Thermodynamic properties of mixtures containing ionic liquids: Activity coefficients at infinite dilution of organic compounds in 1-propyl boronic acid-3-alkylimidazolium bromide and 1-propenyl-3-alkylimidazolium bromide using inverse gas chromatography

Article abstract of DOI:10.1021/je060033f

Activity coefficients at infinite dilution γ∞ of 28 organic compounds in eight room-temperature ionic liquids of the family of 1-propyl boronic acid-3-alkylimidazolium bromide and 1-propenyl-3-alkylimidazolium bromide (with alkyl = methyl, octyl, decyl, or dodecyl) were determined at 323.15 K using inverse gas chromatography. Using all the available γ∞ data, the selectivities at infinite dilution S₁₂ ^∞ were determined. Indeed, such values are extremely useful for the design and the optimization of separation processes.

Full text of DOI:10.1021/je060033f

1274
J. Chem. Eng. Data 2006, 51, 1274-1279
Thermodynamic Properties of Mixtures Containing Ionic Liquids: Activity
Coefficients at Infinite Dilution of Organic Compounds in 1-Propyl Boronic
Acid-3-Alkylimidazolium Bromide and 1-Propenyl-3-alkylimidazolium Bromide
Using Inverse Gas Chromatography
Fabrice Mutelet,*,† Jean-Noe1l Jaubert,† Marek Rogalski,‡ Malika Boukherissa,‡ and Amadou Dicko‡
Institut National Polytechnique de Lorraine, Ecole Nationale Supe´rieure des Industries Chimiques, Laboratoire de
Thermodynamique des Milieux Polyphase´s, 1 rue Grandville, 54000 Nancy, France, and Laboratiore de Chimie et Applications,
Universite´ de Metz, 1 Boulevard Arago, Technopoˆle Metz 2000, 57078 Metz, Cedex 3, France
Activity coefficients at infinite dilution γ∞ of 28 organic compounds in eight room-temperature ionic liquids of
the family of 1-propyl boronic acid-3-alkylimidazolium bromide and 1-propenyl-3-alkylimidazolium bromide
(with alkyl ) methyl, octyl, decyl, or dodecyl) were determined at 323.15 K using inverse gas chromatography.
Using all the available γ∞ data, the selectivities at infinite dilution S1∞2 were determined. Indeed, such values are
extremely useful for the design and the optimization of separation processes.
coefficients at infinite dilution γ∞ give a direct measure of
interactions between unlike molecules in the absence of solute-
solute interactions.
Gas chromatography is widely used for determining thermo-
dynamic properties of pure substances or solvent properties of
binary mixtures. From retention data, the solute activity coef-
ficient at infinite dilution, the gas-liquid partition coefficient,
and other thermodynamic properties of mixing can be easily
obtained. Using these parameters and appropriate models allow
understanding of the intermolecular interactions responsible for
solvation in the stationary phase.17-18
In other respects, numerous approaches were proposed to
characterize the interactions between the solute and the station-
ary phase. Among others, Abraham and co-workers19-22 de-
veloped the linear solvation energy relationship (LSER) to
correlate many physicochemical properties. The LSER equation
can be written as follows:
Introduction
Room-temperature ionic liquids (RTILs) are solvents that may
have great potential in chemical analysis. RTILs have shown
potential as unique solvents with wide range of solubility,
miscibility, and other physicochemical properties accompanied
by an extremely promising nonvolatile behavior. These liquid
salts can be custom synthesized, be water-miscible or water-
immiscible, and are capable of undergoing multiple solvation
interactions with many types of molecules. All RTILs exhibit
a high viscosity and, hence, a relatively low conductivity. The
uses and properties of RTILs have been the focus of many recent
scientific investigations.1-13 The physical and physicochemical
properties of RTILs depend on their large cation (alkylimida-
zolium, alkylpyridinium, alkylphosphonium, quaternary am-
monium) and on their inorganic anion having a delocalized
charge (PF6-, BF4-, and (Tf2)N). It means that RTILs of various
physical properties can be obtained by changing the cation or
the anion. Because they are air and water stable and are able to
solvate a variety of organic and inorganic species, ionic liquids
are emerging as alternative green solvents, namely, as reaction
media for synthesis, catalysis, and biocatalysis.14-16 It has been
observed that the solubility of an ionic liquid in water can
strongly increase replacing the anion. The main factors that
influence physical properties of RTILs are the charge distribution
on the ions but also H-bonding ability, the polarity, and the
dispersive interactions.
log SP ) c + rR2 + sπH2 + a R2H + b â2H + l log L16 (1)
∑
∑
SP is some free energy related solute property such as gas-
liquid partition coefficient, specific retention volume, or adjusted
retention time at a given temperature. The independent variables
in eq 1 are the solute excess molar refraction (R2), the effective
solute dipolarity/polarizability (πH2 ), the effective solute hy-
drogen bond acidity (∑RH2 ), the effective solute hydrogen bond
basicity (∑âH2 ), and the solute gas-liquid partition coefficient
on hexadecane at 25 °C (log L16). The coefficients c, r, s, a, b,
and l are not simply fitting coefficients, but they reflect
complementary properties of the solvent phase. The r coefficient
reflects the tendency of the phase to interact with gaseous solutes
through dispersive-type interactions via electron pairs and π
electrons. The coefficient s is a measure of the phase dipolarity/
polarizability. The coefficient a represents the complementary
property to solute hydrogen bond acidity and is a measure of
the hydrogen bond basicity. Likewise, the coefficient b is a
measure of the phase hydrogen bond acidity. Finally, the
coefficient l is a combination of the work needed to create a
It is important to have reliable experimental phase equilibrium
data for the development of thermodynamic models. Activity
coefficients at infinite dilution (γ∞) are very useful for process
synthesis and design. Such thermodynamic data can be used to
determine gE model parameters and predict phase diagrams of
mixtures containing ionic liquids. Activity coefficients at infinite
dilution can be also used for the selection of solvents for
azeotropic/extractive distillation and liquid extraction. Activity
* Corresponding author. E-mail: mutelet@ensic.inpl-nancy.fr. Telephone:
+33 3 83 17 51 31. Fax: +33 3 83 17 51 52.
† Laboratoire de Thermodynamique des Milieux Polyphase´s.
‡ Universite´ de Metz.
10.1021/je060033f CCC: $33.50 © 2006 American Chemical Society
Published on Web 04/26/2006

Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006 1275
cavity in the phase and the general dispersion interaction energy
between solute and solvent phase. The system constants are
determined by multiple linear regression analysis of experi-
mental log SP(log VN in this work) values for a group of solutes
of sufficient number and variety to establish the statistical and
chemical validity of the model.
The solutes were purchased from Aldrich with a purity higher
than 99.5 %. The solutes were used without any purification
because the gas-liquid chromatography technique separates any
impurities in the column. All support materials were purchased
from Supelco. The ionic liquid was further purified by subjecting
the liquid to a very low vapor pressure for approximately 240
min. This procedure removed any volatile chemicals and water
from the ionic liquid.
In this work, activity coefficients at infinite dilution have been
determined for 29 polar and nonpolar compounds (alkanes,
alkenes, alkynes, cycloalkanes, aromatics, alcohols) in eight
ionic liquids: 1-propyl boronic acid-3-alkylimidazolium bromide
(with alkyl ) methyl, octyl, decyl, or dodecyl) and 1-propenyl-
3-alkylimidazolium bromide (with alkyl ) methyl, octyl, decyl,
or dodecyl) at 323.15 K. The LSER model was then applied to
characterize the nature of solute interactions with ionic liquids.
Experimental Procedure. Inverse chromatography experi-
ments were carried out using a Varian CP-3800 gas chromato-
graph equipped with a heated on-column injector and a flame
ionization detector. The injector and detector temperatures were
kept at 523 K during all experiments. Helium flow rate was
adjusted to obtain adequate retention times. Methane was used
to determine the column hold-up time. Exit gas flow rates were
measured with an Alltech Digital Flow Check Mass Flowmeter.
The temperature of the oven was measured with a Pt 100 probe
and controlled to within 0.1 K. A personal computer directly
recorded detector signals, and corresponding chromatograms
were obtained using Galaxie software.
Column packing of 1 m length containing from (4 to 14) %
of stationary phases (RTIL) on Chromosorb W-AW (60-80
mesh) were prepared using the rotary evaporator technique. After
evaporation of the dichloromethane under vacuum, the support
was equilibrated at 323 K during 6 h. The mass of the packing
material was calculated from the mass of the packed and empty
column and was checked during experiments. The masses of
the stationary phase were determined with a precision of 0.0003
g. A volume of the headspace vapor of samples of (1 to 5) µL
was introduced to be in infinite dilution conditions. Each
experiment was repeated at least twice to check the reproduc-
ibility. Retention times were generally reproducible to within
(0.01 to 0.03) min. To check the stability of the experimental
conditions, such as the possible eluation of the stationary phase
by the helium stream, the measurements of retention times were
repeated systematically every day for three selected solutes. No
changes in the retention times were observed during 3 months
of continuous operation.
Experimental Section
Materials and Chemicals. The ionic liquids were prepared
according to the following procedure.
(a) Synthesis of 1-R-Imidazole. The 1-R-imidazole (with R
) methyl, octyl, decyl, and dodecyl) was obtained by N-
alkylation of 1-H-imidazole; 0.1 mol of 1-H-imidazole and 0.1
mol of 1-bromo-R (with R ) methyl, octyl, decyl, and dodecyl)
were mixed under alkaline medium (K2CO3/KOH) in the
presence of 0.0075 mol of tetrabutylammonium bromide. The
mixture was placed under microwave irradiation (300 W) for 2
min. Then the product was extracted from the mixture using
CH2Cl2. After the removal of the solvent under reduced pressure,
pure 1-R-imidazole was isolated with a yield higher than 92 %
1
and characterized by H NMR.
(b) Synthesis of 1-Propenyl-3-R-imidazolium Bromides.
1-Propenyl-3-R-imidazolium bromides were obtained by adding
0.05 mol of allylbromide to 0.01 mol of 1-R-imidazole under
reflux for 2 h. The mixture was then solved in ethyl ether to
remove alkylbromide in excess, and the residue was filtered
and washed three times in 20 mL of ethyl ether to give in good
yields (over 90 %) the excepted 1-propenyl-3-R-imidazolium
bromides as yellow pastes (melting point < 30 °C). Each
isolated compound was then characterized by 1H NMR. These
1-propenyl-3-R-imidazolium bromides were used as starting
materials in the synthesis of the corresponding 1- propylboronic
acid-3-R-imidazolium bromides.
Calculation
ActiWity Coefficients at Infinite Dilution. The retention data
determined with inverse chromatography experiments were used
to calculate activity coefficients at infinite dilution of the solute
in the ionic liquids.
The standardized retention volume, VN, was calculated with
the following usual relationship:23
Tcol
Tr
Pow
Po
Synthesis of 1- Propylboronic Acid-3-R-Imidazolium Bro-
mides. 1-Propylboronic acid-3-R-imidazolium bromides were
synthesized in two steps by a hydroboration reaction conducted
under argon in CCl4 at 0 °C with a solution of borane methyl
sulfide (11 mol‚L-1), followed by an acidic hydrolysis (1
mol‚L-1 HCl) of the corresponding borane in methanol. Products
were collected in the organic layer by three liquid/liquid (CHCl3/
water) biphasic extractions. Afterwards, chloroform was re-
moved under reduced pressure, and 1-propylboronic acid-3-R-
imidazolium bromides were isolated in pure form in good yield
VN ) JU0tR′
× 1 -
(2)
(
)
The adjusted retention time tR′ was taken as the difference
between the retention time of a solute and that of the methane.
Tcol is the column temperature, U0 is the flow rate of the carrier
gas measured at the room temperature Tr, Pow is the vapor
pressure of water at Tr, and Po is the pressure at the column
outlet. The factor J in eq 2 corrects for the influence of the
pressure drop along the column and is given by eq 3:24
1
(85 %). The H NMR spectrums had confirmed the propenyl
2
Pi
group disappearance, indicating its total reduction. Each structure
- 1
was confirmed by 1H NMR and for R ) octyl and decyl by 13
C
( )
[
]
]
Po
3
2
J )
(3)
NMR also. Only 1-propylboronic acid-3-dodecylimidazolium
bromide was in solid state (melting point ) 32 °C); the three
others were pale yellow liquids.
3
Pi
- 1
( )
[
Po

1276 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
Table 1. Experimental Activity Coefficients at Infinite Dilution of
28 Organic Compounds in the 1-Propenyl-3-alkylimidazolium
Bromide at 323.15 K
Table 2. Experimental Activity Coefficients at Infinite Dilution of
23 Organic Compounds in the 1-Propyl Boronic
Acid-3-Alkylimidazolium Bromide at 323.15 K
γ
∞ values with alkyl )
γ
∞ values with alkyl )
solutes
hexane
heptane
octane
nonane
cyclohexane
benzene
toluene
ethylbenzene
m-xylene
o-xylene
1-hexene
1-hexyne
1-heptyne
2-butanone
2-pentanone
1.4 dioxane
methanol
methyl
octyl
decyl
dodecyl
solutes
hexane
heptane
octane
nonane
cyclohexane
benzene
toluene
ethylbenzene
m-xylene
methyl
octyl
decyl
dodecyl
25.07
49.65
63.02
65.61
15.68
3.60
6.19
11.09
11.10
8.98
16.5
8.27
13.16
2.68
4.32
1.59
0.18
0.27
0.54
0.69
0.33
11.40
0.25
0.33
1.54
1.04
1.83
1.48
8.90
11.36
14.94
17.42
5.66
1.40
1.92
2.56
2.73
2.39
5.93
2.04
2.56
1.46
1.93
1.52
0.17
0.27
0.28
0.36
0.29
4.18
0.14
0.28
0.82
0.98
1.23
0.93
8.39
11.13
14.35
16.91
5.64
1.39
1.85
2.54
2.73
2.34
5.89
2.02
2.58
1.48
1.78
1.39
0.17
0.26
0.28
0.35
0.29
3.54
0.13
0.27
0.80
0.89
1.14
0.89
4.41
5.32
6.37
6.93
3.12
1.08
1.38
1.77
1.85
1.66
3.47
1.56
1.87
1.43
1.62
1.30
0.15
0.23
0.23
0.29
0.23
2.95
0.12
0.25
0.70
0.86
1.12
0.77
18.23
20.25
23.65
26.17
8.15
1.84
2.41
3.28
3.57
3.11
8.32
2.77
3.70
1.88
2.32
1.78
0.04
0.23
0.41
1.23
1.28
1.58
1.20
5.45
7.04
8.81
9.92
3.89
1.33
1.80
2.37
2.11
1.89
3.76
1.77
2.22
1.59
1.67
1.38
0.05
0.12
0.25
0.79
0.95
1.19
0.79
4.65
5.36
6.34
6.99
3.25
1.21
3.36
3.83
1.98
1.78
3.60
1.86
2.20
1.33
1.63
1.32
0.04
0.19
0.34
0.90
1.16
1.51
0.90
77.64
80.97
83.18
18.07
38.76
45.06
48.15
44.02
o-xylene
1-hexene
1-hexyne
1-heptyne
2-butanone
2-pentanone
1.4 dioxane
methanol
chloroforme
dichloromethane
tetrachloromethane
acetonitrile
1-nitropropane
thiophene
42.90
21.80
24.23
11.21
0.05
3.01
3.20
21.21
3.30
15.84
19.37
ethanol
1-propanol
2-propanol
2-methyl-1-propanol
ether
chloroforme
dichloromethane
tetrachloromethane
acetonitrile
1-nitropropane
thiophene
uncertainty of ( 0.002 bar. The masses of the stationary phase
were determined with a precision of ( 0.0003 g. The temper-
ature of the oven was measured with a Pt 100 probe and
controlled to within ( 0.1 K. Taking into account that
thermodynamic parameters are also subject to an error, γ∞values
are estimated to be accurate within ( 3 %. However, the peak
shape of pyridine, 1-nitropropane, and triethylamine on ionic
liquids is asymmetric, resulting in inaccurate activity coef-
ficients. In this case, γ∞ values are estimated to be accurate
within ( 9 %.
where Pi and Po are respectively the inlet and outlet pressures
of gas chromatography column.
The activity coefficient at infinite dilution of a solute 1 in
the stationary phase 2 (RTIL) was calculated with the following
expression:
0
2B13 - V∞
+
1 JPo (4)
1-Propenyl-3-Alkylimidazolium Bromide. Experimental ac-
tivity coefficients at infinite dilution of 28 organic compounds
are listed in Table 1. The γ∞ values for the n-alkanes increase
with an increase in carbon number. The γ∞ values of n-alkanes
are higher than the values obtained with cyclohexane, alkenes,
alkynes, and aromatics. Introduction of a double or triple bond
in the n-alkanes causes a decreasing of the γ∞ values. Cyclization
of the alkane skeleton (e.g., cyclohexane) also reduces the value
of γ∞ in comparison to that of the corresponding linear alkanes
(e.g., hexane). The more polar the solute, the greater the
interaction with the ionic liquid. Aromatics (with its π-delo-
calized electrons) have smaller γ∞ values than n-alkanes,
presumably because of the interaction with the cation species.
In the serie of chloromethanes, it was observed that γ∞ values
strongly increase from dichloromethane to tetrachloromethane.
This behavior already observed with different ionic liquids1,2
indicates that polar compounds have better solubility in the
RTILs when attractive interaction between polar molecules and
the charged ions of the solvent is possible. In the case of
alcohols, the lone pair of electrons on the oxygen atom could
interact with the ionic liquid cation, and the acidic proton is
attracted to oxygen atoms in the anion. γ∞ values of n-alkanols
increase with increasing chain length. γ∞ values of branched
alkanol skeleton are smaller than γ∞ values of the corresponding
linear alcohol. γ∞ values of ketones, pyridine, thiophene,
acetonitrile, and acetone are lower in comparison with those of
the alcohols. In all cases, γ∞ values of solutes decrease with
increasing alkyl chain length of the ionic liquid. In Figure 1,
γ∞ values of some solutes as a function of the logarithm of the
carbon number of the alkyl chain is plotted. It was found that
n2RT
0B11 - V1
ln γ∞1,2 ) ln
- P1
0
(
)
RT
RT
VNP1
n2 is the mole number of the stationary phase component inside
the column, R is the ideal gas constant, T is the temperature of
the oven, B11 is the second virial coefficient of the solute in the
gaseous state at temperature T, B13 is the mutual virial coefficient
between the solute (1) and the carrier gas helium (3), P10 is the
0
probe vapor pressure at temperature T. The values of P1 and
B
11 have been taken from the literature.25 The molar volume of
the solute V10 was determined from experimental densities, and
the partial molar volumes of the solutes at infinite dilution V1∞
0
were assumed to be equal to V1 . Values of B13 have been
estimated using Tsonopolous’s method.26-28 Critical parameters
and acentric factors used for the calculations were taken from
the literature.25,29
Results and Discussion
Activity coefficients at infinite dilution of organic compounds
in ionic liquids are calculated using eqs 2 to 4. The accuracy of
γ∞ values reported in Tables 1 and 2 may be obtained from the
law of propagation of errors. The following measured parameters
exhibit errors and must be taken into account in the errors
calculations with their corresponding standard deviations. Exit
gas flow rates were measured with an uncertainty of ( 0.1 cm3/
min. At a given temperature, each experiment was repeated at
least twice to check the reproducibility. Retention times were
generally reproducible within (0.01 to 0.03) min. The pressure
drop and the inlet and outlet pressures were measured with an

Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006 1277
Figure 1. Plot of activity coefficient at infinite dilution of various solutes as a function of the logarithm of the carbon number of 1-propenyl-3-R-imidazolium
bromide: 4, cyclohexane; 9, toluene; 2, 2-butanone; ], 2-propanol; ×, dichloromethane; b, ether.
Table 3. LSER Descriptors of Solutes Used to Characterize Ionic
Liquids
there is a linear relationship between γ∞ values and the logarithm
of the carbon number of the alkyl chain.
solutes
hexane
heptane
octane
nonane
cyclohexane
benzene
toluene
ethylbenzene
m-xylene
o-xylene
1-hexene
1-hexyne
1-heptyne
2-butanone
2-pentanone
1.4 dioxane
methanol
R2
π2H
∑RH2
∑âH2
log L16
1-Propyl Boronic Acid-3-Alkylimidazolium Bromide. Ex-
perimental γ∞ values of 23 organic compounds are listed in
Table 2. The high values of activity coefficient at infinite
dilution of n-alkanes indicate their low solubility in 1-propyl
boronic acid-3-alkylimidazolium bromide. Activity coefficient
at infinite dilution of n-alkanes increase with an increase in
carbon number. The γ∞ values of n-alkanes are higher than the
values obtained with cyclohexane, alkenes, alkynes, and aromat-
ics. Alkene and aromatic hydrocarbons with polarizable elec-
trons showed stronger interactions with the ionic liquids than
alkanes, probably due to the increasing strength of ion-induced
dipole interactions. Solutes with polar functional groups had
higher solubility due to attractive interactions with ions of the
ionic liquids. In the serie of chloromethanes, the same behavior
is observed as in many ionic liquids. The γ∞ values strongly
increase from dichloromethane to tetrachloromethane. In the
case of methanol, the lone pair of electrons on the oxygen atom
could interact with the ionic liquid cation, and the acidic proton
is attracted to oxygen atoms in the anion. In the serie of 1-propyl
boronic acid-3-alkylimidazolium bromide, alcohols have a high
solubility (γ∞ values close to zero) and no peak was observed
for ethanol and 2-propanol. No linear relationship between γ∞
values and of the logarithm of the carbon number of the alkyl
chain was found for 1-propyl boronic acid-3-alkylimidazolium
bromide.
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.12
0.12
0
0
0
0.43
0.37
0.37
0.33
0.37
0.37
0
0.15
0.10
0
0
0
0
0
2.668
3.173
3.677
4.182
2.964
2.768
3.325
3.778
3.839
3.939
2.572
2.510
3.000
2.287
2.755
2.892
0.970
1.485
2.031
1.764
2.413
2.601
2.015
2.48
0.305
0.610
0.601
0.613
0.623
0.663
0.078
0.166
0.160
0.166
0.143
0.329
0.278
0.246
0.236
0.212
0.217
0.224
0.041
0.425
0.387
0.458
0.237
0.242
0.687
0.10
0.52
0.52
0.51
0.52
0.56
0.08
0.23
0.23
0.70
0.68
0.75
0.44
0.42
0.42
0.36
0.39
0.42
0.25
0.49
0.57
0.38
0.9
0
0.14
0.14
0.15
0.16
0.16
0.07
0.1
0.1
0.51
0.51
0.64
0.47
0.48
0.48
0.56
0.48
0.48
0.45
0.02
0.05
0
ethanol
1-propanol
2-propanol
2-methyl-1-propanol
1-butanol
ether
chloroforme
dichloromethane
tetrachloromethane
acetonitrile
1-nitropropane
thiophene
2.019
2.833
1.739
2.894
2.819
0.07
0
0
0.32
0.31
0.15
0.95
0.57
LSER Characterization. Equation 1 was used to characterize
ionic liquids studied. System constants c, r, s, a, b, and l of
ionic liquids were obtained by multiple linear regression of the
logarithm of the standardized retention volume log VN of 28
solutes. LSER parameters of probes are given in Table 3. The
system constants for 1-propenyl-3-alkylimidazolium bromide
at 323.15 K are summarized in Table 4. LSER model was not
applied on 1-propyl boronic acid-3-alkylimidazolium bromide
because of lack of information on alcohols. Indeed, our data
contain 23 solutes, which is not a sufficient amount to obtain
meaningful values of the system constant.
1-Propenyl-3-Alkylimidazolium Bromide. LSER parameters
of 1-propenyl-3-alkylimidazolium bromide are very similar to
those obtained for the 1-butyl-3-methylimidazolium octyl
sulfate2 and 2-hydroxy-4-morpholinepropanesulfonate.14 The (c
+ l log L16) term gives information on the effect of cohesion
of the ionic liquids on solute transfer from the gas phase. In
general, the ionic liquids are cohesive solvents.14 The l term
decreases with increasing alkyl chain length of the ionic liquid.
The ionic liquid interacts weakly via nonbonding and π-electrons
(r system constant is zero) and is not much different to other
polar nonionic liquids. The dipolarity/polarizability (s system
constants) of this family of ionic liquids is similar as the most
dipolar/polarizable nonionic stationary phases. The hydrogen-
bond basicity of the ionic liquid (a system constants) is
considerably larger than values obtained for nonionic phases
(0 to 2.1),14 and RTILs are not hydrogen-bond acid (b ) 0).
The increase of the alkyl chain length of the ionic liquid causes
a decrease of the hydrogen bond basicity and of the polarity.
SelectiWity at Infinite Dilution for the Ionic Liquids. Activity
coefficients at infinite dilution are important properties used for
the selection of solvents and for the reliable design of thermal
separation processes.30 As an example, the knowledge of the

1278 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
Table 4. LSER Descriptors of Ionic Liquids Determined at 323.15 Ka
system constants
statistics
SD
ionic liquids
r
s
a
b
l
c
F
n
1-propenyl-3-methylimidazolium bromide
0
2.16
(0.09)
1.72
(0.08)
1.73
(0.07)
1.44
(0.07)
5.19
(0.18)
4.96
(0.16)
4.89
(0.17)
4.87
(0.16)
0
0.53
(0.05)
0.57
(0.05)
0.66
(0.04)
0.72
(0.03)
-1.86
0.990
0.089
28
1-propenyl-3-octylimidazolium bromide
1-propenyl-3-decylimidazolium bromide
1-propenyl-3-dodecylimidazolium bromide
0
0
0
0
0
0
-1.60
-1.58
-1.51
0.989
0.990
0.991
0.088
0.089
0.088
28
28
28
a F, multiple correlation coefficient; SD, standard deviation in the estimation; and n, number of solutes.
Table 5. Selectivity S∞12 at Infinite Dilution for Different Ionic Liquids at 323.15 K
hexane (1) +
benzene (2)
hexane (1) +
hexene (2)
hexane (1) +
methanol (2)
2-pentanone (1) +
hexane (1) +
chloroform (2)
ionic liquids
benzene (2)
1-propenyl-3-methylimidazolium bromide
1-propenyl-3-octylimidazolium bromide
1-propenyl-3-decylimidazolium bromide
1-propenyl-3-dodecylimidazolium bromide
1-propyl boronic acid-3-octylimidazolium bromide
1-propyl boronic acid-3-decylimidazolium bromide
1-propyl boronic acid-3-dodecylimidazolium bromide
6.96
6.36
6.03
4.08
9.91
4.09
3.84
1.52
1.50
1.43
1.27
2.19
1.45
1.29
139.28
52.35
49.35
29.4
455.75
109
1.20
1.27
1.38
1.50
1.26
1.26
1.35
100.28
63.57
64.54
36.75
79.26
45.42
24.47
116.25
separation factor at infinite dilution is needed to avoid the
oversizing of a distillation column. Aliphatic hydrocarbons show
an important deviation from Raoult’s law, in contrast to aromatic
compounds, chloroalkane, and alcohols, in both ionic liquids
investigated in this work. This information may be a useful tool
for extractive distillation or extraction. The selectivity that
indicates the suitability of a solvent for separating mixtures of
components 1 and 2 by extractive distillation is given by31
hydrogen-bond basicity parameter (0.51) and benzene has a low
value (0.14), the other LSER parameters being similar for the
two organic compounds. Finally, the LSER treatment and the
analysis of selectivity lead to the same conclusions about the
interactions that are governing partitioning on the two ionic
liquids.
Conclusions
Activity coefficients at infinite dilution of various solutes in
eight ionic liquids have been measured at 323.15 K using inverse
gas chromatography. It has been observed that γ∞ values of polar
and nonpolar solutes decrease with increasing alkyl chain length
of the ionic liquid. It was also found that there is a linear
relationship between γ∞ values and the logarithm of the carbon
number of the alkyl chain of ionic liquids. The S∞12 values
obtained for different binary mixtures indicate that 1-propenyl-
3-alkylimidazolium bromide and 1-propyl boronic acid-3-
alkylimidazolium bromide can play an important role for the
separation of aromatics, chloroalkanes, and alcohols from
alkanes. LSER treatment and the analysis of selectivity or
activity coefficient at infinite dilution lead to the same conclu-
sions about the interactions that are governing partitioning on
the two ionic liquids.
γ∞1/RTIL
γ∞2/RTIL
S∞12
)
(5)
The selectivity value S∞12 relative to the RTILs studied in this
work at 323.15 K are presented in Table 5. The S∞12 values
were calculated from the γ∞ values listed in Tables 1 and 2.
The large S∞12 values show that RTILs can play an important
role in separating aromatics, alcohols, and chloroalkanes from
alkanes. A further analysis of Table 5 shows that selectivity
increases with decreasing length of the alkyl chain for the
different separation problems for imidazolium compounds with
bromide as anion. This behavior was already observed for ionic
liquids with bis(trifluoromethylsulfonyl)imide.32 The S1∞2 val-
ues obtained for different binary mixtures indicate that 1-pro-
penyl-3-alkylimidazolium bromide and 1-propyl boronic acid-
3-alkylimidazolium bromide can play an important role for the
separation of aromatics, chloroalkanes, and alcohols from
alkanes.
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