Indian Journal of Chemistry
Vol. 49A, May-June 2010, pp. 736-742
Density and ultrasonic sound speed measurements for
N,N-dimethylformamide with ionic liquids
Pankaj Attri, P Madhusudhan Reddy & P Venkatesu*
Department of Chemistry, University of Delhi, Delhi 110 007, India
Email: [email protected]/ [email protected]
Received 13 March 2010; accepted 24 March 2010
To understand the molecular interactions between N,N-dimethylformamide with two families of ionic liquids, the
thermophysical properties such as densities and ultrasonic sound speeds have been measured over the entire composition
range at 25 oC under atmospheric pressure. The materials investigated in the present study include two families of ionic
liquids, viz., ammonium salts and imidazolium salts. Diethyl ammonium acetate, triethyl ammonium acetate, triethyl
ammonium dihydrogen phosphate and triethyl ammonium sulphate are the ammonium salts while 1-benzyl-3methylimidazolium chloride belongs to the imidazolium family. The intermolecular interactions and structural effects are
analyzed on the basis of the measured and derived properties. A qualitative analysis of the results are discussed in terms of
the ion-dipole interactions, ion-pair interactions and hydrogen bonding between the ionic liquids and N,N-dimethylformamide molecules.
Keywords: Ionic liquids, Molecular interactions, Thermophysical properties, Density, Ultrasonic studies
The term ionic liquids (ILs) stands for liquids
composed of anions and cations. ILs are a relatively
new class of compounds that are combinations of
different organic salt ions that are liquid, at or
relatively near, room temperature. Due to the ionic
nature of the materials, ionic liquids have essentially
negligible vapor pressure and so can be envisioned as
being useful in a variety of applications.1-2 The
development of neoteric solvents, i.e., ILs, for
chemical synthesis holds great promise for green
chemistry applications.3 ILs were initially synthesized
in the early 20th century and at present, there are over
200 types of ILs prepared. Some of these have been
successfully applied in organic synthesis and other
aspects.4-6 One of the major objectives of the chemical
industry today is to search for safer alternatives of
volatile organic compounds that will minimize air
pollution, climatic changes, and human health-related
problems. ILs exhibit certain desirable physical
properties, wide electrochemical window, wide
thermal window, nonflammability, wide range of
densities and viscosities, high potential for recycling,
and highly solvating capacity for organic compounds.
Their perceived status as “designer”, alternative
“green” solvents has contributed largely to this
interest, i.e., the existence of fluids with no
measurable volatility and are able to selectively
dissolve different types of solute merely by
exchanging one of the ions that form the IL, or even
more subtly, by altering one of the organic residues
within a given ion.
However, their properties and behaviour with
respect to solvating ability can usually be coarsetuned either by changing the chemical nature of the
cation or of the anion. The enhancement of their
characteristics is usually achieved by slightly
changing the cation’s size, typically by altering the
alkyl chain length of its organic residue, while
preserving its chemical nature. Such extensive
deviations in physical and chemical properties are
possible only in ionic liquids. The research on ILs still
is in its preliminary phase; the reasons are (i) pure ILs
and mixtures containing ILs differ noticeably from socalled “normal liquids”, and their behaviour is not
fully understood, and, (ii) new compounds with
unknown properties are being synthesized. It is clear
that availability of the experimental values of
thermophysical parameters has a crucial significance
to find a quantitative description or even a prediction
of some properties. The ILs possess many unique
properties as compared to ordinary fluids. On the
other hand, their structures are various and their
functions are designable. These properties make them
very attractive especially in the emerging field of
ATTRI et al.: MOLECULAR INTERACTIONS BETWEEN DMF AND AMMONIUM/IMIDAZOLIUM ILs
green chemistry. Therefore, they have become most
promising solvents.
Although the number of articles on ILs is
increasing exponentially, there is still a lack of data
on their thermophysical description and molecular
modelling properties. Apparently, the aim is to
achieve exactly the desired chemical and physical
properties by a judicious combination of an anion and
a cation. Apparently, very little physico-chemical data
is available in the literature, which mainly
characterize ILs.7-11 In this context, our aim is to study
closely two key thermophysical properties, density
and ultrasonic sound speed. Till date, there is no
systematic documentation for the ammonium ILs with
other organic molecular solvents. For these reasons,
four ammonium ILs were synthesized in our
laboratory by the simplest methods, which increase its
utility. In spite of their importance and interest,
accurate values for many of the fundamental physicalchemical properties of this class of ILs are either
scarce or even absent. Interactions of ILs with organic
molecular liquids have been rather scarcely
investigated up to now on the basis of thermophysical
properties and are yet to be understood clearly.
The behaviour of ILs with polar solvents is
fascinating and of great fundamental and practical
importance. N,N-Dimethylformamide (DMF) is a
polar solvent used widely in a variety of industrial
processes, including manufacture of synthetic fibers,
leathers, films and surface coatings.12-14 DMF is a
stable compound with a strong electron-pair donating
and accepting ability and is widely used in studies on
solvent reactivity relationships.15-17 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, a high dielectric
constant (µ= 3.24 Debye and ε = 36.71 at 25 ºC)18 and
good donor-acceptor 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 obtain information on
protein systems. The resonance structure of DMF is
shown below (I).
O
C
H
N
O
CH3
C
CH3
H
I
+
N
CH3
CH3
737
The negative pole in DMF is an oxygen atom that
juts out from the rest of the molecule and this oxygen
atom is the best hydrogen bond acceptor. Through
their unshared pairs of electrons, these negatively
charged, well exposed atoms are solvated very
strongly. The positive pole nitrogen atom, on the
other hand, is buried within the molecule. In DMF the
presence of two electrons repelling -CH3 groups make
the lone pair at nitrogen still more perceptible towards
donation.19,20 Thus, it may be argued that the DMF is
actually the donor of nitrogen electron pairs. This fact
is well known as the transport phenomenon and
thermophysical properties of mixed solvents. The
schematic chemical structures of the DMF as well as
ILs are shown in Fig. 1.
The study of the properties and structure of
complex liquid mixtures is necessary from both the
theoretical and experimental point of view. Mixed
solvents are almost ubiquitous in the industry in very
different fields ranging from petrochemistry to
pharmaceutical industries. Thermodynamic properties
of mixed solvents provided by the intrinsic volume of
the molecules have been considered to be a good
measure of solute-solvent interactions. Experimental
data of thermodynamic and thermophysical properties
of liquids and liquid mixtures are fascinating and of
high fundamental, practical importance for the
industry. Moreover, these properties allow one to
draw information on the structure and interactions of
mixed solvents. Although a qualitative connection
between the macroscopic and microscopic features is
feasible, quantitative conclusions are of interest to
both academic and industry communities.
In order to characterize the type and magnitude of
the molecular interactions between DMF with ILs, we
present herein the ultrasonic speed of DMF with
diethyl ammonium acetate (DEAA), triethyl
ammonium actetate (TEAA), triethyl ammonium
phosphate (TEAP), triethyl ammonium sulphate
(TEAS) and 1-benzyl-3-methylimidazolium chloride
([bmim][Cl]) at 25 ºC and at atmospheric pressure
over the full composition range. No effort appears to
have been made in literature to study the molecular
interactions between DMF and ILs in terms of density
and ultrasonic speed. The mixtures of IL and DMF
provide potential industrial applications for the
utilization of both IL and DMF. Further, the study
intends to draw molecular level information from the
macroscopic properties on the molecular interaction
between ILs and DMF.
738
INDIAN J CHEM, SEC A, MAY-JUNE 2010
Fig. 1 – Schematic structures for (a) DMF; (b) DEAA; (c) TEAA; (d) [bmim][Cl]; (e) TEAS; (f) TEAP. [Colour representation: green =
carbon, white = hydrogen, red = oxygen, orange = phosphorus].
Materials and Methods
DMF (Aldrich, puriss > 99.9 %) and ionic liquid
1-benzyl-3-methylimidazolium
chloride
were
purchase from Sigma Chemical Co., USA. The
fluids were degassed with ultrasound and kept in
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 (ρ), refractive indices (n)
and sound speed (u), which were in good agreement
with literature values.18 The purities of the samples
were further confirmed by GLC single sharp peaks.
Rest of ILs were synthesized21,22 in our laboratory,
as given below.
Synthesis of ILs
The synthesis of TEAA 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 mole) was added dropwise into triethyl
amine (1 mole) at 70 ºC over 1 hour. The reaction
mixture was heated at 80 ºC with stirring for 2 hours
to ensure completion of the reaction. The reaction
mixture was then dried at 80 ºC until the weight of the
residue remained constant. The sample analysed by
Karl Fisher titration revealed very low level of water
(below 70 ppm). The yield of TEAA was 98 %.
1
H NMR (CDCl3) δ (ppm): 0.778 (t, 9H), 1.466 (s,
3H), 2.58 (m, 6H), 11.0 (s, 1H).
For the synthesis of triethyl ammonium phosphate
(TEAP), a similar procedure as delineated above for
TEAA was followed with phosphoric acid [anion]
instead of acetic acid. The yield of TEAP was 198 g.
1
H NMR (DMSO-d6) δ (ppm): 1.18 (t, 9H), 3.06 (m,
6H), 6.37 (s, 1H). M. pt.: 92 ºC.
TEAS was synthesized by a similar procedure as
that delineated above for TEAA using sulphuric acid
[anion] instead of acetic acid. The yield of TEAP was
198 g. 1H NMR (CDCl3): 1.3 (t, 9H), 3.16 (m, 6H),
5.04 (s, 1H). M. pt.: 90 oC.
For the synthesis of diethyl ammonium acetate
(DEAA), a similar procedure as that delineated above
for TEAA was followed with diethyl amine [amine]
instead of triethyl amine. The yield of DEAA was
118 g. 1H NMR (CDCl3) δ (ppm): 1.3 (t, 6H), 1.97 (s,
3H), 2.95 (m, 3H), 9.20 (s, 2H).
ATTRI et al.: MOLECULAR INTERACTIONS BETWEEN DMF AND AMMONIUM/IMIDAZOLIUM ILs
Procedure
The density measurements were performed with an
Anton-Paar DMA 4500 M vibrating-tube densimeter,
equipped with a built-in solid-state thermostat and a
resident program with accuracy of temperature of
±0.03 oC. Typically, density precisions are
0.00005 g cm-3. Proper calibration at each temperature
was achieved with doubly distilled, deionized water
and with air as standards. Ultrasonic sound speeds
were measured by a single crystal ultrasonic interferometer (model F-05) from Mittal Enterprises, New Delhi,
India, at 2 MHz frequency at 25 ºC. A thermostatically
controlled, well-stirred circulated water bath with a
temperature controlled to ± 0.01 ºC was used for all
the ultrasonic sound speed measurements. The
uncertainty in sound speed is 0.02 %. 1H (300 MHz)
spectra were recorded on a Bruker 300 NMR
spectrometer in CDCl3 and DMSO-d6 (with TMS for
1
H as internal references). The reactions were
monitored by thin layer chromatography (TLC) using
aluminium sheets with silica gel 60 F254 (Merck).
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 × 10-4.
Mixing of the two components was promoted by
the movement of a small glass sphere (inserted in the
739
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
syringe.
Results and Discussion
To understand the molecular interactions of
N,N-dimethylformamide (DMF) with two families of
ionic liquids (ILs), we have measured the thermophysical properties such as densities (ρ) and ultrasonic
sound velocities (u) over the entire composition range
at 25 °C under atmospheric pressure. The compounds
investigated in the present study include two families
of ILs such as ammonium salts and imidazolium
salts. Diethyl ammonium acetate ([Et2NH][CH3COO],
DEAA),
triethyl
ammonium
actetate
([Et3NH][CH3COO], TEAA), triethyl ammonium
dihydrogen phosphate ([Et3NH][H2PO4], TEAP), and
triethyl ammonium sulphate ([Et3NH][HSO4], TEAS)
are ammonium salts while 1-benzyl-3-methylimidazolium chloride ([bmim][Cl]) belongs to
imidazolium family. The interactions between DMF
and the ILs are shown in Scheme 1. Experimental
values of density and sound velocity at 25 ºC are
collected in Table 1 for ILs, DMF and their mixtures
Schematic depiction of interactions between TEAP + DMF and DEAA + DMF
[green = carbon, white = hydrogen, red = oxygen, orange = phosphorus]
Scheme 1
740
INDIAN J CHEM, SEC A, MAY-JUNE 2010
Table 1 – Mole fraction (x1) of ILs, density (ρ) and ultrasonic sound speed (u) for the ILs with DMF systems
at 25 ºC and at atmospheric pressure
x1
ρ (g cm-3)
u12 (m s-1)
x1
1456
1464
1490
1487
1491
1496
1503
1512
1517
1520
1523
1534
1551
1577
1595
1609
0.0000
0.1080
0.1390
0.1720
0.3271
0.4076
0.4612
0.5900
0.6600
0.7142
0.8142
0.8628
0.9020
0.9600
1.000
0.0000
0.0717
0.1410
0.1740
0.2204
0.2656
0.3300
0.4065
0.4822
0.5300
0.5600
0.5910
0.6620
0.8160
0.9030
0.9600
1.000
DEAA with DMF
0.0000
0.0230
0.1080
0.1390
0.1740
0.2430
0.3320
0.4580
0.5400
0.5930
0.6600
0.7418
0.8310
0.9140
0.9590
1.000
0.94389
0.95239
0.96984
0.97425
0.97866
0.98692
0.99519
1.00533
1.01106
1.01491
1.01927
1.02378
1.02745
1.02766
1.02401
1.02146
ρ (g cm-3)
u12 (m s-1)
TEAA with DMF
TEAP with DMF
0.94389
0.95665
0.95949
0.96211
0.97290
0.97715
0.97920
0.98538
0.98890
0.99224
0.99957
1.00427
1.00823
1.01300
1.01586
1456
1491
1502
1514
1586
1630
1661
1734
1769
1789
1808
1812
1814
1827
1840
TEAS with DMF
0.0000
0.0210
0.0513
0.0960
0.1240
0.1540
0.2171
0.2981
0.3942
0.4870
0.5601
0.6301
0.7110
0.7930
0.8900
0.9544
1.000
0.94389
0.96342
0.97935
1.00005
1.01019
1.01994
1.03642
1.05267
1.06742
1.07947
1.08783
1.09543
1.10321
1.11046
1.11813
1.12283
1.12570
1456
1457
1464
1471
1479
1487
1503
1521
1536
1537
1535
1531
1534
1553
1610
1699
1794
0.0000
0.0121
0.0247
0.1070
0.1380
0.1700
0.3240
0.4160
0.4540
0.4779
0.5062
0.5442
0.94389
0.95449
0.96095
0.99125
1.00148
1.01067
1.04458
1.05948
1.06491
1.06805
1.07114
1.07516
1456
1465
1472
1515
1529
1549
1646
1715
1742
1760
1778
1805
0.94389
0.97063
0.98902
0.99553
1.00429
1.01301
1.02552
1.04167
1.05837
1.06847
1.07534
1.08195
1.09580
1.12021
1.13175
1.13867
1.14289
1456
1455
1456
1448
1453
1465
1488
1538
1613
1662
1694
1728
1801
1868
1853
1855
1874
1.07799
1.08030
1.08302
1.08804
1.09005
1.09247
1.09542
1.09995
1.11019
1.11501
1.11983
1.13450
1833
1873
1905
1952
1967
1978
1992
2013
2051
2062
2077
2096
[bmim][Cl] with DMF
0.5900
0.6580
0.7150
0.8120
0.8361
0.8555
0.8741
0.9024
0.9428
0.9590
0.9737
1.000
741
ATTRI et al.: MOLECULAR INTERACTIONS BETWEEN DMF AND AMMONIUM/IMIDAZOLIUM ILs
2200
1.20
1.15
2000
5
4
-1
u (m s )
3
5
-3
ρ (g cm )
1.10
1.05
4
1
1800
2
3
1600
1
1.00
2
1400
0.95
0.90
0.0
1200
0.0
0.2
0.4
0.6
0.8
0.2
x1
Fig. 2 – Plots of densities for the mixtures of ILs + DMF versus
mole fraction (x1) of ILs [1, (O) DEAA; 2, (∆) TEAA;
3, (□) TEAP; 4, (●) TEAS; 5, (▲) [bmim][Cl]] + DMF (x2) at
25 oC and at atmospheric pressure. [The solid line represents the
smoothness of these data].
over the entire composition range. Virtually, ILs are
miscible with medium- to high-dielectric liquids and
immiscible with low dielectric liquids.23 In the present
study, all ILs are completely miscible in DMF18
(ε = 36.71 at 25 oC), since DMF is a high dielectric
liquid. The effect of the ILs on the densities and
sound velocities in the DMF has been examined. It
was found that the densities or sound velocities of the
mixtures increase with increasing concentrations of
the IL in DMF, as shown in Figs 2 and 3,
respectively. The results in Fig. 2, clearly show that
the density values for the mixture of TEAA with
DMF are lower when compared to the mixture of
DEAA with DMF under the same experimental
conditions. The density generally decreases with
increasing length of an alkyl chain in a cation or anion
as documented earlier for ILs.24,25 Interestingly, this
conclusion is quite consistent with the observations of
the present study.
Density of all the ILs increases as the mole fraction
increases. A drastic change was found in the case of
TEAS and the sudden increase in density may be due
to the increased intermolecular interaction between
the TEAS and DMF. These results support our theory
that there is hydrogen bonding and ion-pair
interaction between the ILs and DMF, which
increases the density with increase in mole fraction of
ILs. In some of the cases like in the case of DEAA
there is decrease in density or we can say that the
density does not increase sharply, which might be due
0.4
0.6
0.8
1.0
x1
1.0
Fig. 3 – Plots of ultrasonic sound speed for the mixtures of ILs +
DMF versus mole fraction (x1) of ILs [1, (O) DEAA;
2, (∆) TEAA; 3, (□) TEAP; 4, (●) TEAS; 5, (▲) [bmim][Cl]] +
DMF (x2) at 25 oC and at atmospheric pressure. [The solid line
represents the smoothness of these data].
to decrease in the ion-pair interaction between the
DEAA and DMF.
Sound speeds are also another important source of
information about the properties of different liquids
and their mixtures. So in order to better understand
the behaviour of newly synthesized ILs, we also
investigated sound speeds. Using interferometer it is
easily observed that there is sharp increase of speed in
[bmim][Cl], whereas there is slight decrease in that of
TEAA. This shows that there is a significant effect of
density on the sound speed; as the density decreases
the sound velocity increases. On the other hand, in the
case of TEAS, which has a high density, the sound
speed is low. Fascinating results are seen in the case
of DEAA, where there is a slight change in sound
speed although its density is also low. This may be
due to the self-interaction occurring between the
DEAA molecules.
Recycling of ionic liquids
A major concern in the use of ILs is their relatively
high cost, which makes their recycling an important
issue for further study.26,27 To solve this problem
herein, we have chosen a low cost and simple
synthesis of ammonium ILs and eventually recycled
and reused the ILs. Since large amounts of ILs were
used in the various measurements, their recovery and
reuse should be possible. In this case, the recovery of
the ILs from binary mixture (ILs+DMF) was realized
by removal of the DMF component by vacuum. No
appreciable change in the physical properties of the
742
INDIAN J CHEM, SEC A, MAY-JUNE 2010
recovered ILs was observed. The ILs can been
recovered and reused by vacuuming each binary
system at least four times without loss of its purity.
Conclusions
Studies on ammonium ionic liquids are scarce in
the literature. Possibly for the first time, herein we try
to explain their properties and compare with those of
the commercially available ILs. This study intends to
map the thermophysical behaviour of two important
families of ILs, viz., imidazolium and ammonium ILs.
It was found that the densities or sound velocities of
the mixtures increase with increasing concentrations
of the IL in DMF.
Acknowledgement
We wish to acknowledge the financial support
from the Department of Science and Technology
(DST), New Delhi, India (SR/SI/PC-54/2008).
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