Research Article pubs.acs.org/journal/ascecg
Aqueous Biphasic System Containing Long Chain AnionFunctionalized Ionic Liquids for High-Performance Extraction Yuanbang Xie, Huabin Xing,* Qiwei Yang, Zongbi Bao, Baogen Su, and Qilong Ren* Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Xihu District, Hangzhou 310027, China S Supporting Information *
ABSTRACT: Ionic liquid-based aqueous biphasic systems (IL-based ABSs) offer a benign alternative process for conventional extraction systems with volatile organic solvents to separate biomass. Designing an IL-based ABS with excellent phase splitting ability, remarkable extraction efficiency, and good biocompatibility remains challenging. In this work, we report a series of novel ABSs using biocompatible ILs composed of long chain carboxylate anions and a cholinium cation that are all derived from biomass. This strategy introduced long alkyl chains into the anions, which not only significantly increased the hydrogen bond (HB) acceptor ability of the carboxylate anions through a remarkable electron-donating effect but also ensured good hydrophobicity for achieving better phase splitting. The developed IL-based ABS demonstrated a relatively broad biphasic area and extraordinary extraction efficiency for amino acids and bioactive compounds with distribution coefficients for tryptophan, phenylalanine, and caffeine of 58.5, 120 (120 was set as a maximum value for the partition coefficient because in some cases the concentration of the extracted material in the salt-rich phase was below the limit of detection), and 120, respectively, which were remarkably higher than those obtained in ABS with conventional ILs. This work shows that the long chain carboxylate anion is critical for the excellent extraction performance of the developed ABS and that their distribution coefficients increased with increasing anion alkyl chain lengths. In addition, liquid crystal structures were observed when the carbon number of the carboxylate anion of the ILs exceeded eight; thus, IL-based ABS with liquid crystal structures were reported for the first time. KEYWORDS: Ionic liquids, Aqueous biphasic system, Hydrogen bond, Extraction, Liquid crystal
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INTRODUCTION In recent years, ionic-liquid-based aqueous biphasic systems (IL-based ABS) have been considered to be promising processes for achieving green and efficient extraction of biomasses, such as amino acids,1−3 antibiotics,4−6 alkaloids,7,8 enzymes,9−12 and proteins.13−15 In a comparison with traditional ABSs, which contain polymers such as polyethylene, ILbased ABSs have relatively lower viscosity,16 faster phase splitting speed, better polarity tunability,17 and other physicochemical properties. Therefore, since Rogers et al.18 first reported a [C4mim]Cl-based ABSs, researchers have shown increasing interest in discovering the formation mechanisms and improving the extraction efficiencies of these novel extraction systems. Bridges et al.19 reported a series of ABSs composed of ILs with different cationic cores, such as imidazolium, pyridinium, quaternary ammonium, and phosphonium cations that were salted out by various inorganic salts. Li and co-workers20 reported equilibrium phase diagrams of imidazolium IL-based ABSs with different cation alkyl side chains, and the ABSs containing imidazolium acetate were used to enrich for tryptophan. The group of Coutiho and Freire21,22 systematically investigated the influence of different cationic © 2015 American Chemical Society
structures of ILs on phase diagram behavior, which indicated that cationic cores have an impact on phase separation due to steric hindrance and charge distribution, and the side chains of the cations displayed multifaceted effects where the imidazolium ring was selected as the model cation. Dieve et al.23 constructed ABSs with several imidazolium alkylsulfate ILs, which showed that increase of the alkyl chain length of the IL anion led to better phase separation. Ventura et al.3 tested a wide variety of IL anions and concluded that ILs with lower hydrogen bond (HB) basicity values more easily undergo phase splitting. However, in some cases, decreasing the HB basicity of the IL resulted in a relatively low extraction capacity of the ABS due to a weak HB interaction between the IL and solutes.24 As research continues, more biodegradable and environmentally friendly systems are universally preferred.25 Using cholinium ILs for IL-based ABS is a beneficial approach that can be utilized to reach the new demand. Shahriari et al.6 employed a series of cholinium ILs with different anions to extract Received: September 13, 2015 Revised: October 25, 2015 Published: October 26, 2015 3365
DOI: 10.1021/acssuschemeng.5b01068 ACS Sustainable Chem. Eng. 2015, 3, 3365−3372
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to meet market demand or from human urine for drug detection. Therefore, with promising applications and distinct chemical structures, phenylalanine, tryptophan, and caffeine were selected as model molecules to evaluate the extraction performance of the developed ABSs.34−36 Due to the unique physicochemical properties of the ILs, extraordinary extraction efficiencies were achieved. It is noteworthy that remarkable extraction efficiencies were obtained in the ABS with [Ch][C7H15COO] (KPhe = 58.5, KTrp ≥ 120, and KCaf ≥ 120), which were remarkably higher than those in the ABS with conventional ILs and those in hydrophobic−water systems. Furthermore, the physicochemical properties and microstructures of the developed ABSs were also investigated using solvatochromic experiments and polarized optical microscopy (POM). Liquid crystal (LC) structures were observed in ILbased ABSs for the first time.
antibiotics, and gratifying results were achieved. The Coutiho and Freire group26,27 introduced polyethylene glycol (PEG) as a salting-out agent instead of inorganic salts to make IL-based ABS greener and more environmentally friendly. However, it is still challenging to construct an IL-based ABS with fine phase splitting ability, excellent extraction efficiency, and low toxicity simultaneously. Some attempts20,28,29 have been made to improve the extraction efficiency and phase splitting ability by increasing the side chain length of the imidazolium cation. Some results showed that overlong cation side chain lengths led to unfavorable extraction results30 and could even significantly increase toxicity.31 Therefore, the development of a green and efficient IL-based ABS is particularly attractive for researchers. In addition, Yang et al.32 prepared a class of phosphonium-based long chain carboxylate ILs and demonstrate the key role of strong HB basicity on extraction. Our approach here is to synthesize ILs by combining cholinium cation with long chain fatty acid anions, which are all derived from biomass, to create ILs that are greener than common ILs.33 Meanwhile, through the significant electrondonating effect, a proper fatty acid chain length can offer potent HB basicity for carboxylate anions to enhance the HB interactions between ILs and solutes.24,32 Additionally, the alkyl chains of the anions are able to provide hydrophobicity in some degree for the ILs to achieve better phase splitting. Besides, previous studies of the structure−property relationship of IL-based ABSs have not paid sufficient attention to influences of anion structures, and the insights about the microstructures and properties of anions have rarely been reported, which limited the design of novel and efficient ABSs and their applications to separation processes. Therefore, four cholinium ILs with increasing anion chain lengths were synthesized and utilized to construct ABSs. Subsequently, relevant phase diagrams were determined for each system. The extraction performances of the developed ABS were evaluated using typical amino acids (phenylalanine and tryptophan) and bioactive compounds (caffeine) as solutes. Phenylalanine, tryptophan, and caffeine (structures shown in Figure 1) are considered hydrophilic small molecules with
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EXPERIMENTAL SECTION
Materials and Reagents. A tripotassium phosphate aqueous solution that was prepared by dissolving tripotassium phosphate trihydrate in purified water was used as a salting-out agent. The inorganic salt, tripotassium phosphate trihydrate (K3PO4·3H2O, ≥ 99.0%), was obtained from Sinopharm Group Co. Ltd. Purified water was purchased from Wahaha Group Co. Ltd. and was used in all experiments. Solutions of phenylalanine, tryptophan, and caffeine were prepared for extraction experiments. DL -Phenylalanine (98.5%) and Ltryptophan (99.0%) were both purchased from Aladdin Reagent Co., Ltd. Caffeine (≥98.0%) was from Zhejiang Shanshan Tea Co. Ltd. (China). 1-Butyl-3-methylimdazolium bromide ([BMIm]Br, 99.0%), 1-butyl3-methylimdazolium chloride ([BMIm]Cl, 99.0%), and 1-ethyl-3methylimdazolium trifluoromethanesulfonate ([EMIm]CF 3SO3 , 99.0%) were from Lanzhou Institute of Chemical Physics. The reagents for IL synthesis include choline (cholinium hydroxide), tetrabutylphosphonium hydroxide ([P4444][OH]), and four fatty acids. Choline (48−50% in water) and [P4444][OH] (40% in water) were purchased from Tokyo Chemical Industry Co., Ltd., and kept in sealed bottles at 8 °C. Hexanoic acid (99.0%) and octanoic acid (99.0%) were both purchased from J&K CHEMICA, while butanoic acid (≥99.5%,) and lauric acid (98.0%) were both purchased from Aladdin Reagent Co., Ltd. IL Synthesis. Cholinium butyrate ([Ch][C3H7COO]), cholinium hexanoate ([Ch][C 5 H 11 COO]), cholinium octanoate ([Ch][C7H15COO]]), cholinium laurate ([Ch][C11H23COO]), and tetrabutylphosphonium laurate ([P4444][C11H23COO]) were synthesized via a neutralization reaction.32,37,38 First, 0.020 mol/L HCl aqueous solution was prepared by diluting 1.000 mol/L HCl standard solution and was used to titrate the exact concentration of choline and [P4444][OH] aqueous solution. For example, for the preparation of [Ch][C3H7COO], a choline aqueous solution and butanoic acid were mixed stoichiometrically in a round flask and stirred for 48 h at room temperature in order to guarantee a complete reaction because the process is exothermic.39,40 Moderate heating was employed if the ambient temperature was below 283 K in order to lower the viscosity and accelerate the reaction rate. As a result, a clear light amber solution was obtained. [Ch][C5H11COO] and [Ch][C7H15COO] were synthesized in the same way. Extra water was added to the solution when [Ch][C11H23COO] was prepared because [Ch][C11H23COO] in aqueous solution at high concentrations turns into a hydrogel near ambient temperature and impedes the reaction. [P4444][C11H23COO] was prepared using the same procedure as [Ch][C11H23COO] except that [P4444][OH] was used as the cation source instead of choline. After complete reaction, a rotary evaporator was employed to remove most of the water in the product at a temperature of 333 K, and then the oil pump was used to remove residual water. After drying the sample for 48 h at 333 K under vacuum, an amber solid was obtained. The water mass fraction of ILs was measured by Karl Fischer
Figure 1. Chemical structures of the selected extraction materials: (a) phenylalanine (Phe), (b) tryptophan (Trp), (c) caffeine (Caf).
specific biological functions. Phenylalanine and tryptophan are both essential amino acids for humans and are important for food and pharmaceutical industry applications. However, their high-performance extraction is challenging because both amino acids contain a hydrophobic aromatic ring and hydrophilic functional groups as well as carboxyl and amino groups. Caffeine is a xanthine alkaloid found in various plants and mainly functions as a central nervous system stimulant in the human body with low health risks. Also, it is of significance to extract caffeine either from plants to produce relevant products 3366
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Figure 2. Chemical structures of the ILs: (a) [Ch][C3H7COO], (b) [Ch][C5H11COO], (c) [Ch][C7H15COO], (d) [Ch][C11H23COO]. Extraction Experiments. All of the extraction experiments were processed in jacketed vessels composed of glass using a thermostatic circulating water bath to control the temperature at 298.15 K which was the same as the temperature during the determination the binodal phase diagrams. [Ch][C3H7COO], [Ch][C5H11COO], and [Ch][C7H15COO] were of the same concentration (1.20 mol/kg) to further compare the extraction effects. Additionally, [Ch][C7H15COO] and [Ch][C11H23COO] were tested at another relatively low concentration (0.66 mol/kg) for extra contrast because [Ch][C11H23COO] is less soluble in water than the other three mentioned ILs. A typical phenylalanine extraction procedure using [Ch][C3H7COO] is described below. A known concentration of [Ch][C3H7COO] aqueous solution was first added to the jacketed vessel, then the K3PO4 aqueous solution with a certain concentration was added, and finally the calculated water was added if necessary. Phenylalanine was quantifiably dissolved in the K3PO4 aqueous solution. The jacketed vessel was shaken for 30 min using a thermostatic rotary shaker with a speed of 200 r/min and was then placed in the circulator bath system still for 12 h. Next, the two phases were carefully separated and weighed. Samples were taken from each of the phases and analyzed using a UV−vis spectrophotometer (Shimadzu UV-2550) after appropriate dilution. The wavelengths detected for phenylalanine, tryptophan, and caffeine were 278.6, 258.0, and 272.8 nm, respectively. The partition coefficient (K) was calculated as follows:
titration, and values were lower than 0.6%. The structures and purities of the ILs were confirmed by NMR spectroscopy (see Supporting Information). Phase Diagrams and Tie-Lines (TLs). The phase diagrams were determined using the cloud point titration method at 298.15 K and atmospheric pressure, as reported in the literature.18 Temperature was controlled by a refrigerated/heating circulator (F25-ME, Julabo, Germany) within ±0.01 K. K3PO4 and ILs were separately dissolved in water to certain concentrations. The inorganic salt was added drop by drop to the IL−aqueous solution until the mixture turned cloudy and the amount of the added salt solution was recorded. Next, pure water was dripped into the mixture until the mixture turned clear again and the amount of added water was recorded. These two steps were repeated several times to construct the entire curve with a relatively wide range of concentrations. The whole procedure above was performed under constant stirring, and the masses were measured using an analytical balance (AL204, METTLER TOLEDO, China) with an uncertainty of ±10−4 g. All of the phase diagrams of the four ILs were constructed at least three times to confirm the relative positions. The TLs were determined by a gravimetric method, and calculations were performed. A mixture of K3PO4- and IL-aqueous solution was prepared, which was in the biphasic region, according to the phase diagrams determined above. The mixture was fully stirred and then allowed to reach equilibrium by phase separation for 24 h at 298.15 K. After careful separation, both the top and the bottom phases were weighed. The TL data were obtained using the lever arm rule to determine the mass relationship between the top phase and the overall system composition. The binodal curve data gained by the experimental process mentioned above were fitted mathematically using eq 1
[IL] = A exp[(B × [salt]0.5 ) − (C × [salt]3 )]
K=
where [IL] and [salt] indicate the mass fraction percentages of the IL and salt, respectively, while A, B, and C are fitting parameters obtained by regression of the experimental binodal data. This equation was initially proposed by Merchuk et al.41 to describe the polymer system but has still worked for IL-based ABS to date. To determine the TLs, eqs 2−5 were introduced with four unknown values, [IL]IL, [salt]IL, [IL]salt, and [salt]salt, in collaboration with the experimental method aforementioned: (2)
[IL]salt = A exp[(B × [salt]salt 0.5 ) − (C × [salt]salt 3 )]
(3)
[IL]M 1−α [IL]IL = − × [IL]salt α α [salt]IL =
[salt]M 1−α − × [salt]salt α α
(6)
Here C refers to the mass fraction of the extraction material of the corresponding phase. Solvatochromic Experiments. The Kamlet−Taft parameters were measured according to the literature procedure.42,43 N,NDiethyl-4-nitroaniline (DENA) and 4-nitroaniline (NA) were employed as probes and dissolved into dichloromethane to a certain concentration. An appropriate amount of probe−dichloromethane solution was added to a small vessel, and then the sample to be detected was added after completely evaporating the dichloromethane. The probe was thoroughly dissolved in the sample prior to the scanning process using a UV−vis spectrophotometer to detect the maximum absorption wavelength. The spectrum scanning was executed at least seven times with a low scanning speed and relatively accurate scanning to guarantee that the standard deviation was within ±0.2 nm. The Kamlet−Taft dipolarity/polarizability π* and hydrogen bond basicity β were calculated using the following equations:44
(1)
3 [IL]IL = A exp[(B × [salt]0.5 IL ) − (C × [salt]IL )]
C IL Csalt
(4)
π * = 8.649 − 0.314 × νDENA
(8)
β = − 0.357 × νNA − 1.176 × π * + 11.12
(9)
νDENA and νNA are the maximum absorption wavenumbers of DENA and NA, respectively. Liquid Crystal Characterization. The LC structures in the ILbased ABS were characterized using a Motic B2 polarized optical microscope with a CCD camera (Panasonic Super Dynamic II WVCP460). The measuring temperature was controlled by a thermal
(5)
Here, the subscripts IL, salt, and M indicate the IL-rich, salt-rich, and mixed phases, respectively. The parameter α is the ratio between the mass of the IL-rich phase and the total mass of the mixture. 3367
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ACS Sustainable Chemistry & Engineering platform with circulating cooling water. Samples were dropped onto a glass slide and covered with another one for POM detection.45−47
longer alkyl chains increase the hydrophobicity that promotes formation of the ABS. However, extension of the alkyl chain also significantly promotes the HB basicity of the anion due to the electron-donating effect of the alkyl group, which enhances the interactions between the IL anions and water molecules (β [Ch][C3H7COO] = 0.393, β [Ch][C5H11COO] = 0.557, and β[Ch][C7H15COO] = 0.884), resulting in a narrower biphasic region. Thus, the eventual macroscopic phase behavior is governed by two opposite microscopic mechanisms that stem from identical changes in anion structure, and between the two mechanisms, we believe that enhancement of the hydrophobic ability is dominant. Additionally, it should be noted that [Ch][C7H15COO] is even harder to be salted-out than [Ch][C3H7COO] and [Ch][C5H11COO] when its concentration is greater than 4.0 molality, according to the phase diagram. The abnormal phenomenon may be caused by aggregation of [Ch][C7H15COO], which will be discussed in the Microstructures and Properties of the Designed ABS section. Partitioning of the Solutes. After phase splitting, ILs gathered in the top phases and K3PO4 in the bottom phases. A maximum value of 120 was set for the partition coefficient because in some cases the concentration of the extracted material in the salt-rich phase was below the limit of detection, which can be regarded as complete extraction. Hydrophobic IL−water systems were employed for comparison, and the results indicated that [BMIm][PF6] and [BMIm][Tf2N] had poor extraction efficiencies for all the three hydrophililic small molecules. Also, in a comparison with with traditional IL-based ABS, the designed cholinium alkanoate IL-based ABS presented a remarkably distinct extraction ability, as shown in Figure 4.
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RESULTS AND DISCUSSION Phase Diagrams. Four cholinium ILs with increasing lengths of fatty acid anions, as shown in Figure 2, were mainly studied in this work: [Ch][C3H7COO], [Ch][C5H11COO], [Ch][C7H15COO], and [Ch][C11H23COO]. The ternary phase diagrams of the four IL-based ABS were determined using the cloud point titration method with K3PO4 as a salting-out agent. The experimental weight fraction data for the ternary systems are detailed in the Supporting Information. The bimodal curves are presented in units of molality to avoid effects caused by the molecular weight of the ILs and thus provide more appropriate comparisons. Additionally, two reported cholinium-based ILs, cholinium chloride ([Ch]Cl) and cholinium succinate ([Ch][Suc]), were included in the illustration for auxiliary illumination. Cholinium cation is widely believed to be less biotoxic and more biodegradable than other common IL cations because it is derived from choline, a B-complex vitamin. However, the relatively short alkyl chains and hydroxyl functional group make cholinium cation more hydrophilic, and thus cholinium-based ILs have lower phase splitting abilities than most other hydrophilic ILs. As shown in Figure 3, when equipped with
Figure 3. Ternary phase diagrams for ABS composed of IL + H2O + K3PO4 at 298.15K: [Ch][C3H7COO] ■, [Ch][C5H11COO] ●, [Ch][C7H15COO] ▲, [Ch][C11H23COO] ▼, [Ch]Cl ⧫,6 and [Ch][Suc] ▶.6
Figure 4. Partition coefficients of phenylalanine (Phe), tryptophan (Trp), and caffeine (Caf) between IL- and salt-rich phases at 298.15 K: (A) [BMIm][PF6], (B) [BMIm][Tf2N], (C) [BMIm]Br, (D) [BMIm]Cl, (E) [EMIm]CF3SO3, (F) [Ch][C3H7COO], (G) [Ch][C5H11COO], (H) [Ch][C7H15COO], (I) [P4444][C11H23COO], (J) [Ch][C 7 H15 COO]*, (K) [Ch][C11 H 23 COO]*. ([BMIm]Br,3,7 [BMIm]Cl,3,7 and [EMIm]CF3SO3,3,7120 means greater than or equal to 120 because of detection limit.)
fatty acid anions, [Ch][C3H7COO], [Ch][C5H11COO], [Ch][C7H15COO], and [Ch][C11H23COO] are much closer to the coordinate axes than the reported ILs [Ch]Cl and [Ch][Suc], which means that they are much easier to be salted-out by K3PO4, leading to decreased consumption of the ILs and salting-out agent as well as lower cost during the separation process. Their phase splitting abilities decreased with the sequences as follows: [Ch][C11H23COO] > [Ch][C7H15COO] > [Ch][C5H11COO] ≈ [Ch][C3H7COO], in accordance with the decreasing alkyl chain lengths of the anions. This is because longer alkyl chains led to a more powerful hydrophobic capacity within the studied range. The four cholinium-based ILs with fatty acid anions are quite close to each other in the phase diagram in contrast to the long distance between the four ILs and the two reported cholinium-based ILs. This is because
For [Ch][C3H7COO], KPhe = 14.2, KTrp = 63.2, and KCaf = 36.4. For [Ch][C5H11COO], KPhe = 20.2, KTrp = 113.0, and KCaf ≥ 120. For [Ch][C7H15COO] with an equal initial concentration of 1.20 mol/kg, KPhe = 58.5, KTrp ≥ 120, and KCaf ≥ 120. As the lengths of the anion alkyl chains increased, the corresponding partition coefficients also increased in parallel with the escalating HB basicity of the designed ILs. For [Ch][C7H15COO], the partition coefficients of the three selected solutes all reached extremely high levels, and the solutes were almost separated via the one-stage extraction process. For [Ch]Cl, the ABS could not be constructed under 3368
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high partition coefficient despite the relatively poor phase separation. In addition, because the three solutes all contain hydrophobic moieties, nonspecific van der Waals interactions also played a part in the extraction process, although its effect is extremely limited compared with those caused by HB interactions and phase equilibria. Microstructures and Properties of the Designed ABS. To further confirm the correlation between the intensity of the interactions and the partition coefficient data, solvatochromic experiments were conducted to measure the Kamlet−Taft parameters, and the dipole/polarizability parameter π* and the HB basicity parameter β of each IL-rich phase were determined. The π* value measures the dipole and polarizability, describing the capability of forming nonspecific van der Waals interactions, while the β value evaluates the intensity of the HB acceptor ability. To examine the solvation environment that the biomolecules inhabit, the parameters of the top phases were measured instead of those of the pure ILs. As shown in Figure 5, the phases with [BMIm]Br, [BMIm]Cl, and [EMIm]CF3SO3 all had adequate π* values
the tested concentration. This result clearly indicates that our approach for improving the extraction performance of ABS is feasible and that alkanoate anions contribute to ultrahigh partition coefficients. Because [Ch][C11H23COO] has a relatively low solubility in water, [P4444][C11H23COO], which has a similar structure as [Ch][C11H23COO], was employed to demonstrate the high extraction efficiency due to the alkanoate anion auxiliarily. As shown in Figure 4, the ABS that contained [P4444][C11H23COO] showed even better results than the one that contained [Ch][C7H15COO]. However, [Ch]+ should be the better choice than [P4444]+ when taking toxicity into consideration. In conclusion, the cholinium alkanoate ILs showed remarkable performance in the extraction experiment and are promising for further research and application in ABS processes. [Ch][C11H23COO] was employed with a lower concentration (0.66 mol/kg) to construct the ABS due to its relatively low solubility in water, and a mixture with 0.66 mol/kg [Ch][C7H15COO] was prepared for contrast. Both systems were tested in an extraction experiment. As seen in Figure 4, the ABS based on [Ch][C7H15COO] showed better results than the ABS with [Ch][C11H23COO], which was opposite to the results obtained in the previous studies. This abnormal result will be discussed in the next section. High-performance extraction is closely related to the favorable interactions between the cholinium alkanoate ILs and the selected solutes.48−50 In this work, multiple influences should be considered. First of all, both of the selected amino acids contain a carboxyl group, and caffeine contains a purine structure in which the H atom in the sixth position possesses relatively strong acidity; therefore, these three substances are all decent HB donors. Furthermore, the fatty acid anions of the designed ILs are excellent HB acceptors, which means that the introduced IL anions can form favorable HB interactions with the three solutes that contain acidic H atoms. With the extension of the fatty acid anions, their HB-accepting ability became stronger due to the electron-donating effect of the alkyl chains (β[Ch][C3H7COO] = 0.393, β[Ch][C5H11COO] = 0.557, and β[Ch][C7H15COO] = 0.884), which led to even higher partition coefficients.24,34 Second, although [Ch][C3H7COO], [Ch][C5H11COO], and [Ch][C7H15COO] were added to the ABS with the same initial concentration of 1.20 mol/kg, their discrepant phase separation abilities resulted in that they had different concentrations in the IL-rich phases. [Ch][C3H7COO] was approximately 1.62 mol/kg in the top phase while the concentrations of [Ch][C5H11COO] (2.08 mol/kg) and [Ch][C7H15COO] (2.01 mol/kg) were quite close. Therefore, there were less available butyrate anions in the top phase to interact with solute molecules, resulting in a relatively lower partition coefficient. Thus, our strategy of extending the alkyl chains of the anions convincingly proved capable of optimizing the phase behavior of IL-based ABS and increased the IL concentrations in the IL-rich phases, thereby increasing the partition coefficients of the solutes. [Ch][C7H15COO] with a more powerful HB-accepting ability (β[Ch][C7H15COO] = 0.884) performed better than [Ch][C5H11COO] (β[Ch][C5H11COO] = 0.557) during extraction, even though the concentrations of these two ILs in the top phases were close. Additionally, for [Ch][C7H15COO] and [Ch][C11H23COO] with initial concentrations of 0.66 mol/kg, lower IL concentrations in the IL-rich phases were achieved, which negatively influenced KPhe and KTrp, as expected. For caffeine, HB interactions were predominant and guaranteed a
Figure 5. Values of solvatochromic parameters of IL-rich phases at 298.2 K: (A) [BMIm]Br, (B) [BMIm]Cl, (C) [EMIm]CF3SO3, (D) [Ch][C3H7COO], (E) [Ch][C5H11COO], (F) [Ch][C7H15COO], (G) [P4444][C11H23COO], (H) [Ch][C7H15COO]*, (I) [Ch][C11H23COO]*.
but deficient β values. When the β value was lower than 0.55, the extraction performance of the IL-based ABS was not satisfactory. Interestingly, in the case of the cholinium alkanoate ILs, the π* values decreased and the β values increased with increased anion chain lengths when the ILs were 1.20 mol/kg in the mixtures. β[Ch][C7H15COO] (0.884) was considerably high and one of the highest values in the upper phases of the ILbased ABS. The β values were consistent with the partition coefficient data and convincingly proved the leading role of HB interactions in this work. For [Ch][C7H15COO] and [Ch][C11H23COO] with initial concentrations of 0.66 mol/kg, the sequences were as follows: π* [ C h ] [ C 7 H 1 5 C O O ] * > π*[Ch][C11H23COO]*, and β[Ch][C7H15COO]* > β[Ch][C11H23COO]*, which were also consistent with their extraction data. According to former research, long alkyl chains on imidazolium cations cause IL self-aggregation.22 In this work, long alkyl chains were introduced into the anions. Therefore, polarizing optical microscopy (POM) was utilized at 298 K to determine the aggregation behavior of the IL ions in each phase, thus making it possible to reveal the underlying extraction mechanism. As a result, the LC structures were 3369
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Additionally, the fatty acid anions possessed strong HB basicity (β[Ch][C3H7COO] = 0.393, β[Ch][C5H11COO] = 0.557, β[Ch][C7H15COO] = 0.884, β[Ch][C7H15COO]* = 0.798, and β[Ch][C11H23COO]* = 0.752), and ABS with [Ch][C7H15COO] was one of the best IL-based ABSs. The four cholinium alkanoates had excellent extraction efficiencies for phenylalanine, tryptophan, and caffeine, which are substances with HB acidity sites, and the partition coefficients increased with the anion alkyl chain length except for [Ch][C11H23COO]. When the anion extended to [C7H15COO]−, the partition coefficients of the tested solutes increased to an entirely new level with KPhe = 58.5, KTrp ≥ 120, and KCaf ≥ 120, which were remarkably higher than those in the ABS with conventional ILs. The results of the solvatochromic experiments clearly showed the superiorities of the novel two-phase system in its properties and structures, strong HB basicity, low polarity, and appropriate aggregation abilities that ensured high extraction efficiency. In addition, LC structures were found in an IL-based ABS for the first time and were considered to be beneficial for extraction. This newly designed IL-ABS series can be utilized to extract other solutes and also provides a reference for the development of green and high-performance methods for solid−liquid extraction, liquid−liquid extraction, and microextraction.
observed in the IL-rich phases of the systems with [Ch][C7H15COO] and [Ch][C11H23COO], respectively (see Figure 6). In the IL-rich phases with [Ch][C7H15COO], a chromatic
Figure 6. POM images of LC structure in IL-rich phase at 298 K: (a) [Ch][C7H15COO], 1.20 mol/kg initially, (b) [Ch][C7H15COO], 0.66 mol/kg initially, (c) [Ch][C11H23COO], 0.66 mol/kg initially.
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ASSOCIATED CONTENT
S Supporting Information *
pinnate structure was obtained when the initial concentration of [Ch][C7H15COO] was 1.20 mol/kg (see Figure 6a). Nevertheless, as the initial concentration was reduced to 0.66 mol/kg, a pyramidal LC structure was barely seen (see Figure 6b). For [Ch][C11H23COO], a sheet LC structure was clearly observed (see Figure 6c). To the best of our knowledge, an LC structure has not previously been studied in an IL-based ABS. Considering the solubilizing ability of the LC structures,51 we believe that the formation of the LC in the [Ch][C7H15COO]based ABS contributed to partitioning of the biomolecules to the IL-rich phase.52 During the extraction procedure, the hydrophobic section of the solutes was attracted to the alkyl chains of the anions via nonspecific van der Waals interactions, and the hydrophilic section of the solutes developed HB interactions with the carboxyl groups of the anions.53−55 The cooperative effects between the HB interactions and the van der Waals interactions led to an excellent extraction capacity. However, in the ABS with [Ch][C11H23COO], a high amount of aggregation with ordered structures decreased the HB basicity of the IL-rich phase (β[Ch][C7H15COO]* = 0.798 and β[Ch][C11H23COO]* = 0.752), which decreased the interaction between the IL anion and solute, which was not favorable for extraction.28 Therefore, an appropriate anion alkyl chain length would be preferable for extraction.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01068. NMR data, phase diagrams data, binodal curve fitting parameters, solvatochromic parameters, and correlations (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Phone/fax: +86 571 87952375. E-mail:
[email protected]. *Phone/fax: +86 571 87952773. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial supports form the National Natural Science Foundation of China (21222601, 21476192, and 21436010), the Zhejiang Provincial Natural Science Foundation of China (LR13B060001), and the Fundamental Research Funds for the Central Universities (2014XZZX003-17).
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CONCLUSIONS In this work, a series of novel IL-based ABS were constructed using bioderived ILs that were composed of fatty acid anions and a cholinium cation. The newly designed ABSs possess excellent phase splitting abilities, remarkable extraction efficiencies, and good biocompatibilities. LC structures were also determined. The relative phase diagrams were determined at 298.15 K and atmospheric pressure. Compared with the reported cholinium ILs, cholinium alkanoates have more powerful phase splitting abilities, and the phase splitting abilities increased with the extension of the anion alkyl chain lengths.
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