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Solubilities of imidazolium-based ionic liquids in aqueous salt solutions at 298.15 K
J. Chem. Thermodynamics 43 (2011) 1174–1177
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Solubilities of imidazolium-based ionic liquids in aqueous salt solutions at 298.15 K Xiao-Ming Peng, Yu-Feng Hu ⇑, Chuan-Wei Jin State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase Behavior & Property Research Laboratory, China University of Petroleum, Beijing 102249, China
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Article history: Received 15 September 2010 Received in revised form 23 January 2011 Accepted 1 March 2011 Available online 6 March 2011 Keywords: Ionic liquids Solubilities Aqueous solutions Inorganic salts
a b s t r a c t The solubilities of ionic liquids in the ternary systems (ionic liquid + H2O + inorganic salt) were reported at 298.15 K and atmospheric pressure. The examined ionic liquids are [C4mim][PF6] (1-n-butyl-3-methylimidazolium hexafluorophosphate), [C8mim][PF6] (1-n-octyl-3-methylimidazolium hexafluorophosphate), and [C8mim][BF4] (1-n-octyl-3-methylimidazolium tetrafluoroborate). The examined inorganic salts are the chloride-based salts (sodium chloride, lithium chloride, potassium chloride, and magnesium chloride) and the sodium-based salts (sodium thiocyanate, sodium nitrate, sodium trifluoroacetate, sodium bromide, sodium iodide, sodium perchlorate, sodium acetate, sodium hydroxide, sodium dihydrogen phosphate, sodium phosphate, sodium tetrafluoroborate, sodium sulfate, and sodium carbonate). The effects of the cations and the anions of the ionic liquids and of the inorganic salts on the solubility of the ionic liquids in the ternary solutions were systematically compared and discussed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ionic liquids (ILs) are room temperature molten salts which consist of large organic cations and typically smaller inorganic or organic anions. They have recently garnered increased attention from scientific community and chemical industry because of their particular properties, such as high solvation ability [1] and coordination properties, general nonflammability [2], wide liquidus range, wide electrochemical window, high thermal stability [3,4], and negligible vapor pressures [5]. In addition, ILs present the prospect of ‘‘designer solvents’’ since their polarities, viscosities, and hydrophobicities can be finely tuned by changing the chemical structures of their constituent ions [6]. The use of ILs as the potential solvents in the process of separation and extraction has got a wide range of active research. Rogers and co-authors [7] have obtained promising results in the liquid– liquid extraction in the (IL + substituted-benzene derivatives + H2O) systems. Fadeev and Meagher [8] found that ILs have potential as extractants in recovery of butyl alcohol from fermentation broth and that the solubility of ILs in water is one of important factors affecting selectivity of butyl alcohol extraction from aqueous solutions. Papaiconomou et al. [9] have successfully used the water-immiscible ILs based on the [BF4] anion in the extraction of metal ions such as Ni2+, Zn2+, Cd2+, and Hg2+ from wastewater. The solubility of ILs in the aqueous stream is an important issue, as the residue ILs can cause undesirable environmental issues due to their potential toxicity and limited biodegradability. The addi⇑ Corresponding author. Tel./fax: +86 10 89732658. E-mail address: [email protected] (Y.-F. Hu). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.03.002
tion of inorganic salts enhanced liquid–liquid phase separation in the systems involving ILs and aqueous phases, as firstly qualitatively revealed by Dupont et al. [10] and then quantitatively confirmed by Gutowski et al. [11]. This approach may constitute a strategy to solve the problems of IL contamination in aqueous ecosystem in the future [10,11]. To the best of our knowledge, the effect of the addition of several inorganic salts on the solubilities of [Cnmim]+-based ILs ([Cnmim]+ = 1-alkyl-3-methylimidazolium) in water has been reported. However, the [Cnmim]+-based ILs that have been investigated are usually limited to n 6 4. Trindade et al. [12] investigated the effect of the addition of the inorganic salt, NaCl, Na2SO4, or Na3PO4, on the liquid–liquid (L–L) phase diagram of the system {H2O + [C4mim][BF4]} and found that these salts trigger salting-out effects, leading to significant upward shifts of the L–L demixing temperatures of the systems. The salting-out effects were also observed by Najdanovic-Visak et al. [13] when they added K3PO4 to aqueous solutions of [C2mim][EtSO4] ([EtSO4] = ethyl sulfate), of [C4mim][MeSO4] ([MeSO4] = methyl sulfate), and of [Cnmim]Cl with n ¼ 4; 8; 10. Freire et al. [14–16] evaluated the effect of a large series of inorganic and organic salts on the mutual solubilities of water and [C4mim][Tf2N] {[Tf2N] = bis(trifuoromethylsulfonyl)imide)} or [C4mim][C(CN)3]. These authors proposed the mechanisms of salting-in- and salting-out-inducing ions in aqueous solutions of ILs and provided a lot of evidences supporting their mechanisms. In the present study, the solubilities of [C4mim][PF6], [C8mim][PF6], and [C8mim][BF4] in the ternary system (IL + inorganic salt + H2O) were determined by ‘‘cloud point’’ method and 1 H NMR spectroscopy. The effects of a series of sodium-based salts and several chloride salts on the solubilities of the selected ILs in water were investigated.
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N-methylimidazole (mass fraction purity > 0.99), 1-chlorobutane (>0.99) and 1-chlorooctane (>0.99) were purchased from Shanghai Jiachen Chemical Co., Ltd. The ILs ([C4mim][PF6], [C8mim][PF6], and [C8mim][BF4]) were prepared and purified according to the methods reported in references [17,18]. To reduce the water content and the volatile compounds to negligible values, the ILs were dried at vacuum and at 343.15 K, using continuous stirring, for a minimum of 48 h. Their mass fraction purities were checked by 1H NMR spectra interpretation using JEOL ECA-600 NMR spectrometer and found to be >0.99. The water content in the ILs was less than 0.00016 mass fractions, as determined by Karl Fischer titration analysis. Double-distilled deionized water was used in all experiments. The following inorganic salts were acquired from Sinopharm Chemical Reagent Beijing Co., Ltd.: sodium chloride (NaCl, AR, >0.998), sodium thiocyanate (NaSCN, AR, >0.985), sodium nitrate (NaNO3, AR, >0.995), potassium chloride (KCl, GR, >0.998), sodium trifluoroacetate (CF3COONa, AR, >0.97), sodium bromide (NaBr, AR, >0.99), sodium iodide (NaI, AR, >0.99), magnesium chloride hexahydrate (MgCl26H2O, AR, >0.98), sodium acetate (NaCO2CH3, AR, >0.99), sodium hydroxide(NaOH, AR, >0.98), sodium dihydrogen phosphate (NaH2PO4, AR, >0.99), sodium phosphate dodecahydrate (Na3PO412H2O, AR, >0.98), and sodium tetrafluoroborate (NaBF4, CP, >0.98). Potassium hexafluorophosphate (KPF6, AR, >0.99) was acquired from Shanghai Jiachen Chemical Co., Ltd. Sodium sulfate (Na2SO4, AR, >0.96) and D-toluene (AR, >0.996) were purchased from Beijing Modern Eastern Fine Chemical. Co., Ltd. Sodium carbonate (Na2CO3, AR, >0.998) was purchased from North of Beijing Fine Chemicals Co., Ltd. Lithium chloride monohydrate (LiClH2O, AR, >0.97) was obtained from Beijing Yili Fine Chemical Co., Ltd. Sodium perchlorate monohydrate (NaClO4H2O, AR, >0.995) was supplied by Beijing Fangshan Yihua Chemical Factory. All the reagents were used without further purification. The apparatus for the cloud point measurements is similar to that used by Marsh et al. [19]. It consists of a jacketed glass vessel that contains a magnetic stirrer connected to a temperature controlled circulating bath (controlled to ±0.01 K). The vessel can be flushed with dry nitrogen and can be sealed to prevent evaporation of the solvent. The temperature was measured with a PT100 thermometer with an evaluated standard uncertainty of ±0.01 K. The circulating bath (TC-502D) was supplied by Brookfield Engineering Laboratories, Inc., which could control the water bath within ±0.01 K. All solutions were prepared by mass using a CPA225D (Sartorius) balance immediately before use. The uncertainty was ±5 105 g. The water contents of the ILs were determined with a ZKF-1 Karl Fischer (KF) Titrator. 1H spectra were collected on a JEOL ECA-600 NMR spectrometer operating at 600.17 MHz. A total of 64 scans were collected for each sample, and the chemical shifts were quoted in parts per million. All NMR experiments were performed at 298.15 ± 0.01 K. 2.2. Experimental procedure
3. Results and discussion The solubilities of the examined ILs in water measured by different authors have been compared and large discrepancies have been observed [21,22]. The possible reasons are as follows: (1) the solubilities of the examined ILs in water are generally small and the equilibration time required to achieve the thermodynamic equilibrium is long. Accordingly, it is difficult to measure the solubilities of the examined ILs accurately; (2) variable methods have been applied to determine the solubilities of the ILs in aqueous solutions, including UV-spectroscopy, thermogravimetry, 1H NMR, Fisher titration, electrospray ionization mass spectrometry, and ion-selective electrodes [20,2326]. Each of them has its own limitation [26] and will contribute to the deviations to different extents. The solubilities measured in the present study are compared with the data reported in the literature [8,21,2530] in figure 1 and table 1. The present results agree well with the data reported by Alfassi et al. [25] for [C4mim][PF6], Chun et al. [30] for [C8mim][PF6], and Anthony et al. [27] for [C8mim][BF4] and their deviations are 11.02%, 6.57% and 6.47%, respectively. In order to check the accuracy of the 1H NMR method, the binary solutions [C4mim][PF6] + H2O (0.01954 mol kg1) and [C8mim][PF6] + H2O (0.001921 mol kg1) were prepared by mass (these molalities are much lower than the solubilities of the two 80 70 60
mlit-mexp /mlit
2.1. Materials and apparatus
ments were to estimate the solubilities of ILs and simultaneously to prepare aqueous solutions saturated with ionic liquids. After the cloud point solubility has been determined, the solution was rested 48 h at 298.15 K and the molality of the IL in IL-saturated solution was determined using the method of Rickert et al. [20], which is described briefly as follows. After centrifugation, a known aliquot of the aqueous phase was dried on a rotary evaporator and the residue taken up in deuterated toluene for NMR analysis. The [Cnmim]+ content of each sample was determined by comparison of the area of the Cn-side chain methylene ((CH2)2) or methyl (CH3) proton peak to that of a series of standards prepared by dissolution of the same IL in D-toluene. In the process of dealing with spectrogram, the area or intensity under each of the peaks specified in the individual monograph was integrated not fewer than five times.
100*
2. Experimental section
50 40 30 20 10 0 -10
The procedure of the ‘‘cloud point’’ measurements is as follows. Each measurement was started with the addition of a known mass of binary salt solution to the vessel at 298.15 K. The IL was then added drop by drop under vigorous stirring until the cloud point was visually observed. This ternary solution was continuously stirred for 48 h. If the real thermodynamic equilibrium has been achieved, then the cloud point solubility was determined. Note that, the solubilities of the ILs in aqueous solutions were determined by 1H NMR. The above-mentioned cloud point measure-
280
290
300
310
320
T/K FIGURE 1. Relative deviations between the experimental molality ILs in water obtained in this work (mexp) and those reported in the literature (mlit): j, [C4mim][PF6] (reference[27]); s, [C8mim][PF6] (reference [27]); /, [C8mim][BF4] (reference [27]); d, [C4mim][PF6] (reference [28]); N, [C4mim][PF6] (reference [21]); 4, [C8mim][PF6] (reference [21]); ., [C4mim][PF6] (reference [25]); , [C4mim][PF6] (reference [26]); J, [C4mim][PF6] (reference [8]); 5, [C8mim][PF6] (reference [8]); I, [C4mim][PF6] (reference [29]); h, [C4mim][PF6] (reference [30]); and }, [C8mim][PF6] (reference [30]).
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TABLE 1 Solubilities of the three ILs in the ternary solutions at T = 298.15 K, molalities, m/ (mol kg1). Salta
m/(mol kg1) [C8mim][PF6]
KCl LiCl MgCl2 NaBr NaI NaOH NaH2PO4 NaNO3 NaBF4 NaCl CH3COONa NaSCN Na2CO3 Na2SO4 Na3PO4 NaClO4 CF3COONa
0.009607 0.006443 0.005733 0.007508 0.00556 0.004806 0.004043 0.006729 0.002475 0.007405 0.005753 0.004976 0.004913 0.003444 0.003399 0.002069 0.00796
Pure H2O
0.005493 0.005879b 0.01033e 0.00705f 0.02072c
[C8mim][BF4]
[C4mim][PF6]
0.05391
0.063 0.05073 0.01167 0.05528 0.06916 0.06496c
0.07146 0.06742 0.06139 0.1004 0.04005 0.04545 0.03541 0.06523 0.07447 0.05784 0.065d 0.08276e 0.06725f 0.07624g 0.07184c 0.07037h 0.08659i 0.06617b
a
The molality of the inorganic salt in the ternary solution was 0.4 and 0.5 mol kg1 for the [C8mim]+-based ILs and the [C4mim]+-based ILs, respectively. b Reference [30]. c Reference [27]. d Reference [25]. e Reference [8]. f Reference [21] g Reference [26]. h Reference [28]. i Reference [29].
ILs in water). Then the molalities of the two solutions were determined using the method of Rickert et al. [20]. The differences between the prepared and measured molalities are 2.54% and 3.73%, respectively. A measurement uncertainty for quantitative NMR of 1.5% can be achieved for a confidence interval of 95% (k = 2) with universal parameters [31]. The typical NMR spectra of the solubility measurements are shown in figures A1 and A2 (supplementary information). The solubilities of [C4mim][PF6] in the ternary solutions [C4mim][PF6] + MiXi + H2O (MiXi = NaSCN, CF3COONa, NaBF4, NaCl, 1 NaClO4 and CH3COONa; mMi Xi 0:5 mol kg ) are greater than its solubility in pure water. In addition, the solubilities of [C4mim][PF6] in these ternary solutions follows the order SCN > CF3 COO > BF By contrary, 4 > Cl > ClO4 > CH3 COO . the addition of Na2CO3, Na2SO4, or Na3PO4 decreases the solubility of [C4mim][PF6] and the solubilities of [C4mim][PF6] in the three 2 3 ternary solutions decrease from SO2 4 through CO3 to PO4 . There3 fore, the PO4 anion induces the strongest salting-out effect but the SCN anion shows the strongest salting-in effect. These results are in accordance with those observed in reference [15]. The solubilities of [C8mim][PF6] in the ternary solutions [C8mim][PF6] + MnClm + H2O (MnClm = KCl, NaCl, LiCl, and MgCl2; 1 mMn Clm 0:4 mol kg ) were determined to explore the effect of inorganic cations on the solubilities of ILs in aqueous solutions. The results are shown in table 1. It is seen that the addition of these salts promotes the solubility of [C8mim][PF6] and that the solubilities of [C8mim][PF6] in the ternary solutions increase in the order Mg2+ < Li+ < Na+ < K+. It is clear that the relative effect of these cations is inversely related to their charge density.
The solubilities of [C8mim][PF6] in the ternary solutions [C8mim][PF6] + Nan Xm + H2O (Nan Xm ¼ NaCl; NaBr; NaI; NaNO3 ; CF3 COONa; CH3 COONa; NaSCN; Na2 CO3 ; NaOH; NaH2 PO4 ; Na2 SO4 ; 1 Na3 PO4 ; NaBF4 ; and NaClO4 ; mNan Xm 0:4 mol kg ) were determined to investigate the effect of different anions on the solubilities of ILs in aqueous solutions. The results are also shown in table 1. It is clear that the effect of the anion on increasing the IL solubility in water follows the order CF3 COO > Br > Cl > NO3 > CH3 COO > I . The order for the salting-out effect of the 2 3 anions is SCN < CO2 3 < OH < H2 PO4 < SO4 < PO4 < BF4 < ClO4 . Note that the anions SCN, BF , and ClO show the salting4 4 in effect in the ternary solutions [C4mim][PF6] + M i X i + H2O. For the purpose of comparison, the solubilities of [C8mim][BF4] in [C8mim][BF4] + Mi Xi + H2O (Mi Xi ¼ Na2 SO4 ; CF3 COONa; NaNO3 ; Na3 PO4 ; or NaClO4 ) were also determined and the results are shown in table 1. The solubilities of [C8mim][BF4] in the ternary 3 solutions decrease in the order SO2 4 > CF3 COO > NO3 > PO4 > ClO4 . The addition of NaNO3 increases the solubility of [C8mim][PF6] but decreases the solubility of [C8mim][BF4]. The interactions of the anion of the IL with the anion of the salt may play a dominant role. NO 3 , with disperse charge, may have the dispersive forces and electrostatic repulsion interaction with the anions of the ILs. It may have a stronger electrostatic repulsion with the anion [BF4] but a larger dispersion forces with the anion [PF6]. It should be noted that hydrolysis occurred in aqueous solutions of the ILs based on the [BF4] and [PF6] anions [32–35]. The pH values of [C8mim][BF4] and [C8mim][PF6] aqueous solutions with and without the presence of NaNO3 are shown in figures A3 and A4 of supplementary information. It is seen that the existence of NaNO3 facilitated the hydrolysis of [BF4] and [PF6]. Various decomposition products may lead to more complex phase behavior in the studied systems. Therefore, we cannot provide an exact explanation for the effect of NaNO3 on the solubilities of [C8mim][PF6] and [C8mim][BF4] at present. It can be seen from table 1 that the salting-out effect of PO3 4 on the solubility of [C8mim][PF6] is more pronounced than those of SO2 4 , H 2 PO4 , and OH , indicating that the salting-out effect of the anion increases with increasing the valence. This phenomenon was also observed in the cases of anions + PEGs (polyethylene glycols) [36]. Both experimental studies [14,15] and molecular simulations [37,38] indicated that the salting-out inducing ions (high charge density) act mainly through an entropic effect resulting from the formation of water-ion hydration complexes which cause the dehydration of the solute and the increase of the surface tension of the cavity. However, in fact, the surface tension increment 3 of SO2 4 was larger than that of PO4 but the hydrate number of the latter is much more and entropic effect is more consumingly. Thus, the results presented in the current study also provide a sample to show that it is difficult to infer the specificity of the contribution of the modification of the interfacial tension of the cavity formed for the mechanism of salting-out [14,15]. The solubilities of [C4mim][PF6] and [C8mim][PF6] in the ternary solutions show that the solubilities of the ILs decrease with increasing the cation chain length. Besides, it is seen from table 1 that the CF3COO and CH3COO anions and the halide ions increase the solubilities of [C4mim][PF6] and [C8mim][PF6], whereas the an2 3 ions CO2 3 , SO4 , and PO4 always exert the opposite effect on the solubilities of the two ILs. Similarly, the solubilities of [C8mim][BF4] in the examined ternary solutions are always greater than the solubilities of its counterpart, [C8mim][PF6]. Therefore, the presence of inorganic salts does not change the dependence of the solubilities of ILs in water on the cation chain length and on the anion structure. For the overall effect of inorganic salts, Na2CO3, Na2SO4, and Na3PO4 always exert the salting-out effect on the solubilities of the examined ILs, implying that carbonates, phosphates, and sulfates can be used to reduce the solubility of ILs in water.
X.-M. Peng et al. / J. Chem. Thermodynamics 43 (2011) 1174–1177
4. Conclusions The solubilities of the ILs [C4mim][PF6], [C8mim][PF6], and [C8mim][BF4] in the ternary solutions IL + H2O + inorganic salt were determined at 298.15 K and atmospheric pressure. The effects of the inorganic salts on the solubilities of ILs were systematically compared and discussed. It was observed that the solubility of the ILs having the same cation increases from [PF6] to [BF4]. For the inorganic salts with a fixed anion, the solubilities of the ILs show a strong dependence on the nature of the inorganic cations. In addition, the inorganic anion with a higher valence is better salting-out agent than anion with a lower valence. Carbonates, phosphates, and sulfates have strong salting-out effect on the solubility of the [Cnmim]+-based ILs. Acknowledgments The authors thank the National Natural Science Foundation of China (20976189 and 21076224, 21036008, and 20925623) and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-06-0088) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2011.03.002. References [1] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [2] M. Smiglak, W.M. Reichert, J.D. Holbrey, J.S. Wilkes, L. Sun, J.S. Thrasher, K. Kirichenko, S. Singh, A.R. Katritzky, R.D. Rogers, Chem. Commun. 24 (2006) 2554–2556. [3] K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M. Pringle, J.L. Scott, Aust. J. Chem. 57 (2004) 145–147. [4] U. Doman´ska, Thermochim. Acta 448 (2006) 19–30. [5] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123–150. [6] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398. [7] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. 16 (1998) 1765–1766. [8] A.G. Fadeev, M.M. Meagher, Chem. Commun. 3 (2001) 295–296. [9] N. Papaiconomou, J. Salminen, J.M. Prausnitz, Book of abstracts, in: 16th Symposium on Thermophysical Properties, Boulder, Colorado, 2006, p. 55. [10] J. Dupont, C.S. Consorti, P.A.Z. Suarez, R.F. de Souza, L.S. Hegedus (Ed.), Organic Syntheses, vol. 79, John Wiley, Chichester, UK, 2002, pp. 236243.
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JCT 10-330
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Solubilities of imidazolium-based ionic liquids in aqueous salt solutions at 298.15 K Xiao-Ming Peng, Yu-Feng Hu ⇑, Chuan-Wei Jin State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase Behavior & Property Research Laboratory, China University of Petroleum, Beijing 102249, China
a r t i c l e
i n f o
Article history: Received 15 September 2010 Received in revised form 23 January 2011 Accepted 1 March 2011 Available online 6 March 2011 Keywords: Ionic liquids Solubilities Aqueous solutions Inorganic salts
a b s t r a c t The solubilities of ionic liquids in the ternary systems (ionic liquid + H2O + inorganic salt) were reported at 298.15 K and atmospheric pressure. The examined ionic liquids are [C4mim][PF6] (1-n-butyl-3-methylimidazolium hexafluorophosphate), [C8mim][PF6] (1-n-octyl-3-methylimidazolium hexafluorophosphate), and [C8mim][BF4] (1-n-octyl-3-methylimidazolium tetrafluoroborate). The examined inorganic salts are the chloride-based salts (sodium chloride, lithium chloride, potassium chloride, and magnesium chloride) and the sodium-based salts (sodium thiocyanate, sodium nitrate, sodium trifluoroacetate, sodium bromide, sodium iodide, sodium perchlorate, sodium acetate, sodium hydroxide, sodium dihydrogen phosphate, sodium phosphate, sodium tetrafluoroborate, sodium sulfate, and sodium carbonate). The effects of the cations and the anions of the ionic liquids and of the inorganic salts on the solubility of the ionic liquids in the ternary solutions were systematically compared and discussed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ionic liquids (ILs) are room temperature molten salts which consist of large organic cations and typically smaller inorganic or organic anions. They have recently garnered increased attention from scientific community and chemical industry because of their particular properties, such as high solvation ability [1] and coordination properties, general nonflammability [2], wide liquidus range, wide electrochemical window, high thermal stability [3,4], and negligible vapor pressures [5]. In addition, ILs present the prospect of ‘‘designer solvents’’ since their polarities, viscosities, and hydrophobicities can be finely tuned by changing the chemical structures of their constituent ions [6]. The use of ILs as the potential solvents in the process of separation and extraction has got a wide range of active research. Rogers and co-authors [7] have obtained promising results in the liquid– liquid extraction in the (IL + substituted-benzene derivatives + H2O) systems. Fadeev and Meagher [8] found that ILs have potential as extractants in recovery of butyl alcohol from fermentation broth and that the solubility of ILs in water is one of important factors affecting selectivity of butyl alcohol extraction from aqueous solutions. Papaiconomou et al. [9] have successfully used the water-immiscible ILs based on the [BF4] anion in the extraction of metal ions such as Ni2+, Zn2+, Cd2+, and Hg2+ from wastewater. The solubility of ILs in the aqueous stream is an important issue, as the residue ILs can cause undesirable environmental issues due to their potential toxicity and limited biodegradability. The addi⇑ Corresponding author. Tel./fax: +86 10 89732658. E-mail address: [email protected] (Y.-F. Hu). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.03.002
tion of inorganic salts enhanced liquid–liquid phase separation in the systems involving ILs and aqueous phases, as firstly qualitatively revealed by Dupont et al. [10] and then quantitatively confirmed by Gutowski et al. [11]. This approach may constitute a strategy to solve the problems of IL contamination in aqueous ecosystem in the future [10,11]. To the best of our knowledge, the effect of the addition of several inorganic salts on the solubilities of [Cnmim]+-based ILs ([Cnmim]+ = 1-alkyl-3-methylimidazolium) in water has been reported. However, the [Cnmim]+-based ILs that have been investigated are usually limited to n 6 4. Trindade et al. [12] investigated the effect of the addition of the inorganic salt, NaCl, Na2SO4, or Na3PO4, on the liquid–liquid (L–L) phase diagram of the system {H2O + [C4mim][BF4]} and found that these salts trigger salting-out effects, leading to significant upward shifts of the L–L demixing temperatures of the systems. The salting-out effects were also observed by Najdanovic-Visak et al. [13] when they added K3PO4 to aqueous solutions of [C2mim][EtSO4] ([EtSO4] = ethyl sulfate), of [C4mim][MeSO4] ([MeSO4] = methyl sulfate), and of [Cnmim]Cl with n ¼ 4; 8; 10. Freire et al. [14–16] evaluated the effect of a large series of inorganic and organic salts on the mutual solubilities of water and [C4mim][Tf2N] {[Tf2N] = bis(trifuoromethylsulfonyl)imide)} or [C4mim][C(CN)3]. These authors proposed the mechanisms of salting-in- and salting-out-inducing ions in aqueous solutions of ILs and provided a lot of evidences supporting their mechanisms. In the present study, the solubilities of [C4mim][PF6], [C8mim][PF6], and [C8mim][BF4] in the ternary system (IL + inorganic salt + H2O) were determined by ‘‘cloud point’’ method and 1 H NMR spectroscopy. The effects of a series of sodium-based salts and several chloride salts on the solubilities of the selected ILs in water were investigated.
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N-methylimidazole (mass fraction purity > 0.99), 1-chlorobutane (>0.99) and 1-chlorooctane (>0.99) were purchased from Shanghai Jiachen Chemical Co., Ltd. The ILs ([C4mim][PF6], [C8mim][PF6], and [C8mim][BF4]) were prepared and purified according to the methods reported in references [17,18]. To reduce the water content and the volatile compounds to negligible values, the ILs were dried at vacuum and at 343.15 K, using continuous stirring, for a minimum of 48 h. Their mass fraction purities were checked by 1H NMR spectra interpretation using JEOL ECA-600 NMR spectrometer and found to be >0.99. The water content in the ILs was less than 0.00016 mass fractions, as determined by Karl Fischer titration analysis. Double-distilled deionized water was used in all experiments. The following inorganic salts were acquired from Sinopharm Chemical Reagent Beijing Co., Ltd.: sodium chloride (NaCl, AR, >0.998), sodium thiocyanate (NaSCN, AR, >0.985), sodium nitrate (NaNO3, AR, >0.995), potassium chloride (KCl, GR, >0.998), sodium trifluoroacetate (CF3COONa, AR, >0.97), sodium bromide (NaBr, AR, >0.99), sodium iodide (NaI, AR, >0.99), magnesium chloride hexahydrate (MgCl26H2O, AR, >0.98), sodium acetate (NaCO2CH3, AR, >0.99), sodium hydroxide(NaOH, AR, >0.98), sodium dihydrogen phosphate (NaH2PO4, AR, >0.99), sodium phosphate dodecahydrate (Na3PO412H2O, AR, >0.98), and sodium tetrafluoroborate (NaBF4, CP, >0.98). Potassium hexafluorophosphate (KPF6, AR, >0.99) was acquired from Shanghai Jiachen Chemical Co., Ltd. Sodium sulfate (Na2SO4, AR, >0.96) and D-toluene (AR, >0.996) were purchased from Beijing Modern Eastern Fine Chemical. Co., Ltd. Sodium carbonate (Na2CO3, AR, >0.998) was purchased from North of Beijing Fine Chemicals Co., Ltd. Lithium chloride monohydrate (LiClH2O, AR, >0.97) was obtained from Beijing Yili Fine Chemical Co., Ltd. Sodium perchlorate monohydrate (NaClO4H2O, AR, >0.995) was supplied by Beijing Fangshan Yihua Chemical Factory. All the reagents were used without further purification. The apparatus for the cloud point measurements is similar to that used by Marsh et al. [19]. It consists of a jacketed glass vessel that contains a magnetic stirrer connected to a temperature controlled circulating bath (controlled to ±0.01 K). The vessel can be flushed with dry nitrogen and can be sealed to prevent evaporation of the solvent. The temperature was measured with a PT100 thermometer with an evaluated standard uncertainty of ±0.01 K. The circulating bath (TC-502D) was supplied by Brookfield Engineering Laboratories, Inc., which could control the water bath within ±0.01 K. All solutions were prepared by mass using a CPA225D (Sartorius) balance immediately before use. The uncertainty was ±5 105 g. The water contents of the ILs were determined with a ZKF-1 Karl Fischer (KF) Titrator. 1H spectra were collected on a JEOL ECA-600 NMR spectrometer operating at 600.17 MHz. A total of 64 scans were collected for each sample, and the chemical shifts were quoted in parts per million. All NMR experiments were performed at 298.15 ± 0.01 K. 2.2. Experimental procedure
3. Results and discussion The solubilities of the examined ILs in water measured by different authors have been compared and large discrepancies have been observed [21,22]. The possible reasons are as follows: (1) the solubilities of the examined ILs in water are generally small and the equilibration time required to achieve the thermodynamic equilibrium is long. Accordingly, it is difficult to measure the solubilities of the examined ILs accurately; (2) variable methods have been applied to determine the solubilities of the ILs in aqueous solutions, including UV-spectroscopy, thermogravimetry, 1H NMR, Fisher titration, electrospray ionization mass spectrometry, and ion-selective electrodes [20,2326]. Each of them has its own limitation [26] and will contribute to the deviations to different extents. The solubilities measured in the present study are compared with the data reported in the literature [8,21,2530] in figure 1 and table 1. The present results agree well with the data reported by Alfassi et al. [25] for [C4mim][PF6], Chun et al. [30] for [C8mim][PF6], and Anthony et al. [27] for [C8mim][BF4] and their deviations are 11.02%, 6.57% and 6.47%, respectively. In order to check the accuracy of the 1H NMR method, the binary solutions [C4mim][PF6] + H2O (0.01954 mol kg1) and [C8mim][PF6] + H2O (0.001921 mol kg1) were prepared by mass (these molalities are much lower than the solubilities of the two 80 70 60
mlit-mexp /mlit
2.1. Materials and apparatus
ments were to estimate the solubilities of ILs and simultaneously to prepare aqueous solutions saturated with ionic liquids. After the cloud point solubility has been determined, the solution was rested 48 h at 298.15 K and the molality of the IL in IL-saturated solution was determined using the method of Rickert et al. [20], which is described briefly as follows. After centrifugation, a known aliquot of the aqueous phase was dried on a rotary evaporator and the residue taken up in deuterated toluene for NMR analysis. The [Cnmim]+ content of each sample was determined by comparison of the area of the Cn-side chain methylene ((CH2)2) or methyl (CH3) proton peak to that of a series of standards prepared by dissolution of the same IL in D-toluene. In the process of dealing with spectrogram, the area or intensity under each of the peaks specified in the individual monograph was integrated not fewer than five times.
100*
2. Experimental section
50 40 30 20 10 0 -10
The procedure of the ‘‘cloud point’’ measurements is as follows. Each measurement was started with the addition of a known mass of binary salt solution to the vessel at 298.15 K. The IL was then added drop by drop under vigorous stirring until the cloud point was visually observed. This ternary solution was continuously stirred for 48 h. If the real thermodynamic equilibrium has been achieved, then the cloud point solubility was determined. Note that, the solubilities of the ILs in aqueous solutions were determined by 1H NMR. The above-mentioned cloud point measure-
280
290
300
310
320
T/K FIGURE 1. Relative deviations between the experimental molality ILs in water obtained in this work (mexp) and those reported in the literature (mlit): j, [C4mim][PF6] (reference[27]); s, [C8mim][PF6] (reference [27]); /, [C8mim][BF4] (reference [27]); d, [C4mim][PF6] (reference [28]); N, [C4mim][PF6] (reference [21]); 4, [C8mim][PF6] (reference [21]); ., [C4mim][PF6] (reference [25]); , [C4mim][PF6] (reference [26]); J, [C4mim][PF6] (reference [8]); 5, [C8mim][PF6] (reference [8]); I, [C4mim][PF6] (reference [29]); h, [C4mim][PF6] (reference [30]); and }, [C8mim][PF6] (reference [30]).
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TABLE 1 Solubilities of the three ILs in the ternary solutions at T = 298.15 K, molalities, m/ (mol kg1). Salta
m/(mol kg1) [C8mim][PF6]
KCl LiCl MgCl2 NaBr NaI NaOH NaH2PO4 NaNO3 NaBF4 NaCl CH3COONa NaSCN Na2CO3 Na2SO4 Na3PO4 NaClO4 CF3COONa
0.009607 0.006443 0.005733 0.007508 0.00556 0.004806 0.004043 0.006729 0.002475 0.007405 0.005753 0.004976 0.004913 0.003444 0.003399 0.002069 0.00796
Pure H2O
0.005493 0.005879b 0.01033e 0.00705f 0.02072c
[C8mim][BF4]
[C4mim][PF6]
0.05391
0.063 0.05073 0.01167 0.05528 0.06916 0.06496c
0.07146 0.06742 0.06139 0.1004 0.04005 0.04545 0.03541 0.06523 0.07447 0.05784 0.065d 0.08276e 0.06725f 0.07624g 0.07184c 0.07037h 0.08659i 0.06617b
a
The molality of the inorganic salt in the ternary solution was 0.4 and 0.5 mol kg1 for the [C8mim]+-based ILs and the [C4mim]+-based ILs, respectively. b Reference [30]. c Reference [27]. d Reference [25]. e Reference [8]. f Reference [21] g Reference [26]. h Reference [28]. i Reference [29].
ILs in water). Then the molalities of the two solutions were determined using the method of Rickert et al. [20]. The differences between the prepared and measured molalities are 2.54% and 3.73%, respectively. A measurement uncertainty for quantitative NMR of 1.5% can be achieved for a confidence interval of 95% (k = 2) with universal parameters [31]. The typical NMR spectra of the solubility measurements are shown in figures A1 and A2 (supplementary information). The solubilities of [C4mim][PF6] in the ternary solutions [C4mim][PF6] + MiXi + H2O (MiXi = NaSCN, CF3COONa, NaBF4, NaCl, 1 NaClO4 and CH3COONa; mMi Xi 0:5 mol kg ) are greater than its solubility in pure water. In addition, the solubilities of [C4mim][PF6] in these ternary solutions follows the order SCN > CF3 COO > BF By contrary, 4 > Cl > ClO4 > CH3 COO . the addition of Na2CO3, Na2SO4, or Na3PO4 decreases the solubility of [C4mim][PF6] and the solubilities of [C4mim][PF6] in the three 2 3 ternary solutions decrease from SO2 4 through CO3 to PO4 . There3 fore, the PO4 anion induces the strongest salting-out effect but the SCN anion shows the strongest salting-in effect. These results are in accordance with those observed in reference [15]. The solubilities of [C8mim][PF6] in the ternary solutions [C8mim][PF6] + MnClm + H2O (MnClm = KCl, NaCl, LiCl, and MgCl2; 1 mMn Clm 0:4 mol kg ) were determined to explore the effect of inorganic cations on the solubilities of ILs in aqueous solutions. The results are shown in table 1. It is seen that the addition of these salts promotes the solubility of [C8mim][PF6] and that the solubilities of [C8mim][PF6] in the ternary solutions increase in the order Mg2+ < Li+ < Na+ < K+. It is clear that the relative effect of these cations is inversely related to their charge density.
The solubilities of [C8mim][PF6] in the ternary solutions [C8mim][PF6] + Nan Xm + H2O (Nan Xm ¼ NaCl; NaBr; NaI; NaNO3 ; CF3 COONa; CH3 COONa; NaSCN; Na2 CO3 ; NaOH; NaH2 PO4 ; Na2 SO4 ; 1 Na3 PO4 ; NaBF4 ; and NaClO4 ; mNan Xm 0:4 mol kg ) were determined to investigate the effect of different anions on the solubilities of ILs in aqueous solutions. The results are also shown in table 1. It is clear that the effect of the anion on increasing the IL solubility in water follows the order CF3 COO > Br > Cl > NO3 > CH3 COO > I . The order for the salting-out effect of the 2 3 anions is SCN < CO2 3 < OH < H2 PO4 < SO4 < PO4 < BF4 < ClO4 . Note that the anions SCN, BF , and ClO show the salting4 4 in effect in the ternary solutions [C4mim][PF6] + M i X i + H2O. For the purpose of comparison, the solubilities of [C8mim][BF4] in [C8mim][BF4] + Mi Xi + H2O (Mi Xi ¼ Na2 SO4 ; CF3 COONa; NaNO3 ; Na3 PO4 ; or NaClO4 ) were also determined and the results are shown in table 1. The solubilities of [C8mim][BF4] in the ternary 3 solutions decrease in the order SO2 4 > CF3 COO > NO3 > PO4 > ClO4 . The addition of NaNO3 increases the solubility of [C8mim][PF6] but decreases the solubility of [C8mim][BF4]. The interactions of the anion of the IL with the anion of the salt may play a dominant role. NO 3 , with disperse charge, may have the dispersive forces and electrostatic repulsion interaction with the anions of the ILs. It may have a stronger electrostatic repulsion with the anion [BF4] but a larger dispersion forces with the anion [PF6]. It should be noted that hydrolysis occurred in aqueous solutions of the ILs based on the [BF4] and [PF6] anions [32–35]. The pH values of [C8mim][BF4] and [C8mim][PF6] aqueous solutions with and without the presence of NaNO3 are shown in figures A3 and A4 of supplementary information. It is seen that the existence of NaNO3 facilitated the hydrolysis of [BF4] and [PF6]. Various decomposition products may lead to more complex phase behavior in the studied systems. Therefore, we cannot provide an exact explanation for the effect of NaNO3 on the solubilities of [C8mim][PF6] and [C8mim][BF4] at present. It can be seen from table 1 that the salting-out effect of PO3 4 on the solubility of [C8mim][PF6] is more pronounced than those of SO2 4 , H 2 PO4 , and OH , indicating that the salting-out effect of the anion increases with increasing the valence. This phenomenon was also observed in the cases of anions + PEGs (polyethylene glycols) [36]. Both experimental studies [14,15] and molecular simulations [37,38] indicated that the salting-out inducing ions (high charge density) act mainly through an entropic effect resulting from the formation of water-ion hydration complexes which cause the dehydration of the solute and the increase of the surface tension of the cavity. However, in fact, the surface tension increment 3 of SO2 4 was larger than that of PO4 but the hydrate number of the latter is much more and entropic effect is more consumingly. Thus, the results presented in the current study also provide a sample to show that it is difficult to infer the specificity of the contribution of the modification of the interfacial tension of the cavity formed for the mechanism of salting-out [14,15]. The solubilities of [C4mim][PF6] and [C8mim][PF6] in the ternary solutions show that the solubilities of the ILs decrease with increasing the cation chain length. Besides, it is seen from table 1 that the CF3COO and CH3COO anions and the halide ions increase the solubilities of [C4mim][PF6] and [C8mim][PF6], whereas the an2 3 ions CO2 3 , SO4 , and PO4 always exert the opposite effect on the solubilities of the two ILs. Similarly, the solubilities of [C8mim][BF4] in the examined ternary solutions are always greater than the solubilities of its counterpart, [C8mim][PF6]. Therefore, the presence of inorganic salts does not change the dependence of the solubilities of ILs in water on the cation chain length and on the anion structure. For the overall effect of inorganic salts, Na2CO3, Na2SO4, and Na3PO4 always exert the salting-out effect on the solubilities of the examined ILs, implying that carbonates, phosphates, and sulfates can be used to reduce the solubility of ILs in water.
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4. Conclusions The solubilities of the ILs [C4mim][PF6], [C8mim][PF6], and [C8mim][BF4] in the ternary solutions IL + H2O + inorganic salt were determined at 298.15 K and atmospheric pressure. The effects of the inorganic salts on the solubilities of ILs were systematically compared and discussed. It was observed that the solubility of the ILs having the same cation increases from [PF6] to [BF4]. For the inorganic salts with a fixed anion, the solubilities of the ILs show a strong dependence on the nature of the inorganic cations. In addition, the inorganic anion with a higher valence is better salting-out agent than anion with a lower valence. Carbonates, phosphates, and sulfates have strong salting-out effect on the solubility of the [Cnmim]+-based ILs. Acknowledgments The authors thank the National Natural Science Foundation of China (20976189 and 21076224, 21036008, and 20925623) and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-06-0088) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2011.03.002. References [1] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [2] M. Smiglak, W.M. Reichert, J.D. Holbrey, J.S. Wilkes, L. Sun, J.S. Thrasher, K. Kirichenko, S. Singh, A.R. Katritzky, R.D. Rogers, Chem. Commun. 24 (2006) 2554–2556. [3] K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M. Pringle, J.L. Scott, Aust. J. Chem. 57 (2004) 145–147. [4] U. Doman´ska, Thermochim. Acta 448 (2006) 19–30. [5] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123–150. [6] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398. [7] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. 16 (1998) 1765–1766. [8] A.G. Fadeev, M.M. Meagher, Chem. Commun. 3 (2001) 295–296. [9] N. Papaiconomou, J. Salminen, J.M. Prausnitz, Book of abstracts, in: 16th Symposium on Thermophysical Properties, Boulder, Colorado, 2006, p. 55. [10] J. Dupont, C.S. Consorti, P.A.Z. Suarez, R.F. de Souza, L.S. Hegedus (Ed.), Organic Syntheses, vol. 79, John Wiley, Chichester, UK, 2002, pp. 236243.
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JCT 10-330