Determination of thermal diffusivities, thermal conductivities, and sound speeds of room-temperature ionic liquids by the transient grating technique

Article abstract of DOI:10.1021/je0600092

We report measurements of thermal diffusivity of several room-temperature ionic liquids (RTILs) using the transient grating method. Measurements are carried out using ionic liquids with small concentrations of an inert dye that is excited by the 532 nm output of a Nd:YAG laser in a grating with a fringe spacings of (92 and 104) μm. The experiments give thermal diffusivities from which thermal conductivities can be determined, sound speeds, and acoustic damping parameters for seven ionic liquids. In this study, we have used combinations of the cation 1-butyl-3-methylimidazolium ([BMIm]⁺) with the anions tetrafluoroborate ([BF₄]^-), hexafluorophosphate ([PF₆]^-), and bis(trifluoromethylsulfonyl)imide ([Tf₂N]^-) and combinations of the anion [Tf₂N]^- with the cations 1-ethyl-3-methylimidazolium ([EMIm]⁺), 1-pentyl-3-methylimidazolium ([PMIm]⁺), 1-hexyl-3-methylimidazolium ([HMIm]⁺), and 1-octyl-3-methylimidazolium ([OMIm]⁺) to determine the effect of anion and cation on the thermophysical properties of the RTILs. Results obtained indicate that the anion exerts a strong influence not only on the sound speed but also on the thermal diffusivity and acoustic damping of the RTILs. For RTILs with the same cation [BMIm]⁺, changing the anion from [BF ₄]^- to either [PF₆]^- or [Tf ₂N]^- leads to decreases in the sound speed, thermal diffusivity, and thermal conductivity. The size of the cation, however, does not significantly influence the sound speed or the thermal diffusivity of the RTILs.

Full text of DOI:10.1021/je0600092

1
250
J. Chem. Eng. Data 2006, 51, 1250-1255
Determination of Thermal Diffusivities, Thermal Conductivities, and Sound
Speeds of Room-Temperature Ionic Liquids by the Transient Grating Technique
Clifford Frez* and Gerald J. Diebold
Department of Chemistry, Brown University, 324 Brook Street, Providence, Rhode Island 02906
Chieu D. Tran and Shaofang Yu
Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201
We report measurements of thermal diffusivity of several room-temperature ionic liquids (RTILs) using the transient
grating method. Measurements are carried out using ionic liquids with small concentrations of an inert dye that
is excited by the 532 nm output of a Nd:YAG laser in a grating with a fringe spacings of (92 and 104) µm. The
experiments give thermal diffusivities from which thermal conductivities can be determined, sound speeds, and
acoustic damping parameters for seven ionic liquids. In this study, we have used combinations of the cation
+
-
-
1
-butyl-3-methylimidazolium ([BMIm] ) with the anions tetrafluoroborate ([BF4] ), hexafluorophosphate ([PF6] ),
-
-
and bis(trifluoromethylsulfonyl)imide ([Tf2N] ) and combinations of the anion [Tf2N] with the cations 1-ethyl-
-methylimidazolium ([EMIm] ), 1-pentyl-3-methylimidazolium ([PMIm] ), 1-hexyl-3-methylimidazolium ([HMIm] ),
and 1-octyl-3-methylimidazolium ([OMIm] ) to determine the effect of anion and cation on the thermophysical
+
+
+
3
+
properties of the RTILs. Results obtained indicate that the anion exerts a strong influence not only on the sound
speed but also on the thermal diffusivity and acoustic damping of the RTILs. For RTILs with the same cation
+
-
-
-
[
BMIm] , changing the anion from [BF4] to either [PF6] or [Tf2N] leads to decreases in the sound speed,
thermal diffusivity, and thermal conductivity. The size of the cation, however, does not significantly influence
the sound speed or the thermal diffusivity of the RTILs.
Introduction
beams in space generated by intersecting two coherent, pulsed
laser beams, referred to as “pump” beams, in a weakly absorbing
Room-temperature ionic liquids (RTILs) are organic salts that
are liquid at room temperature and that have unique chemical
and physical properties, including stability on exposure to air
and moisture, a high solubility power, and extremely low vapor
pressure. Because of these properties, they can serve as a “green”
recyclable alternative to the volatile organic compounds that
are traditionally used as industrial solvents. In the laboratory,
RTILs have successfully been used in a broad spectrum of
applications, including replacing traditional organic solvents in
organic and inorganic syntheses, solvent extractions, liquid-
liquid extractions, electrochemical reactions, and as a medium
for enzymatic reactions.1
The majority of studies have focused on synthesizing RTILs
with new anions and cations and determining their chemical
properties. Unfortunately, information on the RTILs including
their physical properties and the relationships between structure
and physical properties is not widely available despite their
superior physical properties, which make them suitable not only
as green solvents but also as high-performance fluids for use in
a wide range of engineering and materials science applications,
such as high-pressure and high-temperature1-8 lubricants. It is
probable that industrial applications of RTILs in chemistry,
engineering, and material processing are limited because of the
paucity of data on their physical properties.
9
-15
sample.
Following absorption of the laser radiation, the
excited chromophore typically decays on a fast time scale with
respect to the laser pulse width depositing the absorbed optical
energy in the fluid as heat, giving a sinusoidal temperature rise
in the fluid in space and generating counter propagating acoustic
waves. The acoustic standing wave acts as a nearly adiabatic
sound wave and is referred to as the acoustic mode of wave
motion while the slow decay of the sinusoidal temperature
increase is known as the thermal mode of wave motion.16 Both
the acoustic and thermal modes of the wave in the grating have
associated density changes; hence, a recording of the intensity
of diffraction of a continuous “probe” laser beam directed at
the Bragg angle to the grating acts as a monitor of the time
evolution of the density variations induced by optical excitation.
From a knowledge of the optical fringe spacing in the grating,
measurements of the oscillation frequency and decay rate of
the acoustic wave can be used to determine the sound speed
and damping rate of the acoustic wave, while a measurement
of the slow decay of the sinusoidal temperature variation gives
-8
1
2-16
the thermal diffusivity of the fluid.
Here we report a study using the transient grating method9
-16
directed not only at measurement of the physical properties of
RTILs but also toward determining the relationship between
the RTIL structure and their thermophysical properties. In the
Experimental Section, we describe the experimental apparatus,
present data, and give expressions for reduction of the data.
The Discussion section points out the correlations in the
thermophysical properties of the different RTIL samples studied,
elucidating the effects of the different anions or cations on the
thermophysical properties of the RTILs.
Transient grating method is a noncontact method of probing
both photothermal and photoacoustic changes in density
can be used to determine thermophysical properties of solids,
liquids, and gases. The method is based on the formation of a
series of nodes and antinodes in the electric field of the laser
9-11
and
*
Corresponding author. Tel: (401)863-3619. E-mail:frez@brown.edu.
1
0.1021/je0600092 CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/13/2006

Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006 1251
Figure 1. Schematic diagram of the transient grating system.
Table 1. List of Room Temperature Ionic Liquids Used in This Study
Experimental Section
(Hamamatsu, Inc. model R928) whose output was fed to an
inverting operational amplifier (Burr-Brown Inc., model OPA686)
was used to detect the first-order diffracted beam. The photo-
multiplier was equipped with a narrow band interference filter
to block scattered radiation from the pump laser. Signals from
the photomultiplier were averaged with a 1.5 GHz bandwidth
Instrument and Methods. The transient grating apparatus
9
used here is similar to that described by previous researchers,
except that a relatively long laser pulse and a small crossing
angle are used for the pump beams. As shown in Figure 1, the
frequency doubled 532 nm; 10 ns output of a Q-switched Nd:
YAG laser operated at 10 Hz was used as the pump beam. The
(Agilent, Inc., model 545845) digitizing oscilloscope.
The pump laser beam could be attenuated with a half wave
532 nm beam was split into two equal intensity beams by a
plate and Glan Taylor polarizer to give pulse energies of the
order of 10 mJ pulses at the front surface of the cuvette. In
beam splitter and recombined to produce approximately 100
µm optical fringes in a 1.0 cm × 1.0 cm × 4.5 cm glass cuvette.
A continuous, 4 mW, 633 nm He-Ne laser beam was directed
at the Bragg angle, θB, to the grating where θB satisfies
+
-
experiments with [BMIm] [PF6] , laser pulse energies were on
the order of 33 mJ. (See Table 1 for structures of the RTILS
used in this study.) Experiments were carried out over a range
of pulse energies to determine if the recorded waveforms were
affected by laser pulse energy. Because the RTIL samples are
transparent at 532 nm, an absorbing dye tris(1,10-phenanthro-
line) iron(II) sulfate known as ferroin was added to each sample.
sin θ ) λprobe/2Λ
(1)
B
where Λ is the fringe spacing of the grating and λprobe is the
wavelength of the probe beam. A side-on photomultiplier

1
252 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
Specifically, 1.0 µL of 0.025 mol‚L-1 aqueous ferroin solution
[EMIm] [Tf2N]- 1H NMR (DMSO-d6, 300 MHz). δ: 9.09
(1H, s), 7.75 (1H, s), 7.66 (1H, s), 4.18 (2H, q), 3.83 (3H, s),
1.41 (3H, t).
+
was added to 2 mL of each RTIL sample. A control experiment
+
-
was carried out with [BMIm] [BF4] , which has a small
absorbance so that a transient grating experiment could be done
without the addition of a dye. The results of the experiment
agreed, within the experimental error, with a measurement on
the same RTIL containing the 1.0 µL ferroin dye solution used
with the other RTILs.
[PMIm] [Tf2N]- 1H NMR (DMSO-d6, 300 MHz). δ: 9.08
(1H, s), 7.74 (1H, t), 7.67 (1H, t), 4.13 (2H, t), 3.82 (3H, s),
1.76 (2H, m), 1.28 (2H, m), 1.19 (2H, m), 0.85 (3H, t).
+
+
- 1
[HMIm] [Tf2N] H NMR (DMSO-d6, 300 MHz). δ: 9.07
(1H, s), 7.74 (1H, t), 7.67 (1H, t), 4.13 (2H, t), 3.82 (3H, s),
1
.76 (2H, m), 1.25 (6H, broad), 0.84 (3H, t).
[
Experiments were conducted first to determine the fringe
+
- 1
OMIm] [Tf2N] H NMR (DMSO-d6, 300 MHz). δ: 9.07
(1H, s), 7.74 (1H, t), 7.67 (1H, t), 4.12 (2H, t), 3.82 (3H, s),
.76 (2H, m), 1.23 (10H, broad), 0.84 (3H, t). Complete NMR
spacing, performed by using a solution of CoCl2 in methanol
whose sound speed is well-documented.17 The acoustic wave-
1
length Λ (related to the wavenumber K through K ) 2π/Λ) for
two different angles of intersection of the pump beams was
found to be 104 µm and 92 µm. Data for each RTIL sample
was taken several times using different laser fluences; data were
taken at the two different angles of intersection of the pump
beams as well. All experiments were done at 0.1 MPa and
spectra of all seven ionic liquids are presented in the Supporting
Information.
Data Reduction. The theory governing the photoacoustic
effect and its application to the transient grating technique have
been described previously.
δ can be written as
10-16
The density change in the grating
2
96.85 K.
Chemicals. 1-Methylimidazole (mole fraction purity of 99
), 1-chlorooctane (mole fraction purity of 99 %), 1-chloro-
aj E0â
Cp
-K2Rt
-K2σt
δ )
[-e
+ e
cos(cKt)]
(2)
%
pentane (mole fraction purity of 99 %), 1-chlorohexane (mole
fraction purity of 99 %), acetonitrile (mole fraction purity of
9 %), ethyl acetate (mole fraction purity of 99 %), lithium
trifluoromethane sulfonimide (LiTf2N) (mole fraction purity of
9.95 %), and hexafluorophosphoric acid with a mass fraction
of 0.60 in water were purchased from Alfa Aesar. 1-Methylimi-
dazole was distilled under vacuum prior to use. Other chemicals
were used as received.
where the damping parameter σ and the viscous and heat
9
16
conduction lengths lv′ and lh, respectively, are given by
9
1
2
σ ) c[l ′ - (γ - 1)l ]
v
h
4
η + µ
3
Chloride salts of C5, C6, and C8 alkyl imidazolium were
synthesized from corresponding alkyl chloride and 1-meth-
lv′ )
Fc
6
-8
ylimidazole based on methods used in our previous studies.
ø
c
lh )
(3)
For example, octylmethylimidazolium chloride was prepared
by refluxing a 1:1 molar ratio mixture of 1-methylimidazole
and octyl chloride under nitrogen at 333 K for 2 days. Initially,
there were two layers in the mixture. As the reaction proceeded,
it became one single layer. The product was washed twice with
ethyl acetate. The chloride salts were then converted into
The quantities R, c, γ, η, µ, and F are the thermal diffusivity,
sound speed, heat capacity ratio, bulk viscosity coefficient, shear
viscosity coefficient, and density of the fluid, respectively. E0,
aj , â, and C are the energy fluence of the laser beam, the optical
absorption coefficient, the volume expansion coefficient, and
the isobaric heat capacity, respectively, of the fluid. The first
and second terms in eq 2 describe the time dependences of the
thermal and acoustic modes of wave motion, respectively. Since
the decays of the acoustic and thermal mode densities back to
their ambient values take place on such different time scales,
they were recorded on the oscilloscope using different time
bases. The fast oscillations of the acoustic mode were not visible
on the long time base used for recording the thermal mode;
hence, the diffracted light intensity recorded by the photomul-
tiplier, represented by I and I for the two different modes,
p
-
corresponding [Tf2N] salts by metathesis reaction using the
procedure reported in our earlier studies.6 Essentially, a 1:1
molar ratio of alkyl methylimidazolium chloride and LiTf2N
were dissolved in cold water separately. The solutions were then
mixed together and stirred for 2 h at room temperature. As the
reaction proceeded, the homogeneous solution separated into
two layers with a water layer on top and an RTIL layer in the
bottom. The upper water layer was discarded; the RTIL was
further washed with water three times and then dried under
vacuum at 333 K overnight. Similar procedures were used to
-8
+
-
+
-
prepare [C4MIm] [BF4] and [C4MIm] [PF6] from [C4MI-
A
T
+
-
m] [Cl] . Purity of the ionic liquids obtained was verified by
H NMR. The chemical shifts agree with previous research.
Although a reference is not available for [PMIm] [Tf2N] , the
NMR spectra is in excellent agreement with the expected
characteristic peaks of this RTIL. The NMR characteristics of
each compound are as follows:
were fitted separately with the following two expressions:
1
18-24
2
+
-
2π(t + C)
[
-E(t+C)
I (t) ) F -A + e
cos
}) + D1
]
(4)
(5)
A
( {
B
-
H(t+C) 2
I
(t) ) [-G e
] + D
2
T
[
BMIm] [BF4]- 1H NMR (DMSO-d6, 300 MHz). δ: 9.06
+
where the quantities denoted by capital letters are adjustable
parameters used in the least-squares fitting procedure. In the
data fitting, the thermal mode contribution to the signal on a
short time scale is approximated as -1 in eq 4, as its decay is
negligible on a short time scale. Figures 2 and 3 show an
(
1
1H, s), 7.73 (1H, t), 7.67 (1H, t), 4.13 (2H, t), 3.82 (3H, s),
.74 (2H, m), 1.24 (2H, m), 0.88 (3H, t).
+
[
BMIm] [PF6]- 1H NMR (DMSO-d6, 300 MHz). δ: 9.06
(1H, s), 7.73 (1H, t), 7.66 (1H, t), 4.13 (2H, t), 3.82 (3H, s),
1
.74 (2H, m), 1.24 (2H, m), 0.88 (3H, t).
+
-
experimental waveform for the ionic liquid [BMIm] [BF4]
and their least-squares fits using eq 4.
+
[
BMIm] [Tf2N]- 1H NMR (DMSO-d6, 300 MHz). δ: 9.09
(
1
1H, s), 7.74 (1H, t), 7.67 (1H, t), 4.15 (2H, t), 3.83 (3H, s),
.75 (2H, m), 1.25 (2H, m), 0.89 (3H, t).
To ensure the accuracy of the technique and the reliability
of the fitting model, calibration experiments were performed

Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006 1253
Table 2. Experimental Thermal Diffusivities (r), Sounds Speeds (c), and Damping Parameters (σ) of Ionic Liquids Compared with Literature
Sound Speeds and Viscosities
0-
7
R
c
cRef/(m‚s-1
)
10-4
σ
η/(mPa‚s)
1
2
-1
m‚s-1
2
-1
ionic liquid
m ‚s
p ) 0.1 MPa T ) (293.15 ( 0.01) K
m ‚s
T/K
298
298
293.15
293.15
+
-
1570 ( 3a
119.78 ( 1.28d
c
[
[
[
[
[
[
[
BMIm] [BF4]
0.86 ( 0.01
0.75 ( 0.02
0.60 ( 0.01
0.601 ( 0.004
0.58 ( 0.02
0.606 ( 0.009
0.61 ( 0.01
1564 ( 8
1441 ( 4
1227 ( 6
1240 ( 4
1227 ( 2
1232 ( 11
1232 ( 5
3.7 ( 0.4
4.2 ( 0.4
2.4 ( 0.6
2.6 ( 0.7
3.0 ( 0.8
2.3 ( 0.3
3.1 ( 0.7
+
-
1433 ( 3a
BMIm] [PF6]
207.00 ( 11.12
+
-
1238.06 ( 62b
c
BMIm] [Tf2N]
52 ( 3
+
-
c
EMIm] [Tf2N]
31
+
-
PMIm] [Tf2N]
+
-
-
HMIm] [Tf2N]
+
OMIm] [Tf2N]
a
The literature values were taken from ref 26. b The literature values were taken from ref 27. c The literature values were taken from ref 2. d The literature
values were taken from ref 28.
Figure 2. Photomultiplier signal vs time for an experiment with
+
-
[
BMIm] [BF4] showing the acoustic mode of wave motion. The solid
Figure 4. Photomultiplier signal (in arbitrary units) vs time for three
RTILs: (a) [BMIm] [BF4] ; (b) [BMIm] [PF6] ; and (c) [BMIm] [Tf2N] .
+
-
+
-
+
-
trace is the experimental signal and the circles are the model fit using eq 4.
Results and Discussion
The results listed in Table 2 show that the properties of the
anion of the RTILs have a strong effect on the sound speed;
-
-
specifically, changing the anion from [BF4] to either [PF6]
-
or [Tf2N] leads to a decrease in the sound speeds, from (1564
8) m‚s for [BMIm] [BF4] to (1441 ( 4) m‚s for
BMIm] [PF6] and to (1227 ( 6) m‚s for [BMIm] [Tf2N] .
-
1
+
-
-1
(
+
-
-1
+
-
[
A change of the cation in these species, on the other hand, does
not show a significant affect on the sound speed of the RTILs:
the sound speed of the RTILs with C2MIm cations (i.e.,
+
-
-1
[
EMIm] [Tf2N] (1240 ( 4) m‚s ) is similar to that for RTILs
+
-
-1
Figure 3. Photomultiplier signal vs time for an experiment with
with C4MIm cations ([BMIm] [Tf2N] (1227 ( 6) m‚s ), C5-
+
-
[
BMIm] [BF4] showing the thermal mode of wave motion. The ratio of
+
-
-1
MIm cations ([PMIm] [Tf2N] (1227 ( 2) m‚s ), C6MIm
the standard deviation of the fit to the least squares value of the thermal
diffusivity is 0.01.
+
-
-1
cations ([HMIm] [Tf2N] (1232 ( 11) m‚s ), and C8MIm
+
-
-1
cations ([OMIm] [Tf2N] (1232 ( 5) m‚s ). Generally, it
would be expected that an increase in size of the cation by
lengthening the alkyl chain would increase the viscosity and
-1
using CoCl2 in methanol (with an absorbance less than 0.3 cm )
as a standard. It was found that the thermal diffusivity of
methanol determined by the transient grating method was found
2,25
hence the damping of the acoustic mode. Although, the use
of the damping constant appears to be an effective indicator
for measuring large viscosity changes, it does not appear to
discern between small changes in viscosity. More experiments
with different RTILs are required for an accurate assessment.
It is noteworthy that the sound speeds reported here for
-
7
2
-1
to be (1.00 ( 0.03)‚10 m ‚s and is in excellent agreement
with ref 17. In measurements on the RTILs, data were taken at
grating fringe spacings of 104 µm and 92 µm (i.e., with
4
-1
4
-1
wavenumbers of 6.0‚10 m and 6.8‚10 m , respectively).
The data for each chemical species listed in Table 2 are averages
from experiments taken with different laser energy fluences and
fringe spacings.
+
-
+
-
-1
[BMIm] [BF4] and [BMIm] [PF6] agree to within ( 8 m‚s
26
with the results determined by other workers using an acoustic
microcell operating at 0.5 MHz.
As can be seen by inspection of Figure 4, the damping
The discrepancies in thermal diffusivities and sound speeds
of the RTILs had no dependence on changes in either laser
energy fluence or fringe length. Typically, the percent relative
standard deviation of the experiments for each RTIL in
determining the thermal diffusivity is better than ( 4 % and in
determining the sound speed is better than ( 0.5 %. The
standard deviations of the data are reported in Table 2. Some
of the uncertainties and conditions of the reference values were
not found in the literature.
+
-
+
-
constant for [BMIm] [PF6] is similar to that of [BMIm] [BF4]
+
-
but is approximately double that of [BMIm] [Tf2N] . As Table
shows, the character of the anion of the RTIL has a strong
2
effect on the damping parameter σ, while that of the cation exerts
no significant effectsa trend that parallels the relative influence
of the anion and cation on the sound speed, as noted above.
Since σ is a decay parameter that includes energy losses
primarily due to viscosity with typically only a small contribu-
tion from heat conduction, the data suggest that for identical

1
254 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
+
-
+
-
+
-
+
-
Table 3. Experimentally Determined Thermal Conductivities (λ) of [BMIm] [BF4] , [BMIm] [PF6] , [BMIm] [Tf2N] , and [EMIm] [Tf2N]
from Previously Reported Values of Density (G) and Specific Heat Capacity (Cp)a
F/(kg‚m-3)
T/K
Cp/(J‚kg-1‚K-1
)
λRef/(W‚m-1‚K-1
)
ionic liquid
p/MPa
p ) 0.1 MPa T ) 298.15 K
1613 ( 81b
λ/(W‚m-1‚K-1
)
T ) 298 K
0.186 ( 0.001f
+
-
1205.0 ( 0.2b
[
BMIm] [BF4]
0.1
298.15
95.4
93.15
298.15
98
298.15
95
96.4
0.162 ( 0.006
e
e
2
2
1205 ( 2
1555 ( 58
f
g
1201.8
1614
+
-
b
1438 ( 72b
[
BMIm] [PF6]
0.1
0.1
1366.0 ( 0.3
0.109 ( 0.005
0.108 ( 0.004
e
2
1363d
1399 ( 62
+
-
c
1293c
[
BMIm] [Tf2N]
1437.04
2
2
1429d
1279 ( 46e
e
1438.6 ( 0.3
+
-
1520d
1340 ( 52e
[
EMIm] [Tf2N]
295
96.0
0.120 ( 0.005
2
1521(2e
1291g
a
b
The thermal conductivities presented by this work were calculated from the averages of the literature heat capacities and literature densities. The
literature values were taken from ref 26. The literature values were taken from ref 27. The literature values were taken from ref 2. e The literature values
c
d
were taken from ref 5. The literature values were taken from ref 28. g The literature values were taken from ref 29.
f
Supporting Information Available:
1
The complete H NMR spectra of the seven RTILs reported in
this work. This material is available free of charge via the Internet
at http://pubs.acs.org.
Literature Cited
(
(
(
1) Welton, T. Room-temperature ionic liquids. Solvents for synthesis and
catalysis. Chem. ReV. 1999, 99, 2071-2084.
2) Mantz, R. A.; Trulove, P. C. Ionic Liquids in Synthesis; Wasserscheid,
P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2003.
3) Holbrey, J. D.; Reichart, W. M.; Reddy, R. G.; Rogers, R. D. Ionic
Liquids as Green SolVents; Seddon, K. R., Rogers, R. D., Eds.; ACS
Symposium Series 856; American Chemical Society: Washington,
DC, 2003.
Figure 5. Photomultiplier signal vs time for three RTILs. The signal for
all three traces is scaled to unity at time ) 0. The thermal diffusivities of
the RTILs can be seen to decrease with the size of the anion.
(4) Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley: New
York, 2005.
+
-
(5) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, N. V. K.;
Brennecke, J. Thermophysical properties of imidazolium-based ionic
liquids. J. Chem. Eng. Data 2004, 49, 954-964.
6) Tran, C. D.; De Paoli Lacerda, S. H.; Oliveira, D. Absorption of water
by room-temperature ionic liquids: affect of anions on concentration
and states of water. Appl. Spectrosc. 2003, 57, 152-157.
7) Tran, C. D.; De Paoli Lacerda, S. H. Determination of binding
constants of cyclodextrins in room-temperature ionic liquids by near-
infrared spectrometry. Anal. Chem. 2002, 74, 5337-5341.
densities that [BMIm] [PF6] is the most viscous of the RTILs
studied here and that varying the number of carbons on the
cation has little influence on the overall viscosity. Furthermore,
if the bulk viscosity can be taken as negligible to the shear
viscosity, then the experimental quantity σ is related to the bulk
and shear viscosities through σ ) (2/3)‚µ‚F. As listed in Table
(
(
+
-
+
-
2
, viscosities for [BMIm] [PF6] , [BMIm] [Tf2N] , and
+
-
2
[
EMIm] [TF2N] are reported to be 207 mPa‚s, 52 mPa‚s, and
(8) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. The structure of a
room-temperature ionic liquid with and without trace amounts of
water: the role of C-H‚‚‚O and C-H‚‚‚‚F interactions in 1-n-butyl-
3
1 mPa‚s, respectively. This mirrors the trend for σ determined
from the experiments reported here.
3-methylimidazolium tetrafluoroborate. Angew. Chem., Int. Ed. 2003,
4
2, 4364-4366.
As in the case of the sound speed and damping, the character
of the anion in an RTIL exerts strong influence on thermal
diffusivity. The effect of changing anion can be seen in Figure
(
9) Mandelis, A. Non-destructiVe EValuation (NDE); Prentice Hall:
Englewood Cliffs, NJ, 1994.
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distribution of relaxation times in glycerol: time-domain light scat-
tering study of acoustic and mountain-mode behavior in the 20 MHz-3
GHz frequency range. J. Chem. Phys. 1988, 88, 6477-6486.
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5, which plots the decays of the thermal mode for the three
(
(
RTILs with the same cation but with different anions. It is
evident that the decay of the thermal mode becomes slower as
the anion increases in bulk.
Although, the transient grating measurements yield values
for thermal diffusivity for each RTIL directly, the thermal
conductivity must be found from a knowledge of the density
-
1
-1
and specific heat capacity through the relation R ) λ‚F ‚Cp
.
(
14) Chen, H. X.; Diebold, G. J. Production of the photoacoustic effect
and transient gratings by molecular volume changes. J. Chem. Phys.
1996, 104, 6730-6741.
Only a few values of density and heat capacity have been
+
-
+
-
+
-
reported for [BMIm] [BF4] , [BMIm] [PF6] , [BMIm] [Tf2N] ,
+
- 2,5,26-29
(15) Sun, T.; Morais, J.; Diebold, G. J.; Zimmt, M. B. Investigation of
viscosity and heat conduction effects on the evolution of a transient
picosecond photoacoustic grating. J. Chem. Phys. 1992, 97, 9324-
and [EMIm] [Tf2N] .
Based on the reported values of
these two quantities, thermal conductivities for these four RTILs
were calculated, and the results listed in Table 3.
9334.
(
16) Morse, P.; Ingard, K. U. Theoretical Acoustics; McGraw-Hill: New
In summary, we have reported values for the sound speed,
acoustic damping, and thermal diffusivities of several RTILs.
The relative ease of determining these parameters, the small
error found in the resulting data fits, and the agreement of some
of the extracted parameters with conventional methods reaffirms
the value of the transient grating method for determining
thermophysical parameters of fluids.
York, 1968.
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ylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium
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(
(
(
20) Reynolds, J. L.; Erdner, K. R.; Jones, P. B. Photoreduction of
benzophenones by amines in room-temperature ionic liquids. Org. Lett.
2002, 4, 917-919.
(
27) de Azevedo, R. G.; Esperanca, J. M. S. S.; Szydlowski, J.; Visak, Z.
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(
28) Van Valkenburg, M. E.; Vaughn, R. L.; Williams, M.; Wilkes, J. S.
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(
(
(
(
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Received for review January 6, 2006. Accepted April 5, 2006. C.F. and
G.J.D. gratefully acknowledge the support of this research by the U.S.
Department of Energy under Grant ER-13235.
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