Influence of alkyl chain length and temperature on thermophysical properties of ammonium-based ionic liquids with molecular solvent

Article abstract of DOI:10.1021/jp3015386

Mixing of ionic liquids (ILs) with molecular solvent can expand the range of structural properties and the scope of molecular interactions between the molecules of the solvents. Exploiting of these phenomena essentially require a basic fundamental understanding of mixing behavior of ILs with molecular solvents. In this context, a series of protic ILs possessing tetra-alkyl ammonium cation [R₄N]⁺ with commonly used anion hydroxide [OH]^- were synthesized and characterized by temperature dependent thermophysical properties. The ILs [R₄N]⁺[OH]^- are varying only in the length of alkyl chain (R is methyl, ethyl, propyl, or butyl) of tetra-alkyl ammonium on the cationic part. The ILs used for the present study included tetramethyl ammonium hydroxide [(CH₃) ₄N]⁺[OH]^- (TMAH), tetraethyl ammonium hydroxide [(C₂H₅)₄N]⁺[OH]^- (TEAH), tetrapropyl ammonium hydroxide [(C₃H₇)₄N] ⁺[OH]^- (TPAH) and tetrabutyl ammonium hydroxide [(C ₄H₉)₄N]⁺[OH]^- (TBAH). The alkyl chain length effect has been analyzed by precise measurements such as densities (ρ), ultrasonic sound velocity (u), and viscosity (η) of these ILs with polar solvent, N-methyl-2-pyrrolidone (NMP), over the full composition range as a function of temperature. The excess molar volume (V^E), the deviation in isentropic compressibility (Δκ_s) and deviation in viscosity (Δη) were predicted using these properties as a function of the concentration of ILs. Redlich-Kister polynomial was used to correlate the results. A qualitative analysis of the results is discussed in terms of the ion-dipole, ion-pair interactions, and hydrogen bonding between ILs and NMP molecules. Later, the hydrogen bonding features between ILs and NMP were also analyzed using a molecular modeling program with the help of HyperChem 7.

Full text of DOI:10.1021/jp3015386

Article
pubs.acs.org/JPCB
Influence of Alkyl Chain Length and Temperature on Thermophysical
Properties of Ammonium-Based Ionic Liquids with Molecular Solvent
T. Kavitha,† Pankaj Attri,‡ Pannuru Venkatesu,*,‡ R. S. Rama Devi,† and T. Hofman§
†Department of Chemistry, Sri Padmavathi Womens Degree and Post Graduate College, Tirupati 517 502, India
‡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
ABSTRACT: Mixing of ionic liquids (ILs) with molecular solvent can expand the
range of structural properties and the scope of molecular interactions between the
molecules of the solvents. Exploiting of these phenomena essentially require a basic
fundamental understanding of mixing behavior of ILs with molecular solvents. In
this context, a series of protic ILs possessing tetra-alkyl ammonium cation [R4N]+
with commonly used anion hydroxide [OH]− were synthesized and characterized
by temperature dependent thermophysical properties. The ILs [R4N]+[OH]− are
varying only in the length of alkyl chain (R is methyl, ethyl, propyl, or butyl) of
tetra-alkyl ammonium on the cationic part. The ILs used for the present study
included tetramethyl ammonium hydroxide [(CH3)4N]+[OH]− (TMAH), tetra-
ethyl ammonium hydroxide [(C2H5)4N]+[OH]− (TEAH), tetrapropyl ammonium
hydroxide [(C3H7)4N]+[OH]− (TPAH) and tetrabutyl ammonium hydroxide
[(C4H9)4N]+[OH]− (TBAH). The alkyl chain length effect has been analyzed by
precise measurements such as densities (ρ), ultrasonic sound velocity (u), and viscosity (η) of these ILs with polar solvent,
N-methyl-2-pyrrolidone (NMP), over the full composition range as a function of temperature. The excess molar volume (VE),
the deviation in isentropic compressibility (Δκs) and deviation in viscosity (Δη) were predicted using these properties as a
function of the concentration of ILs. Redlich−Kister polynomial was used to correlate the results. A qualitative analysis of the
results is discussed in terms of the ion-dipole, ion-pair interactions, and hydrogen bonding between ILs and NMP molecules.
Later, the hydrogen bonding features between ILs and NMP were also analyzed using a molecular modeling program with the
help of HyperChem 7.
1. INTRODUCTION
In recent years, large quantities of solvents have been used
for numerous processes in academia and industries. However,
because of new environmental regulations, the challenge of
using nonharmful solvents has prompted a great of develop-
ment of innovative products to protect the environment. In this
context, ionic liquids (ILs) emerged as a new class of solvents
considered for the replacement of volatile organic compounds
in chemical processes to reduce both cost and environmental
pollution.1−4 The IL completely consists of the weak co-
ordinating of ions such as an organic cation and the inorganic
or organic anion.5,6 The potential utilization and applications of
ILs have been rapidly increased in all scientific fields by several
researchers.7−16 Apparently, the physicochemical properties
of ILs are quite sensitive toward the structure and nature
of cations and anions.17,18 The variations in thermophysical
properties of ILs, such as density (ρ), speed of sound (u),
viscosity (η), and conductivity, are observed to be very sensitive
to the change in cation, mainly due to microscopic level
interactions between solvent molecules.19
Figure 1. Schematic structures for ILs such as (a) TMAH, (b) TEAH,
(c) TPAH, and (d) TBAH.
Received: February 15, 2012
Revised: March 22, 2012
Published: March 24, 2012
Obviously, an interesting aspect is that ILs being able to
influence on physicochemical properties of mixtures and even a
© 2012 American Chemical Society
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slight variation in their ions that can lead substantial differences in
their properties. However, very little work is known about the
appropriate variation in physicochemical properties upon varying
either an anion or the cation in ILs. The experimental values of
thermophysical properties allow us establishing new predictive
interactions and information about the molecular level structure of
the mixtures to determinate the properties that are necessary for
the industry in a fast reliable and economical way. Therefore, a
deep knowledge of thermophysical properties of liquid mixtures
containing ILs has essentially required for scientific community.
N-Methyl-2-pyrrolidone (NMP) is a dipolar aprotic basic
solvent that is frequently used to recover hydrocarbons from
petrochemical processes by extractive distillation. Moreover,
NMP has large chemical and thermal resistance, low toxicity,
and high selectivity.20 NMP is also used as an extractive agent
for paraffin and aromatic separations. Nowadays, the demand
for the NMP is explicitly and incessantly increasing with the
research and development of the electronics and chemical
industry. It has a large dipole moment (μ = 4.09 D) and a high
dielectric constant (ε = 32.2 at T = 298.15 K).21 Moreover,
NMP is a strong polar liquid and has the potential for use in the
solvent extraction process for separating polar substances from
nonpolar substances.22,23
This work is a part of our extensive ongoing project aimed at
a detailed characterization of ILs in molecular solvents and in
biomolecules.6,8,12−16 Here, we now uncover the character-
ization for the structural variation of cations of ammonium
based ILs as well as molecular interactions with a molecular
solvent through temperature variable properties. 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 properties of
different ILs.12−15,24 The knowledge of thermophysical proper-
ties of mixed solvents of ILs with organic molecular liquid are
paramount for the design of many technological processes and
required for many practical applications.24−27 In spite of the
importance of properties of ILs in different solvent media, only
a small amount of physicochemical data is available in the
literature, which mainly characterizes ILs.28−35 A survey of the
literature reveals that thermophysical properties of pure ILs
Table 1. Solvent Molecular Weight (MW), Purity, Density (ρ),
Speed of Sound (u), and Viscosity (η) for the Solvents Such
As ILs and NMP at 298.15 K
MW
purity
(%)
solvent
NMP
(g mol−1
)
ρ/(g cm−3
)
u/(m s−1
)
η/(mPa s)
a
,
b
c f
,
g
99.13
99
1.02590
1.02374
1.01797
1.00881
0.99594
0.99358
1552
1.66
c
,d
,
e
c,h
1.49
2.77
4.94
6.10
6.69
TMAH
TEAH
TPAH
TBAH
99.15
147.26
203.37
259.50
99
99
99
99
1828
1814
1801
1798
a
b
c
d
1.02590 from ref 21. 1.025590 from ref 36. At 303.15 K. 1.02370
e
f
from ref 21. 1.02395 at 303.15 K, from ref 38. 1527 at 303.15 K,
from ref 38. 1.66 from ref 21. 1.55 at 303.15 K, from ref 38.
g
h
○
Figure 2. Influence of alkyl-chain length on (a) density, (b) ultrasonic sound velocity, and (c) viscosity for ammonium based ILs: ( ) TMAH,
(Δ) TEAH, ( ) TPAH, and ( ) TBAH plotted against temperatures.
□
●
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Figure 3. Densities (ρ) for the mixtures of ILs + NMP as a function of the composition expressed in the mole fraction (x1) of IL for (a) TMAH (1) +
□
○
●
NMP (2); (b) TEAH (1) + NMP (2); (c) TPAH (1) + NMP (2); and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30, ( ) 35, and ( ) 40 °C at
atmospheric pressure. The solid line represents the smoothness of these data.
have been reported extensively implying widespread interest
from both fundamental and industrial points of view. However,
the thermophysical properties of ILs particularly in molecular
solvents have not been explored in a systematic way. In this
context, our aim is to explore thermophysical properties such as
ρ, u, and η for the mixed solvents of ILs and polar solvent to
expand the basic needs for scientific research. Further, to gain
some insight into several aggregations of molecular interactions
of ILs with polar compounds, we have synthesized the ILs of
different combination of cations with commonly used anion for
our study because these novel kinds of ILs appear to be
promising for various applications for the chemical community.
The main goal of the work is to report molecular and struc-
tural information between ILs and NMP based on the tempera-
ture dependence properties of ρ, u, and η as well as derived
properties of excess molar volume (VE), deviation of isentropic
compressibility (Δκs), and deviation in viscosity (Δη). We
report here the synthesis of tetramethyl ammonium hydroxide
[(CH3)4N]+[OH]− (TMAH), tetraethyl ammonium hydroxide
[(C2H5)4N]+[OH]− (TEAH), tetrapropyl ammonium hydro-
xide [(C3H7)4N]+[OH]− (TPAH), and tetrabutyl ammonium
hydroxide [(C4H9)4N]+[OH]− (TBAH). A survey of the litera-
ture reveals that no studies of VE, Δκs, and Δη are reported for
ammonium ILs with NMP. In this paper, we report the
measured results of ρ, u, and η for the mixtures of ILs with
NMP over a complete mole fraction range at 5 °C intervals
over a wide temperature range of 25 − 40 °C. Moreover, the
hydrogen bonding features between ILs and NMP were carried
out to get deeper insight into intermolecular interactions for
the studied compounds. These studies were performed accord-
ing to the semiempirical calculations by using Hyperchem 7.
2. EXPERIMENTAL SECTION
2.1. Materials. NMP (Merck >99% of purity) was stored
over freshly activated 3 Å molecular sieves and was purified by
the standard method described by Riddick et al.21 The purity of
the NMP was verified by measuring the ρ, u, and η which were
in good agreement with the literature values.21,36−38 The purity
of the sample was further confirmed by GLC single sharp
peaks. To our knowledge, the values of ρ, u, and η for studied
ammonium ILs are not available in the open literature, and a
data of comparison for the values of ILs could not be drawn in
the present study. The ILs used in the present study were
synthesized in our laboratory, as given below.
2.2. Synthesis of ILs. Ammonium based protic ILs used in
this study, abbreviated as TMAH, TEAH TPAH, and TBAH,
were synthesized14,39,40 by the procedure as follows.
2.2.1. Synthesis of Tetramethyl Ammonium Hydroxide
(TMAH). The synthesis of this IL was carried out in a 250 mL
round bottomed flask, which was immersed in a water-bath and
fitted with a reflux condenser. Solid potassium hydroxide (40 mmol)
was added to a solution of tetramethyl ammonium bromide
[(CH3)4N]+[Br]− (40 mmol) in dry methylene chloride (20 mL),
and the mixture was stirred vigorously at room temperature for
10 h. The precipitated KBr was filtered off, and the filtrate was
evaporated to leave the crude [(CH3)4N]+[OH]− as a viscous
liquid that was washed with ether (2 × 20 mL) and dried at
70 °C for 5 h to obtain the pure IL. The sample was analyzed
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Figure 4. Ultrasonic sound velocity (u) for the mixtures of ILs + NMP as a function of the composition expressed in the mole fraction (x1) of IL for
□
○
(a) TMAH (1) + NMP (2), (b) TEAH (1) + NMP (2), (c) TPAH (1) + NMP (2), and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30, ( ) 35, and
●
( ) 40 °C at atmospheric pressure. The solid line represents the smoothness of these data.
by Karl Fisher titration and revealed very low levels of water
(below 70 ppm). The yield of TMAH was 68%. H NMR
The schematic chemical structures of ILs are shown in Figure 1.
1
The molecular weight, normal purity, ρ, u, and η of ILs and
NMP, which are studied in the present work, are summarized in
Table 1.
(DMSOd6): δ (ppm) 2.38 (s, 12H), 4.79 (s, OH). HRMS
calculated for C4H13NO (M + −OH) 91.15, found 91.09.
2.2.2. Synthesis of Tetraethyl Ammonium Hydroxide (TEAH).
A similar procedure as delineated above for [(C2H5)4N]+[OH]−
was followed with the exception of the use of [(C2H5)4N]+[Br]−
([cation]) instead of [(CH3)4N]+[Br]−. The yield of TEAH was
78%. 1H NMR (DMSOd6): δ (ppm) 1.01 (t, 12H), 3.06 (q, 8H),
4.43 (s, OH). HRMS calculated for C8H21NO (M+ - OH) 147.26,
found 147.20.
2.3. Methods. 2.3.1. Sample Preparation. Clear binary
mixtures were prepared by weighing the components using a
Mettler Toledo balance with an accuracy of 0.0001 g for all
measurements. The estimated uncertainty on the mole fraction
composition 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 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. A
bubble-free sample was introduced into the sample cell, and the
cell was placed under the sensor plates of the viscometer.
2.3.2. Density Measurements. The density measurements of
various ILs, NMP, and ILs + NMP were carried out using 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 °C. The uncertainties in
the density experimental measurements were 0.00005 g cm−3.
The instrument was calibrated once a day with double distilled,
deionized water and with air at 20 °C as standards. The excess
molar volumes (VE) of ILs with NMP systems over the IL con-
centration range at different temperatures have been deduced
from the ρ of the pure compounds and mixture (ρm) using the
standard equations.24
2.2.3. Synthesis of Tetrapropyl Ammonium Hydroxide (TPAH).
A procedure similar to that above for [(CH3)4N]+[OH]− was
followed with the exception of the use of [(C3H7)4N]+[Br]−
([cation]) instead of [(CH3)4N]+[Br]−. The yield of TPAH
1
was 82%. H NMR (DMSOd6): δ (ppm) 0.8 (t, 12H), 1.46
(m, 8H), 2.92 (t, 8H), 4.56 (s, OH). HRMS calculated for
C12H29NO (M + −OH) 203.36, found 203.25.
2.2.4. Synthesis of Tetrabutyl Ammonium Hydroxide (TBAH).
A procedure similar to that above for [(CH3)4N]+[OH]− was
followed with the exception of the use of [(C4H9)4N]+[Br]−
([cation]) instead of [(CH3)4N]+[Br]−. The yield of TBAH
1
was 82%. H NMR (DMSOd6): δ (ppm) 0.94 (t, 12H), 1.37
(m, 8H), 1.96 (m, 8H), 3.43 (t, 8H), 4.78 (s, OH). HRMS
calculated for C16H37NO (M + −OH) 259.47 found 259.34.
All ILs were degassed and dried over 3 Å molecular sieves
1
and the purity of ILs was confirmed by H NMR analysis
1
as well as GLC single sharp peaks. H (400 MHz) spectra
were recorded on a JEOL 400 NMR spectrometer in DMSO-d6.
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Figure 5. Viscosities (η) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1) of IL for (a) TMAH (1) +
□
○
●
NMP (2), (b) TEAH (1) + NMP (2), (c) TPAH (1) + NMP (2), and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30, ( ) 35, and ( ) 40 °C at
atmospheric pressure. The solid line represents the smoothness of these data.
2.3.3. Ultrasonic Sound Velocity Measurements. A single
crystal ultrasonic interferometer (model F-05) from Mittal
Enterprises, New Delhi, India, at 2 MHz frequency was used for
sound velocity measurements for pure solvents and mixtures at
various temperatures. Prior to measurements, the interferom-
eter was calibrated with double distilled water and purified
methanol. A thermostatically controlled, well-stirred circulating
water bath with a temperature controlled to 0.01 K was used
for all of the ultrasonic sound velocity measurements for pure
components and mixtures. The uncertainty of the sound velo-
city is 0.02%.
2.3.4. Viscosity Measurements. Vibro viscometer (model
SV-10, A&D Company Limited, Japan) was used for viscosity
measurements. The instrument has been provided with two
sensor plates of gold coating. The measurements of η were
taken from the digital display device attached to the vibro
viscometer. Viscosity measurements of the sample were taken
at heating rate 1 °C/15 min for getting the thermodynamic
equilibrium. Viscometer was calibrated with pure water and
purified methanol at various temperatures. A thermostatically
controlled, well-stirred circulated water bath with a temperature
controlled to 0.01 °C was used for all the viscosity measure-
ments. Typically, the viscosities uncertainty is to be 1%.
2.3.5. Hydrogen Bonding Through Simulation Program.
The structures of ILs and of NMP were optimized based on
molecular mechanics and semiempirical calculations using the
HyperChem 7 Molecular Visualization and Simulation pro-
gram. 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 optimiza-
tion using both molecular mechanics and semiempirical
calculations, the Polak-Ribiere routine with 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 energy minimum by enabling molecules to cross
potential barriers.15,41−43 The structures of ILs were optimized
using semiempirical calculations, and single-point calculations
were carried out to determine the total energy and heat of
formation.
3. RESULTS AND DISCUSSION
Virtually, the thermophysical properties of ILs mainly depend
on the nature and structure of ions and the alkyl chain length of
the cation. As it can be seen in Figure 2, ρ, u, and η values of
ILs have been found to decrease linearly with increasing the
temperature. Further, the ρ and u values decrease as the cation
alkyl chain length increases from methyl to butyl while an
opposite trend was observed, in which the values of η increase
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E
Figure 6. Excess molar volumes (V ) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1) of IL for (a)
□
○
●
TMAH (1) + NMP (2), (b) TEAH (1) + NMP (2), (c) TPAH (1) + NMP (2), and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30, ( ) 35, and ( )
40 °C at atmospheric pressure. The solid lines correlated by Redlich−Kister equation.
with increasing the number of carbon atoms in the alkyl chain
length of cation of ILs. This is mainly contributing to anion
accommodation closer to the cation. These observed results are
quite consistent with the existing results,44−46 in which ρ and u
values decrease while η values increase with increasing the
cation alkyl chain length of ILs. Apparently, ILs that possess a
higher cation side chain are accompanied by lower ρ, lower u,
and larger η. Clearly, our results might imply that the cation
size was responsible for the alteration of the thermophysical
properties of protic ILs.
Figures 3−5 and Table 1S reveal the measured values for the
temperature dependence properties ρ, u, and η for a series of
ammonium hydroxide ILs, TMAH, TEAH, TPAH, TBAH, and
NMP, as well as their mixtures as a function of IL concentration
for complete composition range at temperatures from 25 to
40 °C in steps of 5 °C under atmospheric pressure. Since, NMP
has high dielectric constant (ε = 32.2 at T = 298.15 K),21 we
have obtained complete miscible solutions with all studied ILs.
These results allow us to analyze the influence of alkyl chain
length of cation of IL and temperature on the mixed solvents of
IL with NMP. Therefore, the effect of the ILs on ρ, u, and η in
the NMP has been examined as a function of temperature.
Figure 3a illustrates that with the addition of IL to NMP a
significant increase in ρ occurs for the mixtures of TMAH +
NMP up to ∼0.3000 at all experimental temperatures. A drastic
decrease in ρ was found in this mixture as the mole fraction of
IL increases above ∼0.4500 at all temperatures, as shown in
Figure 3a. This may be due to a decrease in the ion pair
interactions between TMAH and NMP. The observed behavior
of ρ for the system TEAH + NMP is a sharp increase up to
∼0.2000 mol fraction of TEAH after an abrupt decrease up to
the mole fraction ∼0.8100. Eventually, a slight increase in the
value of ρ was found for the remaining IL-rich region (Figure 3b).
Within the TPAH + NMP mixture the ρ values increase
significantly upon increasing the mole fraction of TPAH up to
∼0.2100, and later drastic decrease in ρ was found in this mix-
ture of IL at all studied temperatures, as shown in the Figure 3c.
The ρ values for the mixture of TBAH in NMP increase
When the number of carbon atoms in the alkyl chain length
of the cation is increased, the change in investigated properties
is very high from methyl to propyl group of ILs. However, not
much difference was observed in ρ, u and η values among
higher alkyl chain length of cation of ILs from TPAH to TBAH
(Figure 2). Obviously, this phenomena agrees well with recent
reports that show the thermophyiscal properties higher for
cation size of ILs are not much altering.47,48 The highest η of
the IL containing the [TBA]+ cation can be explained by strong
molecular interactions due to hydrogen bonding. The lower η
+
of the [TMA] IL can be explained by the charge delocaliza-
tion in the [OH]−anion combined with a low molecular weight
and size of the IL. The highest viscosity found for TBAH
having the highest molecular weight of all ILs studied within
this work. Usually, low η values for ILs where the cation has
a sufficient side chain mobility and a low molecular weight
of IL.28,49
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Figure 7. Deviation in isentropic compressibility (Δκs) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction
○
(x1) of IL for (a) TMAH (1) + NMP (2), (b) TEAH (1) + NMP (2), (c) TPAH (1) + NMP (2), and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30,
□
●
( ) 35, and ( ) 40 °C at atmospheric pressure. The solid lines correlated by Redlich−Kister equation.
gradually up to ∼0.1900 mol fraction of TBAH after an
abruptly decrease in the value for the rest of mole fraction of
TBAH (Figure 3d). The measured ρ results in Figure 3
explicitly elucidate that the ρ values significantly decrease as the
temperature increases in all four systems. As can be seen from
Figure 3, the lower alkyl chain lengths of cations of ILs are
much denser than higher alkyl chain lengths of ILs.
NMP-rich composition, the u values increase, whereas for the
majority of IL compositions, the behavior is the opposite. As
shown in Figure 4, the u values decrease with increasing the
temperature in all ILs studied systems.
To explain the effect of the alkyl chain length of the cation on
the [R4N]+[OH]− and further to gain deeper insight into the
mechanism present within ILs with NMP, we have also mea-
sured the η values for ILs with NMP under the same experi-
mental conditions. The experimental data are presented in
Figure 5 as a function of IL concentrations at different tempera-
tures. An increase in η is observed when TMAH composition
enhances in mixture of TMAH with NMP at all studied
temperatures. This result has been interpreted by the fact that
the strong columbic interaction between the ions is strengthen-
ing upon mixing with the organic solvent leading to a lower
mobility of the ions. Analogously, the η values for the mixture
of TEAH or TPAH + NMP increase rapidly up to ∼0.5000 or
∼0.4000 mol fraction of TEAH or TPAH, respectively. Later
and abrupt, slight increase is observed for the rest of the IL
composition, whereas in the case of TBAH with NMP, the
η values increase rapidly until the mole fraction of IL is
∼0.4000 and then slightly increase with further addition of IL at
all studied temperatures, as shown in Figure 5d.
The temperature-dependent u values of ILs with NMP show
different behavior for each system as displayed in Figure 4. For
the TMAH with NMP system, a monotonic increase is observed
when the TMAH concentration increases in the mixture at all
studied temperatures as illustrated in Figure 4a. The opposite
behavior is observed for the TEAH with NMP system, in
which, the u values sharply increase up to ∼0.3000. Later, the
values of u decrease with increasing the mole fraction of TEAH
up to ∼0.8100, and finally a sharp increase occurs above the
mole fraction ∼0.8100 at all studied temperatures, as shown in
Figure 4b. On the other hand, Figure 4c show that the values of
u were found to increase initially with increasing the mole
fraction of TPAH in NMP system up to ∼0.5700, and afterward
the u values sharply decrease up to ∼0.9000. From ∼0.9000
onward the u value increases at all experimental temperatures.
With an increase in the concentration of TBAH in the mixture
of TBAH + NMP, u values initially increase rapidly up to
∼0.3400 of IL, and later the u values slightly increase for the
remaining composition of TBAH at all investigated tempera-
tures (Figure 4d). Depending on the mixture composition, a
different type of influence of the cation can be observed. So, for
Further, at a given temperature, the η value of the TMAH +
NMP mixture is found to be lower than those of TEAH,
TPAH, and the TBAH + NMP system. Clearly, the η values
increased with increasing the number of carbon atoms in the
alkyl chain and are likely to be influenced by cation size.
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Figure 8. Deviation in viscosity (Δη) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1) of IL for
□
○
(a) TMAH (1) + NMP (2), (b) TEAH (1) + NMP (2), (c) TPAH (1) + NMP (2), and (d) TBAH (1) + NMP (2) at ( ) 25, (Δ) 30, ( ) 35, and
●
( ) 40 °C at atmospheric pressure. The solid lines correlated by Redlich−Kister equation.
However, in the case of TPAH or TBAH with NMP, the
columbic interactions strengthen as the concentration of IL is
increased, and thereby the viscosity is high at all investigated
concentrations. We have suspect the reason for the high
viscosity in the case of TPAH or TBAH is that the size of cation
is large enough to facilitate the interactions between anion and
cation instead of between IL and molecular liquid. On the other
hand, due to the large gap in the size of anion and cation of
TMAH the interactions are not enough to enhance the visco-
sity values. However, interestingly temperature shows similar
effect on all binary mixtures. One can clearly understand from
Figure 5 that, in all investigated binary systems, η decreases as
the temperature increases. This mainly contributes to the weak
coulomb interactions, and thereby, the mobility of ions increases
and ultimately η values are decreased.
Further, for the sake of clarity and presentation, the values of
VE, Δκs, and Δη for IL with NMP over the whole composition
range at 25 °C are graphically represented in Figure 9. These
properties were correlated with composition (x) using the
Redlich−Kister equation
n
Y = x x ( a (x − x )i)
∑
1
2
i
1
2
(1)
i=0
where Y refers to VE, Δκs, or Δη. ai is adjustable parameter and
can be obtained by least-squares analysis. Values of the fitted
parameters are listed in Table 2 along with the standard devia-
tions of the fit.
Graphical representations in Figure 6 show that the values of
VE are negative across the complete composition range for ILs
with NMP systems at all examined temperatures, except for the
TEAH + NMP system. In the case of the TEAH + NMP mix-
ture, the VE values reveal an inversion in the sign from negative
to positive deviation. The negative excess molar volumes reveal
that the systems have a strong packing effect by hetero-
associations between both molecules through H-bonding. The
structural changes upon mixing are remarkable for IL and
NMP, considering that, despite the dipolar ordering in their
pure compounds, H-bonding is only developed in the presence
of IL molecules. As can be seen in Figure 6a, the VE values for
TMAH + NMP are negative in the entire range of composition,
at all temperatures. It approaches the minimum values in the
NMP-rich region for the TMAH + NMP system at mole
Excess molar volume (VE) deviation in isentropic compres-
sibility (Δκs) and the deviation in viscosities (Δη) were
evaluated from the experimental data according to well-known
thermodynamic expressions.13,24 Isentropic compressibility (κs)
was calculated from Laplace equation. The composition
dependence of the VE, Δκs, and Δη properties represents the
deviation from ideal behavior of the mixtures and provides
an indication of the interactions between IL and NMP. The
values of VE, Δκs, and Δη for the binary mixtures at various
temperatures as function of ILs concentrations are included in
Table 1S. Figures 6−8 display the values of VE, Δκs, and Δη,
respectively, along with the fitted curves for the NMP with ILs
as function of IL concentrations at various temperatures.
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Figure 9. Plot of (a) excess molar volumes (VE), (b) deviation in isentropic compressibility (Δκs), and (c) deviation in viscosity (Δη) for the
□
○
●
mixtures of ( ) TMAH (1) +, (Δ) TEAH (1) +, ( ) TPAH (1), or ( ) TBAH (1) + NMP (2) as function of the composition expressed in the
mole fraction (x1) of IL at 25 °C under atmospheric pressure. The solid lines represent the fit of the experimental data by Redlich−Kister equation.
fractions of TMAH about x1 ≈ 0.39000 at all investigated tem-
peratures. This behavior can be explained in terms of hydrogen
bonding that is certainly more-dependent than columbic inter-
actions. The minimum decreases in VE as the temperature
increases for the TMAH + NMP system, and this can be due to
hydrogen bonds between molecules and IL. This phenomenon
is explicitly explained in Scheme 1. The negative excess molar
volumes reveal that a more efficient packing or attractive inter-
actions occurred when the TMAH mixed with NMP. The
interactions between the NMP molecules and the ions of these
TMAH are due to ion−dipole interactions. This will reduce
the hydrogen bond between the cation and anion in the IL,
which contribute to the negative VE values.
In other words, the results in Figure 6b depict that an
inversion in the sign from negative to positive deviation in the
VE values was observed for the TEAH + NMP mixture, which
implies that the interactions between IL and NMP decrease as
the concentration of the IL increases. Moreover, the VE values
are positive for IL-rich compositions and negative for NMP-rich
compositions for the mixture of TEAH + NMP at all investi-
gated temperatures (Figure 6b). 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 NMP and IL
contributes to a positive deviation. The minimum value occurs
at the mole fraction of about ∼0.2700 and maximum at ∼0.8100
for the system TEAH + NMP at all investigated temperature.
The absolute minimum decreases while the maximum increases
with an increase of temperature in this system. It is noteworthy
to compare the negative deviation of excess volumes of TMAH +
NMP with that of TEAH + NMP, suggesting that there is a
difference of the alkyl chain in the TEAH, leading to variation
in the hydrogen bonding.
The values of VE for TPAH with NMP are negative in the
entire range of composition, at various temperatures. It approaches
the minimum values in the TPAH + NMP system at mole
fractions of this IL of about x1 ≈ 0.3300 at all investigated
temperatures, as seen in Figure 6c. This behavior can be
explained in terms of hydrogen bonding that is certainly more
T-dependent than columbic interactions. From close observa-
tion of Figure 6d, one can clearly see that the values of VE for
NMP with TBAH are negative over the whole concentration
range at all investigated temperatures. This might be due to the
large difference between molar volumes of the NMP and
TBAH imply that it is possible that the relatively small organic
molecules fit into the interstices upon mixing at all studied
temperatures. Therefore, the filling effect of molecular liquids in
the interstices of ILs, and the ion-dipole interactions between
organic molecular liquid and ILs, all contribute to the negative
values of VE. On the other hand, the TBAH + NMP mixture
reveals negative values of VE and minimum at ∼0.3200 as seen
in Figure 6d. The minimum can be due to hydrogen bonds
between molecules and IL.
The negative VE values reveal that a more efficient packing or
attractive interaction occurred when these IL and NMP were
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Table 2. Estimated Parameters of eq 1 and Standard Deviation, σ, for the Systems of ILs with NMP as Function of Temperature
Y
system
T/°C
a0
a1
a2
a3
a4
σ
VE/ cm3 mol−1
TMAH + NMP
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
25
30
35
40
−7.182
−6.760
−6.453
−6.105
−6.198
−5.301
−4.401
−3.586
−9.839
−10.366
−10.978
−12.372
−12.215
−9.971
−9.420
−8.696
−138.67
−155.81
−169.97
−184.38
−58.72
−16.20
24.84
6.603
6.092
−2.262
−0.862
0.060
0.054
0.004
0.005
0.144
0.008
0.007
0.008
0.006
0.010
0.008
0.101
0.139
0.008
0.007
0.002
0.4
6.102
5.999
0.785
−2.931
−0.089
−0.512
1.775
TEAH+ NMP
TPAH + NMP
TBAH+ NMP
TMAH + NMP
TEAH+ NMP
TPAH + NMP
TBAH+ NMP
TMAH + NMP
TEAH+ NMP
TPAH + NMP
TBAH + NMP
13.592
14.911
15.506
15.966
7.945
7.644
4.913
4.223
4.355
2.531
4.291
4.199
2.951
4.731
2.995
0.810
−1.017
−1.919
1.901
7.291
7.231
−9.017
−16.185
−12.230
−10.115
−6.042
6.312
3.555
12.293
11.490
11.749
11.580
117.43
119.63
96.92
3.585
−2.734
2.323
−0.471
−17.36
9.19
ΔKs/ T.Pa−1
0.3
−11.48
−54.00
69.44
−43.952
0.6
102.68
544.05
486.65
512.87
543.43
250.12
228.38
227.80
220.53
261.97
247.54
259.77
260.22
0.003
1.2
−7.470
105.16
153.02
151.48
−60.32
1.7
−34.69
−50.28
−126.43
−3.89
−37.05
−51.06
−110.01
−55.33
260.10
349.79
568.17
1.8
2.1
67.06
3.0
−163.36
−178.23
−192.54
−206.87
−228.98
−251.64
−278.71
−286.84
−0.86
1.2
0.7
1.1
2.7
−92.62
1.0
2.0
54.93
−82.56
0.006
31.82
−134.82
−0.375
5.85
1.3
84.53
3.2
Δη/ mPa.s
−0.276
−0.219
−0.161
−0.004
−2.82
0.003
0.003
0.002
0.004
0.007
0.107
0.004
0.003
0.005
0.003
0.002
0.003
0.008
0.005
0.003
0.002
−0.78
−0.716
0.654
−0.037
−0.009
−0.009
−4.52
−5.91
−5.515
−5.218
−6.88
−7.52
−6.39
−5.68
−7.99
−7.95
−8.21
−7.19
−0.102
−0.151
−0.007
2.13
4.84
4.96
4.51
1.29
0.76
4.16
−8.01
−7.57
7.06
−0.18
1.32
−3.13
−6.38
6.64
−0.76
2.54
−4.30
−2.58
6.17
1.73
6.04
5.45
−0.69
mixed at all cases. The interactions between the NMP mole-
cules and the ions of IL are due to ion−dipole interactions. The
observed negative VE values for the mixture of higher alkyl
chain length of IL + NMP are higher than those for lower alkyl
chain length of IL + NMP under same experimental conditions.
Therefore, higher alkyl chain length of cation ILs interacts with
NMP more strongly than lower alkyl chain length of IL. Thus,
it is important to note that the nature of interactions in the IL +
NMP system is highly dependent on the alkyl chain length of
IL. Interestingly, Figures 10−13 demonstrate the hydrogen
bonding between ILs and NMP clearly appearing, through the
semiempirical calculations which we predicted with the help of
Hyperchem 7.
Figure 7 depicts the Δκs values of ILs + NMP over the full
composition range at various temperatures as a function of ILs
concentration. From Figure 7a, it can be seen that Δκs values
are negative over the whole composition range and at all
experimental temperatures for TMAH + NMP systems. The
minimum values are observed at mole fraction ∼0.4000 for the
TMAH + NMP system. The negative Δκs values of TMAH +
NMP are attributed to the strong attractive interactions due to
the solvation of the ions in these solvents. An inversion in the
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Scheme 1. Schematic Depiction of the Hydrogen Bonding Interactions between ILs + NMP for the Negative Deviation
Figure 10. Hydrogen bonding interaction between TMAH and NMP molecules, which is predicted by semiempirical calculation with the help of
Hyperchem 7.
sign of Δκs values from negative to positive is observed for the
TEAH + NMP system at all investigated temperatures (Figure 7b).
As the concentration of the IL increases and a large portion of
the NMP molecules are solvated, the amount of bulk solvent
decreases causing a decrease in the compressibility. As the mole
fraction increases, the negative deviation increases up to ∼0.2700,
while on further addition of the IL, there is decrease in the
compressibility, and it enters into positive deviation and has a
maximum value at the mole fraction of ∼0.8100 at all temperatures.
The curves in Figure 7c,d show that the Δκs values for the
TPAH or TBAH + NMP systems are negative over the com-
plete composition range and at all temperatures employed
(except TPAH at higher IL composition and at low tempera-
ture). The minimum is approached at mole fractions of
∼0.3000 and ∼0.3200 for the TPAH + NMP and TBAH +
NMP systems, respectively. The negative Δκs values attributed
to the strong attractive interactions due to the salvation of the
ions in these solvents. The negative values of Δκs of the IL with
NMP imply that solvent molecules around solute are less
compressible than the solvent molecules in the bulk solutions.
While on further addition of IL there is a decrease in the
compressibility graph at all temperature ranges. This might be
due to decrease in attraction of NMP and IL molecules in IL
rich concentration region. These discrepancies vary from IL to IL
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and solvent to solvent and also depend on the nature as well as
structural arrangement of IL and solvent.
NMP molecules (Scheme 1). The positive values of Δη for the
investigated binary systems suggest that the η values of associ-
ates formed between unlike molecules are relatively more than
those of the pure component. According to Bearman and
Jones,50 the η of a mixture depends strongly on its entropy,
which is related to the liquid’s structure. Therefore, the Δη
value depends on molecular interactions as well as on the size
and shape of the molecules. The highest Δη was observed for
TBAH with NMP at all studied temperatures. These variations
in Δη could be attributed to relatively larger hydrogen bonding
interactions of the ion with NMP as compared with the rest of
the ions of IL with NMP. The minimum Δη values were observed
in the case of TMAH with NMP, as this system contains more van
der Walls interactions. Indeed, the temperature strongly influences
the Δη. As the temperature increases, the Δη values were
decreased for all of the investigated systems, and this reveals that
the interactions become very weak due to weakening of the
dipolar association by the IL as well as the dissociation of the IL
ion pair. These results are well corroborated with the VE and Δκs
data that as the alkyl chain increases the hydrogen bonding
increases.
An examination of the curves in Figure 8 shows that the Δη
values for ILs + NMP are positive as a function of temperature
except for TBAH + NMP in the very IL-rich region, in
which slightly negative values are observed. The deviations are
observed to be slightly negative between the mole fractions of
0.9300 and 1.000 at all experimental temperatures for the
TBAH (Figure 8d). The positive deviation is characteristic of
mixtures containing hydrogen bonding between ions of IL and
In Figure 9a, it can be observed that the negative VE values
increase with increasing alkyl chain length of IL. Clearly, the
negative VE values for the mixture of higher alkyl chain length
of IL + NMP are higher than those for lower alkyl chain length
of IL + NMP at 25 °C. Therefore, it is important to note that
the nature of interactions in the IL + NMP system is highly
dependent on the alkyl chain length of IL. It is interesting to
note that the Δκs values in the TEAH + NMP mixture show
more negative values at the NMP-rich composition than the
rest of the mixtures at 25 °C (Figure 9b). Furthermore, the
observed positive values at IL-rich compositions show that
there exists no specific interactions between unlike molecules
and also that the compact structure of the polar component
(NMP) due to dipolar association has been broken by the
Figure 11. Hydrogen bonding interaction between TEAH and NMP
molecules, which is predicted by semiempirical calculation with the
help of Hyperchem 7.
Figure 12. Hydrogen bonding interaction between TPAH and NMP molecules, which is predicted by semiempirical calculation with the help of
Hyperchem 7.
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Figure 13. Hydrogen bonding interaction between TBAH and NMP molecules, which is predicted by semiempirical calculation with the help of
Hyperchem 7.
TEAH. This conclusion is consistent with the VE data in the
case of the TEAH system. The curves in Figure 9b show that
the Δκs values for the TMAH, TPAH, or TBAH + NMP systems
*E-mail: venkatesup@hotmail.com; pvenkatesu@chemistry.du.
are negative over the whole composition range at 25 °C (except
ac.in. Tel: +91-11-27666646-142. Fax: +91-11-2766 6605.
TPAH at higher IL composition). The results in Figure 9c
AUTHOR INFORMATION
■
Corresponding Author
Notes
show that the Δη values for ILs + NMP are the positive at
The authors declare no competing financial interest.
25 °C except for TBAH + NMP at the very IL-rich region, in
which a slightly negative value observed. The Δη values
increase with increasing the alkyl chain length of the cation of
IL, except in the case of the TBAH system at 25 °C.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Council of Scientific Industrial
Research (CSIR), New Delhi, through Grant No. 01(2343)/
09/EMR-II, the 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.
4. CONCLUSIONS
This work reports extensive experimental data of thermophysical
properties of four ammonium based ILs as a function of tempera-
ture. Further, to obtain some insight into new generation of green
solvents, we synthesized the novel ammonium hydroxide ILs
namely TMAH, TEAH, TPAH, and TBAH and studied the
influence of IL on the polar solvent, NMP. Variable temperature
measurements for ρ, u, and η for ILs with NMP are performed
changing the temperature from 25 to 40 °C in steps of 5 °C under
room pressure. From these measurements, we have predicted VE,
Δκs, and Δη at each temperature as a function of IL concentration.
The predicted properties were correlated with the Redlich−Kister
type equation. Obviously, fascinating results are obtained for
structure-based and temperature-dependent properties of novel
kinds of ILs with NMP, and the data reveals that higher alkyl chain
ILs have a greater tendency to form hydrogen bonds as compared
to lower alkyl chain ILs. These observed interactions are supported
by our theoretical calculations, which are obtained by Hyperchem 7.
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