structural
Characterization of Trimethyl Ammonium Ionic Liquids with
a Highly Polar Solvent
Pankaj Attri,† Pannuru Venkatesu,*,† and T. Hofman‡
†Department of Chemistry, University of Delhi, Delhi 110 007, India
‡Division of Physical Chemistry, Warsaw, University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland
S Supporting Information
b
ABSTRACT: We report the synthesis and characterization of a series
of an ammonium ionic liquids (ILs) containing acetate, dihydrogen
phosphate, and hydrogen sulfate anions with a common cation. To
characterize the thermophysical properties of these newly synthe-
sized ILs with the highly polar solvent N,N-dimethylformamide
(DMF), precise measurements such as densities (F) and ultrasonic
sound velocities (u) over the whole composition range have been
performed at atmospheric pressure and over wide temperature ranges
(25ꢀ50 °C). The excess molar volume (VE) and the deviation in
isentropic compressibilities (Δks) were predicted using these tem-
perature dependence properties as a function of the concentration of
ILs. The RedlichꢀKister polynomial was used to correlate the results.
The ILs investigated in the present study included trimethylammo-
nium acetate [(CH3)3NH][CH3COO] (TMAA), trimethylammonium dihydrogen phosphate [(CH3)3NH][H2PO4] (TMAP),
and trimethylammonium hydrogen sulfate [(CH3)3NH][HSO4] (TMAS). The intermolecular interactions and structural effects
were analyzed on the basis of the measured and the derived properties. In addition, the hydrogen bonding between ILs and DMF has
been demonstrated using semiempirical calculations with help of Hyperchem 7. A qualitative analysis of the results is discussed in
terms of the ionꢀdipole, ion-pair interactions, and hydrogen bonding between ILs and DMF molecules and their structural factors.
The influence of the anion of the protic IL, namely, acetate (CH3COO), dihydrogen phosphate (H2PO4), and hydrogen sulfate
(HSO4), on the thermophysical properties is also provided.
’ INTRODUCTION
The right choice of the cationꢀanion pair allows the modula-
tion of physicochemical properties, such as density and ultrasonic
sound velocity as well as structural properties of the solvent
media, which is markedly controlled both by cationꢀanion and
ionꢀsolvent interaction.21 The chemical complexity of the
combination of ions of ILs involved in the liquid structure exhibit
challenges for theoreticians and chemists to understand the
thermophysical properties for mixed solvents of ILs with organic
molecular liquids. Studies on thermophysical properties of the
pure ILs and their mixtures with polar compounds have shown
that the choice of anion or cation has strongly influenced on the
molecular characteristics of the different species of ILs.15ꢀ17,22
To expand basic needs for scientific research and gain some
insight into the several aggregations of molecular interactions of
ILs with highly polar compounds, we have synthesized different
combinations of anions with an ammonium cation of new ILs for
our study because this novel kind of ILs appears to be promising
for various chemical industrial applications.
The design and synthesis of ionic liquids (ILs) have been one
of the ways to accelerate the pace of ILs in large-scale industrial
applications and prevent environmental hazards. ILs are per-
forming more promising perspectives in the various fields such as
organic synthesis,1 catalysis or biocatalysis,2ꢀ4 materials science,5
electrochemistry,6 separation technology,7,8 and preserving the
protein stability9,10 due to their unique physicochemical proper-
ties at the laboratory level and even at the industrial scale.11,12
The significant attention that in the past decade the scientific
community has shown toward mainly due to the great versatility
in cationꢀanion combinations and favorable unique properties
such as high ionic mobility, nonflammability, high potential for
recycling, and extremely low vapor pressure.13ꢀ17 For these
reasons, they rapidly gained interest as greener replacements
for traditional volatile organic solvents (VOCs). Moreover, ILs
have become much popular because of the intensified ion
densities, good ionic conductivities, wide electrochemical win-
dow, and good thermal stability, and ILs are used as green
solvents for biochemistry, polymer chemistry, catalysis reactions,
chemical synthesis, and separation science.9,10,16ꢀ20
Received: June 23, 2011
Revised:
July 21, 2011
Published: July 22, 2011
r
2011 American Chemical Society
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Figure 1. Schematic structures for (a) DMF, (b) TMAA, (c) TMAP, and (d) TMAS. Color representation as green = carbon, white = hydrogen, red =
oxygen, and blue = nitrogen.
Thermophysical properties of mixed solvents of ILs with
organic molecular liquids are paramount for the design of
many technological processes and required for any practical
applications.23ꢀ28 Moreover, experimental data of thermody-
namic and thermophysical properties of liquids and liquid
mixtures are fascinating and of high fundamental and practical
importance for the industry. Although a qualitative connection
between the macroscopic and microscopic features is feasible,
quantitative conclusions are of interest to both academic and
industry communities. In spite of the importance of properties of
ILs in different solvent media, only a small amount of physico-
chemical data is available in the literature, which mainly char-
acterizes ILs.29ꢀ34 In this context, our aim is to explore closely
two key thermophysical properties such as density (F) and
ultrasonic sound velocity (u) for the mixed solvents of ILs and
polar solvent. Up to now, there is no systematic documentation
for the trimethyl ammonium ILs with polar solvents.
obtain information on protein systems. The resonance structure
of DMF is shown below.
In DMF, the presence of two electrons repelling ꢀCH3
groups make the lone pair at nitrogen still more perceptible
toward donation.43,44
Thus, it may be argued that the DMF is
actually the donor of nitrogen electron pairs. The schematic
chemical structures of the DMF as well as ILs are shown in
Figure 1.
N,N-Dimethylformamide (DMF) is a polar solvent with a high
boiling point and is used widely in a variety of industrial
processes, including manufacturing of synthetic fibers, leathers,
films, and surface coatings.35ꢀ37 It is also used in the manufactur-
ing of solvent dyes as an important raw material as well as a
solvent in peptide coupling for pharmaceuticals, in the develop-
ment and production of pesticides. DMF is a stable compound
with a strong electron pair donating and accepting ability and is
widely used in studies on solvent reactivity relationships.38ꢀ41
DMF is of particular interest because any significant structural
effects are absent due to the lack of hydrogen bonds. Therefore, it
may be used as an aprotic protophilic solvent with a large dipole
moment (μ) of 3.24 D, a high dielectric constant (ε) of 36.71 at
25 °C,42 and good donorꢀacceptor properties, which enable it to
dissolve a wide range of both organic and inorganic substances. In
addition, DMF can serve as a model compound for peptides to
In view of the wide scope of molecular interactions between
DMF with our newly synthesized ammonium ILs, it is essential to
study the thermophysical properties to obtain deeper insight into
the knowledge of molecular interactions of binary mixtures.
Therefore, the present work main pursues the following: (i) to
analyze the anion effect over wide ranges of temperature, (ii) to
bring in molecular and structural information from an anion/
cation features based on the temperature dependence properties
of F and u as well as derived properties of excess molar
volume (VE) and deviation of isentropic compressibility (Δks),
and (iii) to infer the theoretical calculation of the hydrogen
bonding between ILs and DMF. We report here the synthesis of
trimethyl ammonium ILs, mainly trimethylammonium acetate
[(CH3)3NH][CH3COO] (TMAA), trimethylammonium dihy-
drogen phosphate [(CH3)3NH][H2PO4] (TMAP), and tri-
methylammonium hydrogen sulfate [(CH3)3NH][HSO4]
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Scheme 1. Synthesis of Trimethylammonium Ionic Liquids
Table 1. Solvent Purity, Molecular Weight (MW), Density
(G), and Ultrasonic Sound Velocity (u) for the Solvents such
as ILs and DMF at Various 25 °C and at Atmospheric Pressure
MW
(g molꢀ1
F
u
solvent
)
purity
(g cmꢀ3
)
(m sꢀ1
)
3
3
3
DMF
73.05
119.05
157.02
157.01
99%
99%
99%
99%
0.94385
1.05385
1.35361
1.35361
1456
1544
1672
1672
TMAA
TMAP
TMAS
(TMAS). This study intends to draw molecular level information
from the macroscopic properties on the molecular interaction
between ILs and DMF.
’ METHODS
1. Density Measurements. The density measurements of
pure compounds and ILs + DMF were performed with an
’ EXPERIMENTAL SECTION
Anton-Paar DMA 4500
M vibrating-tube densimeter,
equipped with a built-in solid-state thermostat and a resident
1. Materials. DMF (Aldrich, purity >99.9%) and trimethyla-
mine were purchased from Sigma Chemical Company. The
fluids were degassed with ultrasound, kept in the dark over Fluka
0.3 nm molecular sieves for several days, and purified by
fractional distillation. The purity of the chemical products was
verified by measuring the densities (F), refractive indices (n), and
sound velocity (u), which were in good agreement with the
literature values.42 The purities of the samples were further
confirmed by the gasꢀliquid chromatography (GLC) single
sharp peaks. All ILs were synthesized45,46 in our laboratory, as
given below.
program with accuracy of temperature of (0.03 °C. Typically,
density precisions are 0.00005 g cmꢀ3. The instrument was
3
calibrated once a day with double-distilled, deionized water
and with air as standards. The excess molar volumes (VE)
((0.003 cm3 molꢀ1) of ILs with DMF systems over the IL
3
concentration range at different temperatures have been
deduced from the densities of the pure compounds and
mixture (Fm) using the standard equations.
2. Ultrasonic Sound Velocity Measurements. Ultrasonic
sound velocities were measured by a single crystal ultrasonic
interferometer (model F-05) from Mittal Enterprises, New
Delhi, India, at a 2 MHz frequency at various temperatures.
Prior to measurements, the interferometer was calibrated at
25 °C with double-distilled water and purified methanol. A
thermostatically controlled, well-stirred circulated water bath
with a temperature controlled to (0.01 °C was used for all of
the ultrasonic sound velocity measurements for pure compo-
nents and mixtures. The uncertainty of sound velocity is 0.02%.
3. NMR Measurements. The purity of ILs was proved by 1H
NMR analysis. 1H (400 MHz) spectra were recorded on a JEOL
400 NMR spectrometer in DMSO-d6.
2. Preparation of an Ammonium ILs. All ammonium ILs
used in this work, namely, TMAA, TMAP, and TMAS, of general
type [amine][anion] were synthesized, which is shown schema-
tically in Scheme 1.
3. Synthesis of Trimethylammonium Acetate (TMAA). The
synthesis of TMAA was carried out in a 250 mL round-bottomed
flask, which was immersed in a water bath and fitted with a reflux
condenser. Acetic acid (1 mol) was added dropwise to a
trimethylamine (1 mol) at 70 °C for 1 h. The reaction mixture
was heated at 80 °C under vigorous stirring for 2 h to ensure that
the reaction had proceeded to completion. The reaction mixture
was then dried at 80 °C until the weight of the residue remained
constant. The sample was analyzed by Karl Fisher titration
revealed very low levels of water (below 70 ppm). The yield of
TMAA was 98%. 1H NMR (DMSO-d6): δ (ppm) 2.51 (s, 1H),
2.10 (s, 3H). HRMS calculated for C5H13NO2 119.09, found
119.05.
4. Synthesis of Trimethylammonium Dihydrogen Phos-
phate (TMAP). The similar procedure as that described above for
TMAA was followed with the exception of the use of phosphoric
acid [anion] instead of acetic acid. The yield of TMAP was 97%.
1H NMR (DMSO-d6): δ (ppm) 2.51 (s, 9H), 2.74 (s, 1H).
HRMS calculated for C3H12NO4P 157.05, found 157.02. Melt-
ing point: 116 °C.
5. Synthesis of Trimethylammonium Hydrogen Sulfate
(TMAS). The similar procedure as that delineated above for
TMAA was followed with the exception of the use of sulfuric acid
[anion] instead of acetic acid. The yield of TMAS was 98%. 1H
NMR (DMSO-d6): δ (ppm) 2.52 (s, 9H), 2.75 (s, 1H). HRMS
calculated for C3H11NO4S 157.04, found 157.01. Melting point:
129 °C.
The molecular weight, normal purity, density, and ultrasonic
sound velocities of ILs and DMF, which are studied in the
present work, are summarized in Table 1.
Clear solutions were prepared gravimetrically using a Mettler
Toledo balance with a precision of (0.0001 g. The uncertainty in
solution composition expressed in mole fraction was found to be
less than 5 10ꢀ4. The mixing of the two components was
3
promoted by the movement of a small glass sphere (inserted in
the vial prior to the addition of the ILs) as the flask was slowly and
repeatedly inverted. After mixing the sample, the bubble-free
homogeneous sample was transferred into the U-tube of the
densimeter or the sample cell of ultrasonic interferometer
through a medical syringe.
4. Hydrogen Bonding through Simulation Program. The
structures of ILs and of DMF were optimized based on molecular
mechanics and semiempirical calculations using the HyperChem
7 Molecular Visualization and Simulation program.47 Geometry
optimizations based on molecular mechanics (using the MM+
force field) and semiempirical calculations were used to find the
coordinates of molecular structures that represent a potential
energy minimum. For geometry optimization using both molec-
ular mechanics and semiempirical calculations, the Polakꢀ
Ribiere routine with a root-mean-square (rms) gradient of 0.01
as the termination condition was used. The minimum distance
between solvent molecules and solute atoms was set at 2.3 Å.
Molecular dynamics calculations were used to obtain a lower
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Figure 2. Temperature dependence of densities for the mixtures of ILs with DMF vs mole fraction of IL x1 for (a) TMAA {(x1) 25 °C (O), (x1) 30 °C
(4), (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)},(b) TMAP {(x1) 25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0), (x1)
40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)}, (c) TMAS {(x1) 25 °C (O), (x1) 30 °C (4), or (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C
(2), or (x1) 50 °C (9) + DMF (x2)} at various compositions and at atmospheric pressure. The solid line represents the smoothness of these data.
energy minimum by enabling molecules to cross potential
barriers.48,49 For the structures of ILs optimized using semiem-
pirical calculations, single point calculations were carried out to
determine the total energies and heats of formation.
that the both F and u values reflect the structural properties of
liquids and packing factors of the system. A careful examination
of Figure 2 reveals that the acetate IL system shows lower
densities than HSO4 or H2PO4 anion IL systems. The HSO4
and H2PO4 anion systems display the highest density values due
to increased the size of the anion. For the sake of clarity and
comparison between three ILs, the F and u values of IL + DMF
over the whole composition range at 25 °C have presented in
Figure 4.
Usually, the densities of the ILs containing the same cation
increase with increasing molecular weight of the associated
anion.51 Fredlake and co-workers52 observed that this behavior
is quite consistent for anions and pointed out that anions are
small enough to easily occupy a position closer to the relatively
large cation. The difference in density between IL (different
anions) systems can be explained by the different sizes of the
associated with same cation. In this context, the results in Figure 4
clearly show that the density values of the TMAS + DMF
mixtures are higher, when compared to the mixtures of the
TMAP + DMF and TMAA + DMF. However, the TMAS + DMF
and TMAP + DMF mixtures have almost same density from the
mole fraction of IL 0 to ≈0.2000. The lower densities of TMAA +
DMF than other two systems are due to weaker intermolecular interac-
tions of the relatively larger TMAS or TMAP with DMF. The large
size of the (dihydrogen phosphate ≈ hydrogen sulfate > acetate)
’ RESULTS AND DISCUSSION
Our experimental F and u values for a series of trimethyl
ammonium ILs with DMF were obtained at atmospheric pres-
sure for the whole concentration range at temperatures from 25
to 50 °C in steps of 5 °C. The values of F and u are presented in
Table 1 (Supporting Information), for ILs, DMF, and their
mixtures as a function of IL concentration. Virtually, ILs are
miscible with medium- to high-dielectric liquids and immiscible
with low dielectric liquids.50 In the present study, all ILs are
completely miscible in DMF (ε = 36.71 at 25 °C),42 since DMF is
a high dielectric liquid. It is seen that the F or u (up to ≈0.4000
mole fraction of IL, later the u values decrease) of the mixtures
increase with increasing concentrations of the IL in DMF, as
shown in Figures 2 or 3, respectively. The effect of the ILs on the
F and u in the DMF has been examined at various temperatures.
As it can be observed, the F and u sharply decreases as the
temperature increases in the three systems. There are no
previous F and u data reported in the literature for ammonium
ILs + DMF at various temperatures for comparison. It is obvious
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Figure 3. Temperature dependence of ultrasonic sound velocity for the mixtures of ILs with DMF vs mole fraction of IL x1 for (a) TMAA {(x1)
25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)}, (b) TMAP {(x1) 25 °C (O), (x1) 30 °C
(4), (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)}, (c) TMAS {(x1) 25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0),
(x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)} at various compositions and at atmospheric pressure. The solid line represents the
smoothness of these data.
Figure 4. (a) Densities and (b) ultrasonic sound velocity for the mixtures of ILs with DMF vs mole fraction of IL (x1) for TMAA (O), TMAP (0), and
TMAS (4) with DMF (x2) at 25 °C.
anion shows higher density values than lower sizes of the anion.
However, in the case of the TMAA mixture there is no sharp
increase in F as compared to other two ILs mixtures. In other
words, we can state that the density does not increase sharply
at a mole fraction of IL (0.3000 to 0.6000), which might be due
to a decrease in the ion-pair interaction between these TMAA
and DMF. We observed the F values to be higher in the
TMAS mixture, lower in the TMAA, and moderate in the TMAP
mixture. These discrepancies are varying from IL to IL and
solvent to solvent and also depending on the nature as well as the
structural arrangement of IL and solvent. The density generally
decreases with the increasing size of a cation or anion as
documented earlier for ILs.53,54 However, this conclusion is
not consistent with observations of the present study. As
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Table 2. Estimated Parameters of eq 1 and Standard Deviation, σ for the Systems of ILs with DMF at Various Temperatures
ARTICLE
Y
system
T/°C
a0
a1
a2
a3
a4
σ
VE/(cm3 molꢀ1
)
TMAA + DMF
25
30
35
40
45
50
25
30
35
40
45
50
25
30
35
40
45
50
25
30
35
40
45
50
25
30
35
40
45
50
25
30
35
40
45
50
ꢀ3.4416
ꢀ3.8409
ꢀ4.0758
ꢀ4.3148
ꢀ4.5481
ꢀ4.9934
ꢀ8.8735
ꢀ9.5491
ꢀ10.2493
ꢀ11.1079
ꢀ11.6467
ꢀ12.9151
ꢀ11.2067
ꢀ12.1057
ꢀ12.8528
ꢀ13.6303
ꢀ15.2173
ꢀ16.1652
ꢀ168.78
ꢀ231.95
ꢀ261.52
ꢀ299.01
ꢀ341.92
ꢀ375.21
ꢀ366.91
ꢀ423.23
ꢀ464.37
ꢀ508.42
ꢀ567.37
ꢀ630.76
ꢀ482.15
ꢀ515.46
ꢀ589.78
ꢀ661.82
ꢀ742.34
ꢀ801.05
4.3758
4.335
ꢀ0.5796
ꢀ0.7886
0.1536
0.008
0.019
0.011
0.012
0.010
0.015
0.040
0.059
0.067
0.081
0.088
0.058
0.028
0.034
0.057
0.075
0.055
0.055
0.55
3
3.9618
3.864
0.9883
1.2230
1.5143
1.2613
ꢀ2.2253
ꢀ2.7491
ꢀ2.9160
ꢀ3.9223
0.0303
3.7770
4.0008
12.6845
12.5285
12.5706
12.7476
12.8164
12.3590
15.8373
15.3037
15.3194
15.3711
13.9281
15.5084
439.93
355.28
364.03
354.22
358.16
365.19
261.81
282.17
294.65
351.85
380.22
409.19
336.98
351.62
407.95
459.14
494.69
520.87
ꢀ0.0681
0.4574
TMAS + DMF
TMAP + DMF
TMAA + DMF
TPAH + DMF
TMAS + DMF
ꢀ1.2599
ꢀ2.1527
ꢀ2.5901
ꢀ2.7307
ꢀ3.6176
0.9669
2.6815
ꢀ8.6559
ꢀ1.6982
ꢀ2.6305
ꢀ3.3493
1.9091
0.8131
ꢀ40.59
18.77
3.6264
ꢀ9.7122
ꢀ10.0261
ꢀ8.6173
Δks/(T Paꢀ1
)
3
44.21
59.89
ꢀ127.27
ꢀ193.29
0.65
ꢀ76.13
28.12
1.62
1.08
ꢀ99.13
ꢀ109.83
ꢀ22.91
ꢀ25.06
40.94
2.44
3.04
0.59
0.93
63.12
ꢀ193.65
1.19
ꢀ66.85
83.63
2.62
ꢀ20.82
ꢀ41.41
ꢀ356.85
ꢀ404.95
1.72
66.76
1.92
0.816
1.46
ꢀ67.12
ꢀ74.17
ꢀ112.47
722.67
2.01
2.21
ꢀ67.80
ꢀ46.46
ꢀ408.08
ꢀ566.66
1.66
74.93
1.36
expected, TMAS or TMAP + DMF mixtures show a very slight
difference in their densities, since both of the ILs have the almost
same molar mass. These results execute that the anion of ILs
plays a key role in the density of mixtures.
when compared to the smaller size of acetate anion IL systems
(Figure 3) at all studied temperatures. Therefore, our result
demonstrates that the influence of the anion is significantly
affected on ammonium ILꢀsolvent interactions. As seen from
Figure 4b, the sound velocity of TMAP with DMF at 25 °C
rapidly increases with the increasing composition of IL up to
≈0.4000 mole fraction, and later the u values slightly increased.
On the other hand, in the case of TMAS with DMF there is a
rapid increase in u values up to the mole fraction ≈0.4300 of
TMAS; later the values suddenly decreased up to 0.9900 mole
fraction of IL at 25 °C. The progressive weakening of the
ionꢀion bonds is probably the reason for decrease in u ≈
0.4000 mole fraction of IL, in all cases except the TMAA system.
Fascinating results are seen in the case of TMAA for which its
u values rapidly enhance a ≈0.2000 mole fraction of IL; further a
decrease is noted up to 0.8000 mole fraction of IL, and later the
values sharply increasing up to mole fraction 0.9800 of TMAA.
This may be due to the self-interaction occurring between the
Temperature-dependent u values of ILs with DMF were
measured starting at 25 °C and increasing the temperature in
steps of 5 °C up to 50 °C, and these u data displayed in Figure 3 as
a function of IL concentration under atmospheric pressure. The
values of u were found to decrease with an increase in tempera-
ture, while u values increase with an increase in mole fraction of
IL up to ≈0.4500 except in the mixture of TMAA with DMF,
whereas in the case of TMAA the u values sharply increase
up to ≈0.3200 mole fraction of TMAA. In the case of TMAP or
TMAS + DMF, u values increase up to ≈0.4200 of IL, and later
the u values decrease up to 0.9900 mole fraction of IL at all
investigated temperatures. The anionic sound velocity effect is
most obvious, since we have a common cation, resulting in
significantly higher u values for the larger anion size of IL systems,
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Figure 5. Concentration dependence of excess molar volumes (VE) against the mole fraction of ILs (a) TMAA {(x1) 25 °C (O), (x1) 30 °C (4), (x1)
35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)},(b) TMAP {(x1) 25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0), (x1) 40 °C
(b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)}, (c) TMAS {(x1) 25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or
(x1) 50 °C (9) + DMF (x2)} at atmospheric pressure. Solid (—) lines are correlated by the RedlichꢀKister equation.
TMAA molecules. Figure 4 illustrated that the TMAP IL exhibits
higher u values up to ≈0.4000 mole fraction followed by TMAS
than TMAP and least in TMAA with DMF at 25 °C. This is a
somewhat surprising result, since one would expect at first view
that, as the anion varies from acetate ion to sulfate ion, the overall
contribution increases up to ≈0.5000. This shows that there is a
significant effect of F on the u; as the density increases, the sound
velocity also increases.
mixtures were calculated using the relation from F and u.
The detailed procedure of obtaining of ks and deviation in
isentropic compressibility (Δks) was delineated in our previous
papers.16,17,39 The composition dependence of the VE and Δks
properties represents the deviation from ideal behavior of the
mixtures and provides an indication of the interactions between
IL and DMF. The following RedlichꢀKister expression was used
to correlate these properties:
Excess molar volumes (VE) as well as ultrasonic studies are
known to provide useful insights into solution structural effects
and intermolecular interactions between component molecules.
The extent of deviation of liquid mixtures from ideal behavior is
best expressed by excess functions. Among them, the excess
volumes can be interpreted in three areas, namely, physical,
chemical, and structural effects.16,17,35 Volumetric properties of
binary mixtures of ILs with polar compounds are contribute to
the clarification of the various intermolecular interactions exist-
ing between the different species found in solution. The excess
volumes are determined from the density of pure compounds
(F1 and F2) and mixture (Fm) using a standard equation.39
The ultrasonic studies have been adequately employed in
understanding the nature of molecular interaction in solvent
mixed systems. In the chemical industry, a knowledge of the
ultrasonic and its related properties of solutions is essential in the
design involving chemical separation, heat transfer, mass transfer,
and fluid flow. Isentropic compressibilities (ks) of the binary
n
i
Y ¼ x1x2ð aiðx1 ꢀ x2Þ Þ
ð1Þ
∑
i¼0
where Y refers to VE or Δks. ai are adjustable parameters and can
be obtained by least-squares analysis. Values of the fitted para-
meters are listed in Table 2 along with the standard deviations of
the fit. The values of VE and Δks for the binary mixtures at various
temperatures as a function of IL concentration are included in
Table 1. Figures 5 and 6 display the experimental data for the
binary mixtures, and the fitted curves, along with the excess
properties of VE and Δks, respectively, for the DMF with ILs as a
function of IL concentration at different temperatures.
As can be seen in Figure 5, the VE values have a broad
minimum at the composition around ≈0.3000 mole fraction of
all ILs, for all three binary mixtures at all investigated tempera-
tures. However, we observed that the VE values present a
maximum at x1 ≈ 0.91 for the systems of TMAP or TEAS with
DMF at 25 °C only. The minimum increases, and the maximum
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Figure 6. Concentration dependence of deviation in isentropic compressibilities (4ks) against the mole fraction of ILs (a) TMAA {(x1) 25 °C (O),
(x1) 30 °C (4), (x1) 35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)},(b) TMAP {(x1) 25 °C (O), (x1) 30 °C (4), (x1)
35 °C (0), (x1) 40 °C (b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)}, (c) TMAS {(x1) 25 °C (O), (x1) 30 °C (4), (x1) 35 °C (0), (x1) 40 °C
(b), (x1) 45 °C (2), or (x1) 50 °C (9) + DMF (x2)} at atmospheric pressure. Solid (—) lines are correlated by the RedlichꢀKister equation.
decreases (except TMAA systems) as temperature increases for
all three systems. This behavior can be explained because
hydrogen bonding is certainly more T-dependent than Coulom-
bic interactions. The minimum can be due to hydrogen bonds
between DMF molecules and IL. At higher concentrations of IL,
the dissociation of the ions forming the ILs and loss of dipolar
interactions of DMF. Moreover, the VE values are positive for IL-
rich compositions and negative for DMF-rich compositions for
the mixture of TMAP or TMAS with DMF at 25 °C. The
decrease in the magnitude of the negative VE values with an
increase in IL composition can be attributed to the decrease of
hydrogen bonding. Although it can also be understood as an
increase in the concentration of the IL, a decrease of packing
efficiency between IL and DMF contributes to positive deviation
for TMAP or TMAS with DMF at 25 °C. For this behavior our
assumption is that at a higher concentration of IL some extensive
self-association through hydrogen bonding or ionꢀion interac-
tion might be occurring. This information is represented as
schematically in Scheme 2.
interaction between TMAP or TMAS and DMF decreases as
the concentration of IL increases at this temperature. It can be
seen that the VE values are negative in the entire range of
composition at 30ꢀ50 °C for TMAP or TMAS with DMF. This
might be due to the fact that th large difference between molar
volumes of the DMF and TMAP or TMAS implies that it is
possible that the relatively small organic molecules fit into the
interstices upon mixing at all studied temperatures except 25 °C.
Therefore, the filling effect of organic molecular liquids in the
interstices of ILs and the ionꢀdipole interactions between
organic molecular liquid and ILs all contribute to the negative
values of VE. The negative VE values reveal that a more efficient
packing or attractive interaction occurred when these IL and
DMF were mixed at all cases. DMF (oxygen ion of resonating
structure of DMF) forms a hydrogen bond with cation, while the
other part of the DMF resonating structure (non-hydrogen
bonding) interacts more strongly with the anion of the IL. These
interactions are shown schematically in Scheme 3. The interac-
tions between the DMF molecules and the ions of IL are due to
ionꢀdipole interactions. This will reduce the hydrogen bonding
between the cation and the anion in the IL, which contributes to
the negative VE values. A careful examination of Figure 5 explains
that VE values are significantly at each investigated temperature
over the whole mole fraction range of all ammonium ILs with
DMF systems. The observed VE values for all three systems
The experimental results in Figure 5 show that the values of VE
for DMF with TMAA are negative over the whole concentration
range at all investigated temperatures. Figure 5 parts b and c,
depict that, at lower temperatures (25 °C), the VE values for the
system TMAP or TMAS with DMF exhibits an inversion in the
sign from negative to positive deviation, indicating that the
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appear to increase sharply with increasing temperature, which
shows the deviation from ideal behavior to become pronounced
as the temperature is enhanced. These observations can be
depicted to the inherent complexity of the IL with DMF systems
as far as interactions with in the system are concerned. On the
other hand, the hydrogen bonding between the IL + DMF
(Scheme 3) would result in volume contraction, and thereby
the VE values contribute to the negative sign. Obviously, the
contraction in volume of the mixture may be contributed to the
weakening the hydrogen-bond interactions between IL mole-
cules with DMF particles and, as a result, a slight increase in the
absolute values of VE values with increasing the temperature of
the mixture.
Clearly, the observed negative VE values decrease further with
increasing the temperature in the entire mole fraction range for
all three IL systems. It is interesting note that the VE values in the
TMAP + DMF mixture shows more negative values of VE than
the TMAS + DMF and TMAA + DMF mixtures at 25 °C over the
entire concentration range (Figure 7a), implying that in the
TMAP there are ionꢀdipole interactions and packing effects
with DMF that are stronger than those in the TMAS and TMAA
solution at x1 ≈ 0.2500. Furthermore, the observed positive
values at IL-rich compositions show that there exists no specific
interactions between unlike molecules, and also the compact
structure of the polar component (DMF) due to dipolar
association has been broken by these ILs at rich-IL region
(Figure 7a). The magnitude and sign of VE values are a reflection
of the type of interactions taking place in the mixture, which are
the result of different effects containing the loss of the DMF
dipole interaction from each other (positive VE) and the break-
down of the IL ion-pair (positive VE). The interaction between
the ion-pair of ILs increases as compared to IL + DMF interac-
tions, which leads to a positive contribution at higher IL
concentrations. Interestingly, the hydrogen bonding between
ILs and DMF has predicted using semiempirical calculations with
the help of Hyperchem 7, and those interactions are explicitly
elucidated in Figures 8ꢀ10. Our interpretation of hydrogen
bonding between of IL and DMF molecules (based VE data) is
quite corroborated with our theoretical calculation of hydrogen
bonding of IL + DMF molecules. It is noteworthy to compare
the negative deviation of VE values of TMAA + DMF with
TEAS + DMF and TMAP + DMF, which suggests that there
is a difference of the anion in all ILs, leading to variation in
the hydrogen bond between the ion and DMF molecules
(Figures 8ꢀ10).
Scheme 2. Schematic Depiction of Interactions between
ILs + DMF at Various Temperatures for the Positive
Deviation Where Color Representation Is the Following,
Green = Carbon, White = Hydrogen, Red = Oxygen, and
Blue = Nitrogen
In Figure 6, the Δks values for ILs + DMF at a wide range of
temperatures and atmospheric pressure are illustrated over whole
composition range, as a function of IL mole fraction. It can be
seen from Figure 6 that the Δks values for all the systems are
negative over the entire composition range and at all investigated
temperatures, except in TMAA + DMF systems. However, the
observed Δks values exhibit positive values for the mixture of
Scheme 3. Schematic Depiction of Interactions between ILs + DMF at Various Temperatures for the Negative Deviation Where
Color Representation Is the Following, Green = Carbon, White = Hydrogen, Red = Oxygen, and Blue = Nitrogen
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Figure 7. (a) Excess molar volumes (VE) against the mole fraction of ILs for TMAA (O), TMAP (0), and TMAS (4) at 25 °C and (b) deviation in
isentropic compressibilities (4ks) against the mole fraction of ILs for TMAA (O), TMAP (0), and TMAS (4) at 25 °C.
Figure 8. Schematic depiction of the hydrogen bonding interaction
between TMAA and DMF molecules, which is predicted by a semi-
empirical calculation with the help of Hyperchem 7.
Figure 10. Schematic depiction of the hydrogen bonding interaction
between TMAS and DMF molecules, which is predicted by a semi-
empirical calculation with the help of Hyperchem 7.
the IL mole fraction x1 = 0.3000, ≈0.3700, and ≈0.3800 for the
mixtures of TMAA + DMF, TMAP + DMF, and TMAS + DMF,
respectively. As the concentration of the ILs increase and large
portion of the DMF molecules are solvated, the amount of bulk
solvent decreases which causes a decrease in the compressibility.
The negative Δks of ILs in DMF are also attributed to the strong
attractive interactions due to the solvation of the ions in these
solvents. The negative values of the Δks of an ammonium ILs
with DMF imply that solvent molecules around the solute are less
compressible than the solvent molecules in the bulk solutions. As
the mole fraction of IL increases the negative deviation increases
sharply up to x1 ≈ 0.3300, while with further addition of the ILs
there is a decrease in the compressibility graph at all temperature
ranges. This might be due to a decrease in attraction of DMF and
IL molecules in the IL-rich concentration region, since the
interaction between the IL to IL increases and that between IL
to DMF decreases (Scheme 2).
Figure 9. Schematic depiction of the hydrogen bonding interaction
between TMAP and DMF molecules, which is predicted by a semi-
empirical calculation with the help of Hyperchem 7.
On the other hand, the negative Δks values of DMF-rich
region of TMAA + DMF becomes positive Δks values at higher
IL concentration region. These discrepancies vary from IL to IL
TMAA + DMF at lower temperatures (25 and 30 °C) at the IL-
rich concentration region. A minimum value of Δks is observed at
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(depending on the anion size) and solvent to solvent and also
depend on the nature as well as the structural arrangement of IL
and solvent. The results in Figure 6 show that the negative Δks
values increase with increasing temperature. Figure 7b depicts
that at the lower temperature 25 °C the Δks values for the TMAA
with DMF exhibit an inversion in the sign from negative to
positive deviation, indicating that the interaction between TMAA
and DMF decreases as the concentration of IL increases at this
temperature. The negative Δks values for TMAS + DMF at 25 °C
are higher than those for TMAA or TMAP with DMF mixtures
(Figure 7b). The observed strong attractive interactions between
IL + DMF are in quite good agreement with the theoretical
calculation of hydrogen-bonding interaction of IL + DMF
molecules (Figures 8ꢀ10).
’ ACKNOWLEDGMENT
We are gratefully acknowledged the Council of Scientific
Industrial Research (CSIR), New Delhi, through Grant No.
01(2343)/09/EMR-II, Department of Science and Technology
(DST), New Delhi, India (Grant No. SR/SI/PC-54/2008) and
University Grants Commission (UGC), New Delhi for financial
support.
’ REFERENCES
(1) Baj, S.; Chrobok, A.; Derfla, S. Green Chem. 2006, 8, 292–295.
(2) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351–356.
(3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39,
3772–3789.
(4) Van, R. F.; Sheldon, R. A. Chem. Rev. 2007, 107, 2757–2785.
(5) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.;
Wormald, P.; Morris, R. E. Nature 2004, 430, 1012–1016.
(6) Yoshizawa, M.; Narita, A.; Ohno, H. Aust. J. Chem. 2004, 57,
139–144.
(7) Branco, L. C.; Crespo, J. G.; Afonso, C. A. M. Angew. Chem., Int.
Ed. 2002, 41, 2771–2773.
(8) Dietz, M. L. Sep. Sci. Technol. 2006, 41, 2047–2063.
(9) Attri, P.; Venkatesu, P.; Kumar, A. Phys. Chem. Chem. Phys. 2011,
13, 2788–2796.
’ CONCLUSION
To gain some insight into the new generation of green
solvents, we synthesized the novel trimethylammonium ILs,
namely, TMAA, TMAP, and TMAS, and studied the influence
of the IL, particularly an anion, on the polar solvent. In this paper,
densities and ultrasonic sound velocities for three new ILs with
DMF have been reported at 25ꢀ50 °C under atmospheric
pressure. The performed work intends to map the temperature
effect on the molecular interaction behavior of an ammonium IL
with the DMF molecule. From these measurements, we pre-
dicted VE and Δks at each temperature as a function of IL
concentration. The VE values for ILs + DMF are negative at all
ranges of composition, except TMAP or TMAS with DMF at
25 °C. The negative excess molar volumes reveal that a more
efficient packing or attractive interaction occurred when the IL and
DMF were mixed. The decrease in the magnitude of the negative VE
values with an increase in IL composition can be attributed to the
decrease of hydrogen bonding. The variation in the excess proper-
ties depends upon the hydrogen bonding between ILs and DMF.
These observed interactions are supported by our theoretical
calculations, which are obtained by Hyperchem 7.
The Δks values for all of the systems are negative over the
whole composition range at all studied temperatures, except
TMAA + DMF systems. The negative Δks values of ILs in DMF
are also attributed to the strong attractive interactions due to the
solvation of the ions in these solvents. The values of VE and Δks
were correlated by RedlichꢀKister equations. The values of VE
and Δks values increase with increasing temperature. For the first
time we show the utility of trimethylammonium ILs and motivate
other researchers to explore the different aspects and applications
of these novel ILs. These ILs have some additional character
compared to the conventional organic solvents in that they are
reusable at least five times without loss in their purity.
(10) Attri, P.; Venkatesu, P. Phys. Chem. Chem. Phys. 2011, 13,
6566–6575.
(11) Seddon, K. R. Nat. Mater. 2003, 2, 363–365.
(12) Plechkova, N. V.;Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123–150.
(13) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo,
L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006,
439, 831–834.
(14) Welton, T. Chem. Rev. 1999, 99, 2071–2083.
(15) Attri, P.; Reddy, P. M.; Venkatesu, P. Indian J. Chem. A 2010,
49A, 736–742.
(16) Attri, P.; Reddy, P. M.; Venkatesu, P.; Kumar, A.; Hofman, T.
J. Phys. Chem. B 2010, 114, 6126–6133.
(17) Attri, P.; Venkatesu, P.; Kumar, A. J. Phys. Chem. B 2010,
114, 13415–13425.
(18) Scammells, P. J.; Scott, J. L.; Singer, R. D. Aust. J. Chem. 2005,
58, 155–169.
(19) Seddon, K. R.; Stark, A.; Torres, M. Pure Appl. Chem. 2000,
72, 2275–2287.
(20) Blesic, M.; Lopes, J. N. C.; Gomes, M. F. C.; Rebelo, L. P. N.
Phys. Chem. Chem. Phys. 2010, 12, 9685–9692.
(21) D'Anna, F.; Marullo, S.; Noto, R. J. Org. Chem. 2010, 75,
767–771.
(22) Govinda, V.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Fluid
Phase Equilib. 2011, 304, 35–43.
(23) Mokhtarani, B.; Sharifi, A.; Mortaheb, H. R.; Mirzaei, M.; Mafi,
M.; Sadeghian, F. J. Chem. Thermodyn. 2009, 41, 323–329.
(24) Wagner, M.; Stanga, O.; Schr€oer, W. Phys. Chem. Chem. Phys.
2003, 5, 3943–3950.
(25) Ortega, J.; Vreekamp, R.; Marrero, E.; Penco, E. J. Chem. Eng.
Data 2007, 52, 2269–2276.
’ ASSOCIATED CONTENT
(26) Fr€oba, A. P.; Kremer, H.; Leipertz, A. J. Phys. Chem. B 2010,
112, 12420–12430.
(27) Wang, J.; Tian, Y.; Zhuo, K. Green Chem. 2003, 5, 618–622.
(28) Geppert-Rybczyꢀnska, M.; Heintz, A.; Lehmann, J. K.; Golus, A.
J. Chem. Eng. Data 2010, 55, 4114–4120.
S
Supporting Information. Mole fraction, density, ultra-
b
sonic velocity, isentropic compressibility, and deviation in isen-
tropic compressibility values for the systems of ILs with DMF at
various temperatures and atmospheric pressure. This material is
available free of charge via the Internet at http://pubs.acs.org.
(29) Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. J. Phys.
Chem. B 2007, 111, 6181–6188.
(30) Firestone, A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.;
Dietz, M. L. Langmuir 2002, 18, 7258–7260.
(31) Law, G.; Watson, P. R. Langmuir 2001, 17, 6138–6141.
(32) Zhang, Z.; Wu, W.; Gao, H.; Han, B.; Wang, B.; Huang, Y. Phys.
Chem. Chem. Phys. 2004, 6, 5051–5055.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: venkatesup@hotmail.com; pvenkatesu@chemistry.du.
ac.in. Tel.: +91-11-27666646-142. Fax: +91-11-2766 6605.
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The Journal of Physical Chemistry B
ARTICLE
(33) Iglesias-Otero, M. A.; Troncoso, J.; Carballo, E.; Romaní, L.
J. Chem. Eng. Data 2008, 53, 1298–1301.
(34) Wang, J.; Tian, Y.; Zhao, Y.; Zhuo, K. Green Chem. 2003,
5, 618–622.
(35) Venkatesu, P. Rev. Fluid Phase Equilib. 2010, 289, 173–191.
ꢁ
ꢁ
(36) Zagar, E.; Zigon, M. Polymer 2000, 41, 3513–3521.
(37) Kang, Y. K.; Park, H. S. J. Mol. Struct. (THEOCHEM) 2004,
676, 171–176.
(38) García, B.; Alcalde, R.; Leal, J. M.; Matos, J. S. J. Chem. Soc.,
Faraday Trans. 1997, 93, 1115–1118.
(39) Venkatesu, P.; Lee, M. J.; Lin, H. M. J. Chem. Thermodyn. 2005,
53, 996–1002.
(40) Venkatesu, P.; Rao, M. V. P. J. Chem. Eng. Data 1997, 42, 90–92.
(41) Venkatesu, P.; Rao, M. V. P. J. Chem. Thermodyn. 1998, 30,
207–213.
(42) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic solvents, 4th
ed.; Wiley-Interscience: New York, 1986.
(43) Venkatesu, P.; Rao, M. V. P.; Prasad, D. H. L.; Ravikumar,
Y. V. L. Thermochim. Acta 1999, 342, 73–78.
(44) Venkasteu, P.; Sekhar, C. G.; Rao, M. V. P.; Hofman, T.
Thermochim. Acta 2006, 443, 62–71.
(45) Attri, P.; Pal, M. Green Chem. Lett. Rev. 2010, 3, 249–256.
(46) Weng, J.; Wang, C.; Li, H.; Wang, Y. Green Chem. 2006, 8,
96–99.
(47) Huq, F.; Yu, J. Q. J. Mol. Model. 2002, 8, 81–86.
(48) Jorgensen, W. L.; Chandrasekhas, J.; Madura, J. D.; Impey,
R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926–935.
(49) Schmid, E. D.; Odbek, E. Can. J. Chem. 1985, 63, 1365–1371.
(50) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram,
K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168–1178.
(51) Pinkert, A.; Ang, K. L.; Marsh, K. N.; Pang, S. Phys. Chem. Chem.
Phys. 2011, 13, 5136–5143.
(52) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.;
Brennecke, J. F. J. Chem. Eng. Data 2004, 49, 954–964.
(53) Wilkes, J. S. J. Mol. Catal., A: Chem. 2004, 214, 11–17.
(54) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Fluid Phase Equilib.
2004, 219, 93–98.
10097
dx.doi.org/10.1021/jp2059084 |J. Phys. Chem. B 2011, 115, 10086–10097