Fluid-phase behavior of {1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C6mim][NTf2], + C2-C8 n-alcohol} mixtures: Liquid-liquid equilibrium and excess volumes

Article abstract of DOI:10.1021/je060307z

In the frame of the IUPAC Project entitled Thermodynamics of ionic liquids, ionic liquid mixtures, and the development of standarized systems, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C ₆mim][NTf₂], was selected as the model ionic liquid to perform round-robin measurements on a series of ionic liquid properties. In this work, the temperature-composition liquid-liquid equilibria of this ionic liquid with linear alkan-1-ols, C_nH_(2n+1)OH, with n = 4, 5, 6, and 8, were determined at a nominal pressure of 0.1 MPa and up to 50 MPa for n = 4, 5, and 6. Excess molar volumes, V^E, of mixtures of this same ionic liquid with linear alkan-1-ols, C_nH _2n+1OH, with n ranging from 2 to 5, were measured at a nominal pressure of 0.1 MPa. The consistency between the two sets of experimental data (phase diagrams and V^E) was analyzed from a theoretical perspective; we noticed that as the alkyl chain length, n, of the linear alcohol varies both the pressure dependence of the phase diagrams and the critical excess molar volume change sign.

Full text of DOI:10.1021/je060307z

J. Chem. Eng. Data 2006, 51, 2215-2221
2215
Fluid-Phase Behavior of {1-Hexyl-3-methylimidazolium
Bis(trifluoromethylsulfonyl) Imide, [C6mim][NTf2], + C2-C8 n-Alcohol}
Mixtures: Liquid-Liquid Equilibrium and Excess Volumes‡
Joanna Łachwa,† Pedro Morgado,§ Jose´ M. S. S. Esperanc¸a,† Henrique J. R. Guedes,|
Jose´ N. Canongia Lopes,*,†,§ and Lu´ıs Paulo N. Rebelo*,†
Instituto de Tecnologia Qu´ımica e Biolo´gica, ITQB2, Universidade Nova de Lisboa, Av. Repu´blica, Apartado 127, 2780-901
Oeiras, Portugal, Centro de Qu´ımica Estrutural, Instituto Superior Te´cnico, 1049-001 Lisboa, Portugal, and REQUIMTE,
Departamento de Qu´ımica, Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
In the frame of the IUPAC Project entitled Thermodynamics of ionic liquids, ionic liquid mixtures, and the
deVelopment of standarized systems, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C6mim]-
[NTf2], was selected as the “model” ionic liquid to perform round-robin measurements on a series of ionic liquid
properties. In this work, the temperature-composition liquid-liquid equilibria of this ionic liquid with linear
alkan-1-ols, CnH(2n+1)OH, with n ) 4, 5, 6, and 8, were determined at a nominal pressure of 0.1 MPa and up to
50 MPa for n ) 4, 5, and 6. Excess molar volumes, VE, of mixtures of this same ionic liquid with linear alkan-
1-ols, CnH2n+1OH, with n ranging from 2 to 5, were measured at a nominal pressure of 0.1 MPa. The consistency
between the two sets of experimental data (phase diagrams and VE) was analyzed from a theoretical perspective;
we noticed that as the alkyl chain length, n, of the linear alcohol varies both the pressure dependence of the phase
diagrams and the critical excess molar volume change sign.
mixtures is an important issue. Particularly, studies involving
the liquid-liquid equilibrium (LLE) and volumetric behavior
of mixtures have a double objective: (i) they supply fundamental
data which are paramount for, e.g., mass balance and mass
transfer calculations in separation and extraction processes, and
(ii) they provide insight about the nature of the interactions
between the ionic liquid’s ions and solvent molecules.
In this work, we investigated mixtures of [C6mim][NTf2] with
n-alkan-1-ols of different alkyl chain length (from C2 to C8):
for the shorter alcohols (ethanol to pentan-1-ol), we measured
the excess molar volume (VE) of the (IL + alcohol) binary
mixtures as a function of composition and temperature, whereas
for alcohols with longer chains (butan-1-ol to octan-1-ol), we
mapped the liquid-liquid phase diagram and its pressure
dependence in analogous mixtures. The consistency between
the two sets of experimental data was analyzed from a theoretical
perspective because the sign of the molar excess volume results
must reflect the pressure dependence of the liquid-liquid phase
diagrams.
Introduction
Ionic liquids (ILs) are a class of salts with melting point (mp)
temperatures below 100 °C. Apart from these unusually low
mp temperatures, ILs are characterized by a negligible vapor
pressure,1,2 a broad liquid range,3 and a very rich and complex
behavior as solvents4-7 that can be modified by changing the
nature of the cation or anion. Their classification as “green” or
“designer” solvents (based upon some of the above-mentioned
characteristics) has triggered a wealth of attention from both
the scientific community and industry. Studies as diverse as
applications in synthesis and catalysis, separation methods,
electrochemistry, or analytical methods are currently under
way.8-10 As well, several processes have already been imple-
mented at a pilot or industrial scale.11-13
Ionic liquids usually consist of a large, asymmetric organic
cation coupled with a generally smaller, weakly coordinating
anion. The most commonly used cations are those of the 1-alkyl-
3-methylimidazolium family, [Cnmim]+. On the other hand, bis-
(trifluoromethylsulfonyl)imide, [NTf2]-, has gained in recent
years some importance among the commonly used anions due
to its stability to moisture, air, and high-temperature conditions.
Therefore, this anion was recently selected14 along with the
1-hexyl-3-methylimidazolium cation, [C6mim]+, as a standard
IL reference against which the properties of other ILs can be
compared.
Experimental Section
Chemicals. The ionic liquid used in this project,14 1-hexyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [C6-
mim][NTf2], came from a batch synthesized by Dr. Mark
Muldoon in Professor Joan Brennecke’s laboratories at the
University of Notre Dame, where it underwent several purifica-
tion stages (see Supporting Information). This 50 cm3 batch
labeled as “Tube #8” came from the material being used for
the entire IUPAC Project 2002-005-1-100 (http://www.iupa-
c.org/projects/2002/2002-005-1-100.html) and is known to be
of high purity. It is colorless to the eye, contains less than 10
ppm of halide, measured by an ion-selective electrode, and
contains less than 20 ppm of water, analyzed by coulometric
In this context, a systematic investigation of the thermody-
namic and thermophysical properties of [C6mim][NTf2] and its
‡ Work performed in the frame of the IUPAC Project 2002-005-1-100
entitled Thermodynamics of ionic liquids, ionic liquid mixtures, and the
deVelopment of standardized systems (http://www.iupac.org/projects/2002/
2002-005-1-100.html).
* Corresponding authors. E-mail: luis.rebelo@itqb.unl.pt; jnlopes@ist.utl.pt.
† Instituto de Tecnologia Qu´ımica e Biolo´gica, Universidade Nova de Lisboa.
§ Instituto Superior Te´cnico.
1
| Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa.
Karl Fischer titration. Additionally, H and 19F NMR spectra
10.1021/je060307z CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/03/2006

2216 Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006
(see Supporting Information) show the ionic liquid purity to be
greater than 99.5 % (excluding the water impurity and other
peaks known to originate from the NMR solvent). During the
manipulation of the sample, it is expected that the ionic liquid
increases its water content. Karl Fischer analysis has never
revealed levels of water after measurements greater than 240
ppm. After the initial set of experimentssvolumetric and/or LLE
determinations in (IL + alcohol) binary mixturessthe ionic
liquid was recycled and reused in further experiments as it is
very easy to fully separate the alcohol from the IL by moderate
warming in a vacuum (0.1 Pa) line. During this process, the
alcohol contained in the mixtures was distilled under reduced
pressure at temperatures ranging from room temperature to 120
°C and the IL remaining in the residue was subjected to further
degassing (in conditions similar to those stated above). The
purity of the recovered IL was checked by further analyses
(NMR, Karl Fischer), which showed purification levels similar
to those for the original sample. Additionally, tests comparing
the density of the recycled IL with that of the original sample
revealed no difference within the experimental density uncer-
tainty ((0.01 %).
allowed to re-enter the samples during the mass measurements
and the mixture and transfer processessis exactly the same in
samples obtained at different times. To check this point, different
VE determinations were performed using samples (pure IL and
its mixtures) prepared from a batch that was thoroughly
degassed, and other determinations were done with other
samples obtained from a batch saturated in nitrogen. Although
the density of the pure ILs and their mixtures was different in
the two cases (obviously lower density for the gas-saturated
samples), the VE values were comparable within (0.15 % as
long as the excess property was calculated using exclusively
data from one experiment (degassed) or the other (gas-saturated).
Finally, it was checked whether viscosity corrections15,16 to
the direct, raw density determinations needed to be considered.
It is known that the bistriflamide anion combined with dialkyl-
imidazolium cations yields low-viscosity ionic liquids, as
compared with analogous ionic liquids based on other anions
such as chloride or hexafluorophosphate.17 In the case of
[C6mim][NTf2], the viscosity correction between (293 and 303)
K corresponds to a decrease in density of less than 0.01 % and
was not considered in the calculations.
All n-alcohols were dried with 3 Å molecular sieves. Their
origin was the following: ethanol, 99.8 % (Panreac); 1-propanol,
99.7 % (Sigma-Aldrich); 1-butanol, 99.9 % (Merck); 1-pentanol,
99 % (Sigma-Aldrich); 1-hexanol, 99 % (Sigma-Aldrich);
1-octanol, 99 % (Sigma-Aldrich). Karl Fischer titrations per-
formed on the dried alcohol samples revealed very low levels
of water (less than 10 ppm).
Equipment and Procedure for the Liquid-Liquid Equilib-
rium Measurements. Part of the liquid-liquid equilibrium
temperature data at 0.1 MPa nominal pressure was determined
using a dynamic method with visual detection of the phase
transitions (naked-eye determination of turbidity).
The appropriate mixtures of the ionic liquid and solvent were
placed in Pyrex glass capillaries and heated, for two-phase to
one-phase transitions (or cooled off, for one-phase to two-phase
transitions), very slowly (less than 2 K‚h-1 near the equilibrium
temperature) with continuous stirring. For measurements be-
tween (283 and 343) K, a glass thermostatic bath filled with
water was used, whereas for lower temperatures, ethanol/water
mixtures were used. The capillaries were sealed at both ends,
and the mixture occupied almost the entire internal volume
((0.5 cm3), leaving a small dead volume of gas phase. The
mixtures were gravimetrically prepared, and the error in the mass
fraction composition was estimated to be within (2‚10-5. The
temperature at which the last/first signs of turbidity disappeared/
appeared was taken as the temperature of the liquid-liquid
transition. Because of the slow rate of heating/cooling, no
significant hysteresis in the determination of the phase-transition
temperatures was observed, at least within the uncertainty of
the measurements (cf. below). The temperature was measured
using a platinum resistance thermometer coupled to a Keithley
199 System DMM/Scanner. The thermometer was calibrated
against high-accuracy mercury thermometers ((0.01 K). The
overall temperature of the transition uncertainty in these visual
determinations is obviously higher (estimated to be (0.2 K).
Equipment and Procedure for the Density Measurements.
Atmospheric pressure density measurements were performed
in an Anton-Paar DMA 5000 vibrating-tube densimeter with
temperature stability better than 0.005 K over periods of more
than 10 min in the (293.15 to 303.15) K interval. The calibration
of the apparatus was checked by the standard procedure, i.e.,
by measuring the densities of pure water and air at 298.15 K
and at atmospheric pressure. The composition of the mixtures
was determined by a standard gravimetric method; i.e., they
were prepared by weighing the necessary amount of each
component directly into a small (ca. 5 cm3), sealed glass vial
to an estimated mass fraction uncertainty of (0.02 %. The
density overall uncertainty is estimated to be about 0.01 %.
The thorough 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 homogeneous liquid (ca.
2 cm3) was transferred to a syringe and injected into the
densimeter. To check whether the mixture was well homo-
genized, the vibrating tube was first filled with some of the
contents of the syringe (ca. 0.8 cm3), and a first density
measurement was taken (after the temperature set point was
reached). Another measurement followed when the contents of
the vibrating tube were replaced with the mixture that remained
in the syringe. The agreement between both values is a measure
of the effectiveness of the mixing process.
As for the pressure effects on the liquid-liquid equilibrium
temperature, two apparatuses (with pressure ranges below and
above 5 MPa, respectively, and both working with a He-Ne
laser light scattering technique) were used.
For pressures below 5 MPa, the apparatus and the methodol-
ogy used for the determination of phase transitions have already
been described in detail.18 Here, only a brief description is
provided: the cell (with an internal volume of ca. 1.0 cm3 and
an optical length of ca. 2.6 mm) is a thick-walled Pyrex glass
tube that is connected to a pressurization line and separated from
it by a mercury plug. The intensity of the scattered light is
captured at a very low angle (2° < 2θ < 4°) in the outer portion
of a bifurcated optical cable, whereas transmitted light is
captured in the inner portion of this cable. The intensities of
scattered light (Isc) and transmitted light (Itr) are corrected for
To guarantee the good internal consistency of the excess
molar volume results, the densities of both pure components
used to prepare a given binary mixture were also determined
under experimental conditions similar to those employed during
the density determination of the mixture (during the same series
of density determinations, with all samples being prepared
simultaneously from the same batch of pure components). This
point is crucial in the determination of an accurate value for
the excess volume of a binary mixture because it is impossible
to guarantee that the amount of degassingsand the gas that is

Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006 2217
Table 1. Excess Molar Volume of ([C6mim][NTf2] + n-Alcohol) Mixturesa
d293 pure
kg‚m-3
d298 pure
kg‚m-3
d303 pure
kg‚m-3
xIL
106‚VE
m3‚mol-1
106‚VE
m3‚mol-1
106‚VE
m3‚mol-1
293
298
303
system
run
5
-
[C6mim][NTf2] +
ethanol
1372.6
789.4
1368.0
785.1
1363.4
780.8
0.0354
0.1970
0.3950
0.4977
0.5942
0.7946
0.0998
0.1480
0.2232
0.3009
0.6939
0.4330
0.1769
0.3724
0.4923
0.5971
0.7899
0.2008
0.6764
0.2003
0.3968
0.5031
0.5963
0.7693
0.0511
0.1023
0.1523
0.3013
0.6851
0.1011
0.2010
0.4967
0.3987
0.5988
0.7995
-0.23
-0.34
-0.24
-0.18
-0.12
-0.04
-0.33
-0.34
-0.31
-0.28
-0.06
-0.22
-0.14
-0.08
-0.00
0.04
0.05
-0.14
0.04
-0.01
0.06
0.10
0.11
0.11
-0.06
-0.05
-0.03
0.03
-0.24
-0.35
-0.24
-0.17
-0.11
-0.04
-0.34
-0.34
-0.32
-0.28
-0.05
-0.22
-0.135
-0.068
0.003
0.053
0.052
-0.134
0.046
-0.01
0.07
0.11
0.12
0.11
-0.06
-0.05
-0.03
0.04
0.12
0.03
-0.24
-0.35
-0.24
-0.17
-0.11
-0.04
-0.35
-0.35
-0.32
-0.28
-0.05
-0.22
-0.135
-0.064
0.005
0.056
0.053
-0.134
0.050
-0.01
0.08
0.11
0.12
0.12
-0.06
-0.05
-0.03
0.05
0.12
0.03
[C6mim][NTf2] +
ethanol
6
1
1375.0
789.4
1370.4
785.1
1365.8
780.8
[C6mim][NTf2] +
1-propanol
1376.4
803.5
1371.8
799.5
1367.2
795.5
[C6mim][NTf2] +
1-propanol
[C6mim][NTf2] +
1-butanol
2
3
1375.8
803.5
1374.7
809.6
1371.2
799.5
1370.1
805.7
1366.6
795.5
1365.5
801.9
[C6mim][NTf2] +
1-butanol
4
7
1374.9
809.6
1370.3
805.7
1365.7
801.9
0.11
0.03
0.08
0.20
0.18
0.20
0.16
[C6mim][NTf2] +
1-pentanol
1375.4
814.6
1370.8
810.9
1.366.2
807.3
0.09
0.21
0.19
0.21
0.09
0.21
0.20
0.21
0.16
0.16
a The number of significant digits shown reflects the uncertainty of each of the molar fraction, x, and density, d, measurements (see text).
density fluctuations, reflections, and multiple scattering effects.
over isobaric ones because pressure transmission (isothermal
mode) is many orders of magnitude faster than thermal
equilibration (isobaric mode). Also, the rate at which one is
able to change pressure is much greater than that for temperature.
Nonetheless, all runs had to be performed in the isobaric mode
due to the very low T-p slope presented by the binary mixtures
being studied here.
-1
The cloud point is the point on the least-squares fits of (Isc,corr
)
against pressure (p) or temperature (T) where the slope changes
abruptly. Temperature uncertainty is typically (0.01 K in the
range 240 < T/K < 380. As for pressure, uncertainty in the
manometer readings is (0.01 MPa in the range 0.1 < p/MPa
< 5. The cell can be operated in the isobaric or isothermal mode.
The overall uncertainties in the determination of the transition
temperature and pressure are (0.02 K and (0.1 MPa, respec-
tively.
Results and Discussion
Table 1 contains volumetric data (excess molar volume, VE)
for seven runs corresponding to four (IL + n-alcohol) binary
mixtures at three temperatures (293.15, 298.15, and 303.15 K)
and at a nominal atmospheric pressure of 0.1 MPa. The excess
volume data were calculated taking into account the definition
of an excess property: VE ) Vmix - Videal ) Vmix - x1V1 -
x2V2, where Vmix, Videal, V1, and V2 are the molar volumes of
the mixture, of the hypothetical ideal mixture, and of the two
pure components, respectively, and xi (i ) 1 or 2) are the
corresponding mole fractions. The density of each mixture can
be calculated from the data presented in the table and is not
presented.
Table 1 also reports the densities of the ionic liquid used to
prepare each set of mixtures. Small run-to-run fluctuations (of
(0.15 %) in these latter values reflect the fact that, as discussed
in the previous section, the ionic liquid used in different sets of
mixtures underwent different degrees of degassing and drying.
However, such scattering in the data does not affect the VE
results owing to the cancellation effect implicit in the definition
The second apparatus uses the methodology mentioned above.
The cell body is a stainless steel cylindrical cell19 closed on
both sides with thick sapphire windows. It was used for
experiments in which the pressure was raised to 60 MPa. In
this case, the hydraulic fluid is the pure alcohol under study
that is in contact with a sufficiently long (1/16 in.) stainless-
steel tube filled with the solution (buffer volume), to avoid
contamination during compression/expansion cycles. The total
volume (buffer + optical) of injected solution is typically 1.6
cm3, although the optical volume roughly corresponds to a mere
0.5 cm3. In the case of isothermal runs, temperature uncertainty
is maintained ((0.01 K), but it worsens a bit for isobaric runs.
As for pressure, the uncertainty is (0.1 MPa in this higher-
pressure range. Both cells can be operated in the isobaric or
isothermal mode.
In this work, this apparatus was mainly used for the
determination of phase transitions at pressures different from
atmospheric. Whenever possible, isothermal runs are preferred

2218 Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006
of an excess property, provided an internally consistent proce-
dure is followed (see above, Experimental Section). Judging
from the repeatability of the results within each binary mixture,
including those presenting slightly different density values for
a given pure IL component, the estimated uncertainty of the VE
results is typically (0.03 cm3‚mol-1
.
Irrespective of the nature of the mixture (the size of the
alcohol alkyl chain), all excess molar volumes are small (of
the order of tenths of cm3‚mol-1sless than 0.1 % of the
mixture’s molar volume). Therefore, for practical estimates of
the mixture’s molar volume, and to good approximation, the
ideal mixture model can be used. It was also found that the VE
values for these liquid mixtures are basically insensitive to
temperature. Figure 1 depicts the behavior of the excess volumes
vs composition for these systems at 298.15 K (if the data
obtained at other temperatures were also presented in Figure 1,
the resulting curves would be superimposed).
A more peculiar feature of the curves is their asymmetry and,
in some cases, their change of sign. In fact, the small magnitude
of the VE values could lead one to think that the precision of
the results was simply not enough to conclude anything about
the shape (and sign) of the curves of VE vs composition.
However, such a notion was dismissed because the analysis of
different independent runs for the same mixture (obtained with
samples prepared from different IL batches) produced consistent
results (runs 1 and 2; 3 and 4; 5 and 6, cf. Table 1), and the
trend along the series with increasingly longer n-alcohols also
exhibited a regular behavior (cf. Figure 1b, in which the trend
is plotted for the equimolar condition, VE1/2). In fact, VE
asymmetrical curves are more likely to occur when the two
components of the mixture have a large molar volume differ-
ence, as is indeed the case. Furthermore, it is known that in
Figure 1. (a) Excess molar volume of binary mixtures of ([C6mim][NTf2]
+ n-alcohol) at 298.15 K: circles, ethanol; triangles, 1-propanol; squares,
1-butanol; and rhombs, 1-pentanol. The shaded and nonshaded markers
correspond to two independent runs. The lines represent three-parameter
Redlich-Kister fits and are merely used as guides to the eye. (b) Regular
trend and change of sign of the excess volume, illustrated here for the
equimolar mixtures, VE1/2, vs the number of carbon atoms in the alcohol.
Table 2. Liquid-Liquid Phase Equilibria of [C6mim][NTf2] + CnH(2n+1)OH, in which n is the Number of Carbon Atoms in the Alcohol, at a
Nominal Atmospheric Pressure of 0.1 MPa, wIL and xIL are the Mass and Mole Fraction of the Ionic Liquid in the Mixture, Respectively, and T
is the Temperature of Transition
xIL
wIL
T/K
xIL
wIL
T/K
xIL
wIL
T/K
n ) 4
0.01829
0.02813
0.03987
0.05236
0.06599
0.10107
0.14872
0.20039
0.25011
0.29898
260.11
264.52
266.93
267.97
268.69
0.08011
0.09883
0.10685
0.11947
0.14224
0.34453
0.39830
0.41931
0.45024
0.50024
269.20
269.44
269.45a
269.46
269.30
0.16393
0.19937
0.23802
0.27730
0.32046
0.54202
0.60048
0.65343
0.69843
0.74002
269.01
268.17
266.90
265.04
262.00
n ) 5
0.01142
0.02147
0.02724
0.03372
0.04706
0.06095
0.07807
0.05536
0.10018
0.12444
0.15047
0.20040
0.24781
0.30061
272.94
281.13
283.31
285.08
287.30
288.49
289.10
0.09594
0.11610
0.11622
0.13966
0.16444
0.19134
0.19400
0.35006
0.40000
0.40028
0.45172
0.49972
0.54564
0.54989
289.44
289.62
289.62a
289.64
289.52
289.20
288.94a
0.22635
0.26737
0.31615
0.34027
0.37491
0.44041
0.59758
0.64940
0.70117
0.72359
0.75273
0.79978
288.46
287.04
284.50
282.94
280.58
275.02
n ) 6
0.01188
0.02489
0.03031
0.03868
0.05384
0.07020
0.08838
0.05001
0.10054
0.12040
0.14980
0.19947
0.24844
0.29801
288.41
297.40
299.50
301.76
303.79
304.99
305.75
0.10888
0.12973
0.15293
0.15745
0.18893
0.21692
0.25160
0.34853
0.39494
0.44150
0.45003
0.50495
0.54811
0.59548
306.21
306.30
306.32a
306.33
306.15
305.84
305.11
0.29698
0.34941
0.37049
0.40548
0.47827
0.55978
0.64909
0.70164
0.72044
0.74915
0.80056
0.84774
303.35
300.33
299.17
295.76
290.24
281.78
n ) 8
0.01488
0.03134
0.04779
0.06762
0.08787
0.10969
0.04933
0.10003
0.14707
0.19945
0.24865
0.29739
316.25
325.97
329.66
331.91
332.86
333.32
0.13627
0.16148
0.19309
0.22455
0.26092
0.30860
0.35149
0.39816
0.45118
0.49871
0.54809
0.60528
333.50
333.62
333.46
333.14
332.48
331.23
0.34523
0.40333
0.47166
0.62114
0.71942
0.64431
0.69900
0.75411
0.84923
0.89805
329.32
325.95
321.33
305.79
287.75
a Values obtained from extrapolation of T ) f(p) to p ) 0.1 MPa.

Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006 2219
Table 3. Liquid-Liquid Phase Equilibria of [C6mim][NTf2] + CnH(2n+1)OH, in which n is the Number of Carbon Atoms in the Alcohol, as a
Function of Pressure, wIL and xIL are the Mass and Mole Fraction of the Ionic Liquid in the Mixture, Respectively, and T and p are the
Temperature and Pressure, Respectively, of Transition
T/K
dT/dp
(K/MPa)
-0.027
CnH(2n+1)OH
wIL
xIL
p/MPa
T/K
(p ) 0.1 MPa)
269.45
n ) 4
0.41931
0.10685
10.129
20.101
30.054
40.026
50.258
269.16
268.90
268.66
268.36
268.06
n ) 5
0.40028
0.11622
10.148
20.089
24.965
30.055
39.508
49.981
289.67
289.70
289.72
289.74
289.76
289.83
289.62
0.004
n ) 5
n ) 6
0.54989
0.44150
0.19400
0.15293
10.240
20.316
30.245
40.251
50.141
288.96
289.01
289.09
289.18
289.24
288.94
306.32
0.007
0.023
10.055
14.881
20.176
24.967
30.110
35.077
40.046
45.079
49.926
55.067
306.53
306.65
306.84
306.85
307.01
307.18
307.25
307.36
307.46
307.56
several binary mixtures containing ionic liquids the locus of
maxima/minima in depictions of volumetric- or LLE-related
phenomena against composition is also away from the
equimolar.4,20-23 Doman´ska et al.24 and Arce et al.25 measured
the excess molar volume of mixtures of (ionic liquid + alcohol)
and likewise for the trend found for the L-L temperature-
pressure slopes, the critical VE values also change from negative
to positive between n ) 4 and 5.
for alkylsulfate-based and tetrafluoroborate-based ionic liquids,
respectively, and their results are consistent with ours, exhibiting
asymmetric curves with negative-to-positive trends as the
alcohol alkyl chain increases (see, for instance, Figure 3 of ref
24 or Figure 6 of ref 25).
Liquid-liquid equilibrium data of (IL + n-alcohol) binary
mixtures are shown in Tables 2 and 3. All four mixtures
(containing 1-butanol, 1-pentanol, 1-hexanol, and 1-octanol)
exhibit upper critical solution temperature (UCST) behavior.
The results are also illustrated in Figures 2 and 3 as atmospheric-
pressure temperature-composition diagrams (Figure 2a) and
temperature-pressure plots (Figure 3a) along with the corre-
sponding trends along the n-alcohol family (Figures 2b and 3b).
As previously observed for other cases involving alcohols and
ionic liquids,21,24,26-30 the UCSTs increase regularly as the
alcohol alkyl chain length increases (Figure 2b). As can be
judged by analyzing Figure 2a, the agreement between our
results and those of ref 21 is excellent.
Finally, the consistency of the experimental data can be tested
by correlating the dT/dp and VE results. Pressure effects are
extremely small (see dT/dp values in Table 3) but correlate very
well with the trends observed for VE: dT/dp changes from
negative (-270 K‚Pa-1) to an almost null but positive value
(+40 K‚Pa-1) to a positive value (+230 K‚Pa-1) as n varies
from 4 to 6. On the other hand, VE exhibits a similar trend (from
negative to positive as the alkyl chain length increases) in the
left side of Figure 1. By taking the maximum of each UCST
envelope as the critical concentration and expressing it in molar
fractions, we can conclude that the liquid-liquid critical
concentrations are found at ca. 0.10, 0.11, 0.13, and 0.16, for n
) 4, 5, 6, and 8, respectively. For these critical mole fractions,
Figure 2. (a) Liquid-liquid phase equilibria of ([C6mim][NTf2] +
CnH(2n+1)OH), in which n is the number of carbon atoms in the alkyl chain
of the alcohol and wIL is the weight fraction of the ionic liquid. Experimental
data: solid symbols, this work ; empty symbols, ref 21. The two-phase
regions are inside the envelopes defined by the data pointssupper critical
solution temperature (UCST). (b) Regular trend of L-L critical temperatures
vs the number of carbon atoms in the alcohol.

2220 Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006
Literature Cited
(1) Earle, M. J.; Esperanc¸a J. M. S. S.; Gilea, M. A.; Canongia Lopes, J.
N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A.
The Distillation and Volatility of Ionic Liquids. Nature 2006, 439,
831-834.
(2) Zaitsau, Dz. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Ya. U.;
Tschersich, A.; Verevkin, S. P.; Heintz, A. Experimental Vapor
Pressures of 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfo-
nyl)imides and a Correlation Scheme for Estimation of Vaporization
Enthalpies of Ionic Liquids. J. Phys. Chem. A 2006, 110, 7303-7306.
(3) Baranyai, K. J.; Deacon, G. B.; MacFarlane, D. R.; Pringle, J. M.;
Scott, J. L. Thermal Degradation of Ionic Liquids at Elevated
Temperatures. Aust. J. Chem. 2004, 57, 145-147.
(4) Łachwa, J.; Szydlowski, J.; Najdanovic-Visak, V.; Rebelo, L. P. N.;
Seddon, K. R.; Nunes. da Ponte, M.; Esperanc¸a J. M. S. S.; Guedes,
H. J. R. Evidence for Lower Critical Solution Behavior in Ionic Liquid
Solutions. J. Am. Chem. Soc. 2005, 127, 6542-6543.
(5) Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.;
Swatloski, R. P.; Holbrey J. D.; Rogers, R. D. Controlling the Aqueous
Miscibility of Ionic Liquids: Aqueous Biphasic Systems of Water-
Miscible Ionic Liquids and Water-Structuring Salts for Recycle,
Metathesis, and Separations. J. Am. Chem. Soc. 2003, 125, 6632-
6633.
(6) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. CO2 as a Separation
Switch for Ionic Liquid/Organic Mixtures. J. Am. Chem. Soc. 2002,
124, 10276-10277.
(7) Łachwa, J.; Bento, I.; Duarte, M. T.; Canongia Lopes, J. N.; Rebelo,
L. P. Condensed Phase Behaviour of Ionic Liquid-Benzene Mix-
tures: Congruent Melting of a [Emim][NTf2]‚C6H6 Inclusion Crystal.
Chem. Commun. 2006, 2445-2447.
(8) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids IIIB: Fundamentals,
Progress, Challenges, and OpportunitiessTransformations and Pro-
cesses; American Chemical Society: Washington DC, 2005.
(9) Endres, F.; El Abedin, S. Z. Air and Water Stable Ionic Liquids in
Physical Chemistry. Phys. Chem. Chem. Phys. 2006, 8, 2101-2116.
(10) Dupont, J.; Suarez, P. A. Z. Physico-chemical Processes in Imidazolium
Ionic Liquids. Phys. Chem. Chem. Phys. 2006, 8, 2441-2452.
(11) Seddon, K. R. Ionic Liquids - A Taste of the Future. Nat. Mater.
2003, 2, 363-365.
Figure 3. (a) Liquid-liquid equilibrium temperatures as a function of the
pressure of [C6mim][NTf2] + CnH(2n+1)OH, where n is the number of carbon
atoms in the alkyl chain of the alcohol and wIL is the weight fraction of the
ionic liquid. Experimental data: b, n ) 4 and wIL ) 0.41931; 0, n ) 5
(12) Rogers, R. D.; Seddon, K. R. Ionic Liquids - Solvents of the Future.
Science 2003, 302, 792-793.
and wIL ) 0.40028; ×, n ) 5 and wIL ) 0.54989; 2, n ) 6 and wIL
)
0.44150. (b) Regular trend of critical dT/dp vs n.
(13) Deetlefs, M.; Seddon, K. R. Ionic liquids: Fact and Fiction. Chim.
Oggi 2006, 24, 16-23.
(14) Task Group: Kenneth N. Marsh (Chairman); Joan F. Brennecke,
Michael L. Frenkel, Andreas Heintz, Joe W. Magee, Cor J. Peters,
Luis P. N. Rebelo, and Kenneth R. Seddon (Members). IUPAC
Project: 2002-005-1-100 of the Physical and Biophysical Chemistry
Division (I) entitled Thermodynamics of ionic liquids, ionic liquid
mixtures, and the deVelopment of standardized systems. (http://
www.iupac.org/projects/2002/2002-005-1-100.html)
This correlation can be understood in light of the so-called
Prigogine and Defay equation, which reflects a Clapeyron-type
of relation for binary liquid-liquid equilibria (see, for instance,
refs 31 and 32 and references therein)
(15) Lundstrum, R.; Goodwin, A. R. H.; Hsu, K.; Frels, M.; Caudwell, D.
R.; Trusler, J. P. M.; Marsh, K. N. Measurement of the Viscosity and
Density of Two Reference Fluids, with Nominal Viscosity at T )
298 K and p ) 0.1 MPa of (16 and 29) mPa‚s, at Temperatures
Between (298 and 393) K and Pressures Below 55 MPa. J. Chem.
Eng. Data 2005, 50, 1377-1388.
(16) Fitzgerald, D. Technical Assessment of the Anton Paar DMA5000
density meter H & D Fitzgerald Ltd Publ. 2000 (http://www.densi-
ty.co.uk/ review_of_5000.pdf).
(17) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe,
M. Physicochemical Properties and Structures of Room Temperature
Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium
Cation. J. Phys. Chem. B. 2005, 109, 6103-6110.
(18) de Sousa, H. C.; Rebelo, L. P. N. (Liquid + Liquid) Equilibria of
(Polystyrene + Nitroethane). Molecular Weight, Pressure, and Isotope
Effects. J. Chem. Thermodyn. 2000, 32, 355-387.
(19) Rebelo, L. P. N.; Visak, Z. P.; de Sousa, H. C.; Szydlowski, J.; Gomes
de Azevedo, R.; Ramos, A. M.; Najdanovic-Visak, V.; Nunes da Ponte,
M.; Klein, J. Double Critical Phenomena in (Water plus Polyacryla-
mides) Solutions. Macromolecules 2002, 35, 1887-1895.
(20) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F.
Liquid Phase Behavior of Imidazolium-Based Ionic Liquids with
Alcohols. J. Phys. Chem. B 2004, 108, 5113-5119.
(21) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F.
Liquid Phase Behavior of Imidazolium-Based Ionic Liquids with
Alcohols: Effect of Hydrogen Bonding and Non-polar Interactions.
Fluid Phase Equilib. 2005, 228-229, 303-309.
(22) Najdanovic-Visak, V.; Esperanc¸a, J. M. S. S.; Rebelo, L. P. N.; Nunes.
da Ponte, M.; Guedes, H. J. R.; Seddon, K. R.; de Sousa, H. C.;
Szydlowski, J. Pressure, Isotope, and Water Co-solvent Effects in
Liquid-Liquid Equilibria of (Ionic Liquid plus Alcohol) Systems. J.
Phys. Chem B 2003, 107, 12797-12807.
VE(Tc(p)x)
dT
dp c
= Tc(p)
(1)
(
)
HE(Tc(p)x)
where the subscript c stands for critical conditions and VE and
HE are, respectively, the excess molar volume and the excess
molar enthalpy. Equation 1 is an approximate, simplified version
of the original equality and should only accurately hold under
some restrictive assumptions.31 A system that phase separates
on cooling must present a positive excess enthalpy, HE > 0
(endothermic process upon mixing). Therefore, for a positive
or negative T-p slope of a critical demixing line (see Figure
3), one can anticipate an excess volume with the same sign, VE
> 0 or VE < 0, i.e., an expansion or contraction upon mixing.
At L-L critical conditions, the current family of mixtures
evolves from contraction to expansion upon mixing as the
alcohol alkyl chain increases, and concomitantly, one observes
a change from improved to worsened miscibility as pressure is
applied.
Supporting Information Available:
Synthesis, purification, and 1H NMR and 19F NMR of [C6mim]-
[NTf2]. This material is available free of charge via the Internet at
http://pubs.acs.org.

Journal of Chemical and Engineering Data, Vol. 51, No. 6, 2006 2221
(23) Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z. P.; Nunes. da Ponte,
M.; Szydlowski, J.; Cerdeirin˜a, C. A.; Troncoso, J.; Roman´ı, L.;
Esperanc¸a, J. M. S. S.; Guedes, H. J. R.; de Sousa, H. C. A Detailed
Thermodynamic Analysis of [C4mim][BF4] plus Water as a Case Study
to Model Ionic Liquid Aqueous Solutions. Green Chem. 2004, 6, 369-
381.
(24) Doman´ska, U.; Pobudkowska, A.; Wis´niewska, A. Solubility and
Excess Molar Properties of 1,3-Dimethylimidazolium Methylsulfate,
or 1-Butyl-3-methylimidazolium Methylsulfate, or 1-Butyl-3-meth-
ylimidazolium Octylsulfate Ionic Liquids with n-Alkanes and Alco-
hols: Analysis in Terms of the PFP and FBT Models. J. Solution
Chem. 2006, 35, 311-334.
(25) Arce, A.; Rodil, E.; Soto, A. Physical and Excess Properties for Binary
Mixtures of 1-Methyl-3-octylimidazolium Tetrafluoroborate, [Omim]-
[BF4], Ionic Liquid with Different Alcohols. J. Solution Chem. 2006,
35, 63-78.
(26) Wagner, M.; Stanga, O.; Schro¨er, W. The Liquid-Liquid Coexistence
of Binary Mixtures of the Room-Temperature Ionic Liquid 1-Methyl-
3-hexylimidazolium Tetrafluoroborate with Alcohols. Phys. Chem.
Chem. Phys. 2004, 6, 4421-4431.
(29) Heintz, A.; Lehmann, J. K.; Wertz, C. Thermodynamic Properties of
Mixtures Containing Ionic Liquids. 3. Liquid-Liquid Equilibria of
Binary Mixtures of 1-Ethyl-3-methylimidazolium Bis(trifluorometh-
ylsulfonyl)imide with Propan-1-ol, Butan-1-ol, and Pentan-1-ol. J.
Chem. Eng. Data 2003, 48, 472-474.
(30) Crosthwaite, J. M.; Muldoon, M. J.; Aki, S. N. V. K.; Maginn, E. J.;
Brennecke, J. F. Liquid Phase Behavior of Ionic Liquids with
Alcohols: Experimental Studies and Modeling. J. Phys. Chem. B 2006,
110, 9354-9361.
(31) Rebelo, L. P. N. A Simple g(E)-model for Generating All Basic Types
of Binary Liquid-Liquid Equilibria and their Pressure Dependence.
Thermodynamic Constraints at Critical Loci. Phys. Chem. Chem. Phys.
1999, 1, 4277-4286.
(32) Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z. P.; Nunes. da Ponte,
M.; Troncoso, J.; Cerdeirin˜a, C. A.; Roman´ı, L. Two ways of looking
at Prigogine and Defay’s equation. Phys. Chem. Chem. Phys. 2002,
4, 2251-2259.
(27) Wu, C.-T.; Marsh, K. N.; Deev, A. V.; Boxall, J. A. Liquid-Liquid
Equilibria of Room-Temperature Ionic Liquids and Butan-1-ol. J.
Chem. Eng. Data. 2003, 48, 486-491.
(28) Marsh, K. N.; Deev, A. V.; Wu, A. C.-T.; Tran, E.; Klamt, A. Room-
Temperature Ionic Liquids as Replacements for Conventional Solvents
- A Review. Korean J. Chem. Eng. 2002, 19, 357-362.
Received for review July 6, 2006. Accepted August 30, 2006. This work
was financially supported by FC&T, Portugal, under contract nos. POCI/
QUI/57716/2004 and POCI/EQU/60963/2004. J.Ł. and J.M.S.S.E. are
grateful to FC&T for postdoctoral grants.
JE060307Z