Biphasic Systems That Consist of Hydrophilic Ionic Liquid, Water, and

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Biphasic Systems That Consist of Hydrophilic Ionic Liquid, Water, and Ethyl Acetate: The Effects of Interactions on the Phase Behavior Yifeng Cao, Huabin Xing,* Qiwei Yang, Zhenkang Li, Ting Chen, Zongbi Bao, and Qilong Ren Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The design of a biphasic system is important for the development of liquid−liquid extraction and biphasic catalysis. In this work, we report a series of ternary biphasic systems that consist of hydrophilic ionic liquid (IL), water, and ethyl acetate. The effects of the structure of the ILs (cation, anion, and substituent group), IL/water ratio, and temperature on the phase equilibrium of these biphasic systems were investigated. The mutual solubility was lower in the IL/water−ethyl acetate biphasic system than in the water−ethyl acetate system when ILs with strong hydration ability were introduced, such as [EMIm]OAc, [EMIm]Cl, [EMIm]NO3, [EMIm]EtSO4, and [HOEMIm]Cl. In addition, the hydrogen-bonding basicity of the IL-containing phase could be tuned by changing the IL structure or IL/water ratio. These novel IL/water-ethyl acetate biphasic systems may present another option in addition to hydrophobic IL−water and aqueous biphasic systems for liquid−liquid extraction.

1. INTRODUCTION Recently, ionic liquid (IL)-based biphasic systems have attracted intense interest in the field of liquid−liquid extraction, because of the attractive properties of ILs, such as low volatility, high thermal stability, good salvation ability, a tunable structure, and tunable physiochemical properties.1−5 To date, a large number of IL-based biphasic systems have been developed, including IL−water, alcohols, hydrocarbons and their derivatives, polymers, and even other ILs, and the corresponding phase equilibrium data have been reported in the literature.6−10 These IL-based biphasic systems present a novel platform for studies and allow for liquid−liquid extraction with a large variety of alternatives or even biphasic systems that are superior to traditional ones. Despite the aforementioned favorable attributes, the nature of cation and anion combination also endows ILs with some unfavorable features and limits their applications. For example, the viscosities of ILs are higher than those of traditional organic solvents, which negatively impacts the mixing and transfer properties of biphasic systems. Furthermore, a high-purity hydrophilic IL with a low water content is not easily obtained.11,12 In addition, although the structure and properties of ILs could be tailored,13 precisely controlling the properties of ILs and synthesizing functional ILs with the desired properties are difficult. A practical method to reduce the viscosity while tuning the properties of the IL system is to use IL/diluent mixtures. Organic solvents, water, zwitterionic materials, and the other ILs14−17 could be selected as the diluent to reduce the viscosity of IL-containing systems and tune their physiochemical properties (dipolarity/polarizability, hydrogen-bonding acidity, and basicity).18−20 Because water is a green solvent with unique physicochemical properties and an inevitable impurity for ILs, IL/water mixtures and water-containing IL-based biphasic systems have become an especially promising media/platform for extraction,21 biomass treatment,22,23 and reaction.24 There© 2014 American Chemical Society

fore, the design of novel IL-based biphasic systems with water is of great value to green chemistry. Currently, the most widely studied IL-based biphasic systems that contain water are the IL−water biphasic system and the ILbased aqueous biphasic system. IL−water biphasic systems, which consist of hydrophobic ILs with PF6−, NTf2−, or C(CN)3−, etc. as anions,25 have been widely used to extract metal ions and dyes from water,26−28 recover biomolecules from fermentation liquid,29 pretreat aqueous samples,30 etc. The distribution behavior of the solute strongly depends on the solute and IL nature.31,32 For aqueous biphasic systems, hydrophilic ILs have been proved to form aqueous biphasic systems with the aid of inorganic salts,33−35 saccharides,36 polymers, 9,37 and surfactants.38 Because of their good biocompatibility, IL-based aqueous biphasic systems have been intensively applied to the liquid−liquid extraction of biomaterials, such as enzymes, amino acids and proteins, nucleic acids, and antibodies.39 In addition to hydrophilic IL−water biphasic systems and ILbased aqueous biphasic systems, another novel biphasic system that consists of hydrophilic IL, water, and ethyl acetate (EtAc) was developed and reportedly shows excellent extraction performance for sparingly water- and fat-soluble bioactive compounds and amphiphilic polymers, for which neither hydrophobic IL−water nor aqueous biphasic systems are applicable.40−42 The IL/water-EtAc biphasic system is expected to exhibit a high extraction capacity for compounds with hydrogen-bonding donors and polar groups by utilizing the strong hydrogen-bonding basicity of hydrophilic ILs. 43 Furthermore, the organic phase, EtAc, is a moderately polar solvent, which guarantees a good solubility for polar and Received: Revised: Accepted: Published: 10784

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Table 1. Chemical Structures of Cations and Anions Used in This Studya

a

Legend: [EMIm]+, 1-ethyl-3-methylimidazolium; [EPy]+, N-ethylpyridinium; [EMPyr]+, 1-ethyl-1-methylpyrrolidinium; [BMIm]+, 1-butyl-3methylimidazolium; [HMIm]+, 1-hexyl-3-methylimidazolium; [OMIm]+, 1-octyl-3-methylimidazolium; [EMMIm]+, 1-ethyl-2, 3-dimethylimidazolium; [HOEMIm]+, 1-(2-hydroxylethyl)-3-methylimidazolium; [BzMIm]+, 1-benzyl-3-methylimidazolium; Cl−, chloride; Br−, bromide; OAc−, acetate; NO3−, nitrate; SCN−, thiocyanate; EtSO4−, ethylsulfate; BF4−, tetrafluoroborate; and ClO4−, perchlorate. water was obtained from Hangzhou Wahaha Group Co., Ltd. (China), with a conductivity of 1.75 μS/cm (Leici Model DDS-307A, China). 2.2. Liquid−Liquid Phase Equilibrium Measurements. The phase equilibria of the IL−water−EtAc systems were measured using a static method.44 Known amounts of IL, water, and EtAc were mixed in a self-made jacked glass vessel, whose temperature was maintained by a circulating water bath (Julabo F25-ME) with an uncertainty of ±0.02 K. The mixture was stirred intensely using a magnetic stirrer for at least 30 min to obtain phase equilibrium and then settled at the same temperature until the phases separated. Samples were taken from both the EtAc-rich phase and the IL-rich phase by syringes for Karl Fischer titration measurements and HPLC analysis, respectively. 2.3. Composition Determination. Water contents in both the EtAc-rich phase and the IL-rich phase were determined via Karl Fischer titration on a Metrohm 870 KF Titrino plus device (Sweden). The relative standard deviation (RSD) for each sample was within 3%. The contents of IL and EtAc in the samples were determined using a Waters HPLC equipment system with a Waters Atlantis T3 column (4.6 mm × 250 mm, 5 μm). The mobile phase was methanol at a flow rate of 0.5 mL min−1. The oven temperature and the inner temperature of the refractive index (RI) detector were both maintained at 313.2 K. The concentration of the solute was quantified with an external standard method by direct comparison with reference standards. The RSD for each HPLC measurement is 98% as received and their structures are shown in Table 1. The water contents of the ILs were determined via Karl Fischer titration as follows: [EMIm]Cl (1.02%), [EMIm]Br (0.02%), [EMIm]OAc (0.05%), [EMIm]EtSO4 (0.04%), [EMIm]NO3 (0.02%), [EMIm]SCN (0.15%), [EMIm]BF4 (0.07%), [EMIm]ClO4 (0.54%), [EPy]Br (0.12%), [EMPyr]Br (0.51%), [EMMIm]Br (1.03%), [BMIm]Cl (0.33%), [HMIm]Cl (1.32%), [OMIm]Cl (0.34%), [HOEMIm]Cl (0.42%) and [BzMIm]Cl (1.10%). Ethyl acetate (CR, ≥ 99.8%) and methanol (CR, ≥ 99.5%) were obtained from TEDIA (USA). Purified

v(1)max = 27.52 − 3.182π *

(1)

v(2)max = 1.035v(1)max − 2.80β + 2.64

(2)

where ν(1)max and ν(2)max are the wavelengths of maximum absorbance of dissolved N,N-diethyl-4-nitroaniline and 4-nitroaniline, respectively. 10785

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3. RESULTS AND DISCUSSION The phase equilibrium data of various IL−water−EtAc systems were measured for ILs with different anion structures, cation cores, cationic alkyl chain lengths, and substituent groups. The structures of ILs are shown in Table 1. For ILs with melting points that exceed room temperature (such as [EMIm]Cl), the corresponding solid−liquid−liquid ternary phase diagrams of IL systems that contain these compounds are known to exhibit several phase regions. Given the requirement of liquid−liquid extraction, phase equilibrium data within the liquid−liquid twophase region were measured in this study. The phase equilibrium data are listed in Table S1 in the Supporting Information. The composition of ILs in the EtAc-rich phase was not given because the solubulities of studied ILs are too low (see Table S2 in the Supporting Information) and precise quantification is too difficult. The mutual solubilites of the IL−water−EtAc biphasic system were evaluated according to the mole fraction of water in the EtAc-rich phase and that of EtAc in the IL-rich phase. The data listed in Table S1 in the Supporting Information indicate that a series of ILs could form biphasic systems with water and EtAc over a wide range of molar ratios. More importantly, mutual solubilities lower than that of the water−EtAc biphasic system were obtained in some IL-based biphasic systems, including biphasic systems with [EMIm]OAc, [EMIm]Cl, [EMIm]Br, [EMIm]NO3, [EMIm]EtSO4, [EPy]Br, [EMPyr]Br, [EMMIm]Br, and [HOEMIm]Cl. In other words, introducing these ILs to EtAc−water promotes the formation of a biphasic system. For example, the mole fraction of water in the EtAc-rich phase and that of EtAc in the IL-rich phase were 6.67% and 0.56% under the following conditions: [EMIm]OAc:water: EtAc = 0.08:0.72:0.20 (mole ratio) and temperature (T) = 303.2 K, whereas the corresponding values were 14.33% and 2.02% for the water−EtAc biphasic system. Notably, not only the mutual solubilities of the IL−water− EtAc systems are low, but also their tunable hydrogen-bonding basicities for the IL-rich phase are high. The Kalmet−Taft parameters of some ILs, water, and EtAc are summarized in Table 2.46−50 The hydrogen-bonding basicity of ILs studied in

Figure 1. Hydrogen-bonding basicity (β) versus the mole fraction of IL in IL/water mixture. The data for [EPy]Br are from ref 41. (Reproduced with permission from the Royal Society of Chemistry, Copyright 2012, Herts, U.K.)

the IL−water mixture. Therefore, these low mutual solubilities and tunable hydrogen-bonding basicity make these IL−water− EtAc systems attractive biphasic systems for extraction applications. 3.1. Effect of Anion Structure on the Mutual Solubilities. Commonly used 1-ethyl-3-methylimidazoliumbased ILs with various anions were selected to analyze the effect of anions on the phase equilibrium. The studied anions included BF4−, Br−, Cl−, ClO4−, EtSO4−, NO3−, OAc−, and SCN−. The mole fractions of EtAc in the IL-rich phase and water in the EtAc-rich phase are presented in Figure 2,

Table 2. Solvatochromatic Parametersa of Representative Ionic Liquids, Water, and Ethyl Acetate46−50 solvent

ET(30)

ETN

π*

α

β

[EMIm]OAc [EMIm]NO3 [EMIm]ClO4 [EMIm]BF4 [HOEMIm]Cl water ethyl acetate

49.8 51.5 52.4 53.7

0.590 0.642 0.670 0.710 0.769 1.00

1.09 1.13 1.11 1.03 1.16 1.33 0.55

0.40 0.48 0.56 0.71 0.73 1.12 0.00

0.95 0.66 0.41 0.35 0.68 0.14 0.45

Figure 2. Effect of ionic liquid anion type on the water content in the EtAc-rich phase and EtAc content in the IL-rich phase for the IL− water−EtAc biphasic system. The shared cation of ionic liquids is [EMIm]+. The initial molar ratio of IL:water:EtAc was 0.08:0.72:0.20. The temperature was 303.2 K.

ET(30), polarity scale; ETN, equivalent normalized polarity scale; π*, solvent dipolarity/polarizability; α, hydrogen-bonding acidity; and β, hydrogen-bonding basicity. a

indicating that the type of anion significantly influences the mutual solubilities of the biphasic system. The data in Figure 2 show that the mole fraction of water in the EtAc phase follows a trend of OAc− < Cl− ≈ EtSO4− < Br− ≈ NO3− < SCN− < BF4− < ClO4−, and the content of EtAc in the IL-rich phase follows a similar order of OAc− < Cl− < Br− ≈ EtSO4− < NO3− < SCN− < BF4− < ClO4− at a fixed composition (IL:water:EtAc molar ratio = 0.08:0.72:0.20) and 303.2 K. This order is generally consistent with the reverse order of the Gibbs free energy of hydration and hydrogen-bonding basicity of ions: OAc− > Cl− > Br− > NO3− > SCN− > ClO4−.51 This consistency arises

this work ranges from 0.35 ([EMIm]BF4) to 0.95 ([EMIm]OAc). Besides that, the hydrogen-bonding basicity of IL/water mixture could be tailored by changing the IL concentrations, which is concluded from the data shown in Figure 1, that the hydrogen-bonding basicity (β) of the IL/water mixture increases as the IL concentration increases. Also, the dipolarity/polarizability (π*) (Figure S1 in the Supporting Information) could also be tuned by changing the IL content in 10786

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because the ILs employed in this work are all hydrophilic, and the hydration ability of ILs positively correlates with the hydrogen-bonding basicity of ILs. Lower water content in the EtAc-rich phase was observed for more hydrated anions. In addition, because the BF4− and EtSO4− anions are known to hydrolyze in aqueous phase, their use is not recommended for liquid−liquid extraction. 3.2. Effect of Cation Core and Substituent Groups on the Mutual Solubilities. In addition to the anion, the core structure, substituent group number, and the functional group of the cation can be tailored. In this section, a series of experiments were carried out to investigate the effect of the cation structure on the phase behavior of the biphasic systems. Effect of Cation Core Structure. First, a series of Br−-based ILs with different cation core structures were selected as typical ILs to study the effect of the cation core structure on the mutual solubilities of IL−water−EtAc biphasic systems. The results presented in Figure 3 indicate that the studied cation

Figure 4. Effect of cationic alkyl chain length on the water content in the EtAc-rich phase and EtAc content in the IL-rich phase for the IL− water−EtAc biphasic system. The ionic liquids used were [CnMIm]Cl (n = 2, 4, 6, and 8). The initial molar ratio of IL:water:EtAc was 0.08:0.72:0.20. The temperature was 303.2 K.

303.2 K. The result shows that shorter alkyl chain lengths promote the formation of a biphasic system, which can be attributed to enhanced van der Waals and hydrophobic interactions between IL and EtAc as the cationic alkyl chain length increases, which increases the mutual solubilities of the biphasic system. Effect of C2 Substitution. The interaction between the cation and EtAc is an important factor that determines the solubility of EtAc in the IL-rich phase. The hydrogen atom at the C2 position of the imidazolium-based cation shows a higher hydrogen-bonding acidity than the other H atoms in the cation, and this position might be an important hydrogen-bonding interaction site with EtAc. To test this assumption, the phase equilibrium of the [EMMIm]Br system, which contains an additional methyl group in the C2 position, were determined, and the results were compared with that of the [EMIm]Br system. The mole fractions of EtAc and water in the IL-rich and EtAc-rich phases were plotted against the initial mole fraction of IL (Figure 5). As expected, the mole fraction of EtAc in the [EMMIm]Br-rich phase is lower than that in the [EMIm]Brrich phase.

Figure 3. Effect of cation core structure on the water content in the EtAc-rich phase and EtAc content in the IL-rich phase for the IL− water−EtAc biphasic system. The initial molar ratio of (IL+water):EtAc was 0.8:0.2. The temperature was 303.2 K.

cores slightly influence the mutual solubilities of the biphasic system. The water content in the EtAc-rich phase generally follows a decreasing order of [EMPyr]+ > [EMIm]+ > [EPy]+, which agrees with the decreasing trend of the hydrophobicity of cation cores.25 A decreasing order of [EMIm]+ > [EPy]+ > [EMPyr]+ was observed for the content of EtAc in the IL-rich phase, which implies that IL cations with aromatic structures ([EMIm]+ and [EPy]+) exhibit higher affinity for EtAc. The p−π interaction between the aromatic cation and ethyl acetate should account for this phenomenon. To further verify this explanation, the ethyl group of [EMIm]+ was substituted with a benzyl group. As expected, higher mutual solubilities were observed for IL with an aromatic substituent ([BzMIm]Cl > [EMIm]Cl, see Table S1 in the Supporting Information). Effect of Cationic Alkyl Chain Length. Changing the cationic alkyl chain length is a direct way to tune the hydrophobic properties of ILs. 1-Alkyl-3-methylimidazolium chloride ILs with alkyl chain lengths varying from 2 to 8 were chosen to evaluate the influence of the alkyl chain length on the phase behavior of the biphasic system. The corresponding experimental data are shown in Figure 4. Both the concentration of water in the EtAc-rich phase and EtAc in the IL-rich phase increase as the alkyl chain length increases. Notably, a miscible mixture was observed for [OMIm]Cl when the molar ratio of [OMIm]Cl:water:EtAc was 0.08:0.72:0.20 at

Figure 5. Effect of the C2-methyl substituent group of the imidazolium-based cation on the water content in the EtAc-rich phase and EtAc content in the IL-rich phase for the IL−water−EtAc biphasic system. The initial mole ratio of (IL+water):EtAc was 0.8:0.2. The temperature was 303.2 K. 10787

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dominant.55,56 As the IL concentration increases, the EtAc content in the IL-rich phase increases, as a result of the stronger interactions between cation and EtAc. Therefore, the presence of a minimum in the EtAc in the IL-rich phase against the IL concentration is reasonable as a result of the combined effect of the anion−water and cation−EtAc interactions. 3.4. Effect of Temperature on the Mutual Solubilities. The phase equilibrium data of the [EMIm]Br/water-EtAc biphasic system at temperatures ranging from 283.2 K to 323.2 K were measured, and the results are shown in Figure 7. The

Effect of Hydroxyl Group. Introduction of functional group to the cation of IL can be used as an effective way to tune the physicochemical properties of ILs. Herein, a hydroxyl group was introduced to the [EMIm]+ cation to investigate its effect on the mutual solubilities. Compared with adding a C2-methyl group to the [EMIm]+ cation, a similar mutual solubility change was observed when a hydroxyl group was introduced to the C1ethyl group of the [EMIm]+ cation (see Figure 6). However,

Figure 6. Effect of the hydroxyl functional group attached to the cation on the water content in the EtAc-rich phase and EtAc content in the IL-rich phase for the IL−water−EtAc biphasic system. The ionic liquids used were [EMIm]Cl and [HOEMIm]Cl. The initial mole ratio of (IL+water):EtAc was 0.8:0.2. The temperature was 303.2 K.

Figure 7. Effect of temperature on the water content in the EtAc-rich phase and EtAc content in the [EMIm]Br-rich phase for the [EMIm]Br−water−EtAc biphasic system. The initial molar ratio of [EMIm]Br:water:EtAc was 0.08:0.72:0.20.

the underlying mechanism might be different. The anion will preferentially associate with the hydrogen of the hydroxyl group on the [HOEMIm]+ cation. Consequently, the increased IL/IL interactions induce a difference in the multiple IL−solvent interactions.47,52 In the IL−water−EtAc biphasic system, the IL−water and IL−EtAc interactions, especially the hydrogenbonding interactions, are reduced. As a result, lower solubility of EtAc in the IL-rich phase and increased solubility of water in the EtAc-rich phase were observed for biphasic systems containing hydroxyl ILs. 3.3. Effect of Initial IL Concentration on the Mutual Solubilities. The effect of the initial IL concentration on the mutual solubilities was investigated at a (IL + water):EtAc molar ratio of 0.8:0.2 at 303.2 K. The data in Figures 3, 5, and 6 for ILs [EMIm]Cl, [EMIm]Br, [EPy]Br, [EMPyr]Br, [HOEMIm]Cl, and [EMMIm]Br indicate that as the initial mole fraction of IL in the IL-rich phase increases, the mole fraction of water in the EtAc-rich phase decreases. This relationship arises because the interactions between water and the ions are enhanced as the IL concentration increases. Interestingly, the concentration of EtAc in the IL-rich phase first decreases and reaches a minimum point and then increases when the concentration of IL increases, especially in [EMIm]Cl system (Figure 6). This phenomenon might be explained via the microscopic IL−water interactions. With increasing IL content in the mixture, the environments of both ILs and water are changing all the time.53 At low IL concentration, interactions between anion−water and cation−water exist,54 EtAc−water interactions may determine the EtAc content in the IL-rich phase. Compared with EtAc−water binary system, the EtAc−water interaction strength is reduced by adding an IL. While at high IL concentration, the hydrogen-bonding interaction between the anion, rather than cation, and water is

mutual solubilities of the biphasic system positively correlate with the temperature. As the temperature increases from 283.2 K to 323.2 K, the molar concentration of water in the EtAc-rich phase increases from 7.54% to 11.53%, while that of EtAc in the IL-rich phase changes slightly from 1.39% to 1.49%. According the data shown in Figure 7, the [EMIm]Br/water-EtAc ternary system can form a biphasic system with relatively low mutual solubilities over a wide temperature range.

4. CONCLUSIONS The phase equilibria of various ternary systems containing ionic liquid, water, and ethyl acetate were measured, and the interactions that affect the phase behavior were evaluated. Biphasic systems with low mutual solutilities were obtained for certain ILs, such as [EMIm]OAc, [EMIm]Cl, [EMIm]Br, [EMIm]NO 3 , [EMIm]EtSO 4 , [EPy]Br, [EMPyr]Br, [EMMIm]Br, and [HOEMIm]Cl, over wide ranges of initial IL concentration and temperature. The results indicate that the mutual solubilities of the biphasic system are determined by complex interactions between the cation, anion, water, and EtAc. Anions with higher Gibbs free energy of hydration and cations with shorter substituent alkyl chain length could decrease the solubility of water in the EtAc-rich phase. The content of EtAc in the IL-rich phase could be reduced by increasing the hydrogen-bonding basicity of anion, avoiding using aromatic cations, shortening the cationic alkyl chain length, as well as introducing a methyl group to C2 atom and a hydroxyl group to C1-ethyl group of imidazolium cation. In conclusion, the hydrophilic IL−water−ethyl acetate biphasic system may represent an attractive alternative to conventional biphasic systems and common IL-based biphasic systems for liquid−liquid extraction. 10788

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ASSOCIATED CONTENT

S Supporting Information *

Details of phase equilibrium data of studied IL−water−EtAc, experimental procedure and solubilities of ILs in EtAc, and the dipolarity/polarizability of IL/water mixture versus the mole fraction of IL. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-8795-2375. Fax: +86-571-8795-2375. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21076175 and 21222601), the Zhejiang Provincial Natural Science Foundation of China (No. LR13B060001) and the Fundamental Research Funds for the Central Universities (Nos. 2014XZZX003-17 and 2013FZA4022).

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