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Temperature effect on adsorption of imidazolium-based ionic liquids at liquid–liquid interface
Accepted Manuscript Title: Temperature effect on adsorption of imidazolium-based ionic liquids at liquid-liquid interface Author: Javad Saien Simin Asadabadi PII: DOI: Reference:
S0927-7757(13)00328-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.04.043 COLSUA 18373
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
8-3-2013 18-4-2013 22-4-2013
Please cite this article as: J. Saien, S. Asadabadi, Temperature effect on adsorption of imidazolium-based ionic liquids at liquid-liquid interface, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.04.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature effect on adsorption of imidazolium-based ionic liquids at liquid-liquid interface Javad Saien, Simin Asadabadi
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Highlights
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Department of Applied Chemistry, Bu–Ali Sina University, Hamedan 65174, Iran
The synthesized imidazolium-based ILs act as cationic surface active agent at the interface.
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Surface active properties of the used ILs enhance with their alkyl chain length. The CMC of each IL shows a minimum value at around 303 K.
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The experimental data fit quite well with the Frumkin adsorption isotherm.
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Adsorption tendency, effectiveness and interaction exhibit specific variations with temperature.
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Abstract
Interfacial tension variation and micelle formation by two amphiphilic synthesized ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride ([Cnmim][Cl], n=7 and 8), are
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investigated at conventional temperatures, within 293.2–313.2 K. These ILs significantly reduce the interfacial tension of toluene-water system and the influence is more effective
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for the IL with n=8 because of the relevant more hydrophobicity character. The relevant critical micelle concentrations (CMCs) depend on temperature with a minimum appeared around 303 K, for both the ILs. The Frumkin adsorption isotherm predicts quite well the interfacial adsorption of the ILs for concentrations less than CMC. Accordingly, the adsorption tendency reveals increasing with temperature. However, the saturated interface excess and absolute electrostatic repulsion get their maximum values at around the same temperature of 303 K.
Corresponding author. Tel. /fax: +98 811 8257407. E-mail address: [email protected] (J. Saien).
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Keywords: Ionic liquids, Interfacial tension, Adsorption, Frumkin isotherm, CMC
1. Introduction
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Ionic liquids (ILs) are salts exclusively comprising of ions with low melting points [1]. Recently, there has been an increasing interest in ILs, owing to their distinctive physico-chemical
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properties such as very low vapour pressure, non-flammability, high thermal stability and ion conductivity [2,3]. Different ILs have been designed by altering their cation and anion structure
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to improve their applications in catalysing reactions [4], corrosion [5], gas-liquid absorption [6], membrane [7], liquid-liquid extraction [8] and electrochemistry [9].
A majority of recent literatures are concerned on the properties of a kind of ILs, named
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“imidazolium-based ILs”, with a bulky asymmetric organic cation consisting of nitrogen. These are environmentally friendly fluids and behave as green solvents [10]. For instance, Docherty et
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al. [11], have shown good mineralization of these ILs in sludge samples. ILs also exhibit excellent surface/interface activity and micelle formation, because they possess amphiphilic structure with two hydrophilic and hydrophobic portions. A new opportunity
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is sought by researchers to protect environment by introducing ILs as “novel green surface active agents” [12]. Therefore, their surface properties and surface active behaviour, either pure or in
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different solvents, have attracted much interest [13,14]. Klomfar et al. [13], for instance, have investigated the surface tension of four imidazolium-based ILs with different alkyl chain length
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at different temperatures. They have introduced an empirical surface tension-temperature equation describing the temperature dependency of each IL. In another research, Dong et al. [2] have shown that surface activity of imidazolium-based ILs in aqueous solutions is somewhat greater than that of conventional ionic surfactants with the same carbon number in their hydrophobic chain. Besides, Merrigan et al. [15] have shown that ILs can be used to stabilize dispersions.
Despite the current widespread attention to ILs for frequent applications, they have not yet been much subjected to oil-water systems to investigate the relevant interface properties [16]. It is while; an attractive perspective in ILs capacity is predictable in improving extraction performance (mostly due to altering the system hydrodynamic).
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Accordingly, the present study is conducted, for the first time, to study the influence of different dosages of two amphiphilic synthesized imidazolium-based ILs, 1-heptyl-3methylimidazolium chloride, briefly [C7mim][Cl], and 1-octyl-3-methylimidazolium chloride, [C8mim][Cl], on the interfacial tension (IFT) of toluene-water system. This chemical system is
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widely used in studies of the liquid-liquid extraction process [17,18]. Accordingly, micelle concentration and related parameters of interface activity at different temperatures are obtained. For the recent parameters, the Frumkin adsorption isotherm is employed to correlate the
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experimental data in relation to ILs bulk concentrations, less than critical micelle concentration
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(CMC), at each temperature. 2. Experimental
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2.1. Materials
The toluene-water system was chosen as the recommended chemical system with high IFT [19]. Toluene, 1-methylimidazole, 1-chloroheptane, 1-chlorooctane and ethyl acetate were
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purchased from Merck and Riedel-de Haen with purities more than 99.9, 99, 98, 96 and 99.5%, respectively, and used with no further purification. Fresh deionized water with electrical
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conductivity of 0.07 μS·cm1 was used for aqueous solutions throughout. 2.2. Synthesis and characterization of the ILs
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Each IL, 1-heptyl-3-methylimidazolium chloride and 1-octyl-3-methylimidazolium chloride, was synthesized via a direct reaction between equal molar amounts of 1-methylimidazole and
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appropriate 1-chloroalkane in a round-bottomed flask equipped with a reflux condenser and magnetic stirrer at 70 °C for 72 hours. The reaction was under reflux and nitrogen atmosphere. The obtained viscous ILs were allowed to cool to room temperature and then washed twelve times with ethyl acetate to remove any unreacted reagent. After the last washing, the remaining ethyl acetate was removed by heating to 77 °C [12,20]. Water content was determined by using a Karl Fischer Coulometer (Metrohm, model 831). Maximum mass fractions of water content were 0.0006 and 0.0003 for [C7mim][Cl] and [C8mim][Cl], respectively. 1
H NMR, 13C NMR and mass spectroscopy were utilized to characterize the synthesized ILs.
In addition, chloride titration was conducted and purity more than 99% was obtained for each of 3
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the ILs [21]. The spectra indicate good agreement between products and theoretical structures. The high purity of each IL was confirmed by indicating just the corresponding IL peaks and no ones for reactants and/or by-products. The details of spectra for [C7mim][Cl] are: 1H NMR (400 MHz, CDCl3): δ=0.846 (3H, t), 1.254 (8H, m), 1.905 (2H, m), 4.129 (3H, s),
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4.293 (2H, t), 7.385 (H, d), 7.562 (H, d), 10.652 (H, s). 13C NMR (400 MHz, CDCl3): δ=13.63, 22.05, 25.74, 28.21, 29.92, 31.08, 36.11, 49.54, 121.83, 123.61, 136.95.
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Mass Spectroscopy: Molecular weight of [C7mim][Cl] was obtained equal to 216 g·mol1 in and for [C8mim][Cl]:
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agreement with exact molecular structure weight of 216.75 g·mol1.
1H NMR (400 MHz, CDCl3): δ=0.832 (3H, t), 1.239 (10H, m), 1.892 (2H, m), 4.114 (3H, s),
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4.280 (2H, t), 7.396 (H, d), 7.579 (H, d), 10.601 (H, s).
13C NMR (400 MHz, CDCl3): δ=13.59, 22.04, 25.72, 28.45, 29.85, 31.14, 36.03, 49.44,
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121.78, 123.56, 136.81.
Mass Spectroscopy: Molecular weight of [C8mim][Cl] was obtained equal to 230 g·mol1 in agreement with exact molecular structure weight of 230.78 g·mol1.
2.3. IFT measurements
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All the spectra are presented in supporting information.
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The drop volume method has been used by other investigators [22,23] and was applied to measure the IFT of each sample. The detailed procedure has been explained in our previous
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studies [24,25]. Briefly, for contacting with toluene and measuring the IFT, individual IL solutions were prepared in mass. An Ohaus (Adventurer Pro, AV 264) balance with an uncertainty of ± 0.1 mg, 5 and 10 ( 0.01 and 0.02) cm3 volumetric pipettes and 200 and 250 ( 0.2) cm3 volumetric flasks were used for preparation of required solutions. Concentrations ranging from 1.00×104 to 6.00×101 mol·dm3 with maximum absolute standard deviation of 0.01×101 mol·dm3 were prepared for each IL. The IFT measurements were carried out at five different temperatures for each sample. By using a calibrated thermostat (OPTIMA 740, Japan) with an uncertainty of ± 0.1 K, the continuous organic media and conducting tube were temperature adjusted. To ensure obtaining saturated condition, equal volumes (100.0 cm3) of both aqueous solution and toluene were 4
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chosen, mixed for at least one hour and then left for an another hour to rest. The mutual solubility of toluene and IL solutions was very low and the formation of emulsion of either phase in another was not observed. To measure the density, an oscillating U-tube densimeter (Anton Paar DMA 4500, Austria) was used with uncertainty of ± 0.01 kg·m3.
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In drop volume method, the IFT ( γ ) relationship with drop volume (V), falling off a capillary into the organic phase, is given by Harkins and Brown [26]:
V Δ ρg r 3 r V
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γ
(1)
where ρ is the density difference between the aqueous and organic liquids ( ρw and ρo ), r is the 3
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capillary radius and r
V is a constant which can be obtained from the table of Harkins and
The range of r
3
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Brown or empirical relations [27,28].
V (dimensionless) values in this work was within 0.3416–0.5903 for
[C7mim][Cl] and 0.3422–0.6412 for [C8mim][Cl]. The maximum uncertainty in the IFT
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measurements was determined ± 0.1 mN·m1.
The IFT of pure toluene-water system (binary saturated, without IL) was measured and
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compared with the values reported in the literatures at 298.2 K to examine the reliability and performance of the method. The obtained value of 36.3 mN·m1 was pretty close to the reported
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values of 36.1 and 36.6 mN·m1 [29,30].
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3. Results and discussion 3.1. Experimental results
For investigating the IFT variations, a concentration range of 06.00×101 mol·dm3, from very low to beyond the CMC, was utilized for each synthesized IL. Temperature dependency of the IFT was examined with five temperatures ranging from 293.2 to 313.2 K for each solution. Density of individual IL solutions was changed with increasing concentration and temperature. In this regard, the aqueous and organic phase densities varied within 992.20–1001.23 kg·m3 and 848.13–866.86 kg·m3, respectively. The IFT values were within the range of 34.7–37.2 mN·m1 for the pure system (without any IL), within 8.9–36.6 mN·m1 in the presence of [C7mim][Cl] and within 7.1–36.4 mN·m1 with [C8mim][Cl]. Also generated drop volumes were within 55.4– 5
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285.7 mm3 in the presence of [C7mim][Cl] and 43.2–284.2 mm3 with [C8mim][Cl], corresponding to drop formation times of 95.42–492.36 s and 74.44–489.71 s, respectively. The IFT data and phase densities at different temperatures as well as IL concentrations are given in supporting information.
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To guarantee the attainment of equilibrium condition, the IFT variation with drop formation time (obtained with different aqueous phase flow rates, from 0.00049 to 0.01370 cm3·s1) was measured. The results are presented in Fig. 1 for [C7mim][Cl] and [C8mim][Cl] solutions with
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typical concentration of 8.00×102 mol·dm3 and temperature of 298.2 K. The trend in Fig. 1 can
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be explained concerning the fact that the used ILs may not have sufficient time to adsorb at the interface when the drops form rapidly. Therefore, the dynamic IFT is relevant for short times of drop formation. As the time increases, the ILs have further chance to accumulate at the interface.
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Finally, when the time of drop formation becomes very long, depending on the adsorption kinetic of IL, the interface expansion occurs slowly enough to find equilibrium adsorption and no
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significant change is relevant with further drop formation times. This phenomenon in drop volume method has already been reported in literatures [31,32]. In Fig. 1, drop formation times [C8mim][Cl], respectively.
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of 237.24 and 189.40 s corresponds to the measured values reported here for [C7mim][Cl] and
Fig. 1
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Three important results are accordingly deduced from IFT variations, typically presented in Fig. 2. First, IFT decreases with introducing more ILs into the system. In other words, more
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concentration, more migration of ILs toward the interface and more reduction in the IFT [33]. In comparison with the effect of [C6mim][Cl] on the toluene-water IFT which has been recently published [34], it can be concluded that more methylene bridge (–CH2–) in [C7mim][Cl] and [C8mim][Cl] structures causes more IFT reduction, in order, obviously due to more hydrophobicity in ILs tail (hydrophobic portion) and subsequently higher adsorption [35]. Second, temperature causes internal thermal agitation, attenuation of intermolecular forces at interface and IFT reduction [22]. At last, a nearly linear reduction with temperature is relevant (three of them are typically presented) within the used range. Fig. 2
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In Fig. 2, each curve has a distinct break point at the appropriate CMC. Remarkably, the absence of a minimum on the IFT vs. concentration curve near each CMC is another evidence for the high purity of the synthesized ILs [36,37]. By obtaining the CMC for each IL at different temperatures, it can be concluded that
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temperature has complicate effects on the CMCs. Temperature dependency of the CMC is drawn in Fig. 3. The incidence of the minimum on the curves (at around 303 K for both ILs) can be explained by two competing factors: hydrophobic interaction and dehydration of cationic head
Fig. 3
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the second favors micelle formation at lower concentrations [38,39].
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groups (hydrophilic portion). The first one causes the CMC to appear at higher levels, whereas
It is substantial to note that concentrations corresponding to the CMCs are more than 0.01
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mol·dm3 and based on the non-ideal interaction in bulk solutions at these concentrations, it is essential to exert activity instead of concentration ( C C f ) in which C and f are activity
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and average activity coefficient, respectively [40].
To obtain average activity coefficient of ions, f , the extended Debye-Huckel equation for
logf
A z z I 1 Ba I
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short range columbic interactions can be used [41]:
0.0551 I
(2)
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where A and B are numerical constants depending on relative permittivity (dielectric constant) of water ( ε r ) and absolute temperature. The parameters of z and a are ion charge and radius and I
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(=C+Csalt) is ionic strength expressed in mol·dm3 where C and Csalt denote bulk concentrations of IL and univalent inorganic salt (if there is any), respectively [41]. The used relative permittivity of water and calculated constants A and B at different temperatures [40] are presented in supporting information. 3.2. Theoretical model
In most studies, Langmur [42], Szyszkowski [43] and Frumkin [44] adsorption isotherms have been employed to relate interface adsorption and bulk concentration of ionic surface active agents. Among them, Frumkin isotherm takes into account the interactions between adsorbed species at the interface in addition to the adsorption tendency and effectiveness [45,46]. The 7
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interface equation of state and the corresponding Frumkin adsorption isotherm are expressed for electroneutral interfaces by [47]:
2 RT ln 1 θ βθ 2 ω
bF C C Csalt f 12
(3)
θ exp 2 βθ 1 θ
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Π
(4)
where Π is interfacial pressure ( Π γ0 γ ), γ0 is the IFT for clean chemical system; R and T
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are gas law constant and absolute temperature, respectively. As well, θ ( Γω) is interface layer coverage and Γ and ω are interface excess concentration and partial molar area of adsorbed
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species; bF and β are adsorption equilibrium constant and intermolecular interaction parameter, respectively. When β is equal to zero, ideal behavior of adsorbed species at interface is
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concluded. In this condition, the Frumkin model reduces to the Langmuir isotherm. When the value of β is positive, there is van der Waals attractive interaction between hydrophobic portions
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[48]. However, negative β reflects electrostatic repulsion between hydrophilic portions which is common for ionic substances [49]. Accordingly, Eqs. (3) and (4) were employed to fit the experimental dada in this study. As the simplest convention, the factor 2 stands for the
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dissociation of each ionic surface active substance into its cation and anion. It is while interface is essentially electroneutral in the Gibbs dividing interface definition [50].
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The computations were carried out by using IsoFit software [51], described with more details elsewhere [49]. Parameters of model were adjusted by the software to achieve the best fit
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between the experimental and theoretical values and to provide a minimum for objective function (OF) [49]: m
OF i1
ΔCi Πi Cex,i Πm Π1
(5)
where ΔC i and Π i Π i 1 Π i 1 2 are difference between the experimental and calculated each IL concentration and the interfacial pressure range corresponding to the ith point, respectively. The obtained objective function values, within 0.04500.0670 for [C7mim][Cl] and 0.04460.0587 for [C8mim][Cl], indicate an excellent consistent fitting with the model. In Fig. 2, solid and dashed lines are theoretical curves, obtained based on Frumkin model. 8
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Saturated interface excess, Γ m , is regarded as the maximum interfacial concentration and indicates effectiveness of adsorption [25]. The maximum interface excess is calculated from value of partial molar area ( ω 1 Γ m ). In Table 1, for comparison, the obtained maximum interface excess at different temperatures
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for [C6mim][Cl] [34] and the used ILs in this study are presented. It is sensible to attain higher Γ m by [C8mim][Cl] than by [C7mim][Cl] and [C6mim][Cl] because of longer alkyl chain and
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higher hydrophobicity which lead to more adsorption effectiveness. In other words, IL molecules with extra carbon atom in tail are more compact at interface to prevent hydration of hydrophobic
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portion [33]. What’s more, for each IL, there is an utmost in both presented curves in Fig. 4. Dehydration of head group and then disrupting structured water surrounding the hydrophobic portion with increasing temperature are responsible for the specific given variations [38,52].
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Table 1 Fig. 4
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By obtaining interaction parameter, β , the relation between β and Γ m is revealed. As can be observed from Fig. 5, there is an absolute maximum value in repulsive interaction for both ILs,
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which is consistent with the observed utmost value of Γ m in Fig. 4. Indeed, the highest repulsive interaction at interface is resulted from locating compacted cationic head groups next to each other. It is clear that the highest difference between Γ m values and also between β ones are
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observed at 293.2 K for [C7mim][Cl] and [C8mim][Cl]. At other temperatures, there is no significant difference between β values.
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It can be stated that with increasing temperature and internal agitation, more electrostatic interaction between cationic head groups is relevant; however, when surrounding water molecules around hydrophobic portion is disrupted, less IL transfers into interface causes less repulsion [49].
Fig. 5
The standard free energy of adsorption, Gads , can be obtained by [49]:
b ρ ΔGads 2RTln F 2
(6)
where ρ ( ρw 18 ) is molar concentration of water. In Fig. 6, the variations of adsorption constant, bF , and corresponding absolute standard free energy with temperature for both ILs 9
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show that adsorption tendency increases with temperature. It means that agitation resulting from increasing temperature affects tendency of [C7mim][Cl] and [C8mim][Cl] molecules toward the interface. Besides, from Table 1, it is obvious that [C8mim][Cl] has higher adsorption tendency than others.
4. Conclusions Amphiphilic
synthesized
imidazolioum-based
ILs,
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Fig. 6
[C7mim][Cl]
and
[C8mim][Cl],
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significantly reduce the interfacial tension of toluene-water system at different temperatures. From this perspective, the percentages of the IFT reduction are 76 and 80%, respectively, by
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introducing individual ILs within applied temperatures. Studied interfacial properties of the ILs are different because of changes in hydrophobicity feature relevant to alkyl chain length. The experimental data can satisfactorily fit with the Frumkin adsorption isotherm, which takes
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into account the repulsive interaction of cationic substances. Accordingly, adsorption tendency and effectiveness parameters exhibit a high dependency to temperature and therefore the
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equilibrium constant increases with this parameter incessantly; however, there is an utmost point
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for saturated interface excess with temperature variation. Acknowledgments
The authors wish to acknowledge Iran National Science Foundation (INSF) for financial support of this work via grant 90007855. Nomenclature a = radius (Å) A and B = numerical constants
bF = adsorption equilibrium constant (dm3·mol1) C = concentration (mol·dm3) 10
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C = activity (mol·dm3)
f = average activity coefficient I = ionic strength (mol·dm3) OF = objective function
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r = the capillary radius (mm) R = gas law constant (J·mol1·K1)
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T = temperature (K) V = drop volume (mm3)
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z = ion charge Greek Symbols 1
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γ = interfacial tension (mN·m )
ρ = density (kg·m3) Δ = difference 3
V = Harkins and Brown constant
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r
Π = interfacial pressure (mN·m1) 2
1
θ = interface layer coverage
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ω = partial molar area (m ·mol )
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Γ = interface excess concentration (mol·m2) β = intermolecular interaction parameters
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ε r = relative permittivity of water
Gads = standard free energy of adsorption (kJ·mol1)
ρ = molar concentration of water (mol·dm3) 2
Γ m = maximum interface excess (mol·m )
Subscripts
ads = adsorption F = Frumkin m = maximum
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cr
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[Cl])) on the water/oil interfacial tension as a novel surfactant, Colloids Surf. A 421 (2013) 63-71. [17] A.R. Kelishami, H. Bahmanyar, L. Nazari, M.A. Moosavian, Development of an effective diffusivity model for regular packed liquid extraction columns, Aust. J. Basic & Appl. Sci. 3 (2009) 407-417. [18] A. Rahbar, Z. Azizi, H. Bahmanyar, M.A. Moosavian, Prediction of enhancement factor for mass transfer coefficient in regular packed liquid-liquid extraction columns, Can. J.Chem. Eng. 89 (2011) 508-519. [19] T. Misek, Standard Test Systems for Liquid Extraction, Instn. Chem. Engs. for European Federation of Chemical Engineers, Warwickshire, 1985. [20] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Room temperature ionic liquids as novel media for ‘clean’ liquid-liquid extraction, Chem. Commun. (1998) 1765-1766. [21] D.A. Skoog, D.M. West, F.J. Holler, Fundamentals of Analytical Chemistry, Saunders College Publishing, Fort Worth, 1992. [22] 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 370 (2010) 113-119. [23] B.B. Lee, P. Ravindra, E.S. Chan, New drop weight analysis for surface tension determination of liquids, Colloids Surf. A 332 (2009) 112-120. [24] 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. [25] J. Saien, S. Asadabadi, Synergistic adsorption of triton X-100 and CTAB surfactants at the toluene + water interface, Fluid Phase Equilib. 307 (2011) 16-23. [26] 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. [27] J. Drelich, C. Fang, C.L. White, Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2003. [28] B.B. Lee, P. Ravindra, E.S. Chan, A critical review: Surface and interfacial tension measurement by the drop weight method, Chem. Eng. Comm. 195 (2008) 889-924. [29] A. Bahramian, A. Danesh, Prediction of liquid-liquid interfacial tension in multi-component systems, Fluid Phase Equilib. 221 (2004) 197-205. [30] F. Jufu, L. Buqiang, W. Zihao, Estimation of fluid-fluid interfacial tensions of multicomponent mixtures, Chem. Eng. Sci. 41 (1986) 2673-2679. [31] 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. [32] U. Teipel, N. Aksel, Adsorption behavior of nonionic surfactants studied by drop volume technique, Chem. Eng. Technol. 24 (2001) 393-400. [33] Y. Zhao, X. Yue, X. Wang, D. Huang, X. Chen, Micelle formation by N-alkyl-Nmethylpiperidinium bromide ionic liquids in aqueous solution, Colloids Surf. A 412 (2012) 90-95.
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Ac ce
pt
ed
M
an
us
cr
ip t
[34] 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. [35] 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. [36] M. Aratono, K. Shimamoto, A. Onohara, D. Murakami, H. Tanida, I. Watanabe, T. Ozeki, H. Matsubara, T. Takiue, Adsorption of 1-decyl-3-methylimidazolium bromide and solvation structure of bromide at the air/water interface, Anal. Sci. 24 (2008) 1279-1283. [37] N.V. Sastry, N.M. Vaghela, P.M. Macwan, S.S. Soni, V.K. Aswal, A. Gibaud, Aggregation behavior of pyridinium based ionic liquids in water - Surface tension, 1H NMR chemical shifts, SANS and SAXS measurements, J. Colloid Interface Sci. 371 (2012) 52-61. [38] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New Jersey, 2004. [39] X.W. Li, Y.A. Gao, J. Liu, L.Q. Zheng, B. Chen, L.Z. Wu, C.H. Tung, Aggregation behavior of a chiral long-chain ionic liquid in aqueous solution, J. Colloid Interface Sci. 343 (2010) 94-101. [40] M.R. Wright, An Introduction to Aqueous Electrolyte Solutions, Wiley, England, 2007. [41] P.A. Kralchevsky, K.D. Danov, G. Broze, A. Mehreteab, Thermodynamics of ionic surfactant adsorption with account for the counterion binding: Effect of salts of various valency, Langmuir 15 (1999) 2351-2365. [42] 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. [43] 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. [44] S.I. Karakashev, A.V. Nguyen, J.D. Miller, Equilibrium adsorption of surfactants at the gasliquid interface, Adv. Polym. Sci. 218 (2008) 25-55. [45] K.S. Birdi, Handbook of Surface and Colloid Chemistry, CRC Press, Boca Raton, 2009. [46] V.B. Fainerman, S.V. Lylyk, E.V. Aksenenko, A.V. Makievski, J.T. Petkov, J. Yorke, R. Miller, Adsorption layer characteristics of Triton surfactants: 1. Surface tension and adsorption isotherms, Colloids Surf. A 334 (2009) 1-7. [47] C. Stubenrauch, V.B. Fainerman, E.V. Aksenenko, R. Miller, Adsorption behavior and dilational rheology of the cationic alkyl trimethylammonium bromides at the water/air interface, J. Phys. Chem. B 109 (2005) 1505-1509. [48] E.D. Manev, S.V. Sazdanova, R. Tsekov, S.I. Karakashev, A.V. Nguyen, Adsorption of ionic surfactants, Colloids Surf. A 319 (2008) 29-33. [49] V.B. Fainerman, D. Mobius, R. Miller, Surfactants: Chemistry, Interfacial Properties, Applications, Elsevier, Amsterdam, 2001. [50] V.B. Fainerman, E.H. Lucassen-Reynders, Adsorption of single and mixed ionic surfactants at fluid interfaces, Adv. Colloid Interface Sci. 96 (2002) 295-323. [51] IsoFit(Old), http://www.thomascat.info/Scientific/AdSo/AdSo.htm. [52] A. Firooz, P. Chen, Surface tension and adsorption kinetics of amphiphiles in aqueous solutions: The role of carbon chain length and temperature, J. Colloid Interface Sci. 370 (2012) 183-191.
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List of Captions
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Table 1 Maximum interface excess, Γ m , and adsorption equilibrium constant, bF , for adsorption of ILs at different temperatures; data related to [C6mim][Cl] are given from reference [34].
cr
Fig. 1. IFT variation as a function of drop formation time for [C7mim][Cl] and [C8mim][Cl] concentration of 8.00×102 mol·dm3 at 298.2 K.
an
us
Fig. 2. IFT variation as a function of concentration of [C7mim][Cl] and [C8mim][Cl] at different temperatures; solid and dashed lines correspond to theoretical curves obtained by Frumkin model. Inserted figure shows the IFT variation for different ILs at 293.2 K. Data related to [C6mim][Cl] are given from reference [34]. Fig. 3. CMC variation as a function of temperature for [C7mim][Cl] and [C 8mim][Cl] (CMC is expressed in activity).
M
Fig. 4. Maximum interface excess as a function of temperature for [C7mim][Cl] and [C8mim][Cl]. Fig. 5. Interaction parameter as a function of temperature for [C 7mim][Cl] and [C8mim][Cl].
Ac ce
pt
ed
Fig 6. Adsorption equilibrium constant and absolute standard free energy of adsorption as a function of temperature for [C7mim][Cl] and [C8mim][Cl].
15
Page 15 of 20
Table 1
238 320 1320
Ac ce
pt
ed
M
an
293.2 303.2 313.2
189 282 1090
ip t
293.2 303.2 313.2
98 173 861
cr
293.2 303.2 313.2
bF (dm3·mol1)
us
Γ m 10 6 (mol·m2) [C6mim][Cl] 1.29 2.21 1.33 [C7mim][Cl] 1.88 2.44 1.54 [C8mim][Cl] 2.44 2.83 1.67
T (K)
16
Page 16 of 20
27.0 [C7 mim][Cl] [C mim][Cl] 8
25.0
ip t
(mN·m 1)
23.0
γ
21.0
17.0
15.0 50
100
150
200
an
0
us
cr
19.0
250
300
M
t (s)
Ac ce
pt
ed
Fig. 1
17
Page 17 of 20
Fig. 2
0.22
[C 7 mim][Cl] [C 8 mim][Cl]
ip t
0.2
cr
0.16
us
CMC (mol·dm3)
0.18
0.14
0.1 288.0
293.0
298.0
an
0.12
303.0
308.0
313.0
318.0
M
T (K)
ed
Fig. 3
3.10
2.70
7
106 * Γm (mol·m2)
Ac ce
2.50
8
[C mim][Cl]
pt
2.90
[C mim][Cl]
2.30
2.10
1.90
1.70
1.50 288.0
293.0
298.0
303.0
308.0
313.0
318.0
T (K)
Fig. 4
18
Page 18 of 20
-1.00
[C 7 mim][Cl] [C 8 mim][Cl]
ip t
-1.40
cr
β
-1.80
us
-2.20
-3.00 288.0
293.0
298.0
an
-2.60
303.0
308.0
313.0
318.0
M
T (K)
Fig. 5
1360
60.00
ed
[C8 mim][Cl] [C7 mim][Cl]
50.00
Ac ce
45.00
760
40.00
35.00
-Δ G°ads (kJ·mol1)
bF (dm3·mol1)
960
55.00
pt
1160
560
30.00
360
160 288.0
293.0
25.00
298.0
303.0
308.0
313.0
20.00 318.0
T (K)
Fig. 6
19
Page 19 of 20
Graphical abstract
293.2 K, [C7 mim][Cl] 303.2 313.2 293.2 K, [C8 mim][Cl] 303.2 313.2
ip t
36.0
21.0
Organic
N
an
16.0
[Cnmim][Cl] n=7 or 8 Cl
6.0 1.00E-04
1.00E-03
M
11.0 N
us
Specific IFT and CMC variations are relevant due to surfactant behavior of imidazolium ionic liquids.
Aqueous
1.00E-02
1.00E-01
1.00E+00
pt
ed
C IL (mol·dm3)
Ac ce
γ (mN·m1)
26.0
cr
31.0
20
Page 20 of 20
S0927-7757(13)00328-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.04.043 COLSUA 18373
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
8-3-2013 18-4-2013 22-4-2013
Please cite this article as: J. Saien, S. Asadabadi, Temperature effect on adsorption of imidazolium-based ionic liquids at liquid-liquid interface, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.04.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature effect on adsorption of imidazolium-based ionic liquids at liquid-liquid interface Javad Saien, Simin Asadabadi
cr
Highlights
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Department of Applied Chemistry, Bu–Ali Sina University, Hamedan 65174, Iran
The synthesized imidazolium-based ILs act as cationic surface active agent at the interface.
us
Surface active properties of the used ILs enhance with their alkyl chain length. The CMC of each IL shows a minimum value at around 303 K.
an
The experimental data fit quite well with the Frumkin adsorption isotherm.
M
Adsorption tendency, effectiveness and interaction exhibit specific variations with temperature.
ed
Abstract
Interfacial tension variation and micelle formation by two amphiphilic synthesized ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride ([Cnmim][Cl], n=7 and 8), are
pt
investigated at conventional temperatures, within 293.2–313.2 K. These ILs significantly reduce the interfacial tension of toluene-water system and the influence is more effective
Ac ce
for the IL with n=8 because of the relevant more hydrophobicity character. The relevant critical micelle concentrations (CMCs) depend on temperature with a minimum appeared around 303 K, for both the ILs. The Frumkin adsorption isotherm predicts quite well the interfacial adsorption of the ILs for concentrations less than CMC. Accordingly, the adsorption tendency reveals increasing with temperature. However, the saturated interface excess and absolute electrostatic repulsion get their maximum values at around the same temperature of 303 K.
Corresponding author. Tel. /fax: +98 811 8257407. E-mail address: [email protected] (J. Saien).
1
Page 1 of 20
Keywords: Ionic liquids, Interfacial tension, Adsorption, Frumkin isotherm, CMC
1. Introduction
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Ionic liquids (ILs) are salts exclusively comprising of ions with low melting points [1]. Recently, there has been an increasing interest in ILs, owing to their distinctive physico-chemical
cr
properties such as very low vapour pressure, non-flammability, high thermal stability and ion conductivity [2,3]. Different ILs have been designed by altering their cation and anion structure
us
to improve their applications in catalysing reactions [4], corrosion [5], gas-liquid absorption [6], membrane [7], liquid-liquid extraction [8] and electrochemistry [9].
A majority of recent literatures are concerned on the properties of a kind of ILs, named
an
“imidazolium-based ILs”, with a bulky asymmetric organic cation consisting of nitrogen. These are environmentally friendly fluids and behave as green solvents [10]. For instance, Docherty et
M
al. [11], have shown good mineralization of these ILs in sludge samples. ILs also exhibit excellent surface/interface activity and micelle formation, because they possess amphiphilic structure with two hydrophilic and hydrophobic portions. A new opportunity
ed
is sought by researchers to protect environment by introducing ILs as “novel green surface active agents” [12]. Therefore, their surface properties and surface active behaviour, either pure or in
pt
different solvents, have attracted much interest [13,14]. Klomfar et al. [13], for instance, have investigated the surface tension of four imidazolium-based ILs with different alkyl chain length
Ac ce
at different temperatures. They have introduced an empirical surface tension-temperature equation describing the temperature dependency of each IL. In another research, Dong et al. [2] have shown that surface activity of imidazolium-based ILs in aqueous solutions is somewhat greater than that of conventional ionic surfactants with the same carbon number in their hydrophobic chain. Besides, Merrigan et al. [15] have shown that ILs can be used to stabilize dispersions.
Despite the current widespread attention to ILs for frequent applications, they have not yet been much subjected to oil-water systems to investigate the relevant interface properties [16]. It is while; an attractive perspective in ILs capacity is predictable in improving extraction performance (mostly due to altering the system hydrodynamic).
2
Page 2 of 20
Accordingly, the present study is conducted, for the first time, to study the influence of different dosages of two amphiphilic synthesized imidazolium-based ILs, 1-heptyl-3methylimidazolium chloride, briefly [C7mim][Cl], and 1-octyl-3-methylimidazolium chloride, [C8mim][Cl], on the interfacial tension (IFT) of toluene-water system. This chemical system is
ip t
widely used in studies of the liquid-liquid extraction process [17,18]. Accordingly, micelle concentration and related parameters of interface activity at different temperatures are obtained. For the recent parameters, the Frumkin adsorption isotherm is employed to correlate the
cr
experimental data in relation to ILs bulk concentrations, less than critical micelle concentration
us
(CMC), at each temperature. 2. Experimental
an
2.1. Materials
The toluene-water system was chosen as the recommended chemical system with high IFT [19]. Toluene, 1-methylimidazole, 1-chloroheptane, 1-chlorooctane and ethyl acetate were
M
purchased from Merck and Riedel-de Haen with purities more than 99.9, 99, 98, 96 and 99.5%, respectively, and used with no further purification. Fresh deionized water with electrical
ed
conductivity of 0.07 μS·cm1 was used for aqueous solutions throughout. 2.2. Synthesis and characterization of the ILs
pt
Each IL, 1-heptyl-3-methylimidazolium chloride and 1-octyl-3-methylimidazolium chloride, was synthesized via a direct reaction between equal molar amounts of 1-methylimidazole and
Ac ce
appropriate 1-chloroalkane in a round-bottomed flask equipped with a reflux condenser and magnetic stirrer at 70 °C for 72 hours. The reaction was under reflux and nitrogen atmosphere. The obtained viscous ILs were allowed to cool to room temperature and then washed twelve times with ethyl acetate to remove any unreacted reagent. After the last washing, the remaining ethyl acetate was removed by heating to 77 °C [12,20]. Water content was determined by using a Karl Fischer Coulometer (Metrohm, model 831). Maximum mass fractions of water content were 0.0006 and 0.0003 for [C7mim][Cl] and [C8mim][Cl], respectively. 1
H NMR, 13C NMR and mass spectroscopy were utilized to characterize the synthesized ILs.
In addition, chloride titration was conducted and purity more than 99% was obtained for each of 3
Page 3 of 20
the ILs [21]. The spectra indicate good agreement between products and theoretical structures. The high purity of each IL was confirmed by indicating just the corresponding IL peaks and no ones for reactants and/or by-products. The details of spectra for [C7mim][Cl] are: 1H NMR (400 MHz, CDCl3): δ=0.846 (3H, t), 1.254 (8H, m), 1.905 (2H, m), 4.129 (3H, s),
ip t
4.293 (2H, t), 7.385 (H, d), 7.562 (H, d), 10.652 (H, s). 13C NMR (400 MHz, CDCl3): δ=13.63, 22.05, 25.74, 28.21, 29.92, 31.08, 36.11, 49.54, 121.83, 123.61, 136.95.
cr
Mass Spectroscopy: Molecular weight of [C7mim][Cl] was obtained equal to 216 g·mol1 in and for [C8mim][Cl]:
us
agreement with exact molecular structure weight of 216.75 g·mol1.
1H NMR (400 MHz, CDCl3): δ=0.832 (3H, t), 1.239 (10H, m), 1.892 (2H, m), 4.114 (3H, s),
an
4.280 (2H, t), 7.396 (H, d), 7.579 (H, d), 10.601 (H, s).
13C NMR (400 MHz, CDCl3): δ=13.59, 22.04, 25.72, 28.45, 29.85, 31.14, 36.03, 49.44,
M
121.78, 123.56, 136.81.
Mass Spectroscopy: Molecular weight of [C8mim][Cl] was obtained equal to 230 g·mol1 in agreement with exact molecular structure weight of 230.78 g·mol1.
2.3. IFT measurements
ed
All the spectra are presented in supporting information.
pt
The drop volume method has been used by other investigators [22,23] and was applied to measure the IFT of each sample. The detailed procedure has been explained in our previous
Ac ce
studies [24,25]. Briefly, for contacting with toluene and measuring the IFT, individual IL solutions were prepared in mass. An Ohaus (Adventurer Pro, AV 264) balance with an uncertainty of ± 0.1 mg, 5 and 10 ( 0.01 and 0.02) cm3 volumetric pipettes and 200 and 250 ( 0.2) cm3 volumetric flasks were used for preparation of required solutions. Concentrations ranging from 1.00×104 to 6.00×101 mol·dm3 with maximum absolute standard deviation of 0.01×101 mol·dm3 were prepared for each IL. The IFT measurements were carried out at five different temperatures for each sample. By using a calibrated thermostat (OPTIMA 740, Japan) with an uncertainty of ± 0.1 K, the continuous organic media and conducting tube were temperature adjusted. To ensure obtaining saturated condition, equal volumes (100.0 cm3) of both aqueous solution and toluene were 4
Page 4 of 20
chosen, mixed for at least one hour and then left for an another hour to rest. The mutual solubility of toluene and IL solutions was very low and the formation of emulsion of either phase in another was not observed. To measure the density, an oscillating U-tube densimeter (Anton Paar DMA 4500, Austria) was used with uncertainty of ± 0.01 kg·m3.
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In drop volume method, the IFT ( γ ) relationship with drop volume (V), falling off a capillary into the organic phase, is given by Harkins and Brown [26]:
V Δ ρg r 3 r V
cr
γ
(1)
where ρ is the density difference between the aqueous and organic liquids ( ρw and ρo ), r is the 3
us
capillary radius and r
V is a constant which can be obtained from the table of Harkins and
The range of r
3
an
Brown or empirical relations [27,28].
V (dimensionless) values in this work was within 0.3416–0.5903 for
[C7mim][Cl] and 0.3422–0.6412 for [C8mim][Cl]. The maximum uncertainty in the IFT
M
measurements was determined ± 0.1 mN·m1.
The IFT of pure toluene-water system (binary saturated, without IL) was measured and
ed
compared with the values reported in the literatures at 298.2 K to examine the reliability and performance of the method. The obtained value of 36.3 mN·m1 was pretty close to the reported
pt
values of 36.1 and 36.6 mN·m1 [29,30].
Ac ce
3. Results and discussion 3.1. Experimental results
For investigating the IFT variations, a concentration range of 06.00×101 mol·dm3, from very low to beyond the CMC, was utilized for each synthesized IL. Temperature dependency of the IFT was examined with five temperatures ranging from 293.2 to 313.2 K for each solution. Density of individual IL solutions was changed with increasing concentration and temperature. In this regard, the aqueous and organic phase densities varied within 992.20–1001.23 kg·m3 and 848.13–866.86 kg·m3, respectively. The IFT values were within the range of 34.7–37.2 mN·m1 for the pure system (without any IL), within 8.9–36.6 mN·m1 in the presence of [C7mim][Cl] and within 7.1–36.4 mN·m1 with [C8mim][Cl]. Also generated drop volumes were within 55.4– 5
Page 5 of 20
285.7 mm3 in the presence of [C7mim][Cl] and 43.2–284.2 mm3 with [C8mim][Cl], corresponding to drop formation times of 95.42–492.36 s and 74.44–489.71 s, respectively. The IFT data and phase densities at different temperatures as well as IL concentrations are given in supporting information.
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To guarantee the attainment of equilibrium condition, the IFT variation with drop formation time (obtained with different aqueous phase flow rates, from 0.00049 to 0.01370 cm3·s1) was measured. The results are presented in Fig. 1 for [C7mim][Cl] and [C8mim][Cl] solutions with
cr
typical concentration of 8.00×102 mol·dm3 and temperature of 298.2 K. The trend in Fig. 1 can
us
be explained concerning the fact that the used ILs may not have sufficient time to adsorb at the interface when the drops form rapidly. Therefore, the dynamic IFT is relevant for short times of drop formation. As the time increases, the ILs have further chance to accumulate at the interface.
an
Finally, when the time of drop formation becomes very long, depending on the adsorption kinetic of IL, the interface expansion occurs slowly enough to find equilibrium adsorption and no
M
significant change is relevant with further drop formation times. This phenomenon in drop volume method has already been reported in literatures [31,32]. In Fig. 1, drop formation times [C8mim][Cl], respectively.
ed
of 237.24 and 189.40 s corresponds to the measured values reported here for [C7mim][Cl] and
Fig. 1
pt
Three important results are accordingly deduced from IFT variations, typically presented in Fig. 2. First, IFT decreases with introducing more ILs into the system. In other words, more
Ac ce
concentration, more migration of ILs toward the interface and more reduction in the IFT [33]. In comparison with the effect of [C6mim][Cl] on the toluene-water IFT which has been recently published [34], it can be concluded that more methylene bridge (–CH2–) in [C7mim][Cl] and [C8mim][Cl] structures causes more IFT reduction, in order, obviously due to more hydrophobicity in ILs tail (hydrophobic portion) and subsequently higher adsorption [35]. Second, temperature causes internal thermal agitation, attenuation of intermolecular forces at interface and IFT reduction [22]. At last, a nearly linear reduction with temperature is relevant (three of them are typically presented) within the used range. Fig. 2
6
Page 6 of 20
In Fig. 2, each curve has a distinct break point at the appropriate CMC. Remarkably, the absence of a minimum on the IFT vs. concentration curve near each CMC is another evidence for the high purity of the synthesized ILs [36,37]. By obtaining the CMC for each IL at different temperatures, it can be concluded that
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temperature has complicate effects on the CMCs. Temperature dependency of the CMC is drawn in Fig. 3. The incidence of the minimum on the curves (at around 303 K for both ILs) can be explained by two competing factors: hydrophobic interaction and dehydration of cationic head
Fig. 3
us
the second favors micelle formation at lower concentrations [38,39].
cr
groups (hydrophilic portion). The first one causes the CMC to appear at higher levels, whereas
It is substantial to note that concentrations corresponding to the CMCs are more than 0.01
an
mol·dm3 and based on the non-ideal interaction in bulk solutions at these concentrations, it is essential to exert activity instead of concentration ( C C f ) in which C and f are activity
M
and average activity coefficient, respectively [40].
To obtain average activity coefficient of ions, f , the extended Debye-Huckel equation for
logf
A z z I 1 Ba I
ed
short range columbic interactions can be used [41]:
0.0551 I
(2)
pt
where A and B are numerical constants depending on relative permittivity (dielectric constant) of water ( ε r ) and absolute temperature. The parameters of z and a are ion charge and radius and I
Ac ce
(=C+Csalt) is ionic strength expressed in mol·dm3 where C and Csalt denote bulk concentrations of IL and univalent inorganic salt (if there is any), respectively [41]. The used relative permittivity of water and calculated constants A and B at different temperatures [40] are presented in supporting information. 3.2. Theoretical model
In most studies, Langmur [42], Szyszkowski [43] and Frumkin [44] adsorption isotherms have been employed to relate interface adsorption and bulk concentration of ionic surface active agents. Among them, Frumkin isotherm takes into account the interactions between adsorbed species at the interface in addition to the adsorption tendency and effectiveness [45,46]. The 7
Page 7 of 20
interface equation of state and the corresponding Frumkin adsorption isotherm are expressed for electroneutral interfaces by [47]:
2 RT ln 1 θ βθ 2 ω
bF C C Csalt f 12
(3)
θ exp 2 βθ 1 θ
ip t
Π
(4)
where Π is interfacial pressure ( Π γ0 γ ), γ0 is the IFT for clean chemical system; R and T
cr
are gas law constant and absolute temperature, respectively. As well, θ ( Γω) is interface layer coverage and Γ and ω are interface excess concentration and partial molar area of adsorbed
us
species; bF and β are adsorption equilibrium constant and intermolecular interaction parameter, respectively. When β is equal to zero, ideal behavior of adsorbed species at interface is
an
concluded. In this condition, the Frumkin model reduces to the Langmuir isotherm. When the value of β is positive, there is van der Waals attractive interaction between hydrophobic portions
M
[48]. However, negative β reflects electrostatic repulsion between hydrophilic portions which is common for ionic substances [49]. Accordingly, Eqs. (3) and (4) were employed to fit the experimental dada in this study. As the simplest convention, the factor 2 stands for the
ed
dissociation of each ionic surface active substance into its cation and anion. It is while interface is essentially electroneutral in the Gibbs dividing interface definition [50].
pt
The computations were carried out by using IsoFit software [51], described with more details elsewhere [49]. Parameters of model were adjusted by the software to achieve the best fit
Ac ce
between the experimental and theoretical values and to provide a minimum for objective function (OF) [49]: m
OF i1
ΔCi Πi Cex,i Πm Π1
(5)
where ΔC i and Π i Π i 1 Π i 1 2 are difference between the experimental and calculated each IL concentration and the interfacial pressure range corresponding to the ith point, respectively. The obtained objective function values, within 0.04500.0670 for [C7mim][Cl] and 0.04460.0587 for [C8mim][Cl], indicate an excellent consistent fitting with the model. In Fig. 2, solid and dashed lines are theoretical curves, obtained based on Frumkin model. 8
Page 8 of 20
Saturated interface excess, Γ m , is regarded as the maximum interfacial concentration and indicates effectiveness of adsorption [25]. The maximum interface excess is calculated from value of partial molar area ( ω 1 Γ m ). In Table 1, for comparison, the obtained maximum interface excess at different temperatures
ip t
for [C6mim][Cl] [34] and the used ILs in this study are presented. It is sensible to attain higher Γ m by [C8mim][Cl] than by [C7mim][Cl] and [C6mim][Cl] because of longer alkyl chain and
cr
higher hydrophobicity which lead to more adsorption effectiveness. In other words, IL molecules with extra carbon atom in tail are more compact at interface to prevent hydration of hydrophobic
us
portion [33]. What’s more, for each IL, there is an utmost in both presented curves in Fig. 4. Dehydration of head group and then disrupting structured water surrounding the hydrophobic portion with increasing temperature are responsible for the specific given variations [38,52].
an
Table 1 Fig. 4
M
By obtaining interaction parameter, β , the relation between β and Γ m is revealed. As can be observed from Fig. 5, there is an absolute maximum value in repulsive interaction for both ILs,
ed
which is consistent with the observed utmost value of Γ m in Fig. 4. Indeed, the highest repulsive interaction at interface is resulted from locating compacted cationic head groups next to each other. It is clear that the highest difference between Γ m values and also between β ones are
pt
observed at 293.2 K for [C7mim][Cl] and [C8mim][Cl]. At other temperatures, there is no significant difference between β values.
Ac ce
It can be stated that with increasing temperature and internal agitation, more electrostatic interaction between cationic head groups is relevant; however, when surrounding water molecules around hydrophobic portion is disrupted, less IL transfers into interface causes less repulsion [49].
Fig. 5
The standard free energy of adsorption, Gads , can be obtained by [49]:
b ρ ΔGads 2RTln F 2
(6)
where ρ ( ρw 18 ) is molar concentration of water. In Fig. 6, the variations of adsorption constant, bF , and corresponding absolute standard free energy with temperature for both ILs 9
Page 9 of 20
show that adsorption tendency increases with temperature. It means that agitation resulting from increasing temperature affects tendency of [C7mim][Cl] and [C8mim][Cl] molecules toward the interface. Besides, from Table 1, it is obvious that [C8mim][Cl] has higher adsorption tendency than others.
4. Conclusions Amphiphilic
synthesized
imidazolioum-based
ILs,
us
cr
ip t
Fig. 6
[C7mim][Cl]
and
[C8mim][Cl],
an
significantly reduce the interfacial tension of toluene-water system at different temperatures. From this perspective, the percentages of the IFT reduction are 76 and 80%, respectively, by
M
introducing individual ILs within applied temperatures. Studied interfacial properties of the ILs are different because of changes in hydrophobicity feature relevant to alkyl chain length. The experimental data can satisfactorily fit with the Frumkin adsorption isotherm, which takes
ed
into account the repulsive interaction of cationic substances. Accordingly, adsorption tendency and effectiveness parameters exhibit a high dependency to temperature and therefore the
pt
equilibrium constant increases with this parameter incessantly; however, there is an utmost point
Ac ce
for saturated interface excess with temperature variation. Acknowledgments
The authors wish to acknowledge Iran National Science Foundation (INSF) for financial support of this work via grant 90007855. Nomenclature a = radius (Å) A and B = numerical constants
bF = adsorption equilibrium constant (dm3·mol1) C = concentration (mol·dm3) 10
Page 10 of 20
C = activity (mol·dm3)
f = average activity coefficient I = ionic strength (mol·dm3) OF = objective function
ip t
r = the capillary radius (mm) R = gas law constant (J·mol1·K1)
cr
T = temperature (K) V = drop volume (mm3)
us
z = ion charge Greek Symbols 1
an
γ = interfacial tension (mN·m )
ρ = density (kg·m3) Δ = difference 3
V = Harkins and Brown constant
M
r
Π = interfacial pressure (mN·m1) 2
1
θ = interface layer coverage
ed
ω = partial molar area (m ·mol )
pt
Γ = interface excess concentration (mol·m2) β = intermolecular interaction parameters
Ac ce
ε r = relative permittivity of water
Gads = standard free energy of adsorption (kJ·mol1)
ρ = molar concentration of water (mol·dm3) 2
Γ m = maximum interface excess (mol·m )
Subscripts
ads = adsorption F = Frumkin m = maximum
11
Page 11 of 20
References
Ac ce
pt
ed
M
an
us
cr
ip t
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[34] 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. [35] 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. [36] M. Aratono, K. Shimamoto, A. Onohara, D. Murakami, H. Tanida, I. Watanabe, T. Ozeki, H. Matsubara, T. Takiue, Adsorption of 1-decyl-3-methylimidazolium bromide and solvation structure of bromide at the air/water interface, Anal. Sci. 24 (2008) 1279-1283. [37] N.V. Sastry, N.M. Vaghela, P.M. Macwan, S.S. Soni, V.K. Aswal, A. Gibaud, Aggregation behavior of pyridinium based ionic liquids in water - Surface tension, 1H NMR chemical shifts, SANS and SAXS measurements, J. Colloid Interface Sci. 371 (2012) 52-61. [38] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New Jersey, 2004. [39] X.W. Li, Y.A. Gao, J. Liu, L.Q. Zheng, B. Chen, L.Z. Wu, C.H. Tung, Aggregation behavior of a chiral long-chain ionic liquid in aqueous solution, J. Colloid Interface Sci. 343 (2010) 94-101. [40] M.R. Wright, An Introduction to Aqueous Electrolyte Solutions, Wiley, England, 2007. [41] P.A. Kralchevsky, K.D. Danov, G. Broze, A. Mehreteab, Thermodynamics of ionic surfactant adsorption with account for the counterion binding: Effect of salts of various valency, Langmuir 15 (1999) 2351-2365. [42] 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. [43] 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. [44] S.I. Karakashev, A.V. Nguyen, J.D. Miller, Equilibrium adsorption of surfactants at the gasliquid interface, Adv. Polym. Sci. 218 (2008) 25-55. [45] K.S. Birdi, Handbook of Surface and Colloid Chemistry, CRC Press, Boca Raton, 2009. [46] V.B. Fainerman, S.V. Lylyk, E.V. Aksenenko, A.V. Makievski, J.T. Petkov, J. Yorke, R. Miller, Adsorption layer characteristics of Triton surfactants: 1. Surface tension and adsorption isotherms, Colloids Surf. A 334 (2009) 1-7. [47] C. Stubenrauch, V.B. Fainerman, E.V. Aksenenko, R. Miller, Adsorption behavior and dilational rheology of the cationic alkyl trimethylammonium bromides at the water/air interface, J. Phys. Chem. B 109 (2005) 1505-1509. [48] E.D. Manev, S.V. Sazdanova, R. Tsekov, S.I. Karakashev, A.V. Nguyen, Adsorption of ionic surfactants, Colloids Surf. A 319 (2008) 29-33. [49] V.B. Fainerman, D. Mobius, R. Miller, Surfactants: Chemistry, Interfacial Properties, Applications, Elsevier, Amsterdam, 2001. [50] V.B. Fainerman, E.H. Lucassen-Reynders, Adsorption of single and mixed ionic surfactants at fluid interfaces, Adv. Colloid Interface Sci. 96 (2002) 295-323. [51] IsoFit(Old), http://www.thomascat.info/Scientific/AdSo/AdSo.htm. [52] A. Firooz, P. Chen, Surface tension and adsorption kinetics of amphiphiles in aqueous solutions: The role of carbon chain length and temperature, J. Colloid Interface Sci. 370 (2012) 183-191.
14
Page 14 of 20
List of Captions
ip t
Table 1 Maximum interface excess, Γ m , and adsorption equilibrium constant, bF , for adsorption of ILs at different temperatures; data related to [C6mim][Cl] are given from reference [34].
cr
Fig. 1. IFT variation as a function of drop formation time for [C7mim][Cl] and [C8mim][Cl] concentration of 8.00×102 mol·dm3 at 298.2 K.
an
us
Fig. 2. IFT variation as a function of concentration of [C7mim][Cl] and [C8mim][Cl] at different temperatures; solid and dashed lines correspond to theoretical curves obtained by Frumkin model. Inserted figure shows the IFT variation for different ILs at 293.2 K. Data related to [C6mim][Cl] are given from reference [34]. Fig. 3. CMC variation as a function of temperature for [C7mim][Cl] and [C 8mim][Cl] (CMC is expressed in activity).
M
Fig. 4. Maximum interface excess as a function of temperature for [C7mim][Cl] and [C8mim][Cl]. Fig. 5. Interaction parameter as a function of temperature for [C 7mim][Cl] and [C8mim][Cl].
Ac ce
pt
ed
Fig 6. Adsorption equilibrium constant and absolute standard free energy of adsorption as a function of temperature for [C7mim][Cl] and [C8mim][Cl].
15
Page 15 of 20
Table 1
238 320 1320
Ac ce
pt
ed
M
an
293.2 303.2 313.2
189 282 1090
ip t
293.2 303.2 313.2
98 173 861
cr
293.2 303.2 313.2
bF (dm3·mol1)
us
Γ m 10 6 (mol·m2) [C6mim][Cl] 1.29 2.21 1.33 [C7mim][Cl] 1.88 2.44 1.54 [C8mim][Cl] 2.44 2.83 1.67
T (K)
16
Page 16 of 20
27.0 [C7 mim][Cl] [C mim][Cl] 8
25.0
ip t
(mN·m 1)
23.0
γ
21.0
17.0
15.0 50
100
150
200
an
0
us
cr
19.0
250
300
M
t (s)
Ac ce
pt
ed
Fig. 1
17
Page 17 of 20
Fig. 2
0.22
[C 7 mim][Cl] [C 8 mim][Cl]
ip t
0.2
cr
0.16
us
CMC (mol·dm3)
0.18
0.14
0.1 288.0
293.0
298.0
an
0.12
303.0
308.0
313.0
318.0
M
T (K)
ed
Fig. 3
3.10
2.70
7
106 * Γm (mol·m2)
Ac ce
2.50
8
[C mim][Cl]
pt
2.90
[C mim][Cl]
2.30
2.10
1.90
1.70
1.50 288.0
293.0
298.0
303.0
308.0
313.0
318.0
T (K)
Fig. 4
18
Page 18 of 20
-1.00
[C 7 mim][Cl] [C 8 mim][Cl]
ip t
-1.40
cr
β
-1.80
us
-2.20
-3.00 288.0
293.0
298.0
an
-2.60
303.0
308.0
313.0
318.0
M
T (K)
Fig. 5
1360
60.00
ed
[C8 mim][Cl] [C7 mim][Cl]
50.00
Ac ce
45.00
760
40.00
35.00
-Δ G°ads (kJ·mol1)
bF (dm3·mol1)
960
55.00
pt
1160
560
30.00
360
160 288.0
293.0
25.00
298.0
303.0
308.0
313.0
20.00 318.0
T (K)
Fig. 6
19
Page 19 of 20
Graphical abstract
293.2 K, [C7 mim][Cl] 303.2 313.2 293.2 K, [C8 mim][Cl] 303.2 313.2
ip t
36.0
21.0
Organic
N
an
16.0
[Cnmim][Cl] n=7 or 8 Cl
6.0 1.00E-04
1.00E-03
M
11.0 N
us
Specific IFT and CMC variations are relevant due to surfactant behavior of imidazolium ionic liquids.
Aqueous
1.00E-02
1.00E-01
1.00E+00
pt
ed
C IL (mol·dm3)
Ac ce
γ (mN·m1)
26.0
cr
31.0
20
Page 20 of 20