Adsorption behavior of long alkyl chain imidazolium ionic liquids at the n-butyl acetate + water interface

Article abstract of DOI:10.1016/j.molliq.2015.08.056

The adsorption behavior of amphiphilic long alkyl chain imidazolium ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride {[C_nmim][Cl], n = 12, 14, 16} at the interface of n-butyl acetate + water system was studied. Within IL bulk concentration range of 1.00 · 10^{- 5}-1.00 · 10^{- 2} mol/dm³ and temperature of 293.2-318.2 K, the used ILs act as excellent surfactants and reduce the interfacial tension significantly. The corresponding interfacial activity appeared logically in the order of [C₁₆mim][Cl] > [C₁₄mim][Cl] > [C₁₂mim][Cl]. An almost linear decrease in interfacial tension was also relevant by temperature. For modeling, the Szyszkowski adsorption equation was used to reproduce the experimental data. The consistent fittings confirm the ideal adsorption of the used IL molecules at the interface. Results also show that adsorption effectiveness increases with the alkyl chain length; however, it declines with temperature. On the other hand, adsorption tendency increases with alkyl chain length as well as with temperature. Negative standard free energy values indicate simultaneous adsorption for all cases.

Full text of DOI:10.1016/j.molliq.2015.08.056

Journal of Molecular Liquids 212 (2015) 58–62
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Adsorption behavior of long alkyl chain imidazolium ionic liquids at the
n-butyl acetate + water interface
⁎
Javad Saien , Mona Kharazi, Simin Asadabadi
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 alkyl chain imidazolium ionic liquids (ILs), 1-alkyl-3-
methylimidazolium chloride {[Cnmim][Cl], n = 12, 14, 16} at the interface of n-butyl acetate + water system
was studied. Within IL bulk concentration range of 1.00 · 10−5–1.00 · 10−2 mol/dm3 and temperature of
293.2–318.2 K, the used ILs act as excellent surfactants and reduce the interfacial tension significantly. The cor-
responding interfacial activity appeared logically in the order of [C16mim][Cl] N [C14mim][Cl] N [C12mim][Cl].
An almost linear decrease in interfacial tension was also relevant by temperature. For modeling, the Szyszkowski
adsorption equation was used to reproduce the experimental data. The consistent fittings confirm the ideal ad-
sorption of the used IL molecules at the interface. Results also show that adsorption effectiveness increases
with the alkyl chain length; however, it declines with temperature. On the other hand, adsorption tendency
increases with alkyl chain length as well as with temperature. Negative standard free energy values indicate
simultaneous adsorption for all cases.
Received 11 December 2014
Received in revised form 24 July 2015
Accepted 28 August 2015
Available online xxxx
Keywords:
Imidazolium ionic liquids
Long alkyl chain
n-Butyl acetate–water
Interfacial tension
Szyszkowski equation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Despite a lot of studies on ILs, there is scarce literature on their ad-
sorption at the interfaces. In an attempt, the behavior of ILs at the inter-
Ionic liquids (ILs) are a remarkable group of chemicals that have
found widespread applications in different areas such as catalyzing re-
actions [1], corrosion inhibition [2], gas–liquid absorption [3], liquid
membrane separation [4], and liquid–liquid extraction [5]. Interests in
ILs include their amazing applications and characters with respect to
their properties such as non-flammability, high thermal stability, and
ion conductivity. The desired properties can be provided in ILs by the
alteration in their cation and anion structure [6].
One excellent activity, stemming from amphiphilic nature, is the ILs
surface/interface adsorption. ILs possess cationic hydrophilic (head
group) and hydrophobic (tail) portions, facilitating their ease of adsorp-
tion at the interfaces. Accordingly, interfacial tension (IFT) is frequently
considered which reflects the unbalanced forces at the interface [7].
Materials such as surfactants and electrolytes can alter this property
due to their adsorption and the extent of variation reflects the nature
and the impact level of adsorption. IFT plays an important role in
interphase heat and mass transfer particularly in extraction and emulsi-
fication processes [8].
face of toluene + water was studied [12]. The work was recently
continued on the n-butyl acetate + water system with short alkyl
chain imidazolium-based ILs [13]. This chemical system is known as
an “intermediate IFT system” and is recommended by the EFCE working
party [14] for liquid–liquid extraction studies [15–17]. The used ILs
meaningfully reduced the IFT and the adsorption tendency exhibited in-
creasing with temperature. The present study was aimed to identify the
function of long alkyl chain imidazolium-based ILs namely, 1-dodecyl-
3-methylimidazolium chloride, 1-tetradecyl-3-methylimidazolium
chloride, and 1-hexadecyl-3-methylimidazolium chloride {briefly:
([Cnmim][Cl], n = 12, 14 and 16)} at the n-butyl acetate + water inter-
face. Very low but effective IL concentrations were used here under
conventional temperatures. Accordingly, the experimental data were
reproduced using a theoretical model and the relevant parameters
were obtained.
2. Experimental
In recent years, one of the most widely used ILs is the imidazolium-
based structures that exhibit excellent surface activity [9]. In this regard,
the surface and aggregation behavior of aqueous mixtures containing 1-
alkyl-3-methylimidazolium halides have been studied with a variety of
methods [10,11].
2.1. Materials
In this study, n-butyl acetate + water chemical system was used to
study the influence of ILs adsorption. N-butyl acetate with mass fraction
purity of more than 0.995 was obtained from Merck. The raw materials
for synthesizing and purifying ILs, including: 1-methylimidazole, 1-
dodecylchloride, 1-tetradecylchloride, 1-hexadecylchloride, and ethyl
acetate were also purchased from Merck with mass fraction purities
⁎
Corresponding author.
E-mail address: saien@basu.ac.ir (J. Saien).
http://dx.doi.org/10.1016/j.molliq.2015.08.056
0167-7322/© 2015 Elsevier B.V. All rights reserved.

J. Saien et al. / Journal of Molecular Liquids 212 (2015) 58–62
59
more than 0.999, 0.95, 0.96, 0.96, and 0.995, respectively. All materials
were used without further purification. Fresh deionized water with
electrical conductivity of 0.07 μS/cm was used as aqueous phase. The
specifications of the utilized chemicals are given in Table 1.
another was not observed with the used ILs concentrations (below the
CMCs).
For IFT measurements, experiments were conducted at different
temperatures. The media was thermostated at desired temperatures
using a calibrated thermostat (OPTIMA 740, Japan) with an uncertainty
of 0.1 K. The organic and the aqueous phase samples were withdrawn
to measure their density at different temperatures. Density was mea-
sured by means of an oscillating U-tube densimeter (Anton Paar
DMA4500, Austria) with an uncertainty of 0.01 kg/m3, provided
with automatic viscosity correction.
The performance and reliability of the method was examined based
on the IFT value of pure n-butyl acetate + water system (binary saturat-
ed, without IL) at 298.2 K, and the result was compared with reported
values in the literature. The obtained value of 14.0 mN/m is in
agreement with reported values of 14.1 and 14.4 mN/m [25,26]. The
maximum uncertainty for all measured IFT data was estimated as
0.1 mN/m.
Furthermore, to verifying the achievement of equilibrium condition,
the IFT variation with drop formation time was investigated at different
flow rates. The results with typical ILs solution of 2.50 · 10−3 mol/dm3
at 298.2 K are presented in Fig. 1. When drops are formed rapidly, the ILs
have no enough time to adsorb at the interface, and the dynamic IFT is
dominant during short times of drop formation. As the time increases,
each IL has further chance to continue accumulating at the interface.
Eventually, when the time of drop formation becomes adequate
(more than 30 s for drops in this work), the interface expansion occurs
sufficiently slowly and no significant change appears with more drop
formation times [27–29].
2.2. Synthesis and characterization of the ILs
The long chain imidazolium-based ILs, [C12mim][Cl], [C14mim][Cl],
and [C16mim][Cl], were synthetized according to the previously report-
ed procedure [18]. 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 70 °C. Reactions were performed in solvent-free condition, ni-
trogen atmosphere while protected from the light. The products were
allowed to cool to room temperature. The waxy ILs were then washed
with ethyl acetate for at least ten times, to remove any unreacted re-
agents. After the last washing, the remaining ethyl acetate was removed
by heating to 77 °C. As a preliminary estimation, the quality of products
was tested by halide titration, and purities more than 99% were found.
In addition, the synthesized ILs were characterized by means of FTIR,
1H NMR, 13C NMR and mass spectroscopy. The data of characterization
were in agreement with the expected structure. The details of spectra
are presented in supplementary data as well as chemical formula,
color, and state of the used ILs. To examine the purity, chloride titration
was also employed to determine the chloride content of the ILs [19] and
purity more than 99% was obtained for each of the ILs.
2.3. IFT measurements
The drop volume method was used for determining the IFT values. It
is a reproducible technique, frequently used in the measurements
[20–23]. In this method, the IFT, γ, is calculated from Harkins and
Brown equation [24]:
3. Results and discussion
3.1. Evaluation of experimental results
For each synthesized IL, aqueous phase concentrations ranging from
very low to near CMC were utilized for each IL. In order to obtain tem-
perature dependency, each solution was examined at six different tem-
peratures, within 293.2–318.2 K. Density of each system was changed
with increasing both concentration and temperature. The correspond-
ing aqueous and organic phase densities varied within 991.04–
998.80 kg/m3 and 856.44–882.53 kg/m3. The phase densities and the
IFT data at different temperatures as well as IL concentrations are listed
in the supplementary information.
The measured IFT values were within the range of 13.4–14.0 mN/m
for the pure system (without any IL), within 6.2–7.4 mN/m with
[C12mim][Cl], 4.5–5.7 mN/m with [C14mim][Cl] and 3.4–4.6 mN/m
with [C16mim][Cl] under different temperatures. The IFT reduction
percentage reaches to 55, 67 and 75%, by individual ILs, respectively.
ꢀ
ꢁ
VΔρg
r
r
pffiffiffi
γ ¼
ϕ
ð1Þ
3 V
where V is the aqueous drop volume falling off a capillary into the
organic phase; Δρ is the density difference between the aqueous and or-
ganic liquids (ρw and ρo), g and r are the acceleration of gravity and the
pffiffiffi
capillary radius (0.35 mm in this study), respectively, and ϕðr= 3 VÞ is a
constant (dimensionless) which can be obtained from empirical
relations [21].
Aqueous phase concentrations of ILs, within 1.00 · 10−5–1.00 ·10−2
mol/dm3 were prepared by mass, using an Ohaus (Adventurer Pro, AV
264) balance with an uncertainty of 0.1 mg. In this regard, 200 and
250 cm3 volumetric flasks with an uncertainty of 0.2 cm3 and 5 and
10 cm3 volumetric pipettes with
a maximum uncertainty of
0.02 cm3 were used. Accordingly, the absolute standard deviation of
concentrations did not exceed 0.01 · 10−3 mol/dm3 for all cases.
Prior to experiments, equal volumes of aqueous and organic phases
(100 cm3) were mixed for at least 30 min, and then left to rest for anoth-
er 30 min. It is notable that the mutual solubility of both the organic and
aqueous phases was very low and forming emulsion of either phase in
Table 1
Mass fraction purity and CAS number of the used materials (all Merck products).
Chemical name
Mass fraction puritya Water mass fraction CAS number
n-butyl acetate
1-Methylimidazole
1-Dodecylchloride
1-Tetradecylchloride N0.96
1-Hexadecylchloride N0.96
Ethyl acetate
N0.995
N0.999
N0.95
b0.001
123-86-4
616-47-7
112-52-7
2425-54-9
4860-03-1
141-78-6
N0.995
b0.0005
Fig. 1. IFT variation as a function of drop formation time for ILs: ( ), [C12mim][Cl]; ( ),
a
Informed by the supplier.
[C14mim][Cl]; and ( ), [C16mim][Cl] at concentration of 2.50 · 10−3 mol/dm3 and 298.2 K.

60
J. Saien et al. / Journal of Molecular Liquids 212 (2015) 58–62
In Fig. 2, the IFT variations versus ILs concentration are illustrated.
The results show that presence of ILs leads to IFT reduction signifi-
cantly at constant temperatures. Because of amphiphilic nature of
the ILs, they migrate toward the interface and by increasing concen-
tration, more adsorption occurs which causes more reduction in the
IFT [30]. As can be seen, IFT decreases sharply (average reduction of 50,
61 and 69%) with very low ILs dosages to about 1.00 · 10−3 mol/dm3
and then a mild variation is relevant by approaching to CMC. The
interfacial activity of the used ILs appeared in the order of
[C16mim][Cl] N [C14mim][Cl] N [C12mim][Cl]; revealing that more meth-
ylene bridge (–CH2–) causes to impressive declining in the IFT values. It
is since alkyl chain length is one important factor in the hydrophobicity
nature of ILs [31].
In our recent work [13], effect of short alkyl chain ILs ([Cnmim][Cl],
n = 6, 7 and 8) on the IFT of n-butyl acetate + water system was stud-
ied. The results revealed that used ILs behave similar to surfactants and
reduce the IFT significantly. In order to have comparison between the
interfacial activity of short alkyl chain and long alkyl chain (with twice
carbon atoms in alkyl chain) ILs, the IFT variations with effect of variety
of ILs {[C6 to C16mim][Cl]} are presented in Fig. 3. The results obviously
show significant declining in the IFT values with IL carbon numbers and
that efficiency by long alkyl chains exceeds much of those by short alkyl
chains. In this regard, the maximum percentage of the IFT reduction are
20, 26, and 33%, for ILs with n = 6, 7, and 8, respectively; however, rises
to 55, 67, and 75%, using ILs with n = 12, 14, and 16. It has to mention
that the highest concentration used for long alkyl chain ILs, was only
0.01 mol/dm3 (10% of short alkyl chain ILs). Variations presented in
Fig. 3 also show a remarkably linear decrease with carbon number till
n = 14, and then a lower trend is relevant. The used ILs are different
just due to their alkyl chain length and contain the same cationic seg-
ment and the counter anion. In this regard, the ILs with longer alkyl
chain exhibit more hydrophobicity and consequently more interfacial
activity [32]. As Fig. 4 represents, the required IL amount for declining
IFT to a certain value, strongly decreases with alkyl chain length.
Finally, IFT decreases linearly with temperature in all cases (Fig. 5).
The reduction is about 14% for each concentration within the used tem-
perature range. Since the agitation kinetic of the molecules improves
with increasing temperature, the IFT decreases gradually, as similarly
reported for the effect of imidazolium ionic liquids on the surface
tension [20].
Fig. 3. IFT variation as a function carbon number in alkyl chain of IL with concentration of:
1.00 · 10−4 mol/dm3
1.00 · 10−2 mol/dm3
(
), 1.00 · 10−3 mol/dm3
); all at 298.2 K.
( ( ) and
), 5.00 · 10−3 mol/dm3
(
adsorption equations have been used. Among them, most experimental
adsorption data for ILs have been well fitted by the Szyszkowski
equation, which is obtained from the Gibbs and Langmuir adsorption
equations [36]:
γ ¼ γ0−2RTΓm lnð1 þ KLCILÞ
ð2Þ
where γ0 is the IFT for pure chemical system (CIL = 0), and R and T are
gas law constant and absolute temperature, respectively. Also, Γm, KL,
and CIL are the maximum interfacial concentration (saturated interface),
the Langmuir equilibrium adsorption constant and the IL bulk concentra-
tion, respectively. In a simple convention, the factor 2 stands for the dis-
sociation of each ionic surface active substance into its cation and anion.
Correspondingly, Szyszkowski equation was used here to fit the experi-
mental data, obtained with each IL bulk concentrations at constant tem-
peratures. The coefficient of determination, R2 was calculated using [37]:
N
ꢂ
ꢃ
X
2
γ
cal;i−γexp;i
R2 ¼ 1−i¼1
ð3Þ
N
ꢂ
ꢃ
X
2
γ−γexp;i
3.2. Theoretical model
i¼1
In most studies, to relate the equilibrium IFT values to the substrate
bulk concentration; Langmuir [33], Szyszkowski [34], and Frumkin [35]
where N, γcal, γexp, and γ stand for the number of data, the calculated
IFT by the model, the experimental IFT and the average of all the
Fig. 2. IFT variation as a function of concentration of [C12mim][Cl] at: ( ) 293.2 K, (
)
303.2 K, ( ) 318.2 K, [C14mim][Cl] at: ( ) 293.2 K, ( ) 303.2 K, ( ) 318.2 K and
[C16mim][Cl] at: ( ) 293.2 K, ( ) 303.2 K, ( ) 318.2 K. Solid lines correspond to theoret-
ical curves obtained by the Szyszkowski equation.
Fig. 4. IFT variation as a function of ILs concentration: ( ), [C₆mim][Cl]; ( ),
[C₇mim][Cl]; ( ) [C₈mim][Cl]; ( ), [C12mim][Cl]; ( ), [C14mim][Cl]; ( ) [C16mim][Cl]
at: 298.2 K.

J. Saien et al. / Journal of Molecular Liquids 212 (2015) 58–62
61
experimental data. In this regard, the adsorption parameters including
maximum interface excess, Γm, and the Langmuir equilibrium adsorp-
tion constant, KL, were obtained (Table 2).
The maximum interface excess, Γm, is the concentration at the satu-
rated interface. This value explains effectiveness of adsorption in which
maximum adsorption is achieved. Its variation with temperature is
depicted in Fig. 6 for each of the used ILs. It is clear that with increasing
temperature, a decreasing variation in Γm is observed as a result of
disrupting water molecules around the hydrophobic portion which
causes less transfer of ILs toward the interface [38].
The maximum interface excess increases with methylene bridge
(–CH2–) in the IL tail since longer alkyl chains give higher hydropho-
bicity and leads to more molecules adsorption at the interface [39].
As a result, obtained Γm values for [C16mim][Cl] is more than those
of [C14mim][Cl] and [C12mim][Cl].
Corresponding to Γm, the minimum area occupied by an adsorbed
molecule at the interface, Am, can be obtained from the following
equation [40]:
Fig. 5. IFT variation as a function of temperature in the presence of ILs: ( ),
[C12mim][Cl]; ( ), [C14mim][Cl]; and ( ), [C16mim][Cl] at typical concentration of
2.50 · 10−3 mol/dm3.
1
mNAv
Am
¼
ð4Þ
Γ
appropriate experimental values, respectively. The obtained R2 values,
listed in Table 2 (ranged within 0.9777–0.9894), indicate consistent
fittings with the Szyszkowski model. As well, solid lines in Fig. 2 are the-
oretical curves that show close agreement with the experimental data.
One significant result from this study is the ideal IL behavior at the
interface of n-butyl acetate + water, implied from Szyszkowski equa-
tion. However, in our previous study [27] with toluene + water system,
non-ideal interaction between adsorbed ILs was relevant, principally at-
tributed to electrostatic repulsions. The difference can be ascribed to the
interaction between IL molecules with both organic and aqueous
phases. By considering the utilized IL structure (presented in supple-
mentary data), it is clear that there is an acidic hydrogen on carbon
number 2 between two electronegative nitrogen atoms. As a result,
there is a high chance to form hydrogen bonds between this hydrogen
and electronegative atoms in both n-butyl acetate and water molecules.
This interaction decreases or totally removes the repulsion between
absorbed ILs at the interface causing an ideal monolayer adsorption.
Therefore, the Szyszkowski equation can be adequately used to fit the
in which, NAv is the Avogadro's number.
The calculated Am values, for each IL at different temperatures, are
listed in Table 2. The results indicate that when concentration decreases
at the interface, each molecule occupies more area at higher tempera-
tures. On the other hand, compact monolayers with lesser occupied
areas are formed by the IL molecules with longer alkyl chain length.
The adsorption tendency, on the other hand, is described by Lang-
muir equilibrium adsorption constant, KL. Its variation with tempera-
ture is presented in Fig. 7. The results demonstrate that adsorption
tendency increases with both temperature and alkyl chain length. At
higher temperatures, intensifying of motions occurs; therefore, more
tendencies of the IL molecules toward the interface are expected. As
well, it is evident that [C16mim][Cl] has the highest adsorption tendency
than others because of more hydrophobicity nature and more migration
tendency toward the interface.
Related to equilibrium constant, another important parameter is
standard free energy of adsorption, ΔG∘ads, which can be obtained from
[38]:
ꢀ
ꢁ
KLρ0
2
Table 2
ΔG∘ads ¼ −2RT ln
ð5Þ
Maximum interface excess, Γm, Langmuir equilibrium adsorption constant, KL, minimum
area occupied by a molecule, Am, standard free energy of adsorption, ΔG∘ads, and coefficient
of determination, R2, for adsorption of ILs at different temperaturesa.
where ρ′(=ρw/18) is the molar concentration of water for a given
temperature.
T(K)
106 · Γm
10−3 · KL
1018 · Am
−ΔG∘ads
(kJ/mol)
R2
(mol/m2)
(m3/mol)
(nm2)
[C12mim][Cl]
293.2
298.2
303.2
308.2
88.31
86.72
84.20
81.16
76.91
70.09
2.49726
2.57991
2.63845
2.68925
2.71687
2.72111
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
313.2
318.2
[C14mim][Cl]
293.2
298.2
303.2
308.2
96.37
94.12
90.90
87.36
83.01
78.42
3.02677
3.12832
3.19517
3.23724
3.26677
3.27351
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
313.2
318.2
[C16mim][Cl]
293.2
298.2
303.2
308.2
103.61
101.29
98.25
94.31
90.06
84.61
3.52616
3.62922
3.70326
3.75849
3.78579
3.79901
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
313.2
318.2
Fig. 6. Maximum interface excess as a function of temperature: ( ), [C16mim][Cl]; ( ),
[C14mim][Cl]; and ( ), [C12mim][Cl].
a
The standard uncertainty: u(T) = 0.1 K.

62
J. Saien et al. / Journal of Molecular Liquids 212 (2015) 58–62
[4] X.J. Zhang, J.Y. Wang, Y.Q. Hu, Interfacial tension of n-alkane and ionic liquid systems, J.
Chem. Eng. Data 55 (2010) 4687–4690.
[5] S.H. Ha, N.L. Mai, Y.M. Koo, Butanol recovery from aqueous solution into ionic liquids by
liquid–liquid extraction, Process Biochem. 45 (2010) 1899–1903.
[6] R.L. Gardas, R. Ge, N.A. Manan, D.W. Rooney, C. Hardacre, Interfacial tensions of
imidazolium-based ionic liquids with water and n-alkanes, Fluid Phase Equilib. 294
(2010) 139–147.
[7] R.J. Hunter, Foundations of Colloid Science, Clarendon Press, New York, 1987.
[8] J. Saien, S. Daliri, Improving performance of liquid–liquid extraction with temperature for
mass transfer resistance in both phases, J. Taiwan Inst. Chem. Eng. 45 (2014) 808–814.
[9] S. Aparicio, M. Atilhan, F. Karadas, Thermophysical properties of pure ionic liquids: review
of present situation, Ind. Eng. Chem. Res. 49 (2010) 9580–9595.
[10] G. Law, P.R. Watson, Surface tension measurements of N-alkylimidazolium ionic liquids,
Langmuir 17 (2001) 6138–6141.
[11] I. Goodchild, L. Collier, S.L. Millar, I. Prokeš, J.C.D. Lord, C.P. Butts, J. Bowers, J.R.P. Webster,
R.K. Heenan, Structural studies of the phase, aggregation and surface behaviour of 1-alkyl-
3-methylimidazolium halide + water mixtures, J. Colloid. Interface Sci. 307 (2007)
455–468.
[12] 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.
[13] J. Saien, M. Kharazi, S. Asadabadi, Adsorption effect of room-temperature imidazolium
ionic liquids at n-butyl acetate + water interface under different temperatures, Iran. J.
Chem. Eng. (2015) (in press).
[14] T. Misek, Standard Test Systems for Liquid Extraction, Instn. Chem. Engs. For European
Federation of Chemical Engineers, Warwickshire, 1985.
Fig. 7. Equilibrium adsorption constant as
[C16mim][Cl]; ( ), [C14mim][Cl]; and ( ), [C12mim][Cl].
a function of temperature: ( ),
[15] M.A. Waheed, M. Henschke, A. Pfennig, Mass transfer by free and forced convection from
single spherical liquid drops, Int. J. Heat Mass Transf. 45 (2002) 4507–4514.
[16] 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.
As the final point, the standard free energy of IL adsorption, at
different temperatures, has negative values (Table 2); implying that
the adsorption is spontaneous in all cases.
[17] 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 numeri-
cal investigations, Chem. Eng. Sci. 66 (2011) 426–439.
4. Conclusions
[18] 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.
Established upon the IFT measurements, the adsorption of am-
phiphilic synthesized long chain imidazolium ILs, [C12mim][Cl],
[C14mim][Cl], and [C16mim][Cl] at the n-butyl acetate + water inter-
face was investigated. The used ILs significantly decrease the interfacial
tension of the system due to their surfactant nature. Longer alkyl chain
ILs adsorb at the interface are more effectively related to their higher
hydrophobicity. Accordingly, the maximum IFT reductions were 55,
67, and 75%, respectively, within applied temperature ranges. Besides,
the IFT values were decreased linearly with temperature.
The Szyszkowski adsorption equation was satisfactorily applied to
reproduce the experimental data. Accordingly, IL molecules behave
ideal with no interaction at the interface and the maximum interface ex-
cess and the adsorption tendency show dependency on alkyl chain
length, as well as temperature. Totally, maximum interface excess de-
creases with temperature; however, increases when hydrophobicity of
ILs increase due to longer alkyl chain. It is while the Langmuir equilibrium
adsorption constant increases with either increase of temperature or use
of longer alkyl chain length.
[19] D.A. Skoog, D.M. West, F.J. Holler, Fundamentals of Analytical Chemistry, Saunders College
Publishing, Fort Worth, 1992.
[20] H. Matsubara, A. Onohara, Y. Imai, K. Shimamoto, T. Takiue, M. Aratono, Effect of temper-
ature and counterion on adsorption of imidazolium ionic liquids at air–water interface,
Colloids Surf. A 370 (2010) 113–119.
[21] B.B. Lee, P. Ravindra, E.S. Chan, New drop weight analysis for surface tension determina-
tion of liquids, Colloids Surf. A 332 (2009) 112–120.
[22] 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.
[23] J. Saien, S. Asadabadi, Adsorption and interfacial properties of individual and mixtures of
cationic/nonionic surfactants in toluene + water chemical systems, J. Chem. Eng. Data
55 (2010) 3817–3824.
[24] 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 capillary
height method, J. Am. Chem. Soc. 41 (1919) 499–524.
[25] A. Bahramian, A. Danesh, Prediction of liquid–liquid interfacial tension in multicomponent
systems, Fluid Phase Equilib. 221 (2004) 197–205.
[26] W. Apostoluk, J. Drzymala, An improved estimation of water–organic liquid interfacial
tensions based on linear solvation energy relationship approach, Colloid Interface Sci.
262 (2003) 438–488.
[27] J. Saien, S. Asadabadi, Temperature effect on adsorption of imidazolium-based ionic liquids
at liquid–liquid interface, Colloids Surf. A Physicochem. Eng. 431 (2013) 34–41.
[28] C. Mollet, Y. Touhami, V. Hornof, A comparative study of the effect of readymade and in
situ-formed surfactants on interfacial tension measured by drop volume tensiometry, J.
Colloid Interface Sci. 178 (1996) 523–530.
Acknowledgment
[29] U. Teipel, N. Aksel, Adsorption behavior of nonionic surfactants studied by drop volume
technique, Chem. Eng. Technol. 24 (2001) 393–400.
[30] R.M. Pashley, M.E. Karaman, Applied Colloid and Surface Chemistry, Wiley, Hoboken, 2004.
[31] N.V. Sastry, N.M. Vaghela, V.K. Aswal, Effect of alkyl chain length and head group on sur-
face active and aggregation behavior of ionic liquids in water, Fluid Phase Equilib. 327
(2012) 22–29.
[32] 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.
[33] V.S. Markin, M.I. Volkova-Gugeshashvili, A.G. Volkov, Adsorption at liquid interfaces: the
generalized Langmuir isotherm and interfacial structure, J. Phys. Chem. B 110 (2006)
11415–11420.
[34] J. Saien, S. Akbari, Interfacial tension of hydrocarbon + different pH aqueous phase
systems in the presence of Triton X-100, Ind. Eng. Chem. Res. 49 (2010) 3228–3235.
[35] S.I. Karakashev, A.V. Nguyen, J.D. Miller, Equilibrium adsorption of surfactants at the gas–
liquid interface, Adv. Polym. Sci. 218 (2008) 25–55.
The authors wish to thank the university authorities for providing
the financial support to carry out this work.
Appendix A. Supplementary data
The supplementary information of the ILs analyzing spectra and
their specification and the IFT measurement details are given in the
attached file. Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.molliq.2015.08.056.
References
[36] D.K. Chattoraj, K.S. Birdi, Adsorption and the Gibbs Surface Excess, Plenum Press, New
York, 1984.
[1] A.H. Azizov, R.V. Aliyeva, E.S. Kalbaliyeva, M.J. Ibrahimova, Selective synthesis and the
mechanism of formation of the oligoalkylnaphthenic oils by oligocyclization of 1-hexene
in the presence of ionic-liquid catalysts, Appl. Catal. A Gen. 375 (2010) 70–77.
[2] 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 cor-
rosion inhibition of mild steel in acidic environment, Corros. Sci. 52 (2010) 2088–2097.
[3] Z. Zhou, G. Jing, L. Zhou, Characterization and absorption of carbon dioxide into aqueous
solution of amino acid ionic liquid [N1111][Gly] and 2-amino-2-methyl-1-propanol,
Chem. Eng. J. 204 (2012) 235–240.
[37] M. Otto, Chemometrics: Statistics and Computer Application in Analytical Chemistry,
Wiley, New York, 1999.
[38] V.B. Fainerman, D. Mobius, R. Miller, Surfactants: Chemistry, Interfacial Properties,
Applications, Elsevier, Amsterdam, 2001.
[39] H.E. Ríos, J. González-Navarrete, V. Vargas, M.D. Urzúa, Surface properties of cationic
polyelectrolytes hydrophobically modified, Colloids Surf. A 384 (2011) 262–267.
[40] K. Radzio, K. Prochaska, Interfacial activity of trioctyloamine in hydrocarbon/water
systems with nonorganic electrolytes, J. Colloid Interface Sci. 233 (2001) 211–218.