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Extraction of dimethyl sulfoxide using ionic-liquid-based aqueous biphasic systems
Separation and Purification Technology 124 (2014) 107–116
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Extraction of dimethyl sulfoxide using ionic-liquid-based aqueous biphasic systems Jing Gao, Li Chen, Zong C. Yan ⇑ Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 6 October 2013 Received in revised form 7 January 2014 Accepted 10 January 2014 Available online 24 January 2014 Keywords: Aqueous biphasic systems Ionic liquid Extraction Dimethyl sulfoxide
a b s t r a c t To combine ionic liquid (IL) recovery with cosolvent separation in biomass processing, the quaternary aqueous biphasic systems (ABSs) formed by IL, kosmotropic salt and water in the presence of dimethyl sulfoxide (DMSO) are herein proposed. Three main parameters were evaluated through the phase diagrams and the DMSO partitioning process: the IL cation and anion structure, the salt anion, and the temperature. In all systems and conditions texted, DMSO preferentially dissolves in the IL-rich phase. The results obtained indicate that, the partitioning process is essentially controlled by the IL cation interactions with DMSO. The partition coefficients (KDMSO) displays a maximum for the system formed by [C6mim]Cl. The KDMSO value increases monotonically with the initial concentration of DMSO, and it decreases in the systems with different salts: K3PO4 > K2HPO4 > K2CO3 > KOH. The increase of temperature reduces the partition coefficients. Moreover, densities and pH of both aqueous phases were measured at 298.15 K to evaluate the properties of the systems. The results gathered indicate that the densities and pH values of the two phases can be affected by the nature of IL and its initial concentrations, and the smaller density differences between the fluids, the lower the partition coefficients of DMSO. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Liquid–liquid extraction processes are important techniques used for the recovery and purification steps [1,2]. Aiming at avoiding the use of organic solvents as the extractive phase in traditional liquid–liquid extraction techniques, aqueous biphasic systems (ABSs) which essentially composed of water in both phases were introduced by Albertsson [3]. The ABS formed by mixing of polymer/polymer, polymer/salt or salt/salt in water above critical concentrations can be used for separation of biomolecules [4]. However, most of the aqueous solutions of the phase-forming polymers present high viscosities, and also may alter the kinetics or interfere with the enzyme reaction in some other way [5–7]. In 2003, Rogers and co-workers [8] reported the pioneering research pointing to the possible creation of ABS by the addition of inorganic salts to aqueous solutions of ionic liquids (ILs). Consequently, a wide variety of IL-based ABSs in the presence of inorganic salt [9–12], organic salt [13,14], carbohydrate [15–17], polymer [18–20], amino acid [21], and anionic surfactant [22] have been presented. ILs are salts with a large array of fascinating properties, such as a negligible vapor pressure, low viscosity and high thermal and chemical stability [23]. As ‘‘designer solvents’’, the main advantage of the applicability of IL-based ABSs is the ability ⇑ Corresponding author. Tel.: +86 2013450423151. E-mail address: [email protected] (Z.C. Yan). 1383-5866/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2014.01.015
to tailor their polarities and affinities by a proper manipulation of the cation/anion and their combinations in ILs [24,25]. Therefore, these systems have been extensively studied as successful separation/extraction methods of different compounds and molecules, such as biomolecules [11,26], antibiotics [27], polar solutes [28–31], and biocatalysis [32–34]. Meanwhile, the influence of the IL structure [5,35–40], temperature [20,41,42], pH [34,43], viscosity [22,44], and density [44] on the phase behavior or partition coefficient have been extensively studied. Generally, the use of IL-organic solvent mixtures offers a lower viscosity, provides higher biomass loading, and reduces cost by using less IL [45,46]. Dimethyl sulfoxide (DMSO), a dipolar aprotic solvent, has been widely used as IL’s cosolvent to dissolve starch [47], cellulose [48,49], and lignocellulosic biomass [50]. The previous applications of the mixture of IL and DMSO have been reported, yet, to minimize environmental impacts the recovery of IL together with DMSO has been a problem to solve. Recent advances have shown that the mixed solvents of IL and DMSO which were used as pretreated solvents of lignocellulosic biomass for biogas production were successfully recovered together by forming ABS with K3PO4, and the recovery rates of IL and DMSO were all over 90% [51]. Subsequently, the influence of these dipolar aprotic solvents on the phase behavior of IL-based ABS was further evaluated, and it was shown that biphasic area increases when the concentrations of the dipolar aprotic solvent increase [52]. These literatures deserve further attention of the research community
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working with IL-based ABSs in the presence of dipolar aprotic solvent. Attempting at developing the understanding on the partition of dipolar aprotic solvents in ABSs containing ILs (Fig. 1), and to recovery ILs along with the extraction of its cosolvents, in this work an extensive study was conducted using DMSO as the partitioning molecule. For that purpose, several extraction parameters were studied in quaternary systems composed by imidazolium-based ILs, DMSO, water and inorganic salts, namely the influence of the IL cation, the salt, the temperature of extraction and the concentration of DMSO. Moreover, densities and pH of both monophasic region and biphasic region were measured at 298.15 K to evaluate the properties of the systems. 2. Experimental 2.1. Materials The ionic liquids used in this work to study the formation of ABSs in the presence of dipolar aprotic solvent were the following: 1-ethyl-3-methylimidazolium chloride, [C2mim]Cl; 1-butyl-3methylimidazolium chloride, [C4mim]Cl; 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl; 1-octyl-3-methylimidazolium chloride, [C8mim]Cl; 1-allyl-3-methylimidazolium chloride, [Amim]Cl; 1-butyl-2,3-dimethylimidazolium chloride, [BDmim]Cl; 1-ethyl-3-methylimidazolium bromide, [C2mim]Br; 1-butyl-3methylimidazolium bromide, [C4mim]Br; 1-hexyl-3-methylimidazolium bromide, [C6mim]Br; 1-octyl-3-methylimidazolium bromide, [C8mim]Br. All ionic liquids were supplied by Centre of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, China (purity > 99.0 wt%). Dimethyl sulfoxide, DMSO (purity > 99.0 wt%) was acquired at Sigma Aldrich. The inorganic salts used were K3PO43H2O > 96.0 wt%, K2HPO43H2O > 97.0 wt%, KH2PO4 > 99.5%, K2CO3 P 99.5 wt%, KOH P 99.5 wt%, and KCl P 98 wt%, all from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water was double-distilled and passed by a reverse osmosis system and further treated with a Milli-Q plus 185 water purification apparatus. 2.2. Phase diagrams Aqueous solutions of each salt with variable mass fractions in a range between 40 and 60 wt%, and aqueous solutions of ionic liquid with mass fraction at 80 wt% (the concentration of DMSO in IL was from 0 and 20 wt%) were prepared and used for the determination of the corresponding binodal curves. The phase diagrams were determined through the cloud point titration method at
Fig. 1. Recycling of IL together with DMSO by using ABS method in biomass processing.
various temperatures and atmospheric pressure. The experimental procedure adopted was previously used by us and is described in detail elsewhere [12,52]. The experimental binodal curves were correlated according to the following equation proposed by Merchuk et al. [53]:
Y ¼ a expðbX
0:5
cX 0:3 Þ
ð1Þ
where X and Y are the mass fractions of ionic liquid and salt. The constants a, b and c were obtained by least-squares regression. 2.3. DMSO partitioning The amount of DMSO in each aqueous phase was quantified through material balance method [52]. After the careful separation of both phases, the concentrations of IL and salt in the upper and lower phases were determined by an ion chromatography (Basic IC 792, Methohm, Switzerland). A Karl–Fisher moisture titrator (MKS210, Kyoto Electronics, Kyoto) was used to measure the water content in all samples. The partition coefficients of DMSO (KDMSO), were determined as the ratio of the concentration of DMSO in the ionic liquid and in the inorganic salt aqueous-rich phases, accordingly to:
K DMSO ¼ C T =C B
ð2Þ
where CT and CB are the concentration of DMSO in the top and bottom aqueous-rich phases, respectively. For all the mixtures evaluated, the top layer is the IL-rich phase while the bottom phase is the inorganic salt-rich phase. 2.4. Density and pH measurements The solutions from monophasic region to biphasic region with an increasing mass ratio of K3PO4 to [Cnmim]Cl in the presence of DMSO were prepared. The density (g mL1) of each phase was measured using an automated DES PHOTIME. The pH value was tested by a PHS-25 pH meter equipment. 3. Results and discussion 3.1. Effect of IL structure A broad range of ILs was studied to identify the IL structural features responsible for the formation of ABSs and the extraction of DMSO. The ILs investigated here were [C2mim]Cl, [C4mim]Cl, [C6mim]Cl, [C8mim]Cl, [Amim]Cl, [BDmim]Cl, [C2mim]Br, [C4mim]Br, [C6mim]Br and [C8mim]Br, and all the above ILs were able to promote ABSs with K3PO4 in the presence of DMSO. The structures of the ILs studied are displayed in Fig. 2. The experimental weight fraction data for each phase diagram are reported in Supporting Information. The binodal curves of IL + K3PO4 + water + 20% DMSO are plotted in Fig. 3. The binodal curves of all systems were correlated with Eq. (1), and the respective parameters a, b, c, along with their corresponding standard deviations were obtained by the nonlinear regression. The results are listed in Table 1. From the correlation coefficients (R2) obtained, it is safe to admit that Eq. (1) provides a good description of the experimental data. The phase diagram in Fig. 3 provides information about the concentration of phase-forming components required to form two phases, and the concentration of phase components in the top and bottom phases. Moreover, the closer to the axis origin a binodal curve is, the higher the IL hydrophobicity, and the higher their ability to phase split [25]. Taking into the account the alkyl chain length in [Cnmim]Cl, the size of the ILs with alkyl chains shorter than the hexyl is the dominant effect on the formation of ABSs, while self-aggregate in
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Fig. 2. Ionic structures of the studied ionic liquids and DMSO.
Table 1 Correlation parameters (a, b, and c) obtained by the regression of the experimental binodal data through the application of Eq. (1) (and respective standard deviations, r) for the IL + salt + DMSO + water systems at 298 K and atmospheric pressure. c
R2
106r
IL + DMSO (20%) + K3PO4 + water at 298.15 K [C2mim]Cl 0.53 ± 0.02 2.61 ± 0.11 [C4mim]Cl 0.49 ± 0.02 2.33 ± 0.13 [C6mim]Cl 0.59 ± 0.02 3.02 ± 0.10 [C8mim]Cl 0.52 ± 0.01 2.25 ± 0.07 [BDmim]Cl 0.52 ± 0.01 2.51 ± 0.05 [Amim]Cl 0.87 ± 0.04 3.68 ± 0.17 [C2mim]Br 0.61 ± 0.01 2.63 ± 0.27 [C4mim]Br 0.60 ± 0.01 2.64 ± 0.92 [C6mim]Br 0.58 ± 0.01 2.75 ± 0.06 [C8mim]Br 0.66 ± 0.02 2.91 ± 0.11
37.51 ± 0.45 42.99 ± 0.38 60.24 ± 0.27 61.35 ± 0.90 50.15 ± 0.73 28.29 ± 0.95 48.36 ± 0.86 66.90 ± 0.05 79.68 ± 0.67 94.22 ± 0.57
0.9992 0.9962 0.9994 0.9970 0.9992 0.9911 0.9998 0.9973 0.9992 0.9980
4.01 40.0 2.12 20.0 10.0 51.2 3.40 50.0 10.0 30.0
[C4mim]Cl + salt + water at 298.15 K K3PO4 0.61 ± 0.05 2.27 ± 0.12 K2HPO4 0.91 ± 0.02 3.05 ± 0.20 K2CO3 1.01 ± 0.01 3.32 ± 0.04 KOH 0.89 ± 0.06 2.48 ± 0.17
21.78 ± 0.09 20.80 ± 0.13 42.86 ± 0.81 38.64 ± 0.99
0.9992 0.9978 0.9992 0.9991
1.00 14.4 9.79 4.78
Fig. 3. Phase diagrams of various ILs + 20% DMSO + K3PO4 + water systems at 298 K and atmospheric pressure: (h) [C2mim]Br, (s) [C4mim]Br, (N) [C6mim]Br, (r) [C8mim]Br, () [C2mim]Cl, (J) [C4mim]Cl, (4) [C6mim]Cl, (d) [C8mim]Cl, (H) [Amim]Cl, (+) [BDmim]Cl.
[C4mim]Cl + DMSO (5%) + salt + water at 298.15 K K3PO4 0.49 ± 0.03 3.12 ± 0.08 46.23 ± 0.51 K2HPO4 0.57 ± 0.07 2.99 ± 0.24 12.54 ± 0.54 K2CO3 0.76 ± 0.01 2.99 ± 0.05 42.63 ± 0.88 KOH 1.02 ± 0.05 2.95 ± 0.12 39.30 ± 0.01
0.9936 0.9842 0.9991 0.9983
18.6 28.6 10.0 9.05
aqueous media for the ILs above [C6mim]Cl is a dominant effect [40]. However, when moving from [C2mim]Br to [C8mim]Br, increasing the alkyl chain length of the IL cation enhances the fluid’s overall hydrophobicity and increases the creation ability of ABSs. Moreover, the substitution of the most acidic hydrogen at C2 by an alkyl group could reduce the hydrogen bonding of the IL cation with water. Hence, [BDmim]Cl was more easily phase separated than [C4mim]Cl. On the other hand, the presence of a
[C4mim]Cl + DMSO (20%) + salt + water at 298.15 K K2HPO4 0.51 ± 0.05 3.42 ± 0.16 19.30 ± 0.58 K2CO3 0.63 ± 0.01 2.87 ± 0.02 43.04 ± 0.42 KOH 0.47 ± 0.04 1.08 ± 0.23 55.97 ± 0.90
0.9918 0.9998 0.9967
19.2 2.03 20.0
[C4mim]Cl + DMSO (20%) + K3PO4 + water at various temperatures 288.15 K 0.55 ± 0.02 2.67 ± 0.21 32.56 ± 0.25 0.9997 308.15 K 0.57 ± 0.03 2.43 ± 0.15 20.17 ± 0.42 0.9997 318.15 K 0.60 ± 0.06 2.53 ± 0.11 21.86 ± 0.26 0.9990 328.15 K 0.66 ± 0.04 1.96 ± 0.23 23.68 ± 0.35 0.9901
17.1 7.55 20.0 35.4
Variable
a
b
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double bond (allyl group) at [Amim]Cl decreased the ability of ABS to form. That is because the functionalized group enhance the hydrophilic character [25]. It has been demonstrated that the IL aptitude (hydrophobicity) for creating ABSs was based on their hydrogen bond basicity (b) [38]. The b value is one important parameter of Kamlet–Taft parameters, and is a measure of the ability of the solvent to accept a proton. To investigate the effect of b value of the ILs on the hydrophobicity to form ABSs, the b value of the ILs studied in this work were determined by following the procedure reported by Ohno et al. [54]. The b values are presented in Fig. 4. It can be seen that the increasing alkyl side chain length decreases the b value of IL. Fig. 4 reveals that a close relationship exists between the ability of each IL to form ABS and their hydrogen bond basicity values: the lower the b values is, the more efficient the IL is in creating ABS. Although it was found in the previous work that DMSO preferentially partitions for the IL-rich phase [52], the extraction of a molecule with ABS strongly depends on the ability to manipulate the physical/chemical properties of the phases which are due to the chemical nature of the system. To evaluate the influence of the IL cation and anion structure in the extraction of DMSO, the selected ILs allow the study of the alkyl side chain length effect, as well as the study of additional functional groups. All of these studies were performed at 298.15 K. The partition coefficients are present in Fig. 5 and the results show that KDMSO ranges between 1 and 4 (at approximately the same mass fraction compositions of IL and K3PO4). Regarding the IL structure influence, the ability of ILs to dissolve DMSO at initial concentration of 20% follows the order: [C6mim]Cl > [BDmim]Cl [C8mim]Cl > [C4mim]Cl [C8mim]Br > [C 6 mim]Br > [C 4 mim]Br > [C 2 mim]Cl > [Amim]Cl > [C 2 mim]Br. Although DMSO is a dipolar aprotic solvent, the hydrogen at IL cation was found to form weak hydrogen bond (C–H. . .O) with the oxygen atom of DMSO [55]. Besides, the increase of the alkyl side chain length leads to a decrease on the strength of the coulombic and polar interactions, while increasing the dispersive interactions, between the IL ions [29]. However, increasing the size of the alkyl side chain it increases the IL free volume while decreasing the surface tension of the system [56], and thus decreasing the energy of cavity formation to accommodate a DMSO molecule. Moreover, the reversed micelle formation in the IL-rich phases is also the factor responsible for the lower extraction capacity of the systems with [Cnmim]Cl (n > 6) [57]. Therefore, these contributions act in
Fig. 4. Hydrogen-bond basicity (b) as a function of ILs molar mass.
Fig. 5. Partition coefficients of DMSO in IL (22 wt%) and K3PO4 (35 wt%) ABS as a function of different ILs.
different directions leading to the maximum value on the partition coefficients at [C6mim]Cl. It should be pointed out that for all the ILs studied, there is an increase in the DMSO partition coefficients with the initial concentration from 5% to 20%. That is because the increase in content increases the formation of C–H. . .O between IL cation and the O of DMSO, leading to the higher concentration of DMSO in IL-rich phase. That suggests the initial concentration of DMSO largely influences its extraction. 3.2. Effect of salting-out effect It has been demonstrated that the addition of high chargedensity salts to aqueous solutions of ILs leads to liquid–liquid demixing due to a preferential hydration of the charge density salt over the IL, leading to the salting-out of the IL to the IL-rich phase [58]. To carry out a study with the salts used commonly (K3PO4, K2HPO4, K2CO3, and KOH), the solutions of [C4mim]Cl containing DMSO ranges from 5% and 20% were prepared. The phase diagrams of the various systems are graphically presented in Fig. 6. It should be stressed that the amount of water complexed to K3PO4 and K2HPO4 was removed in the calculations of the mass of salts and added to the water composition of each phase diagram. Fig. 6 reports the binodal curves for K3PO4, K2HPO4, K2CO3, and KOH and allows an evaluation of the salt anion effect in the formation of IL-based ABS in the presence of DMSO with different contents. The larger the two-phase region, the stronger the salt’s ability to induce the ABS formation. When a mass ratio of DMSO to [C4mim]Cl equals in each system, the salting-out ability of the salts follows the order K3PO4 > K2HPO4 > K2CO3 > KOH which is in good agreement with the Hofmeister series. The similar trend for the salting-out effect of the salts was previously observed by others [9,12]. The results suggest that the molar entropy of hydration of the salt ions is the essential driving force for ABS formation, in spite of the addition of dipolar aprotic solvent. However, it should be stressed that the addition of DMSO could enhance the separation of the two aqueous phases, resulting in a lesser amount of salt. For instance, the biphasic region of the [C4mim]Cl + 20% DMSO + K2CO3 + water system was much larger than [C4mim]Cl + K2CO3 + water system, and even larger than [C4mim]Cl + K3PO4 + water system. In summary, the salting-out effect is mainly the result of the formation of water-ion complexes that cause the
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Table 2 The parameters obtained by the regression of the experimental data using Eq. (3).
K3PO4 K2HPO4 KH2PO4 K2CO3 KOH KCl
Fig. 6. Phase diagrams for [C4mim]Cl and selected salts with different concentrations of DMSO at 298 K and atmospheric pressure: (j), K3PO4; (h), K3PO4 + 5% DMSO; (I), K3PO4 + 20% DMSO; (d), K2HPO4; (s), K2HPO4 + 5% DMSO; (), K2HPO4 + 20% DMSO; (N), K2CO3; (4), K2CO3 + 5% DMSO; (|), K2CO3 + 20% DMSO; (), KOH; (}), KOH + 5% DMSO; (+), KOH + 20% DMSO.
dehydration of the IL and induce the ABS formation, but the influence of DMSO can be considerable and should not be neglected in simulations and design calculations. To further evaluate the interactions of salt with water, the influence of different salts with increasing concentrations on the hydrogen bond basicity (b) of water was investigated, and the result is presented in Fig. 7. A general trend in the b values of the salt solutions was verified: an increase in the molar ratio of salt to water leads to an increase in the b values. Table 2 presents the parameters obtained by the regression of the experimental data using Eq. (4):
b ¼ bw þ kM
ð3Þ
where b and bw are hydrogen bond basicity of the salt solutions and water, respectively. M is the molar ratio of salt to water, and k is the slope. As can be seen by the correlation coefficients (R2) obtained, Eq. (3) provides a good description of the experimental b values of the solutions as a function of the salts concentration. It means
Fig. 7. Hydrogen-bond basicity (b) as a function of molar ratio of salt to H2O: (j), K3PO4; (d), K2HPO4; (r), K2CO3; (), KOH; (N), KH2PO4; (}), KCl.
b0
k
R2
0.0887 0.0932 0.1035 0.1012 0.1018 0.1006
120.35 89.77 32.21 66.53 36.71 24.59
0.9856 0.9870 0.9915 0.9938 0.9919 0.9674
that the b values of the salt solutions are linearly related to the molar ratio of the salt to H2O. In general, the salt solutions with larger k values require less salt to induce the separation of the ABS, resulting in a binodal curve closer to the axis and to a large biphasic region. Fig. 8 displays the partition coefficients of DMSO in dependence of the salts. Large organic cations such as [C4mim]+ are very poorly solvated by water but are well-solvated by aprotic solvents [59]. A series of three stock systems with compositions within upper region of the phase diagram containing different salts were studied, and the mass of [C4mim]Cl, salt and water are the same (18/22/62, 19/24/57, and 22/28/50) for each system. The results show that the concentration of [C4mim]Cl in IL-rich phase is in the following order K3PO4 > K2HPO4 > K2CO3 > KOH. The more IL in IL-rich phase is, the more DMSO transferred to IL-rich phase. As shown in Fig. 7, the coefficients decreased according to the predicted order. 3.3. Effect of temperature The influence of temperature on the phase diagrams of [C4mim]Cl-based IL ABS with K3PO4 in the presence of 20% DMSO is shown in Fig. 9. Lower temperatures are favourable for the creation of [C4mim]Cl-based ABS. That is because the higher the temperature, the higher are the salt and IL concentrations required for phase separation. In the present context, the solvation of [C4mim]Cl was enhanced and the ionic interaction was weakened with increasing temperature [60]. On the other hand, hydrogen bond weakening with the increase of the temperature led to weakening of salt solvation. Hence, from the viewpoint of IL-DMSO interactions, it may be concluded that the interactions of IL and DMSO are enhanced at lower temperature and the unfavourable interactions of IL with water (hydration) are reduced. Previously, several literatures reported that the temperature significantly influences the extraction efficiency of proteins [61], amino acid [42], and vanillin [44]. Therefore, aimed at evaluation the temperature influence in the extraction of DMSO (20%), six
Fig. 8. Partition coefficients of DMSO in the ABSs containing [C4mim]Cl (22 wt%) and different salts (35 wt%).
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In order to calculate the DMSO thermodynamic parameters of transfer, such as the standard molar Gibbs energy (Dtr G0m ), the standard molar enthalpy (Dtr H0m ), and the standard molar entropy of transfer (Dtr S0m ), the van’t Hoff approach was used. The plots of ln(KDMSO) versus 1/T for the six ILs studied, in the temperature range from 288.15 K to 328.15 K. These parameters reveal the association equilibrium between the DMSO compositions in two different fluids. The following isochors were used to determine the molar thermodynamic functions of transfer (Eqs. (4)–(6)) [44]:
lnðK DMSO Þ ¼
Fig. 9. Phase diagrams for [C4mim]Cl and K3PO4 with DMSO of 20% at different temperatures and atmospheric pressure: (h), 288 K; (d), 298 K; (N), 308 K; (r), 318 K; (), 328 K.
systems were selected ([C2mim]Cl, [C4mim]Cl, [C6mim]Cl, [C8mim]Cl, [Amim]Cl, and [BDmim]Cl). KDMSO was determined at the following temperatures: 288.15 K, 298.15 K, 308.15 K, 318.15 K, and 328.15 K. The data gathered, and accurate mass fraction compositions for each, are displayed in Fig. 10. The six systems, and at all temperatures, present a KDMSO larger than 1.0. Indeed, the increase of temperature reduces the partition coefficients of DMSO. The results indicate that the temperature has a significant effect in the DMSO partitioning.
Dtr H0m 1 Dtr S0m þ T R R
ð4Þ
Dtr G0m ¼ Dtr H0m T Dtr S0m
ð5Þ
Dtr G0m ¼ RT lnðK DMSO Þ
ð6Þ
where KDMSO is the partition coefficient of DMSO between IL-rich and the salt-rich phases, R is the universal gas constant, and T is the temperature. The plots of ln(KDMSO) versus 1/T exhibit linearity indicating that the molar enthalpy of transfer of DMSO is temperature independent. The molar thermodynamic functions of transfer at 298.15 K, obtained by the linear least-square analysis, are summarized in Table 3. For all the systems studied, Dtr G0m is shown to be negative, which in turn reflects the spontaneous and preferential partitioning DMSO for the IL-rich phase. Besides, the value of Dtr G0m is the lowest in [C6mim]Cl-based ABS when compared to other systems. Moreover, the Dtr H0m negative values reveal that the transfer of DMSO between the IL-rich phase and the salt-rich phase is an exothermic process. The three functions of transfer largely depend on the IL cation, and these results again suggest that the partitioning process is essentially controlled by the cation interactions with DMSO. 3.4. Density and pH value
Fig. 10. Partition coefficients of DMSO in IL (22 wt%) and K3PO4 (35 wt%) ABS as the function of temperature for the ILs: (j), [C2mim]Cl; (h), [C4mim]Cl; (N), [C6mim]Cl; (s), [C8mim]Cl; (), [Amim]Cl; (I), [BDmim]Cl.
Fig. 11 shows the color and phase behavior changes in the quaternary systems composed of [C4mim]Cl, DMSO, KOH, and water at various mass fraction ratios. It can be seen that the IL aqueous solution darkened as the KOH was added. The systems turned from pale yellow to brown, until the two phases were formed. It can also be found that the IL-rich phase was much darker than the KOH-rich phase. The superficial phenomenon observed in Fig. 11 may indicate that physicochemical properties are changed and strongly dependent on the compositions of the systems. The properties, especially, of the upper and lower phase are imperative for the design and scale up of extraction processes. Therefore, attempting at developing the understanding on the physicochemical properties of the quaternary systems and the two phases, the densities and pH value of the quaternary systems composed of various ILs ([C2mim]Cl, [C4mim]Cl, [C6mim]Cl, and [C8mim]Cl), salts (K3PO4, K2HPO4, K2CO3, KOH, KH2PO4, and KCl), DMSO, and water were measured at 298.15 K. Results are displayed in Figs. 12–15.
Table 3 Standard molar thermodynamic functions of transfer of DMSO in the ABS composed of IL + K3PO4 + water at 298.15 K. IL
Dtr Hom (kJ mol1)
Dtr Som (J mol1 K1)
Dtr Gom (kJ mol1)
ln(KDMSO)
R2
[C2mim]Cl [C4mim]Cl [C6mim]Cl [C8mim]Cl [Amim]Cl [BDmim]Cl
4.04 6.18 7.58 4.94 4.72 5.56
9.14 11.08 13.78 6.61 5.40 8.69
1.34 2.90 3.50 3.00 2.38 0.69
1.72 ± 0.25 3.24 ± 0.14 4.11 ± 0.31 3.37 ± 0.18 1.23 ± 0.05 3.32 ± 0.22
0.9879 0.9810 0.9977 0.9899 0.9914 0.9988
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Fig. 11. Image of quaternary systems composed of [C4mim]Cl, DMSO, KOH, and water at different mass fraction ratios: (a) 15/3/0/82; (b) 15/5/5/75; (c) 15/3/10/72; (d) 15/3/20/62; (e) 15/3/27/55.
In Fig. 12, densities (q) as a function of mass fraction of salts are presented. For K3PO4, K2HPO4, K2CO3 and KOH the densities of the systems monotonically increase with the mass fraction of salts increase from 0% to 20% until the two phases were created, while the densities of KOH or KCl solutions increase slightly and then
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remained stable when the saturated solutions were obtained. Moreover, the larger density differences between the fluids, the greater the density gradient force produced by density difference. It was observed that the density difference between the [C4mim]Cl-rich phase and KOH-rich phase is the smallest when compared with K3PO4, K2CO3 and K2HPO4 systems at the same mass fractions. Regarding the salting-out effect for the systems with kosmotropic salts the larger density differences between the two phases, the easier to create ABS. The density data for [Cnmim]Cl-based IL (n = 2, 4, 6, 8) are depicted in Fig. 13. Again, the result shows that the density differences between the two phases increase with the initial concentrations of K3PO4 increase. It should be noted that the density differences of the [C4mim]Cl-based ABS are larger than [C6mim]Cl. The results suggest that the densities of the two phases are dependent on the nature of the ILs and salts, and the initial concentrations of each component. Besides the salting-out ability of inorganic salt, the pH of the aqueous solution plays a crucial role toward the formation of IL-based ABSs. The lg[OH] values of the coexisting phases displayed in Fig. 14, were measured in quaternary systems formed by [Cnmim]Cl (n = 2, 4, 6, 8), DMSO, K3PO4 and water at 298.15 K.
Fig. 12. Experimental density (q) as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (j), homogeneous phase; (d), [C4mim]Cl-rich phase; (N), salt-rich phase.
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Fig. 13. Experimental density gradient of IL-rich phase and K3PO4-rich phase as a function of the mass fraction of K3PO4 in the quaternary systems of [Cnmim]Cl + 5%DMSO + K3PO4 + H2O.
Fig. 15. Experimental lg[OH] of as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (H), KOH; (j), K3PO4; (.), K2CO3; (d), K2HPO4; (), KCl; (N), KH2PO4.
A general trend in the lg[OH] values at the four IL-bases quaternary systems was verified: an increase in the mass ration between K3PO4 and IL leads to an increase in the lg[OH] values. Moreover, in [C2mim]Cl and [C4mim]Cl systems the lg[OH] values of the ILrich phases are higher than the lg[OH] values of the K3PO4-rich phase. However, an opposite trend with the [C6mim]Cl and [C8mim]Cl systems was observed that the lg[OH] values of the IL-rich phases are lower than the lg[OH] values of the K3PO4-rich phase. The pH value of IL-rich phase with salt-rich phase was previously measured by Cláudio and co-workers [38,43]. It was found that the pH values of salt-rich phases were generally lower than the pH values of IL-rich phases [38,43]. However, an opposite trend could also be observed in some other systems composed of different ILs. Even for the same system, such as [C4mim]Br +
KHPO4/H2HPO4 + water system, the opposite result could be obtained for pH values of both phases as the initial concentrations of the IL increased [43]. These results indicate that the pH values of the two phases can be affected by the nature of IL and its initial concentrations. To investigate the influence of the salt on the lg[OH] value, a series of quaternary systems composed of [C4mim]Cl + DMSO + salt + water were prepared, and the pH values of the homogenous solutions and the two phases of ABS were measured. As can be seen in Fig. 15, the lg[OH] values of the systems composed of K3PO4, K2HPO4, K2CO3 or KOH increase with an increase in the mass ratio between the salt and [C4mim]Cl, while the decreased lg[OH] values were observed at the systems composed of KCl and K2HPO4. On the other hand, the lg[OH] values at the
Fig. 14. Experimental lg[OH] as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (j), homogeneous phase; (s), IL-rich phase; (h), K3PO4-rich phase.
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[C4mim]Cl-rich phase (T) are all larger than these at the salt-rich phase (B) in the systems composed of K3PO4, K2CO3 and KOH. Moreover, the difference of the lg[OH] values between two phases for different salts follows the rank: K3PO4 > K2HPO4 > K2CO3 > KOH. The result is agreed well with the trend displayed by the salts to induce ABS and extract DMSO. 4. Conclusions In this context, the IL-based ABSs offer the opportunity to combine IL recycling processes with extraction of cosolvent (DMSO) in the conversion of biomass into added-value chemicals. Aiming at evaluating the effect of the IL cation, the salt anion, the temperature and the concentration of DMSO, the phase diagrams and the partition coefficients of DMSO were determined. For all the studied systems, and at all the conditions analyzed, DMSO preferentially partitions for the IL-rich phases presenting KDMSO > 1. The partition coefficients dependency with the cation alkyl chain length displays a maximum for the system formed by [C6mim]Cl resulting from a decrease in the polar character of the IL cation and an increase in the energy of cavity formation accommodate a DMSO molecule. The substitution of the most acidic hydrogen at C2 by an alkyl group could reduce the formation of hydrogen bonds thus the substitution should weaken the IL and water interactions, and resulted in larger biphasic region. However, the introduction of double bond had a negative impact on the partition coefficients of DMSO. The partition coefficients of DMSO increased monotonically with the initial concentration of the solute added to the system. The partitioning of DMSO was also dependent on the concentrations of IL in both phases. The more IL in IL-rich phase is, the more DMSO dissolved in IL-rich phase. Hence, the KDMSO value in the systems with different salts decreased in the following order K3PO4 > K2HPO4 > K2CO3 > KOH. The increase of temperature reduces the KDMSO value. Moreover, the lg[OH] values of the IL-rich phases are higher than that of the K3PO4-rich phase in [C2mim]Cl and [C4mim]Cl systems. However, an opposite result was observed in the [C6mim]Cl and [C8mim]Cl systems. The density differences between the two phases increase with the initial concentrations of K3PO4 increase. The results obtained improved understanding for industrial applications of the systems here studied. Acknowledgments This research was supported by the National Natural Science Foundation of China (21376088), Project of Production, Education and Research, Guangdong Province and Ministry of Education (2012B09100063, and 2012A090300015), and the Fundamental Research Funds for the Central Universities (2013ZZ0071). The authors would also gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2014. 01.015. References [1] M. Rezaee, Y. Assadi, M.-R. Milani Hosseini, E. Aghaee, F. Ahmadi, S. Berijani, Determination of organic compounds in water using dispersive liquid–liquid microextraction, J. Chromatogr. A 1116 (2006) 1–9. [2] I. Komjarova, R. Blust, Comparison of liquid–liquid extraction, solid-phase extraction and co-precipitation preconcentration methods for the determination of cadmium, copper, nickel, lead and zinc in seawater, Anal. Chim. Acta 576 (2006) 221–228.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Extraction of dimethyl sulfoxide using ionic-liquid-based aqueous biphasic systems Jing Gao, Li Chen, Zong C. Yan ⇑ Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 6 October 2013 Received in revised form 7 January 2014 Accepted 10 January 2014 Available online 24 January 2014 Keywords: Aqueous biphasic systems Ionic liquid Extraction Dimethyl sulfoxide
a b s t r a c t To combine ionic liquid (IL) recovery with cosolvent separation in biomass processing, the quaternary aqueous biphasic systems (ABSs) formed by IL, kosmotropic salt and water in the presence of dimethyl sulfoxide (DMSO) are herein proposed. Three main parameters were evaluated through the phase diagrams and the DMSO partitioning process: the IL cation and anion structure, the salt anion, and the temperature. In all systems and conditions texted, DMSO preferentially dissolves in the IL-rich phase. The results obtained indicate that, the partitioning process is essentially controlled by the IL cation interactions with DMSO. The partition coefficients (KDMSO) displays a maximum for the system formed by [C6mim]Cl. The KDMSO value increases monotonically with the initial concentration of DMSO, and it decreases in the systems with different salts: K3PO4 > K2HPO4 > K2CO3 > KOH. The increase of temperature reduces the partition coefficients. Moreover, densities and pH of both aqueous phases were measured at 298.15 K to evaluate the properties of the systems. The results gathered indicate that the densities and pH values of the two phases can be affected by the nature of IL and its initial concentrations, and the smaller density differences between the fluids, the lower the partition coefficients of DMSO. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Liquid–liquid extraction processes are important techniques used for the recovery and purification steps [1,2]. Aiming at avoiding the use of organic solvents as the extractive phase in traditional liquid–liquid extraction techniques, aqueous biphasic systems (ABSs) which essentially composed of water in both phases were introduced by Albertsson [3]. The ABS formed by mixing of polymer/polymer, polymer/salt or salt/salt in water above critical concentrations can be used for separation of biomolecules [4]. However, most of the aqueous solutions of the phase-forming polymers present high viscosities, and also may alter the kinetics or interfere with the enzyme reaction in some other way [5–7]. In 2003, Rogers and co-workers [8] reported the pioneering research pointing to the possible creation of ABS by the addition of inorganic salts to aqueous solutions of ionic liquids (ILs). Consequently, a wide variety of IL-based ABSs in the presence of inorganic salt [9–12], organic salt [13,14], carbohydrate [15–17], polymer [18–20], amino acid [21], and anionic surfactant [22] have been presented. ILs are salts with a large array of fascinating properties, such as a negligible vapor pressure, low viscosity and high thermal and chemical stability [23]. As ‘‘designer solvents’’, the main advantage of the applicability of IL-based ABSs is the ability ⇑ Corresponding author. Tel.: +86 2013450423151. E-mail address: [email protected] (Z.C. Yan). 1383-5866/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2014.01.015
to tailor their polarities and affinities by a proper manipulation of the cation/anion and their combinations in ILs [24,25]. Therefore, these systems have been extensively studied as successful separation/extraction methods of different compounds and molecules, such as biomolecules [11,26], antibiotics [27], polar solutes [28–31], and biocatalysis [32–34]. Meanwhile, the influence of the IL structure [5,35–40], temperature [20,41,42], pH [34,43], viscosity [22,44], and density [44] on the phase behavior or partition coefficient have been extensively studied. Generally, the use of IL-organic solvent mixtures offers a lower viscosity, provides higher biomass loading, and reduces cost by using less IL [45,46]. Dimethyl sulfoxide (DMSO), a dipolar aprotic solvent, has been widely used as IL’s cosolvent to dissolve starch [47], cellulose [48,49], and lignocellulosic biomass [50]. The previous applications of the mixture of IL and DMSO have been reported, yet, to minimize environmental impacts the recovery of IL together with DMSO has been a problem to solve. Recent advances have shown that the mixed solvents of IL and DMSO which were used as pretreated solvents of lignocellulosic biomass for biogas production were successfully recovered together by forming ABS with K3PO4, and the recovery rates of IL and DMSO were all over 90% [51]. Subsequently, the influence of these dipolar aprotic solvents on the phase behavior of IL-based ABS was further evaluated, and it was shown that biphasic area increases when the concentrations of the dipolar aprotic solvent increase [52]. These literatures deserve further attention of the research community
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working with IL-based ABSs in the presence of dipolar aprotic solvent. Attempting at developing the understanding on the partition of dipolar aprotic solvents in ABSs containing ILs (Fig. 1), and to recovery ILs along with the extraction of its cosolvents, in this work an extensive study was conducted using DMSO as the partitioning molecule. For that purpose, several extraction parameters were studied in quaternary systems composed by imidazolium-based ILs, DMSO, water and inorganic salts, namely the influence of the IL cation, the salt, the temperature of extraction and the concentration of DMSO. Moreover, densities and pH of both monophasic region and biphasic region were measured at 298.15 K to evaluate the properties of the systems. 2. Experimental 2.1. Materials The ionic liquids used in this work to study the formation of ABSs in the presence of dipolar aprotic solvent were the following: 1-ethyl-3-methylimidazolium chloride, [C2mim]Cl; 1-butyl-3methylimidazolium chloride, [C4mim]Cl; 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl; 1-octyl-3-methylimidazolium chloride, [C8mim]Cl; 1-allyl-3-methylimidazolium chloride, [Amim]Cl; 1-butyl-2,3-dimethylimidazolium chloride, [BDmim]Cl; 1-ethyl-3-methylimidazolium bromide, [C2mim]Br; 1-butyl-3methylimidazolium bromide, [C4mim]Br; 1-hexyl-3-methylimidazolium bromide, [C6mim]Br; 1-octyl-3-methylimidazolium bromide, [C8mim]Br. All ionic liquids were supplied by Centre of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, China (purity > 99.0 wt%). Dimethyl sulfoxide, DMSO (purity > 99.0 wt%) was acquired at Sigma Aldrich. The inorganic salts used were K3PO43H2O > 96.0 wt%, K2HPO43H2O > 97.0 wt%, KH2PO4 > 99.5%, K2CO3 P 99.5 wt%, KOH P 99.5 wt%, and KCl P 98 wt%, all from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water was double-distilled and passed by a reverse osmosis system and further treated with a Milli-Q plus 185 water purification apparatus. 2.2. Phase diagrams Aqueous solutions of each salt with variable mass fractions in a range between 40 and 60 wt%, and aqueous solutions of ionic liquid with mass fraction at 80 wt% (the concentration of DMSO in IL was from 0 and 20 wt%) were prepared and used for the determination of the corresponding binodal curves. The phase diagrams were determined through the cloud point titration method at
Fig. 1. Recycling of IL together with DMSO by using ABS method in biomass processing.
various temperatures and atmospheric pressure. The experimental procedure adopted was previously used by us and is described in detail elsewhere [12,52]. The experimental binodal curves were correlated according to the following equation proposed by Merchuk et al. [53]:
Y ¼ a expðbX
0:5
cX 0:3 Þ
ð1Þ
where X and Y are the mass fractions of ionic liquid and salt. The constants a, b and c were obtained by least-squares regression. 2.3. DMSO partitioning The amount of DMSO in each aqueous phase was quantified through material balance method [52]. After the careful separation of both phases, the concentrations of IL and salt in the upper and lower phases were determined by an ion chromatography (Basic IC 792, Methohm, Switzerland). A Karl–Fisher moisture titrator (MKS210, Kyoto Electronics, Kyoto) was used to measure the water content in all samples. The partition coefficients of DMSO (KDMSO), were determined as the ratio of the concentration of DMSO in the ionic liquid and in the inorganic salt aqueous-rich phases, accordingly to:
K DMSO ¼ C T =C B
ð2Þ
where CT and CB are the concentration of DMSO in the top and bottom aqueous-rich phases, respectively. For all the mixtures evaluated, the top layer is the IL-rich phase while the bottom phase is the inorganic salt-rich phase. 2.4. Density and pH measurements The solutions from monophasic region to biphasic region with an increasing mass ratio of K3PO4 to [Cnmim]Cl in the presence of DMSO were prepared. The density (g mL1) of each phase was measured using an automated DES PHOTIME. The pH value was tested by a PHS-25 pH meter equipment. 3. Results and discussion 3.1. Effect of IL structure A broad range of ILs was studied to identify the IL structural features responsible for the formation of ABSs and the extraction of DMSO. The ILs investigated here were [C2mim]Cl, [C4mim]Cl, [C6mim]Cl, [C8mim]Cl, [Amim]Cl, [BDmim]Cl, [C2mim]Br, [C4mim]Br, [C6mim]Br and [C8mim]Br, and all the above ILs were able to promote ABSs with K3PO4 in the presence of DMSO. The structures of the ILs studied are displayed in Fig. 2. The experimental weight fraction data for each phase diagram are reported in Supporting Information. The binodal curves of IL + K3PO4 + water + 20% DMSO are plotted in Fig. 3. The binodal curves of all systems were correlated with Eq. (1), and the respective parameters a, b, c, along with their corresponding standard deviations were obtained by the nonlinear regression. The results are listed in Table 1. From the correlation coefficients (R2) obtained, it is safe to admit that Eq. (1) provides a good description of the experimental data. The phase diagram in Fig. 3 provides information about the concentration of phase-forming components required to form two phases, and the concentration of phase components in the top and bottom phases. Moreover, the closer to the axis origin a binodal curve is, the higher the IL hydrophobicity, and the higher their ability to phase split [25]. Taking into the account the alkyl chain length in [Cnmim]Cl, the size of the ILs with alkyl chains shorter than the hexyl is the dominant effect on the formation of ABSs, while self-aggregate in
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Fig. 2. Ionic structures of the studied ionic liquids and DMSO.
Table 1 Correlation parameters (a, b, and c) obtained by the regression of the experimental binodal data through the application of Eq. (1) (and respective standard deviations, r) for the IL + salt + DMSO + water systems at 298 K and atmospheric pressure. c
R2
106r
IL + DMSO (20%) + K3PO4 + water at 298.15 K [C2mim]Cl 0.53 ± 0.02 2.61 ± 0.11 [C4mim]Cl 0.49 ± 0.02 2.33 ± 0.13 [C6mim]Cl 0.59 ± 0.02 3.02 ± 0.10 [C8mim]Cl 0.52 ± 0.01 2.25 ± 0.07 [BDmim]Cl 0.52 ± 0.01 2.51 ± 0.05 [Amim]Cl 0.87 ± 0.04 3.68 ± 0.17 [C2mim]Br 0.61 ± 0.01 2.63 ± 0.27 [C4mim]Br 0.60 ± 0.01 2.64 ± 0.92 [C6mim]Br 0.58 ± 0.01 2.75 ± 0.06 [C8mim]Br 0.66 ± 0.02 2.91 ± 0.11
37.51 ± 0.45 42.99 ± 0.38 60.24 ± 0.27 61.35 ± 0.90 50.15 ± 0.73 28.29 ± 0.95 48.36 ± 0.86 66.90 ± 0.05 79.68 ± 0.67 94.22 ± 0.57
0.9992 0.9962 0.9994 0.9970 0.9992 0.9911 0.9998 0.9973 0.9992 0.9980
4.01 40.0 2.12 20.0 10.0 51.2 3.40 50.0 10.0 30.0
[C4mim]Cl + salt + water at 298.15 K K3PO4 0.61 ± 0.05 2.27 ± 0.12 K2HPO4 0.91 ± 0.02 3.05 ± 0.20 K2CO3 1.01 ± 0.01 3.32 ± 0.04 KOH 0.89 ± 0.06 2.48 ± 0.17
21.78 ± 0.09 20.80 ± 0.13 42.86 ± 0.81 38.64 ± 0.99
0.9992 0.9978 0.9992 0.9991
1.00 14.4 9.79 4.78
Fig. 3. Phase diagrams of various ILs + 20% DMSO + K3PO4 + water systems at 298 K and atmospheric pressure: (h) [C2mim]Br, (s) [C4mim]Br, (N) [C6mim]Br, (r) [C8mim]Br, () [C2mim]Cl, (J) [C4mim]Cl, (4) [C6mim]Cl, (d) [C8mim]Cl, (H) [Amim]Cl, (+) [BDmim]Cl.
[C4mim]Cl + DMSO (5%) + salt + water at 298.15 K K3PO4 0.49 ± 0.03 3.12 ± 0.08 46.23 ± 0.51 K2HPO4 0.57 ± 0.07 2.99 ± 0.24 12.54 ± 0.54 K2CO3 0.76 ± 0.01 2.99 ± 0.05 42.63 ± 0.88 KOH 1.02 ± 0.05 2.95 ± 0.12 39.30 ± 0.01
0.9936 0.9842 0.9991 0.9983
18.6 28.6 10.0 9.05
aqueous media for the ILs above [C6mim]Cl is a dominant effect [40]. However, when moving from [C2mim]Br to [C8mim]Br, increasing the alkyl chain length of the IL cation enhances the fluid’s overall hydrophobicity and increases the creation ability of ABSs. Moreover, the substitution of the most acidic hydrogen at C2 by an alkyl group could reduce the hydrogen bonding of the IL cation with water. Hence, [BDmim]Cl was more easily phase separated than [C4mim]Cl. On the other hand, the presence of a
[C4mim]Cl + DMSO (20%) + salt + water at 298.15 K K2HPO4 0.51 ± 0.05 3.42 ± 0.16 19.30 ± 0.58 K2CO3 0.63 ± 0.01 2.87 ± 0.02 43.04 ± 0.42 KOH 0.47 ± 0.04 1.08 ± 0.23 55.97 ± 0.90
0.9918 0.9998 0.9967
19.2 2.03 20.0
[C4mim]Cl + DMSO (20%) + K3PO4 + water at various temperatures 288.15 K 0.55 ± 0.02 2.67 ± 0.21 32.56 ± 0.25 0.9997 308.15 K 0.57 ± 0.03 2.43 ± 0.15 20.17 ± 0.42 0.9997 318.15 K 0.60 ± 0.06 2.53 ± 0.11 21.86 ± 0.26 0.9990 328.15 K 0.66 ± 0.04 1.96 ± 0.23 23.68 ± 0.35 0.9901
17.1 7.55 20.0 35.4
Variable
a
b
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double bond (allyl group) at [Amim]Cl decreased the ability of ABS to form. That is because the functionalized group enhance the hydrophilic character [25]. It has been demonstrated that the IL aptitude (hydrophobicity) for creating ABSs was based on their hydrogen bond basicity (b) [38]. The b value is one important parameter of Kamlet–Taft parameters, and is a measure of the ability of the solvent to accept a proton. To investigate the effect of b value of the ILs on the hydrophobicity to form ABSs, the b value of the ILs studied in this work were determined by following the procedure reported by Ohno et al. [54]. The b values are presented in Fig. 4. It can be seen that the increasing alkyl side chain length decreases the b value of IL. Fig. 4 reveals that a close relationship exists between the ability of each IL to form ABS and their hydrogen bond basicity values: the lower the b values is, the more efficient the IL is in creating ABS. Although it was found in the previous work that DMSO preferentially partitions for the IL-rich phase [52], the extraction of a molecule with ABS strongly depends on the ability to manipulate the physical/chemical properties of the phases which are due to the chemical nature of the system. To evaluate the influence of the IL cation and anion structure in the extraction of DMSO, the selected ILs allow the study of the alkyl side chain length effect, as well as the study of additional functional groups. All of these studies were performed at 298.15 K. The partition coefficients are present in Fig. 5 and the results show that KDMSO ranges between 1 and 4 (at approximately the same mass fraction compositions of IL and K3PO4). Regarding the IL structure influence, the ability of ILs to dissolve DMSO at initial concentration of 20% follows the order: [C6mim]Cl > [BDmim]Cl [C8mim]Cl > [C4mim]Cl [C8mim]Br > [C 6 mim]Br > [C 4 mim]Br > [C 2 mim]Cl > [Amim]Cl > [C 2 mim]Br. Although DMSO is a dipolar aprotic solvent, the hydrogen at IL cation was found to form weak hydrogen bond (C–H. . .O) with the oxygen atom of DMSO [55]. Besides, the increase of the alkyl side chain length leads to a decrease on the strength of the coulombic and polar interactions, while increasing the dispersive interactions, between the IL ions [29]. However, increasing the size of the alkyl side chain it increases the IL free volume while decreasing the surface tension of the system [56], and thus decreasing the energy of cavity formation to accommodate a DMSO molecule. Moreover, the reversed micelle formation in the IL-rich phases is also the factor responsible for the lower extraction capacity of the systems with [Cnmim]Cl (n > 6) [57]. Therefore, these contributions act in
Fig. 4. Hydrogen-bond basicity (b) as a function of ILs molar mass.
Fig. 5. Partition coefficients of DMSO in IL (22 wt%) and K3PO4 (35 wt%) ABS as a function of different ILs.
different directions leading to the maximum value on the partition coefficients at [C6mim]Cl. It should be pointed out that for all the ILs studied, there is an increase in the DMSO partition coefficients with the initial concentration from 5% to 20%. That is because the increase in content increases the formation of C–H. . .O between IL cation and the O of DMSO, leading to the higher concentration of DMSO in IL-rich phase. That suggests the initial concentration of DMSO largely influences its extraction. 3.2. Effect of salting-out effect It has been demonstrated that the addition of high chargedensity salts to aqueous solutions of ILs leads to liquid–liquid demixing due to a preferential hydration of the charge density salt over the IL, leading to the salting-out of the IL to the IL-rich phase [58]. To carry out a study with the salts used commonly (K3PO4, K2HPO4, K2CO3, and KOH), the solutions of [C4mim]Cl containing DMSO ranges from 5% and 20% were prepared. The phase diagrams of the various systems are graphically presented in Fig. 6. It should be stressed that the amount of water complexed to K3PO4 and K2HPO4 was removed in the calculations of the mass of salts and added to the water composition of each phase diagram. Fig. 6 reports the binodal curves for K3PO4, K2HPO4, K2CO3, and KOH and allows an evaluation of the salt anion effect in the formation of IL-based ABS in the presence of DMSO with different contents. The larger the two-phase region, the stronger the salt’s ability to induce the ABS formation. When a mass ratio of DMSO to [C4mim]Cl equals in each system, the salting-out ability of the salts follows the order K3PO4 > K2HPO4 > K2CO3 > KOH which is in good agreement with the Hofmeister series. The similar trend for the salting-out effect of the salts was previously observed by others [9,12]. The results suggest that the molar entropy of hydration of the salt ions is the essential driving force for ABS formation, in spite of the addition of dipolar aprotic solvent. However, it should be stressed that the addition of DMSO could enhance the separation of the two aqueous phases, resulting in a lesser amount of salt. For instance, the biphasic region of the [C4mim]Cl + 20% DMSO + K2CO3 + water system was much larger than [C4mim]Cl + K2CO3 + water system, and even larger than [C4mim]Cl + K3PO4 + water system. In summary, the salting-out effect is mainly the result of the formation of water-ion complexes that cause the
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Table 2 The parameters obtained by the regression of the experimental data using Eq. (3).
K3PO4 K2HPO4 KH2PO4 K2CO3 KOH KCl
Fig. 6. Phase diagrams for [C4mim]Cl and selected salts with different concentrations of DMSO at 298 K and atmospheric pressure: (j), K3PO4; (h), K3PO4 + 5% DMSO; (I), K3PO4 + 20% DMSO; (d), K2HPO4; (s), K2HPO4 + 5% DMSO; (), K2HPO4 + 20% DMSO; (N), K2CO3; (4), K2CO3 + 5% DMSO; (|), K2CO3 + 20% DMSO; (), KOH; (}), KOH + 5% DMSO; (+), KOH + 20% DMSO.
dehydration of the IL and induce the ABS formation, but the influence of DMSO can be considerable and should not be neglected in simulations and design calculations. To further evaluate the interactions of salt with water, the influence of different salts with increasing concentrations on the hydrogen bond basicity (b) of water was investigated, and the result is presented in Fig. 7. A general trend in the b values of the salt solutions was verified: an increase in the molar ratio of salt to water leads to an increase in the b values. Table 2 presents the parameters obtained by the regression of the experimental data using Eq. (4):
b ¼ bw þ kM
ð3Þ
where b and bw are hydrogen bond basicity of the salt solutions and water, respectively. M is the molar ratio of salt to water, and k is the slope. As can be seen by the correlation coefficients (R2) obtained, Eq. (3) provides a good description of the experimental b values of the solutions as a function of the salts concentration. It means
Fig. 7. Hydrogen-bond basicity (b) as a function of molar ratio of salt to H2O: (j), K3PO4; (d), K2HPO4; (r), K2CO3; (), KOH; (N), KH2PO4; (}), KCl.
b0
k
R2
0.0887 0.0932 0.1035 0.1012 0.1018 0.1006
120.35 89.77 32.21 66.53 36.71 24.59
0.9856 0.9870 0.9915 0.9938 0.9919 0.9674
that the b values of the salt solutions are linearly related to the molar ratio of the salt to H2O. In general, the salt solutions with larger k values require less salt to induce the separation of the ABS, resulting in a binodal curve closer to the axis and to a large biphasic region. Fig. 8 displays the partition coefficients of DMSO in dependence of the salts. Large organic cations such as [C4mim]+ are very poorly solvated by water but are well-solvated by aprotic solvents [59]. A series of three stock systems with compositions within upper region of the phase diagram containing different salts were studied, and the mass of [C4mim]Cl, salt and water are the same (18/22/62, 19/24/57, and 22/28/50) for each system. The results show that the concentration of [C4mim]Cl in IL-rich phase is in the following order K3PO4 > K2HPO4 > K2CO3 > KOH. The more IL in IL-rich phase is, the more DMSO transferred to IL-rich phase. As shown in Fig. 7, the coefficients decreased according to the predicted order. 3.3. Effect of temperature The influence of temperature on the phase diagrams of [C4mim]Cl-based IL ABS with K3PO4 in the presence of 20% DMSO is shown in Fig. 9. Lower temperatures are favourable for the creation of [C4mim]Cl-based ABS. That is because the higher the temperature, the higher are the salt and IL concentrations required for phase separation. In the present context, the solvation of [C4mim]Cl was enhanced and the ionic interaction was weakened with increasing temperature [60]. On the other hand, hydrogen bond weakening with the increase of the temperature led to weakening of salt solvation. Hence, from the viewpoint of IL-DMSO interactions, it may be concluded that the interactions of IL and DMSO are enhanced at lower temperature and the unfavourable interactions of IL with water (hydration) are reduced. Previously, several literatures reported that the temperature significantly influences the extraction efficiency of proteins [61], amino acid [42], and vanillin [44]. Therefore, aimed at evaluation the temperature influence in the extraction of DMSO (20%), six
Fig. 8. Partition coefficients of DMSO in the ABSs containing [C4mim]Cl (22 wt%) and different salts (35 wt%).
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In order to calculate the DMSO thermodynamic parameters of transfer, such as the standard molar Gibbs energy (Dtr G0m ), the standard molar enthalpy (Dtr H0m ), and the standard molar entropy of transfer (Dtr S0m ), the van’t Hoff approach was used. The plots of ln(KDMSO) versus 1/T for the six ILs studied, in the temperature range from 288.15 K to 328.15 K. These parameters reveal the association equilibrium between the DMSO compositions in two different fluids. The following isochors were used to determine the molar thermodynamic functions of transfer (Eqs. (4)–(6)) [44]:
lnðK DMSO Þ ¼
Fig. 9. Phase diagrams for [C4mim]Cl and K3PO4 with DMSO of 20% at different temperatures and atmospheric pressure: (h), 288 K; (d), 298 K; (N), 308 K; (r), 318 K; (), 328 K.
systems were selected ([C2mim]Cl, [C4mim]Cl, [C6mim]Cl, [C8mim]Cl, [Amim]Cl, and [BDmim]Cl). KDMSO was determined at the following temperatures: 288.15 K, 298.15 K, 308.15 K, 318.15 K, and 328.15 K. The data gathered, and accurate mass fraction compositions for each, are displayed in Fig. 10. The six systems, and at all temperatures, present a KDMSO larger than 1.0. Indeed, the increase of temperature reduces the partition coefficients of DMSO. The results indicate that the temperature has a significant effect in the DMSO partitioning.
Dtr H0m 1 Dtr S0m þ T R R
ð4Þ
Dtr G0m ¼ Dtr H0m T Dtr S0m
ð5Þ
Dtr G0m ¼ RT lnðK DMSO Þ
ð6Þ
where KDMSO is the partition coefficient of DMSO between IL-rich and the salt-rich phases, R is the universal gas constant, and T is the temperature. The plots of ln(KDMSO) versus 1/T exhibit linearity indicating that the molar enthalpy of transfer of DMSO is temperature independent. The molar thermodynamic functions of transfer at 298.15 K, obtained by the linear least-square analysis, are summarized in Table 3. For all the systems studied, Dtr G0m is shown to be negative, which in turn reflects the spontaneous and preferential partitioning DMSO for the IL-rich phase. Besides, the value of Dtr G0m is the lowest in [C6mim]Cl-based ABS when compared to other systems. Moreover, the Dtr H0m negative values reveal that the transfer of DMSO between the IL-rich phase and the salt-rich phase is an exothermic process. The three functions of transfer largely depend on the IL cation, and these results again suggest that the partitioning process is essentially controlled by the cation interactions with DMSO. 3.4. Density and pH value
Fig. 10. Partition coefficients of DMSO in IL (22 wt%) and K3PO4 (35 wt%) ABS as the function of temperature for the ILs: (j), [C2mim]Cl; (h), [C4mim]Cl; (N), [C6mim]Cl; (s), [C8mim]Cl; (), [Amim]Cl; (I), [BDmim]Cl.
Fig. 11 shows the color and phase behavior changes in the quaternary systems composed of [C4mim]Cl, DMSO, KOH, and water at various mass fraction ratios. It can be seen that the IL aqueous solution darkened as the KOH was added. The systems turned from pale yellow to brown, until the two phases were formed. It can also be found that the IL-rich phase was much darker than the KOH-rich phase. The superficial phenomenon observed in Fig. 11 may indicate that physicochemical properties are changed and strongly dependent on the compositions of the systems. The properties, especially, of the upper and lower phase are imperative for the design and scale up of extraction processes. Therefore, attempting at developing the understanding on the physicochemical properties of the quaternary systems and the two phases, the densities and pH value of the quaternary systems composed of various ILs ([C2mim]Cl, [C4mim]Cl, [C6mim]Cl, and [C8mim]Cl), salts (K3PO4, K2HPO4, K2CO3, KOH, KH2PO4, and KCl), DMSO, and water were measured at 298.15 K. Results are displayed in Figs. 12–15.
Table 3 Standard molar thermodynamic functions of transfer of DMSO in the ABS composed of IL + K3PO4 + water at 298.15 K. IL
Dtr Hom (kJ mol1)
Dtr Som (J mol1 K1)
Dtr Gom (kJ mol1)
ln(KDMSO)
R2
[C2mim]Cl [C4mim]Cl [C6mim]Cl [C8mim]Cl [Amim]Cl [BDmim]Cl
4.04 6.18 7.58 4.94 4.72 5.56
9.14 11.08 13.78 6.61 5.40 8.69
1.34 2.90 3.50 3.00 2.38 0.69
1.72 ± 0.25 3.24 ± 0.14 4.11 ± 0.31 3.37 ± 0.18 1.23 ± 0.05 3.32 ± 0.22
0.9879 0.9810 0.9977 0.9899 0.9914 0.9988
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Fig. 11. Image of quaternary systems composed of [C4mim]Cl, DMSO, KOH, and water at different mass fraction ratios: (a) 15/3/0/82; (b) 15/5/5/75; (c) 15/3/10/72; (d) 15/3/20/62; (e) 15/3/27/55.
In Fig. 12, densities (q) as a function of mass fraction of salts are presented. For K3PO4, K2HPO4, K2CO3 and KOH the densities of the systems monotonically increase with the mass fraction of salts increase from 0% to 20% until the two phases were created, while the densities of KOH or KCl solutions increase slightly and then
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remained stable when the saturated solutions were obtained. Moreover, the larger density differences between the fluids, the greater the density gradient force produced by density difference. It was observed that the density difference between the [C4mim]Cl-rich phase and KOH-rich phase is the smallest when compared with K3PO4, K2CO3 and K2HPO4 systems at the same mass fractions. Regarding the salting-out effect for the systems with kosmotropic salts the larger density differences between the two phases, the easier to create ABS. The density data for [Cnmim]Cl-based IL (n = 2, 4, 6, 8) are depicted in Fig. 13. Again, the result shows that the density differences between the two phases increase with the initial concentrations of K3PO4 increase. It should be noted that the density differences of the [C4mim]Cl-based ABS are larger than [C6mim]Cl. The results suggest that the densities of the two phases are dependent on the nature of the ILs and salts, and the initial concentrations of each component. Besides the salting-out ability of inorganic salt, the pH of the aqueous solution plays a crucial role toward the formation of IL-based ABSs. The lg[OH] values of the coexisting phases displayed in Fig. 14, were measured in quaternary systems formed by [Cnmim]Cl (n = 2, 4, 6, 8), DMSO, K3PO4 and water at 298.15 K.
Fig. 12. Experimental density (q) as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (j), homogeneous phase; (d), [C4mim]Cl-rich phase; (N), salt-rich phase.
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Fig. 13. Experimental density gradient of IL-rich phase and K3PO4-rich phase as a function of the mass fraction of K3PO4 in the quaternary systems of [Cnmim]Cl + 5%DMSO + K3PO4 + H2O.
Fig. 15. Experimental lg[OH] of as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (H), KOH; (j), K3PO4; (.), K2CO3; (d), K2HPO4; (), KCl; (N), KH2PO4.
A general trend in the lg[OH] values at the four IL-bases quaternary systems was verified: an increase in the mass ration between K3PO4 and IL leads to an increase in the lg[OH] values. Moreover, in [C2mim]Cl and [C4mim]Cl systems the lg[OH] values of the ILrich phases are higher than the lg[OH] values of the K3PO4-rich phase. However, an opposite trend with the [C6mim]Cl and [C8mim]Cl systems was observed that the lg[OH] values of the IL-rich phases are lower than the lg[OH] values of the K3PO4-rich phase. The pH value of IL-rich phase with salt-rich phase was previously measured by Cláudio and co-workers [38,43]. It was found that the pH values of salt-rich phases were generally lower than the pH values of IL-rich phases [38,43]. However, an opposite trend could also be observed in some other systems composed of different ILs. Even for the same system, such as [C4mim]Br +
KHPO4/H2HPO4 + water system, the opposite result could be obtained for pH values of both phases as the initial concentrations of the IL increased [43]. These results indicate that the pH values of the two phases can be affected by the nature of IL and its initial concentrations. To investigate the influence of the salt on the lg[OH] value, a series of quaternary systems composed of [C4mim]Cl + DMSO + salt + water were prepared, and the pH values of the homogenous solutions and the two phases of ABS were measured. As can be seen in Fig. 15, the lg[OH] values of the systems composed of K3PO4, K2HPO4, K2CO3 or KOH increase with an increase in the mass ratio between the salt and [C4mim]Cl, while the decreased lg[OH] values were observed at the systems composed of KCl and K2HPO4. On the other hand, the lg[OH] values at the
Fig. 14. Experimental lg[OH] as a function of mass fraction of salts in the quaternary systems of [C4mim]Cl + 5%DMSO + salt + H2O: (j), homogeneous phase; (s), IL-rich phase; (h), K3PO4-rich phase.
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[C4mim]Cl-rich phase (T) are all larger than these at the salt-rich phase (B) in the systems composed of K3PO4, K2CO3 and KOH. Moreover, the difference of the lg[OH] values between two phases for different salts follows the rank: K3PO4 > K2HPO4 > K2CO3 > KOH. The result is agreed well with the trend displayed by the salts to induce ABS and extract DMSO. 4. Conclusions In this context, the IL-based ABSs offer the opportunity to combine IL recycling processes with extraction of cosolvent (DMSO) in the conversion of biomass into added-value chemicals. Aiming at evaluating the effect of the IL cation, the salt anion, the temperature and the concentration of DMSO, the phase diagrams and the partition coefficients of DMSO were determined. For all the studied systems, and at all the conditions analyzed, DMSO preferentially partitions for the IL-rich phases presenting KDMSO > 1. The partition coefficients dependency with the cation alkyl chain length displays a maximum for the system formed by [C6mim]Cl resulting from a decrease in the polar character of the IL cation and an increase in the energy of cavity formation accommodate a DMSO molecule. The substitution of the most acidic hydrogen at C2 by an alkyl group could reduce the formation of hydrogen bonds thus the substitution should weaken the IL and water interactions, and resulted in larger biphasic region. However, the introduction of double bond had a negative impact on the partition coefficients of DMSO. The partition coefficients of DMSO increased monotonically with the initial concentration of the solute added to the system. The partitioning of DMSO was also dependent on the concentrations of IL in both phases. The more IL in IL-rich phase is, the more DMSO dissolved in IL-rich phase. Hence, the KDMSO value in the systems with different salts decreased in the following order K3PO4 > K2HPO4 > K2CO3 > KOH. The increase of temperature reduces the KDMSO value. Moreover, the lg[OH] values of the IL-rich phases are higher than that of the K3PO4-rich phase in [C2mim]Cl and [C4mim]Cl systems. However, an opposite result was observed in the [C6mim]Cl and [C8mim]Cl systems. The density differences between the two phases increase with the initial concentrations of K3PO4 increase. The results obtained improved understanding for industrial applications of the systems here studied. Acknowledgments This research was supported by the National Natural Science Foundation of China (21376088), Project of Production, Education and Research, Guangdong Province and Ministry of Education (2012B09100063, and 2012A090300015), and the Fundamental Research Funds for the Central Universities (2013ZZ0071). The authors would also gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2014. 01.015. References [1] M. Rezaee, Y. Assadi, M.-R. Milani Hosseini, E. Aghaee, F. Ahmadi, S. Berijani, Determination of organic compounds in water using dispersive liquid–liquid microextraction, J. Chromatogr. A 1116 (2006) 1–9. [2] I. Komjarova, R. Blust, Comparison of liquid–liquid extraction, solid-phase extraction and co-precipitation preconcentration methods for the determination of cadmium, copper, nickel, lead and zinc in seawater, Anal. Chim. Acta 576 (2006) 221–228.
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