Liquid-liquid interfacial tension of equilibrated mixtures of ionic liquids and hydrocarbons

Article abstract of DOI:10.1007/s11426-012-4663-1

Ionic liquids are possible alternative solvents for the separation of aromatic and aliphatic hydrocarbons by liquid-liquid extraction. Interfacial tension is an important property to consider in the design of liquid-liquid extraction processes. In this work, the liquid-liquid interfacial tension and the mutual solubility at 25 °C have been measured for a series of biphasic, equilibrated mixtures of an ionic liquid and a hydrocarbon. In particular, the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (with the alkyl substituent being ethyl, hexyl or decyl), 1-ethyl-3- methylimidazolium ethylsulfate, and 1-ethyl-3-methylimidazolium methanesulfonate have been selected, as well as the hydrocarbons benzene, hexane, ethylbenzene, and octane. The selected sets of ionic liquids and hydrocarbons allow the analysis of the influence of a series of effects on the interfacial tension. For example, the interfacial tension decreases with an increase in the length of the alkyl substituent chain of the cation or with an increase of the degree of charge delocalisation in the anion of the ionic liquid. Also, the interfacial tension with the aromatic hydrocarbons is markedly lower than that with the aliphatic hydrocarbons. A smaller effect is caused by variation of the size of the hydrocarbon. Some of the observed trends can be explained from the mutual solubility of the hydrocarbon and the ionic liquid. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-012-4663-1

SCIENCE CHINA
Chemistry
•
·
ARTICLES •
SPECIAL ISSUE · Ionic Liquid and Green Chemistry
August 2012 Vol.55 No.8: 1519–1524
doi: 10.1007/s11426-012-4663-1
Liquid-liquid interfacial tension of equilibrated mixtures of
ionic liquids and hydrocarbons
*
RODRÍGUEZ Héctor , ARCE Alberto & SOTO Ana
Department of Chemical Engineering, University of Santiago de Compostela, E-15782, Santiago de Compostela, Spain
Received April 17, 2012; accepted May 10, 2012; published online June 25, 2012
Ionic liquids are possible alternative solvents for the separation of aromatic and aliphatic hydrocarbons by liquid-liquid extrac-
tion. Interfacial tension is an important property to consider in the design of liquid-liquid extraction processes. In this work, the
liquid-liquid interfacial tension and the mutual solubility at 25 °C have been measured for a series of biphasic, equilibrated
mixtures of an ionic liquid and a hydrocarbon. In particular, the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluorome-
thanesulfonyl)imide (with the alkyl substituent being ethyl, hexyl or decyl), 1-ethyl-3-methylimidazolium ethylsulfate, and
1
-ethyl-3-methylimidazolium methanesulfonate have been selected, as well as the hydrocarbons benzene, hexane, ethylben-
zene, and octane. The selected sets of ionic liquids and hydrocarbons allow the analysis of the influence of a series of effects
on the interfacial tension. For example, the interfacial tension decreases with an increase in the length of the alkyl substituent
chain of the cation or with an increase of the degree of charge delocalisation in the anion of the ionic liquid. Also, the interfa-
cial tension with the aromatic hydrocarbons is markedly lower than that with the aliphatic hydrocarbons. A smaller effect is
caused by variation of the size of the hydrocarbon. Some of the observed trends can be explained from the mutual solubility of
the hydrocarbon and the ionic liquid.
ionic liquid, aromatic hydrocarbon, aliphatic hydrocarbon, interfacial tension, liquid-liquid equilibrium
1
Introduction
solvents for reaction and separation processes. Although
many other applications have been suggested beyond their
utilisation as simple solvents (for example, in material sci-
ence or in biological/pharmaceutical areas) [5], this contin-
ues to be a major driver for research within the field [2, 6].
One of the main applications proposed for ionic liquids
in the separations arena is their use as solvents in liq-
uid-liquid extraction processes [2]. Liquid-liquid equilibri-
um data, which are critical for the design of this kind of
processes, have been determined for different systems com-
prising ionic liquids and molecular compounds involved in
specific industrial separation problems. From these data,
key parameters such as selectivity or capacity can be in-
ferred. There are, however, other relevant factors, which can
influence the solvent selection and are necessary for a good
design of solvent extraction units. One such factor is the
interfacial tension between the liquid phases. Ideally, it
In the last years, research on ionic liquids has experienced a
formidable increase. The appealing properties of these salts
with low melting temperatures have attracted the attention
of both industry and academia. It is difficult to generalise
properties common to all substances qualifying as ionic
liquids; however, many of them exhibit a practically negli-
gible volatility, the ability to dissolve a broad range of
compounds, and a reasonably good thermal stability [1–3].
In addition, there exists the possibility of tailoring the prop-
erties of ionic liquids to a significant extent, for a specific
application, by judicious choice of the combination of the
constitutive ions [3, 4]. All these characteristics resulted in
the early suggestion of ionic liquids as promising alternative
*
Corresponding author (email: hector.rodriguez@usc.es)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

1520
Rodríguez H, et al. Sci China Chem August (2012) Vol.55 No.8
would be desirable that the immiscible mixture had a mod-
erate interfacial tension, thus balancing the ease of disper-
sion (of non-equilibrated phases) and the promotion of
phase separation (once the phases are equilibrated) [7].
Compared to the already significant number of liq-
uid-liquid equilibria studied for systems comprising an ionic
liquid, much less attention has been paid to the interfacial
tension in these systems. In the literature, interfacial tension
values can be found mainly for binary systems composed by
an ionic liquid and water [8–12], or by an ionic liquid and a
hydrocarbon [9, 12–16], with a few examples of an ionic
liquid with another kind of molecular compound [9, 13, 17].
There is one further example, for a ternary system compris-
ing an ionic liquid, heptane and thiophene [18]. It can be
seen that hydrocarbons have been some of the preferred
compounds for these studies. This is in good connection
with the relative relevance of research oriented to two in-
dustrial applications that have attracted most interest for the
development of solvent extraction processes based on ionic
liquid technology: desulfurisation of fuels, and separation of
aromatic and aliphatic hydrocarbons [2]. To date, however,
most references have reported the liquid-liquid interfacial
tension of mixtures of ionic liquid and aliphatic hydrocar-
bons, with only one work reporting measurements with an
aromatic hydrocarbon [9].
99.0%) were used as received. All ionic liquids were syn-
thesised according to similar procedures to those previously
reported [20–22], and briefly summarised in the following
paragraphs.
The ionic liquid [C2mim][NTf
2
] was synthesised in two
steps. First, 1-methylimidazole (Aldrich, 99%) was directly
alkylated with bromoethane (Sigma-Aldrich, 98%), at 40 °C
under inert atmosphere, for 48 h, to produce 1-ethyl-
3
-methylimidazolium bromide. Then, an aqueous solution
of the latter was mixed with an aqueous solution of lithium
bis(trifluoromethanesulfonyl)imide (3M, 98%), generating
the hydrophobic [C
mim][NTf ] by metathesis. Dichloro-
2 2
methane (Fluka, 99.9%) was added for a better separation of
the organic and aqueous phases; and the organic phase was
washed several times with fresh water, until no precipitation
was observed upon addition of an aqueous solution of silver
nitrate to the residual washing phase. After removal of di-
chloromethane in a rotary evaporator, the desired ionic liq-
uid was purified under reduced pressure (< 0.1 mbar) at
8
0 °C for no less than one day.
The ionic liquids [C
mim][NTf
were synthesised in an analogous manner to [C
6
2
] and [C10mim][NTf
2
]
2
mim][NTf ],
2
carrying out the alkylation step at a suitable temperature
with 1-chlorohexane (Fluka, > 99%) or 1-chlorodecane
(
Fluka, > 97%), respectively, instead of bromoethane.
3 3
The ionic liquids [C mim][EtSO ] and [C mim][CH SO ]
In this article, we have investigated the interfacial tension,
along with mutual solubility, of biphasic liquid-liquid sys-
tems formed by binary mixtures of ionic liquid and aromatic
hydrocarbon. The C6 hydrocarbons benzene and hexane,
and the C8 hydrocarbons ethylbenzene and octane, have
been selected for this study, to analyse the influence of both
the aromatic/aliphatic character and the number of carbons
of the hydrocarbon. Regarding the ionic liquids, several
members of the homologous series of 1-alkyl-3-methylim-
idazolium bis(trifluoromethanesulfonyl)imide ionic liquids
2
4
2
were prepared directly by alkylation of 1-methylimidazole
with diethylsulfate (Aldrich, > 98%) or ethylmethanesul-
fonate (Fluka, 98.0%), respectively. 1-Methylimidazole was
placed in a two-neck round-bottomed flask with a reflux
condenser attached to the main neck. It was placed in an ice
bath, and then the stoichiometric amount of the alkylating
agent was added carefully and stepwise, through the sec-
ondary neck of the flask. The ice bath and the dropwise ad-
dition were necessary in order to neutralise the high exo-
thermicity of the alkylation reactions. After complete addi-
tion of the reactants, the temperature was risen to ensure
(
abbreviated as [C
n 2
mim][NTf ], with n being the number of
carbons in the substituent alkyl chain) have been selected;
namely those with ethyl, hexyl and decyl substituent chains.
Thus, this set of ionic liquids allows for the analysis of the
influence of the length of the alkyl substituent chain, which
is a characteristic tunable feature of imidazolium-based
ionic liquids [19]. In order to explore the effect of the anion,
two other ionic liquids with a common cation have been
investigated: 1-ethyl-3-methylimidazolium ethylsulfate
total alkylation. As with the [NTf
2
]-based ionic liquids, the
resulting ionic liquid was purified under reduced pressure.
The structure and purity of all ionic liquids was con-
1
13
firmed by H and C NMR spectroscopy. Their chemical
structures are shown in Figure 1. Their water content (which
is an important impurity that can critically affect the proper-
ties and performance of ionic liquids [23]) was found to be
lower than 500 ppm for all ionic liquids, as determined by
Karl-Fischer titration in a MetrOhm 737 KF coulometer.
(
[C
2
mim][EtSO
thanesulfonate ([C
4
]) and 1-ethyl-3-methylimidazolium me-
mim][CH SO ]). These are, in fact, the
2
3
3
first non-fluorinated ionic liquids for which the interfacial
tension with hydrocarbons is reported in the literature.
2
.2 Methods
Binary mixtures of an ionic liquid and a hydrocarbon were
prepared, with a global composition lying in the immiscible
domain. They were placed inside jacketed glass cells, at a
controlled temperature of 25.0 °C (by means of Selecta
Ultraterm 6000383 water bath thermostats, precise to within
0.1 °C), and they were vigorously stirred for a minimum of
2
Materials and methods
2
.1 Materials
Benzene (Sigma-Aldrich, > 99.0 %), hexane (Fluka, >
9
9.0 %), ethylbenzene (Fluka, > 99 %) and octane (Fluka, >

Rodríguez H, et al. Sci China Chem August (2012) Vol.55 No.8
1521
phase; whereas for the hydrocarbon rich phase an uncer-
tainty of 0.5% in mole fraction can be estimated.
Interfacial tensions were experimentally determined in a
Krüss K11 tensiometer, using the method of the Wilhelmy
plate. An especially adapted platinum plate with cylindrical
shape (Krüss accessory reference PL22, with dimensions
1
0 mm height  20 mm base perimeter  0.1 mm width)
was used. This adapted plate is suitable for the performance
of reliable measurements with lower amounts of sample
than the conventional flat plate. The measurements were
performed in accordance with the instructions by the manu-
facturer. Cylindrical open-top glass vessels, with a large
enough diameter as to avoid influence of wall effects, were
used to place the denser and/or lighter phases along the dif-
ferent steps of the measuring procedure. These vessels were
partially immersed in an oil bath at 25.00.1 °C, as meas-
ured by a built-in thermometer, with the temperature being
controlled by means of circulating water from a Julabo F12
cryogenic bath. Each interfacial tension value reported in
this work is the average of ten consecutive immersion
measurements, after two initial immersions that were sys-
tematically discarded. The described method has an esti-
mated uncertainty of ca. 0.3 mN/m.
Figure 1 Chemical structures of the ionic liquids used in this work. (a)
mim][NTf ] (with n being 2, 6, or 10); (b) [C mim][EtSO ]; (c)
mim][CH SO ].
[
C
n
2
2
4
[C
2
3
3
1
.5 h, followed by a settling period of no less than 4 h to
allow complete phase separation. Without disturbance of the
liquid-liquid interface, samples were taken from both phases
in equilibrium, and stored at 25.0 °C. For compositional
analysis, a small aliquot of each equilibrated sample was
dissolved in a suitable deuteriated solvent (either CDCl
Aldrich, 99.8%) or CD OD (Aldrich, +99.8%)), and placed
3
(
3
3 Results and discussion
into an NMR tube which was immediately capped. The rest
of the samples’ volume was used in determining the interfa-
cial tension.
Compositional analysis was carried out by means of H
NMR spectroscopy. The samples were run in a 7.04 T
The liquid-liquid equilibrium and the interfacial tension, at
2
5.0 °C, of the binary mixtures of each hydrocarbon (ben-
zene, hexane, ethylbenzene, or octane) with each ionic liq-
uid ([C mim][NTf ], [C mim][NTf ], [C10mim][NTf ],
mim][EtSO ], or [C mim][CH SO ]) were determined.
1
2
2
6
2
2
(
300 MHz resonance for 1H) Varian Mercury 300 spec-
[
C
2
4
2
3
3
trometer, using a relaxation time of 20 s. The mole fractions
of hydrocarbon and ionic liquid in each case were calculat-
ed from the integrated areas under specific peaks of the
spectra. For the ionic liquids, in general, the peaks corre-
sponding to the methylene group (corresponding to 2H) and
methyl group (3H) bonded to the nitrogen atoms in the im-
idazolium ring were selected. In the particular case of
The compositions of each phase in equilibrium, as well as
the corresponding interfacial tension between both phases,
are reported in Table 1. In all cases the phase rich in ionic
liquid was the lower one, whereas the lighter phase was rich
in the hydrocarbon.
In general, no ionic liquid was detected in the upper
phase, with exception of the mixtures of [C mim][NTf ]
10
2
[
C mim][EtSO ], also the peak of the methylene group (2H)
with the two aromatic hydrocarbons. In these cases, a low
mole fraction of the ionic liquid in the hydrocarbon-rich
phase could be measured. For these two systems, the misci-
bility gap is quite narrow and is located in the hydrocar-
bon-rich region [19], thus yielding two phases in equilibri-
um in which the upper phase is almost pure hydrocarbon
and the “ionic liquid-rich phase” has a hydrocarbon mole
fraction of ca. 0.90 or greater. As a result of such similarity
in the composition of both equilibrated phases, the interfa-
2
4
bonded to the oxygen atom in the anion was included in the
calculations. For benzene, its only peak (6H) was selected.
For ethylbenzene, the peak corresponding to the hydrogen
atoms of the aromatic ring (5H) and the one for the
methylene group (2H) were used. Regarding the linear hy-
drocarbons, their peak for the terminal methyl groups (6H)
was chosen. In the systems with [C mim][NTf ] and
6
2
[
C mim][NTf ], the latter peak was overlapped by the peak
10 2
of the terminal methyl group of the long alkyl substituent
chain of the imidazolium ionic liquid, and therefore the
corresponding algebraic calculations had to be made to sub-
tract this contribution, in order to get the final mole frac-
tions of the analysed mixture. With these sets of peaks se-
lected, the maximum error of the results obtained may be
estimated at 2% in mole fraction for the ionic liquid rich
cial tension of the systems benzene + [C10mim][NTf
2
] and
ethylbenzene + [C10mim][NTf
be precisely measured.
2
] is very low, and could not
2 2
For all four systems hydrocarbon + [C mim][NTf ] and
for the two systems of [C10mim][NTf ] with the C6 hydro-
2
carbons, the equilibrium composition obtained in this work
can be compared to previous results reported in the litera-

1522
Rodríguez H, et al. Sci China Chem August (2012) Vol.55 No.8
Table 1 Mole fraction of ionic liquid (xIL) in the lower and upper phases
in equilibrium at 25.0 °C, as well as the corresponding interfacial tension
(), for binary systems hydrocarbon + ionic liquid
x
IL, upper
phase
x
IL, lower
phase
Hydrocarbon
Ionic liquid
 (mN/m)
Benzene
[C10mim][NTf
2
]
0.004
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.003
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.047
0.135
0.241
0.553
0.704
0.644
0.917
0.972
0.987
0.970
0.107
0.260
0.502
0.876
0.950
0.794
0.989
0.986
0.999
0.997
< 0.1
0.5
[
C
C
6
mim][NTf
2
]
[
2
mim][NTf
2
]
1.2
[C
2
mim][EtSO
4
]
3.6
[
[
[
[
C
2
mim][CH
3
SO
3
]
]
]
]
5.2
Hexane
[C10mim][NTf
2
]
0.3
[
C
C
6
mim][NTf
mim][NTf
2
]
4.7
[
2
2
]
11.0
14.2
18.3
< 0.1
0.8
[
C
2
mim][EtSO
mim][CH SO
2
[C10mim][NTf ]
4
]
n 2
Figure 2 Interfacial tension  of systems [C mim][NTf ] + hydrocarbon,
C
2
3
3
at 25 °C, as a function of the number of carbon atoms in the alkyl substitu-
ent chain of the ionic liquid (n). Hydrocarbon: ●, benzene; ♦, ethylbenzene;
■, hexane; ▲, octane. Lines (solid for C6 hydrocarbons; dashed for C8
hydrocarbons) are mere guides to the eye.
Ethylbenzene
Octane
[
C
C
6
2
mim][NTf
2
]
[
mim][NTf
2
]
2.8
[
C
2
mim][EtSO
mim][CH SO
[C10mim][NTf
4
]
6.7
C
2
3
3
9.2
aromatic hydrocarbon, the minimum interfacial tension is
practically reached, as a result of the similarity in composi-
tion of the phases in equilibrium. In fact, the miscibility gap
entirely disappears with a slight increase in the length of the
alkyl substituent chain, as it has been previously reported
2
]
0.7
[
C
C
6
mim][NTf
mim][NTf
2
]
5.5
[
2
2
]
11.7
16.0
19.5
[C
2
mim][EtSO
4
]
C
2
mim][CH SO
3
3
2
for the system [C12mim][NTf ] + benzene [19].
The effect of variation of the anion can be observed in
ture [19, 24]. All values agree within the estimated experi-
mental uncertainty, with the only exception of the composi-
tion of the lower phase for the system hexane +
Figure 3 for the systems involving ionic liquids with the
+
common [C
2
mim] cation. For each group of these systems
with a common hydrocarbon, the interfacial tension de-
creases according to the following ordered series of anions:
2
[C10mim][NTf ], whose deviation is slightly larger, and
probably related to the effect of different impurities (associ-
ated with different sources of the components) that may
influence the liquid-liquid equilibrium. Comparison with
literature data is also possible for some interfacial tension
values. In particular, Gardas et al. [12] have reported inter-



[
CH
3
SO
3
] > [EtSO
4
] > [NTf
2
] . No interfacial tensions of
2
other hydrocarbon + [C mim]-based ionic liquid could be
found in the literature for expansion of this series. In any
case, the series follows the same trend than the degree of
charge delocalisation of the anions. Thus, the most polar of
facial tensions at 298 K for the systems [C
hexane, [C mim][NTf ] + octane, [C mim][NTf
and [C mim][NTf ] + octane. All their values agree with
2
mim][NTf
2
] +

the anions ([CH
3
SO
3
] ) is the one leading to the highest
2
2
6
2
] + hexane,

interfacial tensions; and conversely, the [NTf ] anion, with
2
6
2
a strong delocalisation of its charge, leads to the lowest in-
terfacial tensions.
ours within experimental uncertainty.
n 2
By focusing on the systems involving the [C mim][NTf ]
From Figures 2 and 3 it is also clear that the aromatic
hydrocarbons lead to lower interfacial tensions than their
aliphatic counterparts. This can be related to the greater
solubility of the aromatic hydrocarbon in the ionic liquid
phase (see the lower values of xIL in the lower phase in Ta-
ble 1). Thus, a substantial presence of the hydrocarbon in
both phases may cause a reduction of the tension at their
interface. One further effect that can be inferred from Fig-
ures 2 and 3 is the increase in the interfacial tension with an
increase of the molecular weight of the hydrocarbon within
the aromatic or the aliphatic category, without exception.
That is, the interfacial tension of any of the systems
ethylbenzene + ionic liquid is greater than the one for the
corresponding systems benzene + ionic liquid; and, analo-
gously, for the systems octane + ionic liquid the interfacial
tension is greater than for the corresponding systems hexane
+ ionic liquid. This effect is less pronounced than that of the
ionic liquids, the influence of the length of the alkyl sub-
stituent chain of the ionic liquid cation on the interfacial
tension can be analysed. Figure 2 shows the evolution as a
function of the number of carbons in the alkyl substituent
chain. A clear decrease is observed in the interfacial tension
with an increase in the length of the alkyl substituent, re-
gardless of the type of hydrocarbon present in the system.
This is in agreement with the results by Gardas et al. for
systems of aliphatic hydrocarbon + [C mim][NTf ] systems
n
2
[12], as well as with those by Hu and co-workers for sys-
tems of aliphatic hydrocarbon + [C mim][PF ] or
n
6
[
C mim][BF ] ionic liquid [15, 16]. This decrease of inter-
n 4
facial tension with increase of the alkyl substituent length is
interpreted as a result of the increased van der Waals inter-
action existing between the alkyl chain of the ionic liquid
and the hydrocarbon. For the systems [C mim][NTf ] +
1
0
2

Rodríguez H, et al. Sci China Chem August (2012) Vol.55 No.8
1523
ionic liquid is slightly over ambient temperature, although it
tends to form a subcooled liquid [30]; thus, the indicated
surface tension at 25 °C would be the one corresponding to
subcooled [C
2 3 3
mim][CH SO ]. For the hydrocarbons, the
following values were used [31]: 28.20 mN/m for benzene,
2
2
8.48 mN/m for ethylbenzene, 17.94 mN/m for hexane, and
1.18 mN/m for octane.
The values of  calculated by means of eq. (1) for the
systems studied in this work are reported in Table 2. These
values are in the range of 0.80–0.90 for the mixtures of the
2
[C mim]-based ionic liquids and the aliphatic hydrocarbons,
whereas the remaining values are greater than 0.90 and
reach or slightly surpass the unity for the systems compris-
2
Figure 3 Interfacial tensions of systems [C mim][A] + hydrocarbon, at


2
5 °C, where [A] is the anion of the ionic liquid: [NTf
2
] (black, left bars


of each bar group), [EtSO
4
] (light gray, central bars), or [CH
3
SO
3
] (dark
gray, right bars). The specific hydrocarbon is indicated over each group of
bars.
2
ing the ionic liquid [C10mim][NTf ]. As indicated before,
values lower than unity would correspond to apolar/polar
interfaces, while unity values would correspond to apo-
lar/apolar interfaces. Thus, the indicated evolution of the
parameter  can be interpreted as a transformation from an
apolar/polar interface to an interface with a greater apo-
lar/apolar character. A first effect responsible for this evolu-
tion is the increase of the length of the alkyl side chain in
aromatic versus aliphatic nature of the hydrocarbon, since,
for example, for a given ionic liquid the interfacial tension
in the system with ethylbenzene is still notably lower than
that for the equivalent system with hexane (as it can be di-
rectly observed in Figure 2 for the case of the
n 2
the [C mim][NTf ] ionic liquids, which increases the apolar
n 2
[C mim][NTf ] ionic liquids). A similar evolution with the
contribution of the ionic liquid, thus generating a more apo-
lar/apolar interface with the hydrocarbons. A second effect
to consider is the high solubility of the aromatic hydrocar-
bons in any of the ionic liquids tested, which causes the
ionic liquid phase to have an important apolar contribution
at the interface, even in the case of ionic liquids with short
alkyl substituents. Since values very close to unity have
been obtained for  in the systems involving the aromatic
hydrocarbons, in particular in those with benzene, and in the
systems involving the ionic liquids with a long alkyl sub-
degree of unsaturation was previously observed by Zhu et al.
for mixtures of hydrocarbons with the ionic liquid
1
-butyl-3-methylimidazolium hexafluorophosphate [9].
In trying to relate the interfacial tension of the systems
hydrocarbon + ionic liquid with the surface tensions (i.e.,
interfacial tension with air) of the pure compounds, a simple
expression originally proposed by Girifalco and Good [25]
has been recently applied to binary mixtures of an alkane or
an alkene with an ionic liquid [13–16]. Generalising for the
combination of an ionic liquid with any type of hydrocarbon,
the expression is:
2
stituent (in particular in those with [C10mim][NTf ]), it is
postulated that reasonably good estimations can be made for
the interfacial tension of systems of this kind from
knowledge of the surface tension of the two pure com-
pounds involved, without any extra information.
ILHC  IL HC  2  IL HC
where IL-HC is the interfacial tension of the binary mixture;
IL and HC are the surface tensions (or, in other words, the
interfacial tensions with air) of the ionic liquid and the hy-
drocarbon, respectively; and  is the interaction parameter,
or proportionality factor. This parameter is closely related to
the contribution from polar interaction to the interfacial ten-
sion, and its value is lower than, equal to, or greater than
unity for apolar/polar, apolar/apolar, and polar/polar inter-
faces, respectively [13, 25, 26]. For application of eq. (1),
values of the surface tensions of ionic liquids and of hydro-
(1)

4
Conclusions
Interfacial tensions for a series of binary systems formed by
an ionic liquid and a hydrocarbon with mutual immiscibility
carbons were taken from the literature. For [C
mim][NTf ] and [C mim][NTf ], respective values of
6.68, 32.04 and 31.73 mN/m were obtained by interpola-
tion for 25 °C from the work by Carvalho et al. [27]. For
mim][EtSO ], the value of 46.96 mN/m reported by
Gómez et al. was used [28]. Regarding [C mim][CH SO ],
2 2
mim][NTf ],
Table 2 Interaction parameter  for binary systems hydrocarbon + ionic
liquid
[
C
2
2
2
2
3
Hydrocarbon
Ionic liquid
benzene
ethylbenzene
hexane
octane
[
C
2
4
2
3
3
[C10mim][NTf
2
]
1.00
0.99
0.99
0.98
0.97
1.00
0.99
0.96
0.94
0.92
1.03
0.94
0.85
0.87
0.83
1.01
0.92
0.83
0.83
0.80
a value of 50.16 mN/m was estimated at 25 °C from a
method using the value at 20 °C and density measurements
at different temperatures, reported by Hasse et al. [29]. It
must be noted that the normal melting temperature of this
[C
6
mim][NTf
2
mim][NTf ]
2
]
[C
2
[C
2
mim][EtSO
4
]
[
C
2
mim][CH
3
3
SO ]

1524
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