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

Journal of Molecular Liquids 220 (2016) 136–141

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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

Article history: 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

a b s t r a c t 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. 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. © 2016 Elsevier B.V. All rights reserved.

1. Introduction 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 volatility [1]. Accordingly, emphasis has been placed on two major relevant parts: application of ILs in the fields such as reactions catalyzing [2], corrosion inhibition [3], electrochemistry [4] and liquid–liquid extraction [5]; and characterizing ILs properties for development of structureproperty 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]. ⁎ Corresponding author. E-mail address: [email protected] (J. Saien).

http://dx.doi.org/10.1016/j.molliq.2016.04.028 0167-7322/© 2016 Elsevier B.V. All rights reserved.

Considering these cases, many investigations have been concerned on utilizing different structure ILs. The most widely used ILs contain in 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 coordinating ability. Li et al. [16] demonstrated that counter anion of an IL with weaker hydration has more potential to cause higher surface activity (of water solutions) and ILs with chloride counter anion were found to exhibit lower activity than that with bromide. There are scarce literatures 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

J. Saien, M. Kharazi / Journal of Molecular Liquids 220 (2016) 136–141

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 demonstrated 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 [C16 mim][Cl], 1-hexadecyl-3-methylimidazolium bromide, [C16mim][Br], 1-hexadecyl-3-methylimidazolium iodide, [C16mim][I], 1tetradecyl-3-methylimidazolium chloride, [C14mim][Cl], 1-tetradecyl-3methylimidazolium bromide, [C14mim][Br], and 1-dodecyl-3methylimidazolium 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 system of n-butyl acetate + water was chosen since it is a recommended system 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. Experimental 2.1. Materials N-butyl acetate and the raw materials for synthesizing and purifying ILs, including 1-methylimidazole, 1-dodecylchloride, 1tetradecylchloride, 1-tetradecylbromide 1-hexadecylchloride, 1hexasdecylbromide, 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. 2.2. Synthesis and characterization of the ILs 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 Table 1 Mass fraction purity of the used materials (All Merck products). Chemicals

Purity%

n-Butyl acetate 1-Methylimidazole 1-Dodecylchloride 1-Tetradecylchloride 1-Tetradecylbromide 1-Hexadecylchloride 1-Hexadecylbromide 1-Hexadecyliodide Ethyl acetate

N99.5 N99.9 N95.0 N96.0 N96.0 N96.0 N96.0 N96.0 N99.5

137

343.2 K. Reactions were carried out in solvent-free condition, nitrogen atmosphere and also protected from light. The produced ILs were authorized 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 N 99% 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 structure. In Table 2, the abbreviation, chemical formula, color and state of the used ILs are presented. 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. Solutions 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 confidence). 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 (OPTIMA 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. In the drop volume method, IFT, γ, is calculated from Harkins and Brown equation [25]: γ¼

  vΔρg r ffiffiffi ϕ p 3 r v

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 and organic liquids phases, ρw and ρo, and gravitational acceleration, 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 measured the IFT of six pure chemicals in contact with water at 293.2 K, and compared with those from the literature. The average deviation between 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

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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 interface and drops were formed fast. However, by reducing flow rate, drop formation time increases and each IL has further chance to continue adsorption at the interface. Eventually, when the time of drop formation 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 accordingly 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 within 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. 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 2.50 · 10−3 mol.dm−3 and temperature of 298.2 K.

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 between the provided data here with those obtained with sodium dodecyl 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 approves that bigger counter anion sodium halide salts are weaker hydrated 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 previous 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 indicate that long alkyl chain ILs decrease the IFT higher than short alkyl chains due to more hydrophobicity nature and therefore more interfacial activity [15]. For instance, the maximum IFT reduction in the

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.

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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 percentages 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 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 interfacial 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]. • 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.

used to fit the experimental data. It is based on the Gibbs and Langmuir adsorption equations [36]:

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 proposed [35]. Among them, Szyszkowski equation has been conventionally

Fig. 5. IFT variation as a function of ILs concentration at 298.2 K.

γ ¼ γ0 −2RTΓ m ln ð1 þ K L C Þ

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 equilibrium 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 coefficient of determination, R2, which is a criterion of agreement between calculated and experimental IFT was calculated from: N  2 X γcal;i −γ exp;i

R2 ¼ 1− i¼1N 2 X γ−γ exp;i

ð3Þ

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 parameters 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. One of the consequential results, revealed from Szyszkowski equation, 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 interaction between the IL molecules with both organic and aqueous phases. By considering the used ILs structure, it is clear that there is acidic hydrogen 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 decreases or even disappears the repulsion between adsorbed ILs at the interface 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 adsorption tendency by the Langmuir equilibrium adsorption constant, KL. These parameters are listed in Table 3.

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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, ΔG∘ads, and coefficient of determination, R2, for adsorption of ILs at different temperatures. KL (dm3·mol−1)

1018 × Am (m2)

‐ΔG∘ads (kJ·mol−1)

R2

[C12mim][Cl] 293.2 88.31 298.2 86.72 303.2 84.20 308.2 81.16 313.2 76.91 318.2 70.09

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

[C14mim][Cl] 293.2 96.37 298.2 94.12 303.2 90.90 308.2 87.36 313.2 83.01 318.2 78.42

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

T (K)

106 × Γm ,L (mol·m−2)

[C14mim][Br] 293.2 98.25 298.2 95.44 303.2 92.24 308.2 88.35 313.2 84.06 318.2 79.49

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

[C16mim][Cl] 293.2 103.61 298.2 101.29 303.2 98.25 308.2 94.31 313.2 90.06 318.2 84.61

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

[C16mim][Brl] 293.2 104.91 298.2 102.04 303.2 98.85 308.2 95.07 313.2 90.68 318.2 85.79

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

[C16mim][I] 293.2 105.97 298.2 103.21 303.2 100.16 308.2 96.49 313.2 92.19 318.2 87.49

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

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 temperature, decreasing in Γm is observed as a result of disrupting surrounding 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]: Am ¼

1 Γ m NAv

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 tendencies 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. Finally, another important parameter, relevant to equilibrium constant, is standard free energy of adsorption, ΔG∘ads, which can be obtained from [39]: ΔG∘ads ¼ −2RT ln

  K L ρ0 2

ð5Þ

where ρ′ (ρw/18) is molar concentration of water at a given temperature. The standard free energies of ILs adsorption at different temperatures 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. 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.

ð4Þ

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 temperatures, 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

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 interface 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 influence of hydrocarbon chain length is, of course, more than counter anion under a certain temperature. IFT decreases almost linearly with temperature 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 interface excess decreased with temperature; however, rose when anion polarizability and hydrophobicity of the ILs increased. It is while, the Langmuir adsorption constant increased with all investigated parameters regularly, as a consequence of the ease of migration toward the interface. The supplementary information of the phase densities, drop formation times and IFTs for each IL concentration under different temperatures 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. Acknowledgments The authors wish to thank the university authorities for providing the financial support to carry out this work. 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 npropyl 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-3methylimidazolium hexafluorophosphate ionic liquid and montmorillonite on microcellular foaming behavior of poly (methylmethacrylate) by supercritical CO2, Ind. Eng. Chem. Res. 52 (2013) 11988–11995. [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 solution (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.

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