Phase equilibrium, volumetric, and interfacial properties of the ionic liquid, 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide and 1-Octene

Article abstract of DOI:10.1021/je900697w

Biphasic systems involving ionic liquids (ILs) form the basis of many applications in extractions and reactions. In this work, the liquid-liquid phase equilibrium, density, molar volume, excess molar volume, and interfacial and surface tensions are measured for saturated and subsaturated mixtures of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf ₂N]) and 1-octene at four different temperatures, (10, 25, 50, and 75) °C. Many of the properties of the IL phase change significantly with the increasing concentration of 1-octene. The nonrandom two-liquid (NRTL) activity coefficient model was utilized to model the mutual solubility of two liquids. 1-Octene is fairly soluble in the [HMIm][Tf₂N], but the IL solubility in the 1-octene is very small. The excess molar volume for the mixture of ILs and 1-octene is slightly negative for all isotherms and becomes more negative with an increase in temperature. The air-liquid surface tension decreases with increasing 1-octene concentration, and the saturated [HMIm][Tf₂N] + 1-octene interfacial tension decreases significantly at higher temperatures.

Full text of DOI:10.1021/je900697w

J. Chem. Eng. Data 2010, 55, 1611–1617
1611
Phase Equilibrium, Volumetric, and Interfacial Properties of the Ionic Liquid,
1
-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide and 1-Octene
†
Azita Ahosseini, Brent Sensenich, Laurence R. Weatherley, and Aaron M. Scurto*
Department of Chemical & Petroleum Engineering and NSF-ERC Center for Environmentally Beneficial Catalysis, University of
Kansas, Lawrence, Kansas 66045
Biphasic systems involving ionic liquids (ILs) form the basis of many applications in extractions and reactions.
In this work, the liquid-liquid phase equilibrium, density, molar volume, excess molar volume, and interfacial
and surface tensions are measured for saturated and subsaturated mixtures of 1-hexyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)amide ([HMIm][Tf N]) and 1-octene at four different temperatures, (10, 25, 50,
2
and 75) °C. Many of the properties of the IL phase change significantly with the increasing concentration
of 1-octene. The nonrandom two-liquid (NRTL) activity coefficient model was utilized to model the mutual
2
solubility of two liquids. 1-Octene is fairly soluble in the [HMIm][Tf N], but the IL solubility in the 1-octene
is very small. The excess molar volume for the mixture of ILs and 1-octene is slightly negative for all
isotherms and becomes more negative with an increase in temperature. The air-liquid surface tension
decreases with increasing 1-octene concentration, and the saturated [HMIm][Tf
tension decreases significantly at higher temperatures.
2
N] + 1-octene interfacial
1
. Introduction
properties must be known. Olefin and IL systems have applica-
9
,10
tions in olefin extraction
hydrogenation, and so forth).
5a,n11d-1c3atalysis (hydroformylation,
Numerous applications of ionic liquids (ILs) have been
demonstrated in the literature and industry. The physico-
chemical properties of ILs can be molecularly designed to
positively affect reactions, extractions, and materials process-
ing. In addition, their lack of significant vapor pressure
eliminates potential air pollution, which is a considerable
advantage over conventional organic solvents. Many current
and potential applications utilize ILs in biphasic scenarios,
for example, liquid-liquid, liquid-vapor, liquid-solid, and
so forth. For instance, biphasic extractions and reactions have
In this work, thermodynamic properties of liquid-liquid equilib-
rium, density, and interfacial and surface tension are measured for the
system of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-
2
amide ([HMIm][Tf N]) and 1-octene. Focus is placed on quantifying
the effect of 1-octene concentration on these various properties.
2
. Experimental Methods
2.1. Synthesis of the ILs. 1-Hexyl-3-methylimidazolium
1
-3
been reported.
As with any multiphase system, a quantita-
bis(trifluoromethylsulfonyl)amide ([HMIm][Tf N]) was prepared
2
tive understanding of the phase equilibrium thermodynamics,
mass transport properties, and interphase mass transfer rates
are necessary to properly design and understand these
systems. However, only limited data in any one of these areas
is currently available in the literature.
by anion exchange from the corresponding bromide salt of the
imidazolium cation ([HMIm][Br]) with lithium bis(trifluorom-
ethylsulfonyl)amide (Li[Tf N]) in deionized water as described
2
14,15
in the literature.
The bromide salt of the imidazolium cation
was prepared from a quaternization reaction of 1-methylimi-
dazole with a slight excess amount of the corresponding
1-bromohexane in acetonitrile at 40 °C under an argon
atmosphere with stirring for three days. Caution: This reaction
is highly exothermic, and adequate solVent Volumes and/or
cooling must be proVided during the reaction. The bromide salt
of 1-hexyl-3-methyl-imidazolium was purified with activated
charcoal (10 %), stirring for 24 h. Acetonitrile was added to
reduce the viscosity of the IL, and the mixture was filtered.
The mixture was then passed through a plug of Celite (depth )
7 cm, diameter ) 3 cm) and through a short column (height )
20 cm, diameter ) 2.5 cm) of acidic alumina. The solvent was
removed on a rotary evaporator under reduced pressure at
40 °C and then connected to a high vacuum (< 0.013 Pa) at 50
2
We have recently initiated a complete study of biphasic mass
transfer in a model system of 1-n-hexyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)amide ([HMIm][Tf
see Figure 1). [HMIm][Tf N] is a common IL and chosen as
a model IL by the IL working group of IUPAC. We have
utilized [HMIm][Tf N] in various systems for homogeneously
2
N]) and 1-octene
(
2
4
2
5,6
7,8
catalyzed reactions and in phase equilibria and extractions.
As will be discussed below, the solubility of [HMIm][Tf N] in
-octene is very small, but the 1-octene solubility in the IL phase
2
1
is moderate. Thus, mass transfer will occur in virtually one
direction and thus simplify the mass transfer study in the
biphasic system. However, to model the mass transfer of
1-octene into the IL under certain flow conditions, for example,
falling IL droplets, and so forth, the complete concentration and
temperature dependent thermodynamic and mass transport
°C for at least 48 h. [HMIm][Tf N] was synthesized by anion
exchange between the [HMIm][Br] and the Li[Tf N]. The denser
2
hydrophobic IL phase was decanted and washed with water for
*
Corresponding author. Phone: +1 (785) 864-4947; fax: +1 (785)
64-4967; e-mail: ascurto@ku.edu.
Current address: Neutrogena Corporation, 5760 W. 96th St., Los Angeles,
6
to 8 times. The IL was then dried under vacuum at approxi-
8
†
mately 80 °C. Great care was taken to adequately remove any
of the bromide impurities and water as these contaminants
CA 90054.
1
0.1021/je900697w  2010 American Chemical Society
Published on Web 12/22/2009

1
612 Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010
2
Figure 1. Structure of 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf N]) and 1-octene.
1
6
may significantly change the physical properties. The puri-
fied samples were stored and handled in Schlenk tubes and
procedures under dry argon.
( 0.1 °C. Measuring range for surface and interfacial tensions
1
-
was between (1 and 999) mN·m with a resolution of ( 0.1
-
1
mN·m . The unit operates using the ring method where a
platinum/iridium alloy ring immersed below either the liquid
(for surface tension) or below the liquid-liquid interface (for
interfacial tension) is raised slowly while measuring the force.
The force required to raise the ring just at the interface is at a
maximum. This maximum force is directly related to the surface
or interfacial tension with knowledge of the density. Ten
measurements were taken and reported together with the average
value and the standard deviation. The surface tension of
1H NMR chemical shifts (relative to TMS internal standard)
and coupling constants J/Hz: δ ) 8.65 (s, 1H), 7.39 (d, 2H, J
4.19), 4.17 (q, 2H, J ) 7.4), 3.93 (s, 3H), 1.87 (m, 4H), 1.32
6H, m) 0.87 (t, 3H, J ) 6.53). Analysis calculated for
: C, 32.2; H, 4.28; N, 9.39; S, 14.33. Found:
C, 32.21; H, 4.27; N, 9.25; S, 14.19. From NMR and elemental
analysis, the estimated purity is 99 + %. The residual bromide
mass fraction is less than 8·10 , and the water mass fraction
)
(
12 19 3 6 2 4
C H N F S O
-6
-4
-1
was always less than 3.5 ·10 .
.2. Phase Equilibrium Measurements. The liquid-liquid
equilibrium envelope at ambient pressure of 1-octene and the
deionized water at 25 °C was measured as 69.0 mN ·m which
1
7
-1
2
compared with a literature report of 71.99 mN ·m . The
interfacial tension between octane and water was measured as
51.0 mN ·m compared with 51.2 mN ·m from Zeppieri et
-
1
-1
IL was determined in this study. The amount of 1-octene in
1
8
[
HMIm][Tf
2
N] was measured by a cloud-point method where
al.
one component was continuously yet slowly added to the other
component until the mixture became cloudy. The amount of
each pure component was measured with a Mettler Toledo
XS205 dual range balance, with an accuracy of 0.01 mg for
the range as a maximum of 81 g and 0.1 mg for the range as a
maximum of 220 g. A Branson model 2510 ultrasonic bath was
used to mix the solutions. Temperature control at (10 and 25)
2.5. Analysis. Elemental analysis was performed by Desert
1
Analytics Transwest Geochem. H NMR spectra were recorded
on a Bruker 400 NMR spectrometer using trimethylsilyl (TMS)
1
as a reference for H chemical shifts. Gas chromatography
analyses of the product mixtures were carried out on a Varian
CP-3800 series gas chromatograph equipped with a flame
ionization detector (GC-FID for quantitative analyses) and a
chrompack capillary column CP-Sil 5 CB (25 m, 0.32 mm, 1.2
µm). The water content was determined by a Mettler Toledo
DL32 Karl Fisher coulometer, and the Br content was measured
by a Cole Parmer bromide electrode (27502-05) read with an
Oakton Ion 510 series meter. The amount of IL in the 1-octene
phase at different isotherms was measured using Jobin-Yvon
ICP-OES spectrometer (JY 2000) by following the sulfur
emissions from the anion in a nonaqueous solvent (ethanol).
The ICP parameters were: power ) 1350 W; argon pressure )
°
C was provided by a Fisher Scientific Isotemp 3016 circulating
chiller with a resolution of ( 0.1 °C. For higher temperatures,
samples were heated and stirred on an aluminum vial holder
on an IKA RET basic C hot plate with ETS-D4 fuzzy logic
controller and resolution of ( 0.1 °C. To prevent the loss of
1
-octene because of evaporation, a septum cap was used.
The amount of [HMIm][Tf N] in the octene phase was
2
measured by inductively coupling plasma/optical emission
spectroscopy (ICP-OES; Jobin-Yvon-Horiba JY2000). The ICP-
OES is very sensitive to sulfur concentration as found in the
anion, even in nonaqueous solvents as configured here. The IL
and 1-octene were mixed and stirred for one day (24 h) at the
four different isotherms (10 °C, 25 °C, 50 °C, or 75 °C) using
an oil bath with the temperature controller above. Four samples
were taken from the 1-octene phase and mixed with ethanol to
maintain a single phase at ambient temperature. Volumetric
pipettes and flasks were used for all measurements. The samples
were analyzed using the ICP-OES for the sulfur concentration
which is in the anion.
-
1
6
20.5 kPa for plasma and nebulizer; plasma flow: 18 L ·min ;
1
-
-1
sheath flow; 0.8 L·min ; auxiliary flow ) 0.3 L·min ;
nebulizer type ) concentric glass nebulizer; nebulizer flow )
-
1
1
.63 L ·min ; nebulizer pressure 1.19 bar; and sample pump
-1
flow ) 0.3 mL ·min .
.6. Materials. Argon (extra dry grade, 99.998 %) was
obtained from Airgas, Inc. 1-Octene (CAS 111-66-0), 98 %,
-methylimidazole, (CAS 616-47-7), 99 + %, lithium bis(tri-
2
1
fluoromethylsulfonyl)amide (CAS 90076-65-6), 99.95 %, and
acetonitrile (CAS 75-05-8), g 99.9 %, were purchased from
Sigma-Aldrich. 1-Bromohexane (CAS 111-25-1), 99 + %, was
obtained from Acros. 1-Methyl-imidazole and 1-bromohexane
were vacuum-distilled and used immediately in the synthesis.
2
.3. Density. Density was measured using an Anton Parr
densitometer (DMA 4500) which is based on U-tube oscillation.
To perform a measurement, the sample was loaded into the
measuring cell. Excess sample was used to first purge the U-tube
of air and then to provide an accurate solution in the measuring
2
.7. Modeling. The NRTL (nonrandom two-liquid) activity
19
coefficient model was utilized to correlate the experimental
data. The equations are given as:
-3
cell. Uncertainty is within ( 0.0001 g ·cm . The system was
periodically checked with distilled water at 20 °C.
2
.4. Surface and Interfacial Tension. The Kr u¨ ss EasyDyne
G21
2
τ G
12 12
2
ln γ ) x τ21
+
+
(1)
(2)
tensiometer was used in this work to measure surface tension
and interfacial tension of the liquids. Temperature was main-
tained with the Kr u¨ ss thermoset jacket TJ1020 connected to a
laboratory circulating pump. The instrument can operate in a
temperature range of (-10 to 100) °C with the resolution of
1
2
(
(
)
)
2
[
x + x G
]
]
1
2
21
(x + x G )
2
1
12
2
G12
τ G
21 21
2
ln γ ) x τ12
2
1
2
[
x + x G
2 1
12
(x + x G )
1 2 21

Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010 1613
G12 ) exp(-R τ )
(3)
(4)
Table 2. Regressed NRTL Parameters and Performance for the
12
21
Solution of Mixtures of 1-Octene (1) + [HMIm][Tf
2
N] (2)
G21 ) exp(-R τ )
12
12
t/°C
where R12 is related to nonrandomness in the mixture and is set
equal to 0.2. The binary interaction parameters (BIPs), τ12 and
10
25
50
75
τ
12
4.8985
0.3532
–0.0053
–0.0043
0.0775
0.0112
3.8672
0.3354
–0.0174
–0.0074
3.7535
0.3095
–0.0288
–0.019
3.1465
0.291
–0.0168
–0.0135
τ
21, were fitted to the data at each temperature using CHEMCAD
τ21
a
I
software version 6.1.2 and using the BIP regression algorithm
dev. (x
dev. (x
AARD (x
AARD (x
1
)
)
a
II
1
(
Complex method) and the objective function:
b
I
1
)
)
b
II
2
NP
1
p,calc
p,exp 2
min
(xi
- xi
)
(5)
a
P,pred
P).
∑
∑
Deviation ) (x
1
– x
1
p)1 i)1
b
N
I,pred
1
I
1
II,pred
1
II
1
3
. Results and Discussion
.1. Liquid-Liquid Equilibrium. The mutual solubility of
-octene and [HMIm][Tf N] was measured at (10, 25, 50, and
5) °C, and the values are listed in Table 1. The uncertainties
x
(T ) - x (T )
x
(T ) - x (T )
1
N
i
i
i
i
AARD )
+
∑
I
1
II
[
|
| |
|
]
3
i)1
x (T )
x (T )
i
1
i
1
7
2
of Table 1 represent the standard deviations in the cloud point
measurement with at least three replications. For the IL in the
octene, the uncertainties are from the propagation of the standard
deviations in the ICP-OES signal on the resulting mole fraction
solubility. As shown in Figure 2, appreciable quantities of the
1
-octene in several imidazolium ILs and showed that the
solubility increases with the cation of 1-butyl-3-methylimida-
zolium ([BMIm]) in the order of: [BF ] < [PF ] < [OTf] < [Tf N]
where [OTf] is the trifluoromethylsulfonate anion. However,
they do not report the quantitative solubility. They also showed
that for a given anion, longer alkyl chains yield a higher
4
6
2
1
-octene are soluble in the IL phase. For instance at 25 °C,
the solubility is approximately 0.2 mole fraction. However, the
solubility of [HMIm][Tf N] in 1-octene was very low (< 0.0001
mole fraction). Even at 75 °C, the solubility of the IL in the
-octene phase is only about 0.045 mole fraction. This produces
solubility of 1-octene. Other reports indicate that the smaller
-hexene is slightly more soluble in ILs than 1-octene.
20,22,24,25
2
1
For instance, the solubility of 1-hexene in [BMIm][PF
6
] at
0 °C is approximately 0.12 mole fraction. Most imidazolium
ILs have much larger solubility for olefins compared with
alkanes as the solubility of n-hexane in [BMIm][PF ] at 41.3
C is 0.058 mole fraction. However, the solubility of most
1
26
3
a highly asymmetric liquid-liquid region. This general phase
behavior has interesting ramifications for mass transfer in a
biphasic system. When 1-octene and [HMIm][Tf
tacted, mass transfer will only be in virtually one direction
especially at lower temperatures, that is, 1-octene dissolution
6
N] are con-
25
2
°
imidazolium ILs in the alkene or alkane phase is very low:
usually much less than 0.01 mole fraction at ambient condi-
tions. This large difference in alkene/alkane solubility and low
solubility of the IL is being exploited for olefin extraction.
The molar excess enthalpy, H , can be computed from the
in [HMIm][Tf
2
N]. The NRTL model was used to correlate the
27
data. The model accurately correlates the 1-octene mutual
solubility data at the temperatures investigated, albeit with slight
undercorrelation of the 1-octene solubility in each phase. The
model performance and BIPs are listed in Table 2.
28,29
E
E
NRTL activity coefficient/G model from the regressed param-
eters.
There are a few studies in literature for n-alkene solubility
2
0-22
23
E
in ILs.
Stark et al. determined the relative solubility of
∂G
E
E
H ) G - T
(6)
(
)
∂
T
P,x
Table 1. Experimental Liquid-Liquid Equilibrium of 1-Octene (1)
+
From this relationship, the excess enthalpy is positive
throughout the composition range and indicates that mixing is
2
[HMIm][Tf N] (2)
3
0-33
II
a
I
b
t/°C
x
1
x
1
endothermic.
The model predicts that the excess enthalpy
increases with temperature at a given composition. Nebig
1
2
5
7
0
5
0
5
0.16 ( 0.01
0.19 ( 0.01
0.23 ( 0.02
0.28 ( 0.02
0.9999 ( 0.0001
0.9999 ( 0.0004
0.9986 ( 0.0009
0.9552 ( 0.0008
3
0
et al. measured the excess enthalpy for this [HMIm][Tf N] +
2
1-octene system at 140 °C and found that the excess enthalpy
is positive at this much elevated temperature.
a
II
b
I
3.2. Density. While the density of many different pure ILs
x
1
is the [HMIm][Tf
2
N]-rich phase.
x
1
indicates the 1-octene-rich
phase.
has been extensively studied, fewer studies exist for mixtures.
In this work, the density of pure and mixtures of
[
HMIm][Tf
measured at (10, 25, 50, and 75) °C and listed in Table 3.
The density of pure [HMIm][Tf N] is reported in literature
2
N] and 1-octene to their saturation point was
2
at different temperatures from several sources. At 75 °C, the
maximum absolute deviation between the literature sources
-3
34,35
is 0.0029 g · cm .
For pure 1-octene at 25 °C, the
-3
36
reported density is 0.711 g · cm , which is exactly equal
to our result. The density of the IL decreases with the
increasing mole fraction of 1-octene as seen in Figure 3. The
change is not a strictly linear decrease. At 75 °C, the density
difference between pure [HMIm][Tf
2
N] and a mixture of
[
HMIm][Tf N] with a mole fraction of 0.28 1-octene (satura-
2
tion) is only approximately -4.3 %. The temperature
dependence of density at each composition is linearly
dependent between (10 and 75) °C and shown in Figure 4.
2
Figure 2. Liquid-liquid equilibrium of 1-octene (1) + [HMIm][Tf N] (2).
The line is the correlation by the NRTL model.

1
614 Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010
Table 3. Density of Mixtures of 1-Octene (1) + [HMIm][Tf
2
N] (2)
density F [g·cm-3]
x
1
10 °C
25 °C
50 °C
75 °C
0
0
0
0
0
0
0
0
0
1
1.3854 ( 0.0004
1.3721 ( 0.0008
1.356 ( 0.005
1.34 ( 0.01
1.3707 ( 0.0006
1.358 ( 0.006
1.342 ( 0.002
1.328 ( 0.007
1.326 ( 0.003
1.348 ( 0.002
1.336 ( 0.002
1.319 ( 0.006
1.306 ( 0.006
1.303 ( 0.002
1.324 ( 0.002
1.313 ( 0.005
1.30 ( 0.01
.05 ( 0.0001
.10 ( 0.0001
.139 ( 0.0001
.154 ( 0.0001
1.282 ( 0.006
1.279 ( 0.011
1.34 ( 0.01
a
.16 ( 0.01
1.340 ( 0.009
a
.19 ( 0.01
1.3204 ( 0.0001
a
.23 ( 0.02
1.300 ( 0.002
0.69 ( 0.01
a
.28 ( 0.02
1.267 ( 0.012
0.668 ( 0.012
.0
0.7235 ( 0.0006
0.7112 ( 0.0006
a
This number reflects the saturation value at each isotherm.
Table 4. Excess Molar Volume of Mixtures of 1-Octene (1) +
[
2
HMIm][Tf N] (2) at Four Temperatures
t ) 10 °C
V
m
VE
3
-1
3
-1
x
1
cm ·mol
(
cm ·mol
(
0
0
0
0
0
0
1
322.93
313.85
305.29
298.62
295.35
293.97
155.08
0.02
0.14
0.13
0.13
0.13
0.13
0.02
0
.05 ( 0.0001
.1 ( 0.0001
–0.69
–0.85
–0.98
–1.73
–2.11
0
0.14
0.14
0.14
0.14
0.14
.139 ( 0.0001
.154 ( 0.0001
a
.16 ( 0.01
t ) 25 °C
0.03
0
0
0
0
0
0
1
326.40
317.03
308.44
301.75
298.41
290.61
157.77
0
2
Figure 3. Density of 1-octene (1) + [HMIm][Tf N] (2). Dashed line
indicates saturation limit. 2, 10 °C; b, 25 °C; 9, 50 °C; [, 75 °C.
.05 ( 0.0001
.1 ( 0.0001
0.14
0.14
0.14
0.14
0.03
0.03
–0.94
–1.10
–1.22
–2.03
–3.76
0
0.15
0.19
0.19
0.19
0.13
.139 ( 0.0001
.154 ( 0.0001
a
.19 ( 0.01
t ) 50 °C
0
0
0
0
0
0
1
331.92
322.45
313.78
307.00
303.69
284.82
162.58
0.03
0.15
0.14
0.14
0.14
0.13
0.15
0
.05 ( 0.0001
.1 ( 0.0001
–1.01
–1.21
–1.38
–2.15
–8.15
0
0.20
0.14
0.14
0.14
0.14
.139 ( 0.0001
.154 ( 0.0001
a
.23 ( 0.02
t ) 75 °C
0
0
0
0
0
337.92
328.06
319.35
312.61
309.47
279.05
167.89
0.15
0.15
0.15
0.15
0.15
0.13
0.15
0
.05 ( 0.0001
.1 ( 0.0001
–1.35
–1.57
–1.68
–2.27
–11.26
0
0.21
0.20
0.20
0.20
0.20
.139 ( 0.0001
.154 ( 0.0001
a
Figure 4. Density of the 1-octene (1) + [HMIm][Tf
and composition of 1-octene. Line is of smoothed data. 1-octene mole
fraction, x : b, 0; 9, 0.05; 2, 0.1; 1, 0.154; [, 1.
2
N] (2) with temperature
0.28 ( 0.02
1
1
a
Saturated value.
3
.2.1. Excess Molar Volume. From the density data and
volume of the solution indicates the deviation from ideal solution
behavior at the same temperature and pressure.
E
the composition of the mixture, the excess molar volume (V )
can be computed from:
As can be seen in Figure 5, the excess molar volume for the
mixture of ILs and 1-octene is slightly negative for all isotherms
below approximately 0.15 mole fraction of octene. Above this
mole fraction, the excess molar volume becomes more negative
until the saturation point is reached. The excess molar volume
x M + x M
x1M
1
E
1
1
2
2
V ) V - x V - x V )
-
-
m
1
1
2 2
Fm
F1
x2M2
E
(7)
slightly increases (more negative) with temperature, (∂V /∂T)
P,x
F2
<
0; thus, these contraction effects become a bit more
where V
i
and F
i
are the molar volumes and densities, respec-
N] (2), and their
are their molar masses and
pronounced with temperature. These trends may indicate that,
as small amounts of 1-octene are added, the intermolecular
forces are dominated by IL-IL interactions and that the 1-octene
may simply occupy the spaces between the ions or by weak
dispersion interactions between the 1-hexyl- group of the cation
and 1-octene. However, as the mixture becomes more concen-
trated in the 1-octene, the compounds may begin to have
tively, of pure 1-octene (1), pure [HMIm][Tf
mixture (m), respectively; M and x
i i
2
mole fractions of component i, respectively. A full error anal-
ysis was performed to determine the inherent errors in the excess
molar volume at each point; the maximum uncertainty for excess
3
-1
molar volume is about 0.2 cm ·mol (Table 4). Excess molar

Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010 1615
2
Figure 6. Surface tension of 1-octene (1) + [HMIm][Tf N] (2) and their
mixtures. 2, 10 °C; b, 25 °C; 9, 50 °C; [, 75 °C.
2
Figure 5. Excess molar volume of 1-octene (1) + [HMIm][Tf N] (2). The
last point of the isotherm is the value at saturation. Smoothed line for visual
Table 6. Interfacial Tensions and Model Interaction Parameters for
aid. 2, 10 °C; b, 25 °C; 9, 50 °C; [, 75 °C.
1
2
-Octene (1) + [HMIm][Tf N] (2) at Different Temperatures
Table 5. Surface Tension σ/mN·m- of 1-Octene (1) +
1
t
σ
IT
[
HMIm][Tf
2
N](2) and Their Mixtures
10 °C 25 °C
31.2 ( 0.1 30.8 ( 0.2 28.9 ( 0.4 27.6 ( 0.2
–
1
°
C
mN·m
φ
x
1
50 °C
75 °C
10
25
50
75
13.3 ( 1.3
13.0 ( 1.3
9.7 ( 1.3
4.4 ( 1.3
0.76
0.76
0.82
0.93
0
0
0
0
0
0
0
0
1
.05 ( 0.0001 30.9 ( 0.1 30.3 ( 0.1 28.8 ( 0.6 27.4 ( 0.3
.1 ( 0.0001 29.8 ( 0.1 29.4 ( 0.0 28.7 ( 0.5 27.2 ( 0.6
.15 ( 0.0001 27.8 ( 0.1 27.4 ( 0.1 26.6 ( 0.2 26.3 ( 0.6
a
.16 ( 0.01
27.7 ( 0.1
a
.19 ( 0.01
27.0 ( 0.1
a
some ILs and found that increasing the length of 1-alkyl chain
on imidazolium cations decreases the surface tension. Freire
.23 ( 0.02
25.5 ( 0.4
a
.28 ( 0.02
25.3 ( 0.4
22.5 ( 0.1 21.2 ( 0.1 18.5 ( 0.1 15.7 ( 0.1
4
5
et al. also mentioned the effect of increasing alkyl chain of
cations and its branching on decreasing surface tension of ILs.
They also explain the effect of size of anions on the surface
tension. The larger anions decrease the surface tension due to
more charge delocalization resulting in weaker interactions
between ions. The hydrogen bonding often increases surface
tension because of the high cohesiveness and interactions
a
Reflects the saturation value at each isotherm.
stronger interactions possibly between the unsaturated regions
of the cation and 1-octene, which results in a volume contraction.
3
7,38
Wang et al.
mixtures of [BMIm][PF
acetonitrile, dichloromethane, 2-butanone, and dimethylforma-
also observed similar negative trends for the
4
5
6
] and organic compounds such as
between ions.
Here, we have measured the surface tension of mixtures of
3
9
mide. Heintz et al. explained that the negative values of the
[HMIm][Tf N] and 1-octene to their saturation point at (10, 25,
2
molar excess volume between 4-methyl-N-butylpyridinium
50, and 75) °C, and the data are listed in Table 5. As can be
tetrafluoroborate [4MBP][BF
4
] upon mixing with methanol
seen from the result in Figure 6, the surface tension decreases
as the 1-octene concentration increases; the pure component
surface tension of 1-octene is much lower than the IL. However,
this effect diminishes percentage wise at the higher temperatures.
molecules are most likely stabilized by ion-dipole interactions
in the mixture. However, positive excess molar volumes have
been reported for [BMIm][BF
4
] + H
2
O and [EMIm][BF
4
2
]+ H O
1
6
46
mixtures by Seddon et al.
In a recent experimental thermodynamic and molecular
dynamics simulation study of CO + imidazolium-based ILs
Kimura et al. measured the surface tension of 1-octene (23.4
-
1
47
mN·m at 20 °C), which was close to the value that Jasper
-
1
2
reported (21.8 mN ·m ) compared with our data at 25 °C of
-1
48
4
0
by Cadena et al., it was suggested that the anions and cations
of the ILs form a strong network with the interstices available
in the fluid. Therefore, it is possible that the relatively small
organic molecules fit into the interstices upon mixing. In
21.2 mN ·m . Domanska and Marciniak measured the surface
tension of pure [HMIm][Tf N] at 25 °C with a value of 31.2
2
-1
mN·m which compares with the value measured in the current
4
1
addition, it was found from molecular dynamics simulations
that the non-hydrogen bonding polar solutes, such as the organic
compounds investigated in this work, interact much more
strongly with the cations of the ILs by ion-dipole interactions.
The filling effect of organic compounds in the interstices of
ILs, and the ion-dipole interactions between the organic
compound and the imidazolium ring of the ILs all contribute
to the negative values of the molar excess volumes.
3
.3. Surface Tension. There are a several studies in the
literature concerning surface tension of pure ILs. Huddleson et
42
al. measured surface tension for several ILs and demonstrated
that these values are higher than for organic solvents such as
hexane and toluene but not as high as water at the same
4
3
temperature. Law and Watson also measured the surface
tension of some N-alkylimidazolium ILs and showed the
decreasing trend of these values with temperature. Dzyuba and
Figure 7. Interfacial tension of the saturated 1-octene (1) + [HMIm][Tf
(2) system with temperature compared to the pure component surface
tensions. 2, [HMIm][Tf N]; b, 1-octene; 9, IL/1-octene.
2
N]
4
4
Bartsch studied the structural effect on surface tension for
2

1
616 Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010
-
1
49
work of 30.8 mN ·m . Coutinho and co-workers measured
(3) Scurto, A. M.; Aki, S.; Brennecke, J. F. CO2 as a separation switch
-
1
for ionic liquid/organic mixtures. J. Am. Chem. Soc. 2002, 124 (35),
the surface tension at 50 °C with a value of 30.6 mN · m ,
1
0276–10277.
compared with 28.9 mN ·m- from this study.
1
(
4) Thermodynamics of ionic liquids, ionic liquid mixtures, and the
3
.4. Interfacial Tension. The interfacial tension of mixtures
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23.
2
(
5) Ahosseini, A.; Ren, W.; Scurto, A. M. Hydrogenation in Biphasic Ionic
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measured at (10, 25, 50, and 75) °C and are listed in Table 6.
Figure 7 illustrates that the interfacial tension decreases with
temperature to a small extent at (10 and 25) °C and more
significantly at the higher temperatures. Matsuda et al.
investigated the system of [HMIm][PF ] saturated with hexane
and decane and reported interfacial tensions at 25 °C of (8.36
and 9.76) mN ·m , respectively. These amounts are lower than
the interfacial tensions reported for the ([HMIm][Tf N] +
-octene) system in current study.
The interfacial tension between two liquids has been often
2
Expanded Liquids and Near-Critical Media; Hutchenson, K. W.,
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Washington, DC, 2008.
5
0
(
6) Ahosseini, A.; Ren, W.; Scurto, A. M. Understanding Biphasic Ionic
6
2
Liquid/CO Systems for Homogeneous Catalysis: Hydroformylation.
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-1
(7) Ren, W.; Scurto, A. M. Global phase behavior of imidazolium ionic
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2
1
(
8) Ren, W.; Scurto, A. M. Phase Equilibria of Imidazolium Ionic Liquids
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related to their air-liquid surface tension. An expression has
been proposed by van Oss and recently applied to IL-alkane
5
1
(
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(
(
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IL-Oct
IL
Oct
IL Oct
σ
) σ + σ - 2φ
√
σ σ
(8)
IT
Biphasic Ionic Liquids/CO
2.
12) Ahosseini, A.; Ren, W.; Scurto, A. M. Understanding Biphasic Ionic
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2
Systems. Chem. Today 2007, 25 (2), 40–
where φ is the interaction parameter which, from other systems
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one for nonpolar/polar systems, and greater than one for polar
4
(
2
5
0
Ind. Eng. Chem. Res. 2009, 48 (9), 4254–4265.
+
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(
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2
(
21-24), 2459–2477.
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. Conclusions
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JE900697W