A comparative study on the interface behavior of different counter anion long chain imidazolium ionic liquids

Article abstract of DOI:10.1016/j.molliq.2016.04.028

The adsorption behavior of amphiphilic long-chain imidazolium ionic liquids (ILs), n-alkyl-3-methylimidazolium halides, [C_nmim][X], n = 12, 14, 16 and X = Cl^-, Br^-, I^-, at the interface of n-butyl acetate + water system was studied. The ILs bulk concentration range of 1.00·10^{- 5}-1.00·10^{- 2} mol·dm^{- 3} and temperature of 293.2-318.2 K were applied. The interfacial tension of the system significantly decline due to the excellent action of ILs like surfactants. Under the same conditions, the interfacial activity appears reasonably in the order of [C₁₆mim][I] > [C₁₆mim][Br] > [C₁₆mim][Cl] ? [C₁₄mim][Br] > [C₁₄mim][Cl] ? [C₁₂mim][Cl], consistent with alkyl chain length and the kind of halide anions. Molecules of the used ILs exhibit ideal adsorption within the used concentrations and the data were satisfactorily reproduced by the Szyszkowski adsorption model. Consistently, effectiveness of adsorption increases with the anion polarization and the number of carbon atoms in alkyl chain; however, temperature leads this parameter to decrease. On the other hand, adsorption tendency increases with either of anion polarization, alkyl chain length and temperature. Meanwhile, negative standard free energies were consistent and showed spontaneous adsorptions.

Full text of DOI:10.1016/j.molliq.2016.04.028

Journal of Molecular Liquids 220 (2016) 136–141
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
A comparative study on the interface behavior of different counter anion
long chain imidazolium ionic liquids
⁎
Javad Saien , Mona Kharazi
Department of Applied Chemistry, Bu–Ali Sina University, Hamedan 65174, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The adsorption behavior of amphiphilic long-chain imidazolium ionic liquids (ILs), n-alkyl-3-methylimidazolium
halides, [Cnmim][X], n = 12, 14, 16 and X = Cl−, Br−, I−, at the interface of n-butyl acetate + water system was
studied. The ILs bulk concentration range of 1.00·10−5–1.00·10−2 mol·dm−3 and temperature of 293.2–318.2 K
were applied. The interfacial tension of the system significantly decline due to the excellent action of ILs
like surfactants. Under the same conditions, the interfacial activity appears reasonably in the order of
[C16mim][I] N [C16mim][Br] N [C16mim][Cl] ≫ [C14mim][Br] N [C14mim][Cl] ≫ [C12mim][Cl], consistent with
alkyl chain length and the kind of halide anions. Molecules of the used ILs exhibit ideal adsorption within the
used concentrations and the data were satisfactorily reproduced by the Szyszkowski adsorption model. Consis-
tently, effectiveness of adsorption increases with the anion polarization and the number of carbon atoms in
alkyl chain; however, temperature leads this parameter to decrease. On the other hand, adsorption tendency
increases with either of anion polarization, alkyl chain length and temperature. Meanwhile, negative standard
free energies were consistent and showed spontaneous adsorptions.
Received 14 December 2015
Received in revised form 18 February 2016
Accepted 10 April 2016
Available online xxxx
Keywords:
Ionic liquids
Adsorption
Counter anion
Interfacial tension
Szyszkowski
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Considering these cases, many investigations have been concerned
on utilizing different structure ILs. The most widely used ILs contain in
Ionic liquids (ILs) are a class of materials that have gained much
attention over recent decades due to their amazing applications and
properties such as high ion conductivity, wide liquidus range, excellent
solvation, high thermal stability, non-flammability and negligible vola-
tility [1]. Accordingly, emphasis has been placed on two major relevant
parts: application of ILs in the fields such as reactions catalyzing [2], cor-
rosion inhibition [3], electrochemistry [4] and liquid–liquid extraction
[5]; and characterizing ILs properties for development of structure-
property relationships that can be considered for ILs molecular design.
It is since the properties of ILs such as hydrophobicity, viscosity, density,
ion selectivity, chemical and electrochemical stability can be changed
by altering/modifying cations and/or anions to be tailored for specific
applications [1].
Due to the amphiphilic nature, ILs have shown excellent properties
in the colloids and interface of phases, and been introduced as new
class of cationic surfactants [6–8], especially as dispersants' emulsifiers
and foaming agents [9–10].
ILs can also adsorb at the interface of crude oil and harsh saline water
leading to significant decrease in interfacial tension (IFT), a task which
is not observed from conventional surfactants [11]. In this regard, one
major ILs application has been introduced in upstream petroleum
industries for the enhanced oil recovery (EOR) [12,13].
their structure, asymmetric N-containing organic cations of imidazole,
pyridine and pyrrole. Sastry et al. [14], for instance, used ILs with
imidazolium, pyrrolidinium and piperidinium ionic head groups
consisting of long hydrocarbon tails and studied their influence on the
surface activity of water. Among them, imidazolium-based ILs have
been introduced as extraordinary surface active agents. Fig. 1 shows
the chemical structure of imidazolium-based ILs which consists of a
non-polar hydrophobic tail and a polar hydrophilic cationic head, briefly
introduced as [Cnmim]. The heteroatom ring in the IL head group consists
of two nitrogen and three carbon atoms. This bulky structure gives the ILs
more hydrophobic nature and consequently more interfacial activity
than surfactants [15].
In addition to alkyl chain, the counter anion can alter the adsorption
of ILs [8,16]. The anion is considered to be primarily responsible for
many of the physical properties of ILs such as hydrophobicity and coor-
dinating ability. Li et al. [16] demonstrated that counter anion of an IL
with weaker hydration has more potential to cause higher surface activ-
ity (of water solutions) and ILs with chloride counter anion were found
to exhibit lower activity than that with bromide. There are scarce litera-
tures focusing on the adsorption behavior of ILs with different counter
anions at the interface of oil-water systems.
Recently we investigated the IFT variations and adsorption behavior
with alkyl length of three long-chain and three short-chain imidazolium
ILs at different temperatures [17,18]. The IFT of n-butyl acetate–water
system was changed due to the adsorption of short and long alkyl
⁎
Corresponding author.
E-mail address: saien@basu.ac.ir (J. Saien).
http://dx.doi.org/10.1016/j.molliq.2016.04.028
0167-7322/© 2016 Elsevier B.V. All rights reserved.

J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141
137
343.2 K. Reactions were carried out in solvent-free condition, nitrogen
atmosphere and also protected from light. The produced ILs were au-
thorized to cool to room temperature. The waxy solid products were
washed with ethyl acetate, at least ten times, to remove any unreacted
reagent. After that, the remaining ethyl acetate was removed by heating
to 350.2 K. Therefore, purity of the ILs was tested by halide titration, as a
preliminary estimation and purity N99% was found for each of them. So,
the synthesized ILs were characterized by means of mass spectroscopy,
FTIR, 13C NMR and 1H NMR and the results showed the expected struc-
ture. In Table 2, the abbreviation, chemical formula, color and state of
the used ILs are presented.
Fig. 1. Chemical structure of 1-alkyl-3-methylimidazolium halide (X denotes a halide atom)
ionic liquids.
chain imidazolium-based ILs at different temperatures. It was demon-
strated that the surface activity of the imidazolium ILs with longer
chain is superior. To extent this investigation regarding counter
anion influence, we utilized in this work six long alkyl chain
imidazolium ILs of 1-hexadecyl-3-methylimidazolium chloride,
briefly [C16mim][Cl], 1-hexadecyl-3-methylimidazolium bromide,
[C16mim][Br], 1-hexadecyl-3-methylimidazolium iodide, [C16mim][I], 1-
tetradecyl-3-methylimidazolium chloride, [C14mim][Cl], 1-tetradecyl-3-
methylimidazolium bromide, [C14mim][Br], and 1-dodecyl-3-
methylimidazolium chloride, [C12mim][Cl]. A comprehensive comparison
is made among these ILs to study how the counter anions in conjunction
with different alkyl chain lengths behave in the adsorption of ILs and is
presented by monitoring the IFT variations. For this aim, the chemical sys-
tem of n-butyl acetate + water was chosen since it is a recommended sys-
tem by the European Federation of Chemical Engineers [19] as an
intermediate IFT system for liquid-liquid extraction studies [20,21]. The
variations were followed under different temperatures for each of the
used ILs. The modeling of experimental data was performed and the
relevant adsorption parameters were accordingly obtained and discussed.
2.3. IFT measurements
The drop volume method was used for determining the IFTs. Details
of drop-forming device and the procedure were similar to the those
explained in our previous works [23,24].
Extensive aqueous concentrations of synthesized ILs, ranging from
very low to near critical micelle concentration (CMC) were utilized for
each IL. Prior to the IFT measurements, aqueous phase ILs solutions
within concentration range of (1.00·10−5–1.00·10−2) mol·dm−3 of
individual ILs were prepared for contacting with organic phase. Solu-
tions were prepared in mass by means of an Ohaus (Adventurer Pro,
AV 264) balance, having an uncertainty of 0.1 mg (0.95 level of confi-
dence). The absolute standard deviation of concentrations did not
exceed 0.01 · 10−3 mol·dm−3 for all cases. It is notable that the mutual
solubility of both the organic and aqueous phases was very low and that
emulsion formation was not observed. In order to find the temperature
dependency, each sample solution was examined at six temperatures
within (293.2–318.2) K. The contacting media and conducting tube to
the capillary were thermostated by using a calibrated thermostat (OPTI-
MA 740, Japan) with an uncertainty of 0.1 K (0.95 level of confidence).
Aqueous and organic phase densities were changed with increasing
both temperature and concentration; so, the density of each phase
was measured by means of an oscillating U-tube densimeter (Anton
Paar DMA 4500, Austria), provided with automatic viscosity correction
with uncertainty of 0.01 kg·m−3 (0.95 level of confidence). The density
of the aqueous and organic phases vary within 991.04–998.80 kg·m−3
and 856.44–882.53 kg·m−3, respectively.
2. Experimental
2.1. Materials
N-butyl acetate and the raw materials for synthesizing and
purifying ILs, including 1-methylimidazole, 1-dodecylchloride, 1-
tetradecylchloride, 1-tetradecylbromide 1-hexadecylchloride, 1-
hexasdecylbromide, 1-hexadecyliodide and ethyl acetate, were all
purchased from Merck Company and used without further purification.
The list of chemicals and mass fraction purities are given in Table 1.
Fresh deionized water with electrical conductivity of 0.07 μS·cm−1
was used for the preparation of solutions throughout the experiments.
In the drop volume method, IFT, γ, is calculated from Harkins and
Brown equation [25]:
ꢀ
ꢁ
vΔρg
r
r
pffiffiffi
3 v
γ ¼
ϕ
ð1Þ
where v is drop volume falling off a capillary (with r radius) into the
organic phase; Δρ and g are the density difference between the aqueous
2.2. Synthesis and characterization of the ILs
and organic liquids phases, ρw and ρo, and gravitational acceleration,
Six long chain imidazolium-based ILs, [C16mim][I], [C16mim][Br],
[C16mim][Cl] and [C14mim][Br], [C14mim][Cl], and [C12mim][Cl] were
synthetized according to the previously reported procedure [22].
In an abbreviated manner, equal molar amounts of 1-methylimidazole
and appropriate 1-alkylhalide were mixed and stirred rigorously in a
round-bottomed flask equipped with a reflux condenser for 48 h at
pffiffiffi
respectively. The dimensionless constant ϕðr= 3 vÞ can be extracted
from empirical relation [26]. The maximum uncertainties for drop
formation time, drop volumes were respectively obtained to 1 s and
0.001 cm3. The uncertainty for γ was estimated to be 0.1 mN·m−1
(0.95 level of confidence).
The reliability of the method was examined by measuring the IFT of
pure n-butyl acetate + water system (binary saturated, without IL) at
298.2 K. The obtained value of 14.0 mN·m−1 was close to the reported
values of (14.1 and 14.4) mN·m−1 in the literature [27,28] with 0.7%
and 2.9% relative differences, respectively. In a previous work [20],
using the same set-up and the procedure as used in this work, we mea-
sured the IFT of six pure chemicals in contact with water at 293.2 K, and
compared with those from the literature. The average deviation be-
tween measured values and the literature reported values was 3.3%.
In order to choose a suitable flow rate certifying the achievement
of drops equilibrium conditions which guarantees the equilibrium
adsorption, the IFT variation with drop formation time was followed.
Among the used concentrations, a typical mean concentration of
Table 1
Mass fraction purity of the used materials (All Merck products).
Chemicals
Purity%
n-Butyl acetate
N99.5
N99.9
N95.0
N96.0
N96.0
N96.0
N96.0
N96.0
N99.5
1-Methylimidazole
1-Dodecylchloride
1-Tetradecylchloride
1-Tetradecylbromide
1-Hexadecylchloride
1-Hexadecylbromide
1-Hexadecyliodide
Ethyl acetate

138
J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141
Table 2
Abbreviated and chemical formula, color and state of used ILs.
IL
Abbreviation formula
Chemical formula
Color and state
1-Dodecyl-3-methylimidazolium chloride
1-Tetradecyl-3-methylimidazolium chloride
1-Tetradecyl-3-methylimidazolium bromide
1-Hexadecyl-3-methylimidazolium chloride
1-Hexadecyl-3-methylimidazolium bromide
1-Hexadecyl-3-methylimidazolium iodide
[C12mim][Cl]
[C14mim][Cl]
[C14mim][Br]
[C16mim][Cl]
[C16mim][Br]
[C16mim][I]
C16H31ClN2
C18H35ClN2
C18H35BrN2
C20H39ClN2
C20H39BrN2
C20H39IN2
White waxy solid
White waxy solid
White waxy solid
White waxy solid
White waxy solid
Yellow waxy solid
2.50 · 10−3 mol·dm−3 was chosen for each of the ILs and different
aqueous phase flow rates ranging from (0.00005 to 0.0025) cm3·s−1
(uncertainty of 0.000005 cm3·s−1 with 0.95 level of confidence) were
applied at 298.2 K. As demonstrated in Fig. 2, by applying high flow
rates (low drop formation times), dynamic IFTs were appeared because
the ILs did not find sufficient time to complete adsorption at the inter-
face and drops were formed fast. However, by reducing flow rate,
drop formation time increases and each IL has further chance to contin-
ue adsorption at the interface. Eventually, when the time of drop forma-
tion becomes adequate (N25 s in this work), the interface expansion
occurs sufficiently slow to find equilibrium conditions and no change
is relevant with higher drop formation times, indicating the equilibrium
IFT [29–31]. The slow enough flow rate of 0.0001 cm3·s−1 was accord-
ingly chosen for all experiments. The minimum drop formation time
(57 s) was long enough to reach the equilibrium condition with no IFT
variation. Each of the measurements was repeated at least 15 times at
a specified temperature and ILs concentration. The repeatability was
good (less than of 0.05 s for drop formation times).
Experimental data including phase densities, drop formation times
and IFTs for each IL concentration under different temperatures are
given in Supplementary information. The measured IFT values were with-
in the range of 13.4–14.0 mN·m−1 for the pure system (absence IL),
within 6.2–7.4 mN·m−1 with [C12mim][Cl], 4.5–5.7 mN·m−1 with
[C14mim][Cl], 4.4–5.6 mN·m−1 with [C14mim][Br], 3.4–4.6 mN·m−1
with [C16mim][Cl], 3.3–4.5 mN·m−1 with [C16mim][Br] and 3.1–
4.3 mN·m−1 with [C16mim][I]. The maximum IFT reductions were 55.4,
67.6, 68.4, 74.4, 76.3 and 78.7%, respectively.
increasing concentration, more adsorption occurs which causes the
IFT to decline more [32]. As can be seen in Fig. 3, IFT decreases sharply
with low dosages of ILs to about 1.00 · 10−3 mol·dm−3 and then a
mild variation is evident by approaching to CMC. A comparison be-
tween the provided data here with those obtained with sodium dode-
cyl sulfate (SDS) surfactant and the same chemical system [20],
indicate that for a typical concentration of 2.5 · 10−4 mol·dm−1
and temperature of 293.2 K, except [C12mim]Cl with the approximate
same effect, other ILs act stronger than SDS (9.1 to 19.2% more).
• Second; the IFT reduction depends on the kind of counter anion. With
typical concentration of 2.50 · 10−3 mol·dm−3 and temperature of
298.2 K, the IFT values with [C16mim][I], [C16mim][Br] and
[C16mim][Cl] are 3.7, 3.9 and 4.0 mN·m−1 and with [C14mim][Br],
[C14mim][Cl] are 4.9 and 5.1 mN·m−1 and with [C12mim][Cl] is
6.8 mN·m−1 (Fig. 3). The molecular dynamic simulation study ap-
proves that bigger counter anion sodium halide salts are weaker hy-
drated and have more tendency to adsorb at the surface [33]. In
view of this, Dong et al. [15] measured the surface tension of aqueous
solutions containing individual [C12mim][BF4] and [C12mim][Br] and
reported that more reduction in the surface tension is relevant to
[C12mim][BF4] because of the bigger counter anion. In agreement
with these results, the difference between IFTs, here, can be attributed
to the weaker bulk hydration of the larger I− then Br− ions, compared
with Cl−, which causes their ease of adsorption, respectively.
• Third; it is obvious that more methylene bridge (–CH2–) in cationic
structure of ILs of [C16mim]+ in comparison with [C14mim]+ and
[C12mim]+, causes impressive decline in the IFT values because of
more hydrophobicity character of longer chain ILs [14]. In our previ-
ous work, the effect of short-alkyl chain ILs ([Cnmim][Cl], n = 6, 7
and 8) on the IFT of n-butyl acetate + water system was studied
[17]. To compare the interfacial activity of short and long alkyl chain
ILs, the IFT of the system with [C6 to C16mim][Cl] ILs at temperature
of 298.2 K are presented in Fig. 4. The obtained results obviously indi-
cate that long alkyl chain ILs decrease the IFT higher than short alkyl
chains due to more hydrophobicity nature and therefore more interfa-
cial activity [15]. For instance, the maximum IFT reduction in the
3. Results and discussion
3.1. Evaluation of experimental results
The important revealed results from obtained IFT data are classified
in the following items:
• First; the presence of ILs leads to significant reductions in the IFT. Due
to amphiphilic nature of ILs, they migrate into the interface and by
Fig. 2. IFT variation as a function of drop formation time for different ILs at concentration of
Fig. 3. IFT variation as a function of ILs concentration at 298.2 K. Solid lines correspond to
theoretical curves obtained by the Szyszkowski model.
2.50 · 10−3 mol.dm−3 and temperature of 298.2 K.

J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141
139
Fig. 4. IFT variation as a function carbon number in alkyl chain of IL for different
concentration at 298.2 K.
Fig. 6. IFT variation as a function of temperature for different ILs at typical concentration of
2.50 · 10−3 mol·dm−3
.
presence of ILs with n = 6, 7 and 8 were achieved 18.7, 25.2 and 32.4%
at 298.2 K, respectively. However, in the present study, these percent-
ages increased to 55.4, 67.6 and 74.4% for C12, C14 and C16 ILs, all with
Cl− anion, also 68.4 and 76.3% for C14 and C16 ILs with Br− and exceeds
to 78.7% for C16 IL with I−. It has to mention that the bulk concentration
used for long alkyl chain ILs (n = 12, 14 and 16) were within
(1.00 · 10−5–1.00 · 10−2) mol·dm−3 whereas those for short alkyl
used to fit the experimental data. It is based on the Gibbs and Langmuir
adsorption equations [36]:
γ ¼ γ0−2RTΓm lnð1 þ KLCÞ
ð2Þ
where γ0 is the IFT for pure chemical system (C = 0), R and T are gas law
constant and absolute temperature. Also, Γm, KL and C are the maximum
interfacial concentration at saturated interface, the Langmuir equilibri-
um adsorption constant and the IL bulk concentration, respectively.
The factor 2, in this equation, stands for the dissociation of each ionic
surface active substance into its cation and anion. This equation was
employed to fit the experimental data at constant temperatures. The co-
efficient of determination, R2, which is a criterion of agreement between
calculated and experimental IFT was calculated from:
chain ILs (n = 6, 7 and 8) were within (1.00 · 10−4–1.00 · 10−1
)
mol·dm−3 [17,18]; i.e. the corresponding concentrations for long
chain ILs were about 10% of short chain ILs. As Fig. 5 shows, the required
IL concentration for declining IFT to a certain value is strongly decreased
with the alkyl chain length. Based on these, the adsorption and the in-
terfacial activity of the used ILs are reasonably appeared in the order
of [C16mim][I] N [C16mim][Br] N [C16mim][Cl] ≫ [C14mim][Br] N
[C14mim][Cl] ≫ [C12mim][Cl].
N
ꢂ
ꢃ
X
2
• Finally, by increasing temperature, the kinetic of molecules and number
of collisions increase, causing attenuation of intermolecular forces at the
interface and IFT diminishes gradually, similar to surface tension [34].
This reduction is about 15% for each concentration within the used
temperature range and the variations are almost linear in all cases as
presented by Fig. 6.
γcal;i−γexp;i
R2 ¼ 1− i¼1
ð3Þ
N
ꢂ
ꢃ
X
2
γ−γexp;i
i¼1
where N, γcal, γexp and γ are respectively the number of data used in the
fitting, the calculated IFT by the model, the experimental IFT and the
average of the appropriate experimental values. The obtained parame-
ters for equilibrium IFT values are listed in Table 3. In general, the R2
values vary within the range of (0.9871 to 0.9999) showing excellent
consistent fittings with the model. The solid lines in Fig. 3, which are
based on Szyszkowski equation, demonstrate the goodness of fitting
with experimental data.
3.2. Theoretical model
In order to relate the equilibrium IFT values to the bulk concentration
of a surface active agent, numerous adsorption isotherms have been pro-
posed [35]. Among them, Szyszkowski equation has been conventionally
One of the consequential results, revealed from Szyszkowski equa-
tion, is the ideal behavior of ILs at the n-butyl acetate + water interface.
However, in our previous study [31] where toluene + water system was
used, non-ideal interaction, mainly electrostatic repulsion, between
adsorbed ILs was appeared. The difference can be attributed to the inter-
action between the IL molecules with both organic and aqueous phases.
By considering the used ILs structure, it is clear that there is acidic hy-
drogen on carbon number 2, between two electronegative nitrogen
atoms in ILs head group ring (Fig. 1). There is therefore a high chance
to form hydrogen bonds between this hydrogen and electronegative
atoms in both water and n-butyl acetate molecules. This interaction de-
creases or even disappears the repulsion between adsorbed ILs at the in-
terface leading to ideal behavior [37,38]. Therefore, the Szyszkowski
equation proficiently fits the data. In this equation, interfacial activity
of adsorbed species are characterized by two important parameters;
the effectiveness of adsorption reflected by interface excess, Γm, and ad-
sorption tendency by the Langmuir equilibrium adsorption constant, KL.
These parameters are listed in Table 3.
Fig. 5. IFT variation as a function of ILs concentration at 298.2 K.

140
J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141
Table 3
IFT parameters, maximum interface excess, Γm, Langmuir adsorption equilibrium constant,
KL, minimum area occupied by a molecule, Am, standard free energy of adsorption, ΔGa∘ds
and coefficient of determination, R2, for adsorption of ILs at different temperatures.
,
T (K)
106×Γm,L
KL
1018×Am
‐ΔG∘ads
R2
(mol·m−2
)
(dm3·mol−1
)
(m2)
(kJ·mol−1
)
[C12mim][Cl]
293.2
298.2
303.2
308.2
313.2
318.2
88.31
2.50
2.58
2.64
2.69
2.72
2.72
1.06
1.10
1.16
1.21
1.27
1.31
264.45
267.64
271.12
276.91
282.24
287.66
0.9796
0.9791
0.9784
0.9789
0.9777
0.9781
86.72
84.20
81.16
76.91
70.09
[C14mim][Cl]
293.2
298.2
303.2
308.2
313.2
318.2
96.37
3.03
3.13
3.19
3.24
3.27
3.27
0.91
0.98
1.04
1.09
1.13
1.17
306.25
311.82
317.86
324.19
336.44
343.61
0.9888
0.9885
0.9889
0.9884
0.9886
0.9878
94.12
90.90
87.36
83.01
78.42
Fig. 7. Maximum interface excess as a function of temperature for different ILs.
The variation of adsorption tendency criterion, KL, with temperature
is illustrated in Fig. 8. The adsorption tendency increases by both the
anion polarizability and alkyl chain length. Besides, it is evident that
[C16mim][I] has the highest adsorption tendency than others because
of more tendency of I− to adsorb more than Br− and Cl−. Also this figure
shows that at higher temperatures, intensifying the motion, more ten-
dencies of the IL molecules toward the interface is provided. Therefore,
anion polarizability, alkyl chain length as well as temperature increase
the adsorption tendency in all cases.
[C14mim][Br]
293.2
298.2
303.2
308.2
313.2
318.2
98.25
3.16
3.25
3.32
3.36
3.38
3.38
0.85
0.90
0.96
1.02
1.07
1.11
312.76
318.31
326.66
335.16
344.67
352.91
0.9999
0.9998
0.9997
0.9995
0.9997
0.9998
95.44
92.24
88.35
84.06
79.49
[C16mim][Cl]
293.2
298.2
303.2
308.2
313.2
318.2
103.61
3.53
3.63
3.70
3.76
3.79
3.80
0.81
0.86
0.90
0.98
1.06
1.13
355.14
361.86
369.75
377.48
385.19
396.73
0.9876
0.9871
0.9883
0.9894
0.9880
0.9888
101.29
98.25
94.31
90.06
84.61
Finally, another important parameter, relevant to equilibrium
constant, is standard free energy of adsorption, ΔG∘ads, which can be
obtained from [39]:
ꢀ
ꢁ
KLρ0
2
[C16mim][Brl]
ΔG∘ads ¼ −2RT ln
ð5Þ
293.2
298.2
303.2
308.2
313.2
318.2
104.91
3.69
3.79
3.87
3.92
3.95
3.96
0.74
0.79
0.85
0.90
0.98
1.06
364.15
373.81
386.57
391.26
398.84
404.38
0.9999
0.9997
0.9993
0.9996
0.9993
0.9997
102.04
98.85
95.07
90.68
85.79
where ρ′ (ρw/18) is molar concentration of water at a given tempera-
ture. The standard free energies of ILs adsorption at different tempera-
tures are presented in Table 3. The results show that all values for
ΔG∘ads are negative, which means that adsorption is spontaneous in all
cases. The free energy variations are reasonably like the adsorption
constant.
[C16mim][I]
293.2
298.2
303.2
308.2
313.2
318.2
105.97
3.90
3.99
4.07
4.13
4.15
4.17
0.67
0.72
0.77
0.83
0.89
0.97
379.28
387.49
396.20
403.55
410.73
418.62
0.9999
0.9998
0.9996
0.9997
0.9998
0.9997
103.21
100.16
96.49
92.19
87.49
4. Conclusions
By means of the IFT measurements, the adsorption of amphiphilic
long chain imidazolium ILs with different halide anions at the n-butyl
acetate + water interface was investigated under different temperatures.
The variation of Γm with temperature for each of the ILs is depicted in
Fig. 7. More Polarizable anion (weaker hydration) as well as longer alkyl
chain leads to higher Γm values. Obviously, the presence of more carbon
atoms in the cation of [C16mim]+ gives higher hydrophobicity which
leads to more molecules adsorption than [C14mim]+ and [C12mim]+
at constant temperatures. Further, it is clear that with increasing tem-
perature, decreasing in Γm is observed as a result of disrupting surround-
ing water molecules around hydrophobic portion which causes less
staying of ILs at the interface [39].
Corresponding to Γm, the minimum area occupied by an adsorbed
molecule at the interface, Am, is obtained from [40]:
1
mNAv
Am
¼
ð4Þ
Γ
where NAv is the Avogadro's number. The calculated Am at different
temperatures are listed in Table 3. The results show that compact
mono layers with lesser occupied area are formed at the n-butyl
acetate + water interface by ILs with more polarizable anion and longer
alkyl chain. Also, when interface concentration declines at higher tem-
peratures, each molecule occupies more area.
Fig. 8. Langmuir equilibrium adsorption constant as a function of temperature for different
ILs.

J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141
141
[12] S. Lago, M. Francisco, A. Arce, A. Soto, Enhanced oil recovery with the ionic liquid
trihexyl(tetradecyl)phosphonium chloride: a phase equilibria study at 75 °C, Energy
Fuel 27 (2013) 5806–5810.
[13] A. Zeinolabedini Hezave, S. Dorostkar, S. Ayatollahi, M. Nabipour, B. Hemmateenejad,
Effect of different families (imidazolium and pyridinium) of ionic liquids-based sur-
factants on interfacial tension of water/crude oil system, Fluid Phase Equilib. 360
(2013) 139–145.
[14] N.V. Sastry, N.M. Vaghela, V.K. Aswal, Effect of alkyl chain length and head group on
surface active and aggregation behavior of ionic liquids in water, Fluid Phase Equilib.
327 (2012) 22–29.
[15] B. Dong, N. Li, L. Zheng, L. Yu, T. Inoue, Surface adsorption and micelle formation of
surface active ionic liquids in aqueous solution, Langmuir 23 (2007) 4178–4182.
[16] H. Li, Y. Imai, T. Takiue, H. Matsubara, M. Aratono, Effect and mixing of counter
anions at the surface of aqueous solution of imidazolium-based ionic liquids, Colloids
Surf. A Physicochem. Eng. Asp. 427 (2013) 26–32.
[17] J. Saien, M. Kharazi, S. Asadabadi, Adsorption behavior of short alkyl chain imidazolium
ionic liquids at n-butyl acetate + water interface: experiments and modeling, Iran. J.
Chem. Eng. 12 (2015) 59–74.
[18] J. Saien, M. Kharazi, S. Asadabadi, Adsorption behavior of long alkyl chain
imidazolium ionic liquids at the n-butyl acetate + water interface, J. Mol. Liq. 212
(2015) 58–62.
[19] T. Misek, Standard Test Systems for Liquid Extraction, Instn. Chem. Engs. For
European Federation of Chemical Engineers, Warwickshire, 1985.
[20] J. Saien, S. Akbari, Variations of interfacial tension of the n-butyl acetate + water
system with sodium dodecyl sulfate from (15 to 22) °C and pH between 6 and 9,
Chem. Eng. Data 53 (2008) 525–530.
[21] K. Bäumler, M. Wegener, A.R. Paschedag, E. Bänsch, Drop rise velocities and fluid
dynamic behavior in standard test systems for liquid/liquid extraction-experimental
and numerical investigations, Chem. Eng. Sci. 66 (2011) 426–439.
Results obviously indicate that function of ILs at the interface depends on
both the anion, and the alkyl chain length. The results show that ILs with
more polarizable anion and longer alkyl chain are adsorbed at the inter-
face more effectively. The effect of anions in ILs appeared in the order of
I− N Br− N Cl−, in contrary to the order of their hydration level. The influ-
ence of hydrocarbon chain length is, of course, more than counter anion
under a certain temperature. IFT decreases almost linearly with temper-
ature within the range of (293.2–318.2) K for all cases.
The Szyszkowski adsorption equation was adequately applied to fit
the experimental data. Accordingly, IL molecules behaved ideally at
the interface and the obtained maximum interface excess and the
adsorption tendency demonstrated dependency on the polarizability
of anions, alkyl chain length and temperature. Totally, maximum inter-
face excess decreased with temperature; however, rose when anion po-
larizability and hydrophobicity of the ILs increased. It is while, the
Langmuir adsorption constant increased with all investigated parame-
ters regularly, as a consequence of the ease of migration toward the
interface.
The supplementary information of the phase densities, drop forma-
tion times and IFTs for each IL concentration under different tempera-
tures are given in the file. Supplementary data associated with this
article can be found in the online version, at http://dx.doi.org/10.1016/
j.molliq.2016.04.028.
[22] S. Asadabadi, J. Saien, V. Khakizadeh, Interface adsorption and micelle formation of
ionic liquid 1-hexyl-3-methylimidazolium chloride in the toluene + water system,
J. Chem. Thermodyn. 62 (2013) 92–97.
[23] J. Saien, A. Rezvani Pour, S. Asadabadi, Interfacial tension of n-hexane-water system
under influence of magnetite nanoparticles and sodium dodecyl sulfate assembly at
different temperatures, J. Chem. Eng. Data 59 (2014) 1835–1842.
[24] J. Saien, S. Asadabadi, Alkyl chain length, counter anion and temperature effects on
the interfacial activity of imidazolium ionic liquids: comparison with structurally
related surfactants, Fluid Phase Equilib. 386 (2015) 134–139.
Acknowledgments
The authors wish to thank the university authorities for providing
the financial support to carry out this work.
[25] W.D. Harkins, F.E. Brown, The determination of surface tension (free surface energy),
and the weight of falling drops: the surface tension of water and benzene by the capil-
lary height method, J. Am. Chem. Soc. 41 (1919) 499–524.
[26] B.B. Lee, P. Ravindra, E.S. Chan, A critical review: surface and interfacial tension mea-
surement by the drop weight method, Chem. Eng. Commun. 195 (2008) 889–924.
[27] A. Bahramian, A. Danesh, Prediction of liquid-liquid interfacial tension in multi-
component systems, Fluid Phase Equilib. 221 (2004) 197–205.
[28] W. Apostoluk, J. Drzymala, An improved estimation of water–organic liquid interfa-
cial tensions based on linear solvation energy relationship approach, Colloid Inter-
face Sci. 262 (2003) 438–488.
[29] C. Mollet, Y. Touhami, V. Hornof, A comparative study of the effect of ready-made
and in situ-formed surfactants on interfacial tension measured by drop volume
tensiometry, J. Colloid Interface Sci. 178 (1996) 523–530.
[30] U. Teipel, N. Aksel, Adsorption behavior of nonionic surfactants studied by drop
volume technique, Chem. Eng. Technol. 24 (2001) 393–400.
[31] J. Saien, S. Asadabadi, Temperature effect on adsorption of imidazolium-based ionic
liquids at liquid–liquid interface, Colloids Surf. A Physicochem. Eng. Asp. 431 (2013)
34–41.
[32] R.M. Pashley, M.E. Karaman, Applied Colloid and Surface Chemistry, Wiley, Hoboken,
2004.
[33] P. Jungwirth, D.J. Tobias, Molecular structure of salt solutions: a new view of the
interface with implications for heterogeneous atmospheric chemistry, J. Phys. Chem.
B 105 (2001) 10468–10472.
[34] H. Matsubara, A. Onohara, Y. Imai, K. Shimamoto, T. Takiue, M. Aratono, Effect of
temperature and counterion on adsorption of imidazolium ionic liquids at air-
water interface, Colloids Surf. A Physicochem. Eng. Asp. 370 (2010) 113–119.
[35] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New Jersey, 2004.
[36] D.K. Chattoraj, K.S. Birdi, Adsorption and the Gibbs Surface Excess, Plenum Press,
New York, 1984.
[37] R.A. Demel, P. Joos, Interaction between lecithins and cholesterol at the air–water
and oil–water interfaces, Chem. Phys. Lipids 2 (1968) 35–46.
[38] A. Zdziennicka, Surface behavior of triton X-165 and short chain alcohol mixtures,
Langmuir 26 (2010) 1860–1869.
References
[1] M.C. García-Alvarez-Coque, M.J. Ruiz-Angel, A. Berthod, S. Carda-Broch, On the use of
ionic liquids as mobile phase additives in high-performance liquid chromatography:
a review, Anal. Chim. Acta 883 (2015) 1–21.
[2] A.H. Azizov, R.V. Aliyeva, E.S. Kalbaliyeva, M.J. Ibrahimova, Selective synthesis and
the mechanism of formation of the oligoalkyl naphthenic oils by oligocyclization of
1-hexene in the presence of ionic-liquid catalysts, Appl. Catal. A Gen. 375 (2010)
70–77.
[3] N.V. Likhanova, M.A. Domínguez-Aguilar, O. Olivares-Xometl, N. Nava-Entzana, E.
Arce, H. Dorantes, The effect of ionic liquids with imidazolium and pyridinium cations
on the corrosion inhibition of mild steel in acidic environment, Corros. Sci. 52 (2010)
2088–2097.
[4] S. Theivaprakasam, D.R. Macfarlane, S. Mitra, Electrochemical studies of n-methyl n-
propyl pyrrolidinium bis (trifluoromethanesulfonyl) imide ionic liquid mixtures
with conventional electrolytes in LiFePO4/Li cells, Electrochim. Acta 180 (2015)
737–745.
[5] D.X. Chen, X.K. Ouyang, Y.G. Wang, L.Y. Yang, C.H. He, Liquid–liquid extraction of
caprolactam from water using room temperature ionic liquids, Sep. Purif. Technol.
104 (2013) 263–267.
[6] E. Rilo, M. Domínguez-Pérez, J. Vila, L.M. Varela, O. Cabeza, Surface tension of four
binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic
liquid + water or + ethanol), J. Chem. Thermodyn. 49 (2012) 165–171.
[7] T. Wang, L. Wang, Y. Jin, P. Chen, W. Xu, L. Yu, Study the effect of substituent position
in aromatic counterion to self-aggregation of cationic surface active ionic liquid in
aqueous medium, J. Mol. Liq. 204 (2015) 90–94.
[8] J. Jiao, B. Han, M. Lin, N. Cheng, L. Yu, M. Liu, Salt-free catanionic surface active ionic
liquids 1-alkyl-3-methylimidazolium alkylsulfate: aggregation behavior in aqueous
solution, J. Colloid Interface Sci. 412 (2013) 24–30.
[9] H. Xu, F. Tong, J. Yu, L. Wen, J. Zhang, J. He, Synergistic effect of 1-dodecyl-3-
methylimidazolium hexafluorophosphate ionic liquid and montmorillonite on
microcellular foaming behavior of poly (methylmethacrylate) by supercritical CO2,
Ind. Eng. Chem. Res. 52 (2013) 11988–11995.
[39] V.B. Fainerman, D. Mobius, R. Miller, Surfactants: Chemistry, Interfacial Properties,
Applications, Elsevier, Amsterdam, 2001.
[40] K. Radzio, K. Prochaska, Interfacial activity of trioctyloamine in hydrocarbon/water
systems with nonorganic electrolytes, J. Colloid Interface Sci. 233 (2001) 211–218.
[10] E.B. Silva, D. Santos, D.R.M. Alves, M.S. Barbosa, R.C.L. Guimarães, B.M.S. Ferreira, R.A.
Guarnieri, E. Franceschi, C. Dariva, A.F. Santos, M. Fortuny, Demulsification of heavy
crude oil emulsions using ionic liquids, Energy Fuel 27 (2013) 6311–6315.
[11] A. Zeinolabedini Hezave, S. Dorostkar, S. Ayatollahi, M. Nabipour, B. Hemmateenejad,
Dynamic interfacial tension behavior between heavy crude oil and ionic liquid solu-
tion (1-dodecyl-3-methylimidazolium chloride ([C12mim][Cl] + distilled or saline
water/heavy crude oil)) as a new surfactant, J. Mol. Liq. 187 (2013) 83–89.