Solubility of 1-alkyl-3-ethylimidazolium-based ionic liquids in water and 1-octanol

Article abstract of DOI:10.1021/je700693z

The liquid-liquid phase equilibria (LLE) of the following imidazolium ionic liquids (ELs), {1-ethyl-3-ethylimidazolium, or 1-butyl-3-ethylimidazolium, or 1-hexyl-3-ethylimidazolium bis{trifluoromethyl)sylfonyl}imide [EEIM][Tf ₂N], or [BEIM][Tf₂N], or [HEIM][Tf₂N], + water or + 1-octanol), and {1-butyl-3-ethylimida-zolium hexafluorophosphate [BEEM][PF₆] + water or + 1-octanol}, have been measured. The effect of anion type ([Tf₂N]- compared to [PF₆]-) and the effect of structural components of an ionic liquid including alkyl chain length on the cation and the ethyl substituent instead of methyl at the cation on the physical-chemical properties of the EL and on the phase behavior were studied. An upper critical solution temperature (UCST) was observed in every system. An increase in me alkyl chain length increases the mutual solubility with 1-octanol and partly with water. Ionic liquids with the ethyl substituent on the cation [HEEM][Tf₂N] show higher solubility in 1-octanol in comparison wim the methyl substituent. After the synthesis, the characterization and purity of the compounds were obtained by nuclear magnetic resonance (NMR), elemental analysis, water content (Fischer mediod), and differential scanning microcalorimetry (DSC). From DSC, the melting point, enthalpy of fusion, and the temperatures of glass transition of all the investigated ionic liquids were determined. The experimental LLE data were correlated by means of the NRTL equation.

Full text of DOI:10.1021/je700693z

1126
J. Chem. Eng. Data 2008, 53, 1126–1132
Solubility of 1-Alkyl-3-ethylimidazolium-Based Ionic Liquids in Water and
1-Octanol
Urszula Doman´ska,* Anna Re˛kawek, and Andrzej Marciniak
Physical Chemistry Division, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
The liquid–liquid phase equilibria (LLE) of the following imidazolium ionic liquids (ILs), {1-ethyl-3-
ethylimidazolium,or1-butyl-3-ethylimidazolium,or1-hexyl-3-ethylimidazoliumbis{(trifluoromethyl)sylfonyl}imide
[EEIM][Tf2N], or [BEIM][Tf2N], or [HEIM][Tf2N], + water or + 1-octanol}, and {1-butyl-3-ethylimida-
zolium hexafluorophosphate [BEIM][PF6] + water or + 1-octanol}, have been measured. The effect of
anion type ([Tf2N]- compared to [PF6]-) and the effect of structural components of an ionic liquid including
alkyl chain length on the cation and the ethyl substituent instead of methyl at the cation on the physical-
chemical properties of the IL and on the phase behavior were studied. An upper critical solution temperature
(UCST) was observed in every system. An increase in the alkyl chain length increases the mutual solubility
with 1-octanol and partly with water. Ionic liquids with the ethyl substituent on the cation [HEIM][Tf2N]
show higher solubility in 1-octanol in comparison with the methyl substituent. After the synthesis, the
characterization and purity of the compounds were obtained by nuclear magnetic resonance (NMR), elemental
analysis, water content (Fischer method), and differential scanning microcalorimetry (DSC). From DSC,
the melting point, enthalpy of fusion, and the temperatures of glass transition of all the investigated ionic
liquids were determined. The experimental LLE data were correlated by means of the NRTL equation.
butanol, 1-pentanol)17 and many other ILs with different anions
Introduction
and cations in alcohols.5–7,18 As in many published results, the
All popular imidazolium salts such as [CnMIM][PF6] or
alkyl chain length increasing on the alcohol was found to cause
[CnMIM][Tf2N] present usually the phase split in water and
an increase in the UCST of the system, and increasing the alkyl
1-octanol at room temperature. A miscibility gap was observed
chain length on the cation results in a decrease in the UCST of
for [EMIM][Tf2N], [BMIM][Tf2N], and [HMIM][Tf2N] in
the system. There are numerous publications concerning the
water1–5 and for [HMIM][Tf2N] in 1-octanol6,7 or for pyridinium
suitability of ionic liquids as entrainers for separation of water
ILs in alcohols.8 Liquid–liquid equilibrium (LLE) were mea-
and alcohols.19–21 Ionic liquids based on the 1-alkyl-3-meth-
sured for [EMIM][PF6] and [BMIM][PF6] with water at the
ylimidazolium cation, [CnMIM]-, are among the most popular
temperature range (315 to 323) K.9 It was found that the mutual
and commonly studied and can be largely exploited for
solubility increases with increasing temperature. An increase
liquid–liquid separation purposes using ILs in spite of traditional
in the alkyl chain length from butyl- to octyl- for [BMIM][PF6],
volatile organic liquids. As for the anions, bis{(trifluoro-
[C8MIM][PF6], and [C8MIM][BF4] decreases mutual solubility
methyl)sulfonyl}imide [Tf2N]- is superior compared to the more
of IL with water at T ) 295 K.10 Lengthening the alkyl chain
commonly investigated hexafluorophosphate, [PF6]-, and tet-
length for [EMIM][Tf2N], [BMIM][Tf2N], and [HMIM][Tf2N]
rafluoroborate, [BF4]-, because it is hydrolytically stable, is less
imidazolium ILs also decreases mutual solubility.2–4 As ob-
viscous, and does not decompose to give HF.
served in the other properties, changing the anion from [PF6]-
to [BF4]- increases the mutual solubilities substantially.10 The
Processes that currently involve homogeneous mixtures of
water and volatile organic compounds might be improved by
solubility of the IL in water is mainly determined by the anion
substitution of organic solvent with hydrophilic IL. It has been
of the IL. Typical imidazolium ILs based on halide-, ethanoate-,
shown previously that water can decrease the viscosity and
nitrate-, and trifluoroacetate-based anions are fully water-solu-
ble.2,11 Salts with [BF4]- and [CF3SO3]- anions are partly soluble
density of ILs, which can be satisfactorily used in new
technologies.
in water. For more polar anions, like in [MMIM][CH3SO4] or
This paper is a continuation of our systematic study of ionic
liquid solubility measurements.12,18,22,23 In this work, we report
the mutual solubility of ILs with an ethyl substituent instead of
methyl on the cation: [EEIM][Tf2N], or [BEIM][Tf2N], or
[HEIM][Tf2N] and [BEIM][PF6] with water and 1-octanol.
Some of these ILs were studied earlier in various fields:
[EEIM][Tf2N] in electrochemistry,24 [HEIM][Tf2N]25 and
[HEIM][PF6]26 as lubricants for the contact of steel/steel. These
ILs have showed excellent tribiological performance and were
superior to the ionic liquids of alkylimidazolium tetrafluorobo-
rate and the conventional high-temperature lubricants.25 Un-
fortunately, the ILs with the [Tf2N]-anion indicate the occurrence
[BMIM][CH3SO4], complete miscibility with water was ob-
served.12
A systematic investigation of the mixtures of [BMIM][PF6]
with alcohols was presented very early.13–15 Methanol was found
completely miscible with [BMIM][PF6] at temperatures above
273 K. A systematic decrease in solubility was observed with
an increase of the alkyl chain length of an alcohol. The same
conclusions resulted from the solubility measurements of
[EMIM][Tf2N] in alcohols (1-propanol, 1-butanol, and 1-pen-
tanol)16 and from those of [C8MIM][BF4] with alcohols (1-
* Corresponding author. Tel.: +48-22-6213115. E-mail: ula@ch.pw.edu.pl.
10.1021/je700693z CCC: $40.75
2008 American Chemical Society
Published on Web 04/16/2008

Journal of Chemical & Engineering Data, Vol. 53, No. 5, 2008 1127
Table 1. List of Investigated Ionic Liquids, Names, Structure, and Abbreviations of Names
Table 2. Thermophysical Constants of Pure Salts Determined from
DSC Data
chloride with varying numbers of carbon atoms in a round-
bottom flask under a nitrogen atmosphere to produce 1-alkyl-
3-ethylimidazolium chloride, [EEIM]Cl, [BEIM]Cl, and [HE-
IM]Cl. These substances were purified by mixing with activated
carbon to remove colored compounds. 1-Alkyl-3-ethylimida-
zolium chloride was reacted with lithium bis{(trifluoro-
methyl)sulfonyl}imide (LiTf2N) in water in a round-bottom flask
at 343 K under reflux with strong stirring for at least 3 h. After
reaction, mixtures were transferred to the separator where the
organic phase was washed by water until the mass fraction of
chloride was less than 3 ·10-6. The aqueous phases were tested
for the presence of chloride ion using aqueous 0.1 M silver
nitrate solution.
Next, the organic layer was collected and solvent was
removed under vacuum with a rotary evaporator. Later, a flask
with ionic liquid was placed under high vacuum with stirring
at 343 K. After 24 h, the water content was measured using
Karl Fischer titration. In all cases, the mass fraction of water
was less than 2.3·10-4. 1-Butyl-3-methylimidazolium chloride
was reacted with potassium hexafluorophosphate with equimolar
proportions in water during 2 h. Next, the same procedures were
used as described above. The prepared ILs were characterized
by their 1H NMR and elemental analysis. The 1H NMR spectra
were recorded on a Varian model XL 300 spectrometer at 300
MHz with tetramethylsilane as the standard. Finally, the ILs
were dissolved in the water–methanol solution (1:1) to test the
chloride ion with the Mohr method (titration with aqueous silver
nitrate solution in the presence of 5 % K2CrO4). The ILs were
found to contain from (0 to 0.02) mass % with an accuracy (
0.005.
Tfus,1
∆fusH1
Ttr,1(g)
∆trCp(g)
compound
K
kJ ·mol-1
K
J·K-1 ·mol-1
[EEIM][Tf2N] 262.6 ( 0.1 20.4 ( 0.3 205.9 ( 0.1
19.5 ( 0.5
154 ( 0.5
153 ( 0.5
87 ( 0.5
[BEIM][Tf2N]
[HEIM][Tf2N]
[BEIM][PF6]
–
–
–
–
–
–
180.9 ( 0.1
184.8 ( 0.1
192.9 ( 0.1
of a complicated tribiochemical reaction with iron with the
formation of FeS, organic fluoride, and others25 and that with
[PF6]- with the formation of FePO4 and FeF2 surface com-
pounds.26 It was interesting to check the other possibilities of
these ethyl analogues in extraction and separation processes,
for example.
The melting point, glass transition temperature, and enthalpy
of fusion were determined by differential scanning calorimetry
(DSC) for each ionic liquid investigated. Density was measured
by an Anton-Paar (type DMA 602) apparatus.
Experimental Section
Materials. Four imidazolium-based ILs were synthesized. The
list of chemicals used in the synthesis of ILs in this study,
including CAS number, source and grade, and purification
method (if any) is as follows: 1-H-imidazole (288-32-4, Koch-
Light Laboratory, 99 %), bromoethane (74-96-4, Aldrich 98 %),
1-chlorobutane (109-69-3, Aldrich 99.5 % redistilled), 1-chlo-
rohexane (544-10-5 Aldrich 99 %), lithium bis{(trifluoromethyl)-
sulfonyl}imide (90076-65-6, Aldrich 97 %). 1-H-Imidazole was
reacted with an excess of an alkyl halide (ethylbromide) in a
round-bottom flask under a nitrogen atmosphere to synthesize
the 1-ethylimidazole. Chloride salts were prepared by the
nucleophilic substitution reaction of 1-ethylimidazole by alkyl
Twice distilled and degassed water was used for solubility
measurements. 1-Octanol was acquired from Sigma-Aldrich
Chemie GmbH, Stenheim, Germany. Prior to use, it was
Table 3. Molar Mass and Experimental and Calculated Densities of Ethyl- and Methyl-Substituted ILs at 298.15 K
F(298.15 K)
[CnEIM]exptl
g·cm-3
F(298.15K)
[CnEIM]calcd
g·cm-3
1.470b
1.402e
1.344b
1.328e
F(298.15K)
[CnMIM]lit
g·cm-3
compound
M
[EEIM][Tf2N]
[BEIM][Tf2N]
[HEIM][Tf2N]
[BEIM][PF6]
405.12
433.14
461.16
298.21
1.4760, 1.452a
1.4018
1.519c, 1.5174d
1.436c, 1.4442d, 1.43704f
1.373c, 1.3715d, 1.37081f
1.369c, 1.3674d, 1.3603h
1.3454, 1.343g
1.3276
a Density at ambient temperatures (291 to 303) K from ref 24. b Densities calculated from ref 27. c Densities of 1-alkyl-3-methylimidazolium-based
ILs from ref 29. d Densities of 1-alkyl-3-methylimidazolium-based ILs from ref 30. e Densities calculated from ref 28. f Densities of 1-alkyl-3-methyl-
imidazolium-based ILs from ref 31. g Density at ambient temperature from ref 25. h Densities of 1-alkyl-3-methylimidazolium-based ILs from ref 32.

1128 Journal of Chemical & Engineering Data, Vol. 53, No. 5, 2008
Table 4. Experimental Liquid-Liquid Phase Equilibrium
Temperatures, TLLE, in Function of the IL Mole Fraction, x1, for
{IL (1) + Water (2)} Binary Systems
Table 5. Experimental Liquid-Liquid Phase Equilibrium
Temperatures, TLLE, in Function of the IL Mole Fraction, x1, for
{IL (1) + 1-Octanol (2)} Binary Systems
x1
T1LLE/K
x1
T1LLE/K
x1
T1LLE/K
x1
T1LLE/K
[EEIM][Tf2N]
[EEIM][Tf2N]
0.8797
0.8154
0.6991
0.6707
0.6483
0.6019
0.5771
0.5487
0.4937
0.4613
0.4131
0.3958
0.3699
268.8
284.5
320.3
323.0
325.9
333.2
338.4
340.7
345.3
347.5
349.9
354.6
364.8
0.3496
0.3137
0.2838
0.2307
0.2173
0.1815
0.1640
0.1468
0.1335
0.1193
0.0321
0.0247
377.1
388.2
397.8
407.4
410.6
417.0
424.5
432.6
437.1
441.9
446.6
372.5
0.9269
0.8247
0.7600
0.6051
0.5227
0.4546
0.4053
0.3502
293.5
310.8
328.4
358.2
367.3
373.8
376.7
379.9
0.3133
0.2873
0.2611
0.2195
0.1893
0.1456
0.1035
0.0392
380.8
381.3
381.9
382.2
382.0
381.8
381.1
335.4
[BEIM][Tf2N]
0.7780
0.7100
0.6442
0.5756
0.5198
0.4961
0.4240
0.3690
284.6
299.6
317.9
329.6
336.1
338.5
343.4
346.7
0.3273
0.2785
0.2231
0.1695
0.1176
0.0689
0.0376
349.1
350.9
352.3
353.2
353.3
350.8
346.3
[BEIM][Tf2N]
0.7764
0.7531
0.6983
0.6496
0.5955
0.5437
298.75
305.04
320.05
332.2
343.79
355.71
0.4945
366.75
353.35
343.55
332.25
319.65
308.35
0.00033
0.00027
0.00023
0.00019
0.00015
[HEIM][Tf2N]
0.6898
0.6494
0.6355
0.6165
0.5653
0.5058
0.4578
0.4268
0.3667
259.6
282.5
293.0
302.0
309.1
313.5
316.4
318.5
321.9
0.3314
0.2911
0.2658
0.2420
0.1988
0.1275
0.0766
0.0557
0.0467
323.6
325.1
326.1
326.5
327.5
328.4
327.8
327.0
325.1
[HEIM][Tf2N]
0.8327
0.8171
0.7716
0.7365
0.7196
0.6796
0.6334
300.85
306.65
321.05
331.05
335.95
345.45
356.75
0.5877
367.95
368.65
349.45
340.35
331.05
321.05
0.000131
0.000088
0.000072
0.000054
0.000041
[BEIM][PF6]
[BEIM][PF6]
0.9181
0.8724
0.8163
0.7049
0.6406
0.5674
0.5005
0.4383
0.3870
0.3385
270.1
289.3
310.9
330.4
336.8
344.2
349.7
353.0
354.6
355.3
0.2935
0.2341
0.1564
0.1232
0.1074
0.0866
0.0659
0.0503
0.0395
356.1
356.0
354.5
353.3
351.9
350.4
348.3
346.3
342.7
0.9086
0.8340
0.7814
0.6735
0.5953
294.9
303.8
318.9
331.6
343.6
0.5368
0.4896
0.4355
0.3758
0.3245
363.3
374.3
384.6
409.6
426.0
fractionally distilled to ensure that the water content was
1.6·10-4 in mass fraction. The solvent was stored over freshly
activated molecular sieves of type 4 Å (Union Carbide).
The names of substances under study, chemical formulas, and
the abbreviations are presented in Table 1.
Elementary analysis results are presented in Table 1S of the
Supporting Information.
Differential Scanning Calorimetry (DSC). The enthalpy of
fusion, melting temperature, and glass transition temperatures
were measured by DSC (Perkin-Elmer Pyris 1). Measurements
were carried out at a scan rate of 10 K ·min-1, with a power
sensitivity of 16 mJ·s-1 and with a recorder sensitivity of 5
mV. The DSC was calibrated with a 99.9999 % purity indium
sample. The uncertainty of the calorimetric measurements was
estimated to be ( 1.5 %. The uncertainty of the enthalpy of
melting was ( 0.3 kJ ·mol-1, of measured temperatures Tfus,1
and Ttr,1(g) was ( 0.1 K, and of half the Cp of glass transition
was ( 0.5 J · K-1 · mol-1. The thermophysical properties are
shown in Table 2.
Density Measurements. The densities of all ILs were mea-
sured using an Anton Paar DMA 602 vibrating-tube densimeter
thermostatted at T ) (298.15 ( 0.01) K. The densimeter’s
calibration was performed at atmospheric pressure using doubly
distilled and degassed water, specially purified benzene (CHEMI-
PAN, Poland 0.999), and dried air. The vibrating-tube temper-
ature was measured by an Anton Paar DM 100-30 digital
thermometer and was regulated to better than ( 0.01 K using
a UNIPAN 60 thermostat and 202 Temperature Control System
(UNIPAN, Poland). The densities of ILs are listed in Table 3.
Elemental Analysis. Elemental analysis was carried out using
the Perkin-Elmer Series II model 2400 CHNS/O Analyzer.
Water Content. Water content was analyzed by the Karl
Fischer titration technique (method TitroLine KF). Samples of
all compounds were dissolved in methanol and titrated with a
step of 2.5 µL. The results are presented in Table 2S of the
Supporting Information.
Liquid–Liquid Equilibria Measurements. Liquid–liquid equi-
librium temperatures were determined using a dynamic method
previously described.12,22,23 Appropriate mixtures of IL and
solvent were placed, under nitrogen in a drybox, into a Pyrex
glass cell and heated slowly (less than 2 K ·h-1 near the
equilibrium temperature). The mixture was continuously stirred
within the cell and was placed in a glass thermostat filled with
water. The temperature of the liquid bath was varied slowly
until the one phase was obtained. The equilibrium temperature
was observed visually over an increasing temperature range,
although the observation of the “cloud point” with decreasing
temperature proved to be very difficult. The effect of precooling
and the kinetics of the phenomenon of binary phase creation
were probably the reasons that the temperature of the cloud point
was not repeatable during our analysis. The total mass and
volumes of sample were different from point to point, and they
were from about (1 to 20) g in the Pyrex cell. Mixtures were
prepared by mass ratio, and the uncertainty in the mole fraction
was estimated to be ( 0.0005. The experiment (i.e., the heating

Journal of Chemical & Engineering Data, Vol. 53, No. 5, 2008 1129
Figure 1. Liquid–liquid phase equilibria diagrams for binary systems.{IL
(1) + water (2)}: 9, [BEIM][Tf2N]; 2, [HEIM][Tf2N]; O, [EMIM][Tf2N],
ref 3; 0, [BMIM][Tf2N], ref 3; 4, [HMIM][Tf2N], ref 3; +, UCST for
[BEIM][Tf2N]; and ×, UCST for [HEIM][Tf2N] calculated from NRTL
parameters. Solid line, NRTL equation.
Figure 3. Liquid–liquid phase equilibria diagrams for binary systems.
{[BEIM][PF6] (1) + solvent (2)}: b, water; 9, 1-octanol; ×, UCST for
water calculated from NRTL parameters. Solid line, NRTL equation; O,
[BMIM][PF6] + water.3
Figure 4. Liquid–liquid phase equilibria diagrams for binary systems. {IL
(1) + 1-octanol (2)}: 9, [HMIM][Tf2N], refs 6 and 8; 2, [HEIM][Tf2N].
Solid line, NRTL equation.
Figure 2. Liquid–liquid phase equilibria diagrams for binary systems. {IL
(1) + 1-octanol (2)}: b, [EEIM][Tf2N]; 9, [BEIM][Tf2N]; 2, [HEIM]-
[Tf2N]. Solid line, NRTL equation.
(see general remarks, GRS 1 of Supporting Information and
Figures 1S to 4S of Supporting Information showing the spectra
of investigated ILs).
Elemental analysis results verify a high purity of the
compounds because they are similar to the calculated values.
The difference between found and calculated values is the
highest for [EEIM][Tf2N] and is equal to 3.05 % for the
hydrogen. The smallest difference is for [BEIM][PF6] for
hydrogen, and it is equal to 0.01 %.
The temperatures of glass transition were in the range from
(180 to 205) K for the measured ILs. This is a typical result for
ILs. ILs synthesized by us were liquid at room temperature.
Only the [EEIM][Tf2N] IL exhibits very low melting temper-
ature (262.6 K) and enthalpy of melting 20.46 kJ·mol-1 (see
Table 2). For all ILs under study, the characteristic peak of the
“relaxation” just before the glass transition effect was noted
and cooling cycle) was repeated three to four times to determine
accurate and reproducible values of each solubility point. The
uncertainty of the data taken on the disappearance of two phases
was ( 0.1 K, and that for the cloud point was ( 0.5 K. The
differences of the two temperatures of disappearance of the two
phases and the cloud point were about 0.5 K. Temperatures were
measured with a P550 electronic thermometer (DOSTMANN
electronic GmbH) with the probe totally immersed in the
thermostatting liquid.
Results and Discussion
The quick and trivial method of synthesis permit us to receive
the ethylimidazolium-based ILs with a unique feature. The NMR
technique of 1H NMR of the pure salts does not have unexpected
signals, i.e., from unremoved solvents or unreacted intermediates

1130 Journal of Chemical & Engineering Data, Vol. 53, No. 5, 2008
Table 6. Correlation of the Liquid-Liquid Equilibria Data by Means of the NRTL Equation: Parameters (g12 - g22 ) a12 + b12T), (g21 - g11
) a21 + b21T), and Deviation σx
NRTL parametersa
g12 - g22 (J·mol-1
)
g21 - g11 (J·mol-1
a21
)
system
a12
b12
b21
σx
[BEIM][Tf2N] + water
[HEIM][Tf2N] + water
[BEIM][PF6] + water
[EEIM][Tf2N] + 1-octanol
[BEIM][Tf2N] + 1-octanol
[HEIM][Tf2N] + 1-octanol
[BEIM][PF6] + 1-octanol
19030
20295
28435
17126
21740
18238
15220
-61.95
-60.91
-85.50
-50.65
-73.86
-67.70
-47.76
-430.0
6923
72.78
57.79
0.0084
0.0064
0.0149
0.0569
0.0090
0.0181
0.0253
89.12
661.8
61565
76112
33428
-215.2
27.88
-136.13
-194.84
-66.11
a R ) 0.2.
(see Figure 5S of Supporting Information, DSC diagram for
[HEIM][Tf2N], as an example).
discerned. The solubility of IL is much higher in 1-octanol. The
comparison is also presented with the solubility of [BMIM][PF6]
in water,4 and for the [PF6]- anion, the solubility of ethylimi-
dazolium IL is lower. Replacing the [Tf2N]- anion by the [PF6]-
anion significantly increases the solubility in water at any
particular temperature. The mole fraction of the UCST point
for the [PF6]- anion is shifted to the higher IL mole fraction in
water and to the lower mole fraction in 1-octanol.
The results of LLE of [HEIM][Tf2N] in 1-octanol were
compared with that previously measured for [HMIM][Tf2N] data
in Figure 4.6,8 The results presented by Łachwa et al.7 are
similar. A comparison of the solubility of these two ILs in
1-octanol exhibits higher UCST for the methyl analogue; the
effect of the ethyl vs methyl group on the IL solubility shows
the same trend in the interaction with alcohol as was observed
previously for the alkyl substituent in position 1.
The measured densities of the new compounds are compared
with those predicted by the Ye and Shreeve Group Contribution
Method27 and with the experimental data from the literature of
1-alkyl-3-methyl-substituted imidazolium-based salts in Table
3. A modified model, presented recently by Gardas and
Coutinho,28 provides excellent predictions for our experimental
data. The new model28 predicts the measured densities within
less than 0.2 %. All ethylimidazolium bis{(trifluoromethyl)syl-
fonyl}imides and [BEIM][PF6] have revealed lower densities
than those ILs with a methyl substituent.29–32
The liquid–liquid equilibria for four new ionic liquids,
[EEIM][Tf2N], [BEIM][Tf2N], [HEIM][Tf2N], and [BEIM]-
[PF6], in water and 1-octanol have been determined. The
solubilities are listed in Tables 4 and 5, for water and 1-octanol,
respectively. The tables include direct experimental results of
the equilibrium temperatures, T1LLE, as a function of x1, the
mole fraction of the IL in equilibrium for the investigated
systems.
It is noticed that the lengthening of the aliphatic chain at the
1 or 3 position on the cation results in the same effects of
interaction with solvent and/or has the same influence on
packing effects.
Experimental phase diagrams of LLE investigated in this work
are not easy to interpret. These ILs are complicated and highly
interacting molecules, especially with water and alcohols. The
solubility of ILs in 1-octanol is much higher than in water. It
was observed for many systems previously. By extending the
alkyl chain from the ethyl group to a hexyl group in the ethyl-
substituted imidazolium cation, the UCST decreases and the
ionic liquid-alcohol mutual solubilities increase. Different kinds
of interactions between the IL and an alcohol are responsible
for the solubility, but the dominate are the increasing van der
Walls forces of the longer alkyl chain on the cation with alkyl
portion of the alcohol.2,5–8
Liquid–Liquid Equilibrium Correlation. For liquid–liquid
equilibrium, the results were correlated with the NRTL model.33
The experimental data were correlated using the binary param-
eter NRTL model with the equations described by us
earlier.18,22 The NRTL R parameter was set to a value of R )
0.2. The temperature-dependent model adjustable parameters
(g12 - g22 ) a12 + b12T) and (g21 - g11 ) a21 + b21T) were
found by minimization of the objective function OF
The phase diagrams of the three 1-alkyl-3-ethylimidazolium
bis{(trifluoromethyl)sulfonyl}imides {IL + water} are shown
in Figure 1 and those of {IL + 1-octanol} in Figure 2. A
comparison between the measured data and the data recently
published for the equivalent methylimidazolium compounds in
water is presented in Figure 1 and Figure 3. The effect of ethyl
vs methyl group on the hydrophobicity and on the temperature
dependence of the solubility in water is similar for all
compounds studied, and in general, the higher hydrophobicity
of the new ethyl-substituted ILs makes them less soluble in water
than the corresponding methyl-substituted compounds (see
Figure 1 for bis{(trifluoromethyl)sulfonyl}imides2,3 + water and
Figure 3 for hexafluorophosphates3 + water). In water, the
influence of alkyl chain is not the same as for 1-octanol. The
shape of the equilibrium curve of ILs in water (see Figure 1) is
not unusual for the imidazolium salts. It is similar to the results
of LLE for [MMIM][CH3SO4] and [BMIM][CH3SO4] in dif-
ferent solvents.12,23
In 1-octanol, the upper critical solution temperature decreases
with an increase of the alkane chain of the cation exactly in the
same pattern as was observed for the 1-alkyl-3-methylimida-
zolium-substituted cation6,8 or pyridinium cation with different
substituent groups (in alcohols).7 On occasion, it was impossible
to detect visually the mutual solubility of ILs with the solvent
in the solvent-rich phase. Spectroscopic or other techniques are
necessary for the aforementioned mixtures. The solubility curves
of ILs in the solvent were detected only by few experimental
points (see Table 5).
n
2
1
i
OF )
∆ x 2 + ∆ x/
(1)
(
)
(
[
)
i
]
∑
1
i)1
where n is the number of experimental points and ∆x is defined
as
∆x ) x - xexptl
(2)
With reference to Figure 3, the difference between the
solubilities of [BEIM][PF6] in water and 1-octanol may be
The root-mean-square deviation of mole fraction was defined
as follows

Journal of Chemical & Engineering Data, Vol. 53, No. 5, 2008 1131
n
1⁄2
(5) 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.
(6) Crosthwaite, J. M.; Aki, S. N. V.; 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.
(7) Łachwa, J.; Morgado, P.; Esperanca, J. M. S. S.; Guedes, H. J. R.;
Lopes, J. N. C.; Rebelo, L. P. N. Fluid-Phase Behavior of {1-Hexyl-
3-methylimidazolium Bis(trfluoromethylsulfonyl) Imide, [C6mim]-
[NTf2], +C2-C8 n-Alcohol} Mixtures: Liquid-Liquid Equilibrium and
Excess Volumes. J. Chem. Eng. Data 2006, 51, 2215–2221.
(8) 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.
(9) Wong, D. S. H.; Chen, J. P.; Chang, J. M.; Chou, C. H. Phase
Equilibria of Water and Ionic Liquids [emim][PF6] and [bmim][PF6].
Fluid Phase Equilib. 2002, 194–197, 1089–1095.
(10) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solution Thermody-
namics of Imidazolium-Based Ionic Liquids and Water. J. Phys. Chem.
B 2001, 105, 10942–10949.
(11) Seddon, K. R.; Stark, A.; Torres, M.-J. Influence of Chloride, Water,
and Organic Solvents on the Physical Properties of Ionic Liquids. Pure
Appl. Chem. 2000, 72, 2275–2287.
(12) 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-
methylimidazolium Octylsulfate Ionic Liquids With n-Alkanes and
Alcohols: Analysis in Terms of the PFP and FBT Models. J. Solution
Chem. 2006, 35, 311–334.
x
- x 2
)
i
(
∑
exptl
i)1
(
)
σx )
(3)
n - 1
For the investigated mixtures, it was very difficult to detect
visually the mutual solubility of ILs in the solvent-rich phase.
These data were calculated previously for some ILs by
COSMO-RS23 within x1 ≈ 1·10-4. From the experimental data
of the methyl-substituted imidazolium IL solubility in water,
the average solute mole fraction was x1 ≈ 8.3·10-4 for
[EMIM][Tf2N];3 x1 ≈ 3.2·10-4 for [BMIM][Tf2N];3 x1
≈
9.7·10-5 3
;
and x1 ≈ 1.4·10-3 for [BMIM][PF6].4 It was
assumed that the solubility in the dilute IL region was in the
range of a few experimental points measured in our experimental
work (see Tables 4 and 5). The NRTL parameters and the
corresponding standard deviations are reported in Table 6. The
UCSTs, calculated by the NRTL, are presented in Figures 1
and 3. For the systems presented in this work, the average root-
mean-square deviation, σx, is 0.0198. The results of the
correlations are plotted in Figures 1 to 4. Positive deviations
from ideality were found. The values of activity coefficients of
the IL in the saturated solution, calculated from the NRTL
correlation, were higher than one.
(13) Marsh, K. N.; Deev, A.; Wu, A. C.-T.; Tran, E; Klamt, A. Room
Temperature Ionic Liquids as Replacements for Conventional Sol-
vents-A Review. Kor. J. Chem. Eng. 2002, 19, 357–362.
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Equilibria of Room Temperature Ionic Liquids and Butan-1-ol.
J. Chem. Eng. Data 2003, 48, 486–491.
(15) Sahandzhieva, K.; Tuma, D.; Breyer, S.; Kamps, A. P.-S.; Maurer,
G. Liquid-Liquid Equilibrium in Mixtures of the Ionic Liquid 1-n-
Butyl-3-methylimidazolium Hexafluorophosphate and an Alcohol.
J. Chem. Eng. Data 2006, 51, 1516–1525.
(16) Heintz, A.; Lehmann, J. K.; Wertz, Ch. Thermodynamic Properties
of Mixtures Containing Ionic Liquids. 3. Liquid-Liquid Equilibria of
Binary Mixtures of 1-Ethyl-3-methylimidazolium Bis(trifluoro-
methylsulfonyl)imide with Propan-1-ol, Butan-1-ol, and Pentan-1-ol.
J. Chem. Eng. Data 2003, 48, 472–474.
(17) Heintz, A.; Klasen, D.; Lehmann, J. K.; Wertz, Ch. Excess Molar
Volumes and Liquid-Liquid Equilibria of the Ionic Liquid 1-Methyl-
3-octyl-imidazolium Tetrafluoroborate Mixed with Butan-1-ol and
Pentan-1-ol. J. Solution Chem. 2005, 34, 1135–1144.
Conclusions
The ethyl-substituted imidazolium ionic liquids revealed lower
densities and are less soluble in water and more soluble in
1-octanol than the corresponding methyl compounds. The
temperature dependences are similar for all compounds studied.
The higher hydrophobicity of the ethyl-substituted ILs was
observed as expected for the longer alkane chain. It is interesting
to note here the observed trend of solubilities for IL-alcohol
binary systems, where the ILs-alcohols mutual solubilities
increase with the cation alkyl chain length. This behavior is
due to an increase in extension of the van der Waals interactions
between the alkyl chains of both alcohols and ILs.
The ILs incorporating the ethyl substituent on the cation lead
to lower densities than the one with the methyl substituent. From
a practical standpoint, this means that normally high dense and
viscous 1-alkyl-3-methylimidazolium-based ionic liquids can be
circumvented simply by changing the alkane chain on the cation.
(18) Doman´ska, U.; Marciniak, A. Solubility of Ionic Liquid [emim][PF6]
in Alcohols. J. Phys. Chem B 2004, 108, 2376–2382.
(19) Chapeaux, A.; Simoni, L. D.; Stadtherr, M. A.; Brennecke, J. F. Liquid-
Liquid Equilibrium of Ionic Liquids with Water and Alcohols, 2nd
International Congress on Ionic Liquids (COIL-2); Yokohama,
Japan, August 5–10, 2007.
Supporting Information Available:
(20) Najdanovic-Visak, V.; Rebelo, L. P. N.; da Ponte, M. N. Liquid-liquid
behaviour of ionic liquid-1-butanol-water and high pressure CO2-
induced phase changes. Green Chem. 2005, 7, 443–450.
Elemental analysis, water content, general remarks regarding
nuclear magnetic resonance (GRS 1), DSC diagram, and tables
including liquid–liquid equilibrium data (Tables 3S and 4S). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) Hu, X. S.; Yu, J.; Liu, H. Z. Liquid-Liquid Equilibria of the System
1-(2-Hydroxyethyl)-3-methylimidozolium Tetrafluoroborate or 1-(2-
Hydroxyethyl)-2,3-dimethylimidozolium Tetrafluoroborate + Water
+ 1-Butanol at 293. 15 K. J. Chem. Eng. Data 2006, 51, 691–695.
(22) Doman´ska, U.; Marciniak, A. Phase Behaviour of 1-Hexyloxymethyl-
3-methyl-imidazolium and 1,3-Dihexyloxymethyl-Imidazolium Based
Ionic Liquids with Alcohols, Water, Ketones and Hydrocarbons: the
Effect of Cation and Anion on Solubility. Fluid Phase Equilib. 2007,
260, 9–18.
(23) Doman´ska, U.; Pobudkowska, A.; Eckert, F. (Liquid + Liquid) Phase
Equilibria of 1-Alkyl-3-methylimidazolium Methylsulfate with Alco-
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(24) Hagiwara, R.; Ito, Y. Room Temperature Ionic Liquids of Alkylimi-
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(25) Lu, Q.; Wang, H.; Ye, Ch.; Liu, W.; Xue, Q. Room Temperature Ionic
Liquid 1-Ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide
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This research has been supported by the Warsaw University of
Technology (Grant of the Faculty).
JE700693Z