Liquid–Liquid Equilibria of the Aqueous Two-Phase Systems of Ionic

Article pubs.acs.org/jced

Liquid−Liquid Equilibria of the Aqueous Two-Phase Systems of Ionic Liquid 1‑Butyl-3-methylimidazolium Tetrafluoroborate and Sodium Dihydrogen Phosphate/Disodium Hydrogen Phosphate or Their Mixtures Hekun Lv,†,‡ Zhenxi Jiang,† Yanhong Li,† and Baozeng Ren*,† †

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, Henan, 450000, China Department of Chemical Engineering, Henan Institute of Engineering, Zhengzhou, Henan, 450000, China

‡

ABSTRACT: Liquid−liquid equilibria of the aqueous twophase systems of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate and sodium dihydrogen phosphate/ disodium hydrogen phosphate or their mixtures have been determined at 298.15 K, 303.15 K, and 308.15 K. The nonlinear equation developed by Merchuk was used to correlate the experimental binodal data. The two-phase regions gradually expand with the increase of the percentage of Na2HPO4 in the salt mixtures. It was also shown that the two-phase region decreases along with the temperature increases. The tie-line data were correlated by Othmer−Tobias and Bancraft equations. It was found that these equations fit the experimental data well. The temperature dependence on tie lines has been discussed. The effect of the concentration of the salt mixtures on binodal curves and tie-line data has also been discussed.

been studied.18−21 When purifying some active enzymes with IL + buffer ATPS's, it was found that enzymes were enriched in the IL-enriched top phase with stabilized performance.21 For developing and optimizing the extraction process, liquid−liquid equilibrium (LLE) data of ATPS's with different compositions at different temperatures are necessary. Several phase diagrams and LLE data of IL-bassed ATPS's have been determined and correlated.10,22−24 It was found that both ILs and inorganic salts have an effect on the phase equilibrium of the IL-based ATPS's: the salting-out strength of the kosmotropic salts follows the Hofmeister series; for ILs, the number of alkyl groups at the cation, the length of cation side alkyl chain, and the double bonds, aromatic rings, hydroxyl groups on this alkyl chain, and the change of anion could influence the formation of ATPS's.10,17,23 However, there are only a few reports on IL-based ATPS's, and very limited experimental data of their phase diagram have been reported. Furthermore, there are still some questions on the phase behavior of IL-based ATPS's, like the [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS, one of our research systems. Wang et al. postulated the inability of Na2HPO4 to induce the phase separation of the tetrafluoroboratebased IL,25 but phase separation does occur for this system.2 This may because of the low solubility of Na2HPO4, which cannot

1. INTRODUCTION Aqueous two-phase systems (ATPS's), which were usually composed of two or more polymers, a polymer and a salt, have been recognized as an economical and efficient downstream processing method.1−3 Due to the absence of traditional volatile organic solvents (VOCs), ATPS's are considered to be environmentally friendly.4 Ionic liquids (ILs) are thought to be another alternative to VOCs due to their “green” characteristics such as the negligible vapor pressure and high thermal and chemical stability.5,6 ILs may be used directly in the separation process and could also be designed and tuned by the different combinations of cations and anions to allow for different separation techniques.5 In recent years, ionic liquid based (IL-based) ATPS's have attracted much attention.3,7,8 IL-based ATPS's, which were usually composed of hydrophilic ILs and kosmotropic salts, were found by Gutowski and his co-workers for the first time.9 However, some hydrophobic but water-soluble ILs, such as 1-butyl3-methyl-imidazolium trifluoromethanesulfonate ([Bmim][CF3SO3]), could also produce ATPS's with proper salts.10 As certain ILs could maintain the protein structure (and enzymatic activity),11 IL-based ATPS's can be suitable solvents for the extraction of such biomolecules.12 IL-based ATPS's have been used in the separation, concentration, and purification of biomaterials,3,13,14 drug molecules,15 and small organic species,7,16 with higher yields than traditional processes.17 NaH2PO4 and Na2HPO4 are chosen as the studied salts because their mixtures are widely used as buffer in biotechnology. Several IL + buffer ATPS's have © 2012 American Chemical Society

Received: December 1, 2011 Accepted: August 1, 2012 Published: August 22, 2012 2379

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

provide enough ions for the formation of ATPS's.8 The studied IL, [Bmim][BF4], was found to be unstable in the absence of water in recent years.26 However, a study on [Bmim][BF4]-based ATPS's could still help us to understand IL-based ATPS's. However, systematic reports on the liquid−liquid equilibrium for IL-based ATPS's systems are very necessary. In this work, we reported liquid−liquid equilibrium data for the [Bmim][BF4] + sodium phosphate salts (NaH2PO4, Na2HPO4 or their mixtures) + water ATPS's at 298.15 K, 303.15 K, and 308.15 K. The binodal curves were analyzed by the Merchuk equation, and the tie-line data were correlated by the Othmer−Tobias and Bancroft equations. The temperature dependence on the phase diagram and the tie lines has been discussed. The relationship for the fitting parameters of Merchuk equation with the molecule weight M of the salts or salt mixtures was also discussed.

2.2. Experimental Procedure. Concentrated Na2HPO4 and NaH2PO4 solutions with a known concentration were first prepared by weight. Three (NaH2PO4 + Na2HPO4) solutions were prepared, with different mole ratios (nNaH2PO4:nNa2HPO4= 0.5, 1, and 2). The binodal curves were obtained by cloud-point titration28 at a fixed temperature using aqueous inorganic salt (NaH2PO4, Na2HPO4 or their mixtures) stock solutions prepared above and a series of aqueous IL ([Bmim][BF4]) solutions. A glass tube, which was placed in thermostat bath, was used to determine the phase diagrams. The temperature was controlled by using a water thermostat (model DC-2006, Shanghai BiLang Instrument, China), with a precision of ± 0.05 K. A salt solution was first placed into the tube, and then the [Bmim][BF4] solution was carefully added into the sample

2. EXPERIMENTAL SECTION 2.1. Chemicals. The IL 1-butyl-3-methylimidazolium tetrafluoroborate was purchased from LiHua Pharmaceutical Co. Ltd., with a quoted purity of ≥ 99.0 %. IL was dried under vacuum at about 343 K27 for at least 1 day, and this procedure was repeated always immediately prior to its use. The IL was stored in desiccators. Na2HPO4·12H2O and NaH2PO4·2H2O were purchased from Kermel Chem. Ltd., with a purity of ≥ 99.0 %, and used as received. All other reagents were of analytical grade, and double-distilled deionized water was used throughout the experiment.

Table 2. Binodal Data for the [Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) ATPS at Different Temperaturesa T = 298.15 K

Table 1. Binodal Data for the [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) ATPS at Different Temperaturesa T = 298.15 K

a

T = 303.15 K

T = 308.15 K

100 w1

100 w2

100 w1

100 w2

100 w1

100 w2

73.30 70.06 66.99 61.18 54.35 49.36 45.25 41.72 38.25 36.10 34.74 31.60 29.95 28.13 25.10 23.30 20.93 19.57 17.97 15.17 13.24 11.92 10.93 9.35 8.29 7.43 6.74 6.11 5.54

0.21 0.26 0.29 0.39 0.57 0.73 0.89 1.04 1.21 1.32 1.39 1.62 1.73 1.88 2.20 2.40 2.79 3.06 3.39 4.26 5.07 5.63 6.07 7.24 8.44 9.06 9.73 10.58 11.45

79.06 71.28 57.04 44.09 36.36 28.24 25.00 22.00 19.40 17.44 15.83 14.18 11.98 9.77 8.52

0.13 0.21 0.55 1.14 1.61 2.24 2.50 3.00 3.45 3.92 4.32 5.01 6.03 7.04 7.98

82.82 75.49 64.99 56.49 51.79 45.12 39.07 36.80 33.02 29.86 25.54 22.98 20.32 17.64 15.61 12.87 10.82 9.24

0.21 0.30 0.51 0.69 0.89 1.21 1.56 1.81 2.04 2.38 2.93 3.31 3.80 4.37 5.01 5.89 7.10 8.01

a

T = 303.15 K

T = 308.15 K

100 w1

100 w2

100 w1

100 w2

100 w1

100 w2

62.61 58.38 54.89 49.34 42.50 37.85 34.77 32.14 28.87 25.19 24.81 21.46 18.65 17.36 15.51 14.31 12.06 9.51 8.66

0.65 0.83 1.05 1.40 1.92 2.41 2.82 3.09 3.52 4.39 4.55 5.13 6.23 7.01 7.91 8.66 11.17 12.78 14.35

81.83 76.73 69.24 60.72 55.38 52.27 47.00 44.06 40.56 38.73 34.50 30.34 26.73 23.45 20.23 18.12 15.45 14.44

0.38 0.50 0.72 1.10 1.40 1.74 2.15 2.42 2.67 2.96 3.40 3.93 4.55 5.30 6.32 7.18 8.34 9.37

71.34 65.45 61.73 57.01 54.30 49.65 46.67 41.64 35.72 32.51 29.28 24.70 20.68 17.56 16.18 15.09 13.48 12.18

0.71 1.01 1.23 1.55 1.74 2.1 2.45 3.09 4.03 4.73 5.13 6.41 7.71 8.94 9.5 10.3 10.87 11.75

Standard uncertainties δ are δ(w) = 0.001 and δ(T) = 0.05 K.

Figure 1. Plot of experimental and correlated binodal curves calculated from the Merchuck equation for [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS's at different temperatures. ●, 298.15 K; ▲, 303.15 K; ■, 308.15 K; ○,experimental results measured by Wang et al.;25 , correlated binodal curves calculated from the Merchuck equation.

Standard uncertainties δ are δ(w) = 0.001 and δ(T) = 0.05 K. 2380

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

Figure 2. Plot of experimental and correlated binodal curves calculated from the Merchuck equation for [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS's at different temperatures. ●, 298.15 K; ▲, 303.15 K; ■, 308.15 K; , correlated binodal curves calculated from the Merchuck equation.

Figure 3. Plot of experimental and correlated binodal curves calculated from the Merchuck equation for [Bmim][BF4] (1) + NaH2PO4 (2) + water (3), [Bmim][BF4] (1) + Na2HPO4 (2) + water (3), and [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + water (3) ATPS's at T = 298.15 K. ●, NaH2PO4; ■, Na2HPO4; ⧫, nNaH2PO4:nNa2HPO4 = 0.5; ▲, nNaH2PO4:nNa2HPO4 = 1; ▼, nNaH2PO4:nNa2HPO4 = 2; , correlated binodal curves calculated from the Merchuck equation.

Table 3. Binodal Data for the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) ATPS with Different Mole Ratios of Salts at T = 298.15 Ka nNaH2PO4:nNa2HPO4 = 0.5

a

nNaH2PO4:nNa2HPO4 = 1

nNaH2PO4:nNa2HPO4 = 2

100 w1

100 w2

100 w1

100 w2

100 w1

100 w2

76.73 68.83 59.60 52.41 47.00 42.40 37.79 35.17 32.91 31.49 30.10 28.41 27.09 24.41 22.41 18.23 17.43 16.40 16.22 15.63 14.85 14.26 13.33 12.59 11.83 10.74 9.93 8.08 7.06

0.17 0.28 0.48 0.71 0.93 1.15 1.43 1.59 1.74 1.84 1.96 2.12 2.27 2.62 2.95 3.92 4.16 4.39 4.59 4.86 5.15 5.41 5.88 6.29 6.72 7.40 8.06 9.60 11.78

73.80 67.44 58.75 50.52 44.08 38.78 35.03 31.28 28.94 26.09 23.82 20.59 19.73 18.33 16.92 15.85 14.71 13.63 12.87 11.38 10.45 9.18 8.06

0.25 0.37 0.57 0.88 1.19 1.52 1.77 2.06 2.40 2.78 3.13 3.61 3.83 4.25 4.75 5.20 5.69 6.30 6.89 7.72 8.43 10.06 11.56

76.35 63.95 51.49 43.61 37.92 34.94 31.23 28.19 24.75 22.90 22.08 21.19 18.86 15.26 12.95 10.65 8.69

0.26 0.55 1.13 1.61 2.10 2.47 2.87 3.27 3.90 4.30 4.47 4.67 5.32 6.52 7.98 10.01 12.09

Table 4. Values of Parameters of Equation 1 for the [Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) and [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) ATPS's T 298.15 303.15 308.15 298.15 303.15 308.15

a

b

c

R2

[Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) 112.90 −0.7148 −5.73·10−5 0.998 115.90 −0.6929 1.78·10−4 0.998 119.33 −0.5992 1.58·10−4 0.999 [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) 114.65 −1.0204 −3.72·10−4 0.997 109.49 −0.9012 1.36·10−4 0.998 126.41 −0.9389 −7.65·10−5 0.999

SDa 0.6586 1.0645 0.4299 0.6007 1.1334 0.5176

SD = (∑i=1n(100w1exp − 100w2cal)2/n)0.5, where w1exp is the mass fraction of [Bmim][BF4] in Tables 1 and 2 and w1cal is the mass fraction of [Bmim][BF4] caculated from eq 1. n is the number of binodal data, respectively.

a

and sufficient data for the construction of a binodal curve were obtained. The system was operated under constant stirring. All of the solutions were analyzed by mass. An analytical balance (model LA 120S, Sartorius) was used though the experiment, with a precision of ± 1.0·10−4 g. The maximum uncertainty of the mass fraction for both IL and salt was within ± 0.001. A series of [Bmim][BF4] ATPS's with known compositions were prepared in glass tubes to determine the tie lines. The sample tube was placed in thermostat bath for at least 15 min to reach temperature equilibrium. Then after sufficient mixing, the sample tube was placed in a thermostat bath for at least 12 h. Afterward, centrifuging (model 80-2, Nanjing Medical Instrument Factory, China) at 2000 rpm still operated for about 5 min in each test to ensure the complete phase separation. Both the upper and the lower phase are clear, and an obvious phase boundary exists. Samples of the top and bottom phases were pipetted and diluted conveniently with water for the analysis of [Bmim][BF4] by ultraviolet−visible (UV−vis) spectroscopy (model UV-1750, Shimadzu) at 211 nm.29 Absorbance was always maintained at < 0.8. The mass fraction of Na2HPO4 and

Standard uncertainties δ are δ(w) = 0.001 and δ(T) = 0.05 K.

tube until one further drop made the mixture turbid or cloudy. Afterward, a few drops of water were added to the sample tube to get a clear one-phase system. The procedure was repeated, 2381

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

NaH2PO4 was determined by titration with HCl and NaOH, respectively, using indicators and a pH meter (model pH-211, Hanna). All experiments were repeated several times, and the uncertainty of mass fraction of IL and salt was about ± 0.001. We should pay attention that, when determining the tine-line data, the salt concentration may rather high, even higher than the solubility of Na2HPO4. Because of the low solubility of Na2HPO4,

supersaturated Na2HPO4 and (NaH2PO4 + Na2HPO4) solutions were made. Concentrated Na2HPO4 and (NaH2PO4 + Na2HPO4) solutions with known concentrations were first prepared at a higher temperature (about 310 K), and then the solutions were slowly cooled, so that the supersaturated salt solutions were made. To avoid the recrystallization of the salts, the sample tubes were washed by acetone for several times, and all solutions were filtered through a filter of 0.45 μm pore size.

Table 5. Values of Parameters of Equation 1 for the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) ATPS's at T = 298.15 K

3. RESULTS AND DISCUSSION 3.1. Binodal Data. The binodal data for [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS's and [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS's were determined experimentally at different temperatures (T = 298.15 K, 303.15 K, and 308.15 K) and are given in Tables 1 and 2 and also shown in Figures 1 and 2. For the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + water (3) ATPS, the binodal data were determined at 298.15 K and are shown in Table 3 and Figure 3. For [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS's, a comparison between the binodal data measured by Wang et al.25 and this work have been made and shown in Figure 1, which shows a good agreement with each other. For the studied systems, a nonlinear equation developed by Merchuk et al. was used to fit the binodal curves:30

a

b

SDa

R2

c

[Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 0.5) 113.79 −0.9268 −3.08·10−4 0.997 0.6086 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 1) 114.15 −0.8986 −2.98·10−4 0.999 0.4499 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 2) −8.67·10−5

−0.7674

113.78

0.997

0.6218

SD = − 100w2cal)2/n)0.5, where w1exp is the mass fraction of [Bmim][BF4] in Tables 1 and 2 and w1cal is the mass fraction of [Bmim][BF4] caculated from eq 1. n is the number of binodal data, respectively. a

(∑i=1n(100w1exp

w1 = a exp(bw2 0.5 − cw2 3)

where 1 and 2 represent IL and salt, respectively. Recently, the Merchuk equation has been successfully used for the reproduction of binodal data of some IL-based ATPS's.22,31,32 The fitting parameters a, b, and c as well as the corresponding standard deviations (SDs) are given in Tables 4 and 5. The binodal curves reproduced from the Merchuk equation are also shown in Figures 1 to 3, which imply that the Merchuk equation fits the binodal curves of the investigated systems well. As shown in Figure 1 and 2, with the increase of the

Table 6. Values of Parameters of Equation 2 for the Fitting Parameters of the Merchuk Equation Correlated of Binodal Data a 0

a a1 R2 SD

104.4 7.192·10−2 0.844 0.2263

104·c

b 0

b b1 R2 SD

−2

−1.473·10 1.060 0.946 2.625·10−2

10 ·c 104·c1 R2 SD 4 0

(1)

18.61 −0.1592 0.888 5.133·10−5

Table 7. Tie-Line Data for the [Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) and [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) ATPS's at Different Temperaturesa total T 298.15 K

303.15 K

308.15 K

298.15 K

303.15 K

308.15 K

a

100 w1

top 100 w2

36.26 36.95 37.80 36.24 38.80 36.75 36.76 38.87 36.82

12.84 11.66 10.33 12.79 10.73 11.59 12.75 10.87 10.76

39.30 40.20 41.16 35.94 36.13 38.89 36.03 36.33 38.89

8.83 7.55 6.05 8.72 8.01 8.55 8.97 7.94 8.55

100 w1

bottom 100 w2

100 w1

[Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) 63.74 0.62 3.17 63.08 0.64 3.77 62.33 0.65 4.54 62.37 0.96 4.11 60.33 1.20 4.94 61.00 0.96 3.77 61.34 1.32 4.95 59.63 1.52 5.25 57.90 1.66 5.78 [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) 67.60 0.27 2.71 65.01 0.32 2.88 63.05 0.35 3.71 63.56 0.41 3.30 61.12 0.53 3.52 65.64 0.39 3.00 62.56 0.60 3.50 60.12 0.71 3.92 64.64 0.42 4.12

100 w2

TLL

S

27.35 25.32 23.40 27.65 25.40 26.50 27.97 26.41 24.75

66.20 64.24 62.11 64.09 60.45 62.67 62.37 59.81 57.01

−2.27 −2.54 −2.40 −2.18 −2.29 −2.24 −2.12 −2.18 −2.26

19.86 18.24 15.97 18.69 17.83 19.30 18.88 18.18 19.69

67.78 64.65 61.36 62.97 60.15 65.43 61.82 58.85 63.51

−3.31 −3.46 −3.79 −3.30 −3.33 −3.31 −3.23 −3.22 −3.14

Standard uncertainties δ are δ(w) = 0.001 and δ(T) = 0.05 K. 2382

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

Table 8. Tie-Line Data for the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) ATPS's at T = 298.15 Ka total 100 w1

top

100 w2

100 w1

bottom

100 w2

100 w1

100 w2

S

TLL

[Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 0.5) 45.53 44.45 42.55

6.10 65.04 0.31 5.65 18.06 5.98 63.99 0.32 5.81 17.54 6.12 63.27 0.36 6.04 16.58 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + (nNaH2PO4:nNa2HPO4 = 1)

61.99 −3.34 60.67 −3.37 59.48 −3.52 Water (3)

31.56 32.12 34.10

11.33 63.63 0.45 4.03 20.08 10.58 63.00 0.45 4.66 19.26 9.22 62.13 0.42 5.11 18.68 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + (nNaH2PO4:nNa2HPO4 = 2)

62.75 −3.12 61.30 −3.10 59.87 −3.03 Water (3)

44.78 45.61 42.05 a

7.94 8.39 7.46

65.38 67.89 62.65

0.50 0.45 0.55

5.99 5.76 6.69

21.85 22.96 18.97

63.11 66.08 58.91

Figure 4. Plot of the tie-lines of the [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS at different temperatures. ●, 298.15 K; ▲, 303.15 K; ■, 308.15 K.

−2.78 −2.76 −3.04

Standard uncertainties δ are δ(w) = 0.001 amd δ(T) = 0.05 K.

temperature, the two-phase region decreases. This may because of the decreased solubility of IL in water or the increased phaseforming ability in the studied system.33 As a result of the water interaction with salt and IL, a decrease in temperature makes the structure of water similar to that of a kosmotropic ion and therefore can promote the phase-forming ability in the investigated system.33,34 As can be seen from Figure 3, Na2HPO4 has a stronger phase separation ability than NaH2PO4, meaning that a smaller amount of Na2HPO4 could induce phase separation, which could be related to the Gibbs free energies of hydration (ΔGhyd) of anions of the inorganic salts.17,35 This order follows the Hofmeister series for the strength of the kosmotropic salts. It could also be interpreted by the valence of the anions and the hydrated water molecules.22 For the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + water (3) ATPS's, with the increasing molar ratio of Na2HPO4, it can be seen that the two-phase areas are expanded, with the gradual approach to the phase boundary of the [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS's. The relationship for the fitting parameters of the Merchuk equation, a, b, and c, were expressed in linear forms: a = a 0 + a1M

(2a)

b = b0 + b1M

(2b)

c = c 0 + c1M 0

1

0

Figure 5. Plot of the tie-lines of the [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS at different temperatures. ●, 298.15 K; ▲, 303.15 K; ■, 308.15 K.

(nNaH2PO4:nNa2HPO4 = 2), 6.99 (nNaH2PO4:nNa2HPO4 = 1), and 7.28 (nNaH2PO4:nNa2HPO4 = 0.5), respectively. After being used to produce ATPS's, the pH values have no obvious change. There is no obvious difference of pH value between the two phases for the studied systems. 3.2. Liquid−Liquid Equilibrium (LLE) Data. The tie-line compositions for [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS's and [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS's at different temperatures (T = 298.15 K, 303.15 K, and 308.15 K) are given in Tables 7 and 8 and presented in Figures 4 and 5. For the investigated systems, the IL is enriched in the top phase, and the salt is enriched in the bottom phase, which is in accordance with the most of other researchers.9 However, the inversion of IL and salt rich phases has been observed for highly fluorinated anions.10,36 As the tie line length increases, a higher concentration of [Bmim][BF4] and lower concentration of salt between the two phases was obtained. The tie line length and the slope of the tie line could be described as follows:

(2c) 1

0

1

where a , a , b , b , c , and c are independent adjustable parameters; M is the molecule weight of the salts or the average molecule weight of the salt mixtures, which is 119.97 (NaH2PO4), 141.93 (Na2HPO4), 127.33 (nNaH2PO4:nNa2HPO4 = 2), 130.99 (nNaH2PO4:nNa2HPO4 = 1), and 134.64 (nNaH2PO4:nNa2HPO4 = 0.5), respectively. These fitting parameters, along with the corresponding standard deviations (SDs), are given in Table 6. It was shown that the fitting parameters of Merchuk equation could be successfully described by eq 2, which could help determine the parameters of the Merchuk equation with known molecule weights M of such salt mixtures. The pH values of the three buffers with different NaH2PO4/Na2HPO4 ratios are 6.68

TLL = [(w1t − w2 b)2 + (w2 t − w1b)2 ]0.5 2383

(3)

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

phase increased. For the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + water (3) ATPS (see Figure 6), it could also be found that with the increasing mole ratio of Na2HPO4 of the salt mixture, the slope of the tie-lines gradually increases. Equations developed by Othmer−Tobias (5) and Bancroft (6) have been used to correlate the tie-line compositions:37

(5)

⎛w b⎞ ⎛ w t ⎞r ⎜⎜ 3 b ⎟⎟ = k 2⎜ 3t ⎟ ⎝ w1 ⎠ ⎝ w2 ⎠

(6)

where 1, 2, and 3 represent IL, salt, and water; t and b represent the top and bottom phases, respectively; k1, n, k2, and r are the fit parameters. These two models have been successfully used to correlate the tie-line compositions of some IL-based ATPS's.22,32,38 The values of the parameters k1, n, k2, and r of equations, which can be obtained by linear least-squares regression of the plots log[(1 − w1t)/w1t] against log[(1 − w2b)/w2b] and log[w3b/w2b] against log[w3t/w1t], are given in Tables 9 and 10. It is suggested that these two models can be successfully used for the correlation of the tie-line data of the [Bmim][BF4] + sodium phosphate salts ATPS's.

Figure 6. Plot of the tie-lines of the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + water (3) ATPS at 298.15 K. ⧫, nNaH2PO4:nNa2HPO4 = 0.5; ■, nNaH2PO4:nNa2HPO4 = 1; ◊, nNaH2PO4:nNa2HPO4 = 2.

S = (w1t − w1b)/(w2 t − w2 b)

⎛ 1 − w b ⎞n ⎛ 1 − w1t ⎞ 2 ⎟⎟ ⎟ = k1⎜⎜ ⎜ t b ⎝ w1 ⎠ ⎝ w2 ⎠

(4)

where 1 and 2 represent IL and salt; t and b represent the top and bottom phase, respectively. The results are shown in Tables 7 and 8. As can be seen from the results, for most of the systems, the tie-lines are mainly parallel to each other. One could know the composition of the top and the bottom phases for any given total composition. For [Bmim][BF4] (1) + Na2HPO4 (2) + water (3) ATPS's and [Bmim][BF4] (1) + NaH2PO4 (2) + water (3) ATPS's, as can be seen from Figures 4 and 5, with the increase of the temperature, the slope of the tie lines decreased. It means that, when temperature increases, the concentration of IL in the top phase decreased and the concentration of salt in the bottom

4. CONCLUSION Liquid−liquid equilibrium data for the aqueous two-phase systems of the IL 1-butyl-3-methylimidazolium tetrafluoroborate and sodium dihydrogen phosphate/disodium hydrogen phosphate or their mixtures have been determined at different temperatures (T = 298.15 K, 303.15 K, and 308.15 K). Na2HPO4 is shown to have a stronger ability for phase separation of the kosmotropic salts studied than NaH2PO4, which was discussed on the basis of Gibbs free energy of hydration of salt

Table 9. Values of Parameters of Equations 5 and 6 for the [Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) and [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) Different Temperaturesa

a

SDa

R2

T

k1

n

298.15 K 303.15 K 308.15 K

0.4682 0.2963 0.2795

0.2075 0.7442 0.8607

298.15 K 303.15 K 308.15 K

0.1734 0.0305 0.0348

0.7390 1.9895 1.9579

k2

[Bmim][BF4] (1) + NaH2PO4 (2) + Water (3) 0.996 0.71 32.7891 0.963 1.02 0.5056 0.997 0.56 0.6869 [Bmim][BF4] (1) + Na2HPO4 (2) + Water (3) 0.957 1.70 9.7659 0.997 0.66 9.0340 0.994 0.56 6.6792

r

R2

SDa

4.4602 5.3475 4.5540

0.996 0.981 0.997

0.49 0.81 0.58

1.2520 3.5809 3.6879

0.963 0.994 0.971

2.02 0.51 1.08

SD = (∑i=1n(100wi,j,calt − 100wi,j,expt)2 + (100wi,j,calb − 100wi,j,expb)/2n)0.5, where i = 1 and j = 2, respectively. n is the number of tie-lines.

Table 10. Values of Parameters of Equations 5 and 6 for the [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) ATPS's at T = 298.15 Ka k1

n

0.1862

0.7066

0.2152

0.7053

SDa

R2

SDa

0.921 0.90 9.3017 1.2644 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 1)

0.923

1.23

0.968 0.47 7.3119 1.1458 [Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 2)

0.966

0.58

0.940

2.41

R2

k2

r

[Bmim][BF4] (1) + (Na2HPO4 + NaH2PO4) (2) + Water (3) (nNaH2PO4:nNa2HPO4 = 0.5)

0.1636 a

SD =

0.8964 n

(∑i=1 (100wi,j,calt

0.938

2.35

− 100wi,j,exp ) + (100wi,j,cal − t 2

b

6.6061

100wi,j,expb)/2n)0.5,

1.0131

where i = 1 and j = 2, respectively. n is the number of tie-lines.

2384

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

(15) Liu, Q.; Yu, J.; Li, W.; Hu, X.; Xia, H.; Liu, H.; Yang, P. Partitioning Behavior of Penicillin G in Aqueous Two Phase System Formed by Ionic Liquids and Phosphate. Sep. Sci. Technol. 2006, 41 (12), 2849−2858. (16) Han, J.; Wang, Y.; Yu, C.; Li, C.; Yan, Y.; Liu, Y.; Wang, L. Separation, concentration and determination of chloramphenicol in environment and food using an ionic liquid/salt aqueous two-phase flotation system coupled with high-performance liquid chromatography. Anal. Chim. Acta 2011, 685 (2), 138−145. (17) Li, Z.; Pei, Y.; Wang, H.; Fan, J.; Wang, J. Ionic liquid-based aqueous two-phase systems and their applications in green separation processes. Trends Anal. Chem. 2010, 29 (11), 1336−1346. (18) Rodriguez, O.; Madeira, P. P.; Macedo, E. A. Gibbs free energy of transfer of a methylene group in buffer plus ionic liquid biphasic systems. Ind. Eng. Chem. Res. 2008, 47 (15), 5165−5168. (19) Dreyer, S.; Salim, P.; Kragl, U. Driving forces of protein partitioning in an ionic liquid-based aqueous two-phase system. Biochem. Eng. J. 2009, 46 (2), 176−185. (20) Zafarani-Moattar, M. T.; Hamzehzadeh, S. Effect of pH on the phase separation in the ternary aqueous system containing the hydrophilic ionic liquid 1-butyl-3-methylimidazolium bromide and the kosmotropic salt potassium citrate at T = 298.15 K. Fluid Phase Equilib. 2011, 304 (1−2), 110−120. (21) Dreyer, S.; Kragl, U. Ionic liquids for aqueous two-phase extraction and stabilization of enzymes. Biotechnol. Bioeng. 2007, 99 (6), 1416−1424. (22) Han, J.; Yu, C. L.; Wang, Y.; Xie, X. Q.; Yan, Y. S.; Yin, G. W.; Guan, W. X. Liquid-liquid equilibria of ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate and sodium citrate/tartrate/ acetate aqueous two-phase systems at 298.15 K: Experiment and correlation. Fluid Phase Equilib. 2010, 295 (1), 98−103. (23) Neves, C. M. S. S.; Ventura, S. P. M.; Freire, M. G.; Marrucho, I. M.; Coutinho, J. A. P. Evaluation of Cation Influence on the Formation and Extraction Capability of Ionic-Liquid-Based Aqueous Biphasic Systems. J. Phys. Chem. B 2009, 113 (15), 5194−5199. (24) Li, Z. Y.; Pei, Y. C.; Liu, L.; Wang, J. J. (Liquid + liquid) equilibria for (acetate-based ionic liquids plus inorganic salts) aqueous two-phase systems. J. Chem. Thermodyn. 2010, 42 (7), 932−937. (25) Wang, Y.; Xu, X.; Yan, Y.; Han, J.; Zhang, Z. Phase behavior for the [Bmim]BF4 aqueous two-phase systems containing ammonium sulfate/sodium carbonate salts at different temperatures: Experimental and correlation. Thermochim. Acta 2010, 501 (1−2), 112−118. (26) Freire, M. G.; Neves, C. M. S. S.; Marrucho, I. M.; Coutinho, J. A. P.; Fernandes, A. M. Hydrolysis of Tetrafluoroborate and Hexafluorophosphate Counter Ions in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2009, 114 (11), 3744−3749. (27) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation. Green Chem. 2001, 3 (4), 156−164. (28) Ananthapadmanabhan, K. P.; Goddard, E. D. A correlation between clouding and aqueous biphase formation in polyethylene oxide/inorganic salt systems. J. Colloid Interface Sci. 1986, 113 (1), 294−296. (29) Shimojo, K.; Goto, M. Solvent Extraction and Stripping of Silver Ions in Room-Temperature Ionic Liquids Containing Calixarenes. Anal. Chem. 2004, 76 (17), 5039−5044. (30) Merchuk, J. C.; Andrews, B. A.; Asenjo, J. A. Aqueous two-phase systems for protein separation: Studies on phase inversion. J. Chromatogr., B 1998, 711 (1−2), 285−293. (31) Wu, B.; Zhang, Y. M.; Wang, H. P. Aqueous Biphasic Systems of Hydrophilic Ionic Liquids + Sucrose for Separation. J. Chem. Eng. Data 2008, 53 (4), 983−985. (32) Deive, F. J.; Rivas, M. A.; Rodríguez, A. Sodium carbonate as phase promoter in aqueous solutions of imidazolium and pyridinium ionic liquids. J. Chem. Thermodyn. 2011, 43 (8), 1153−1158. (33) Li, C. X.; Han, J.; Wang, Y.; Yan, Y. S.; Pan, J. M.; Xu, X. H.; Zhang, Z. L. Phase Behavior for the Aqueous Two-Phase Systems

constituent ions. The phase diagram could be satisfactorily reproduced with the Merchuk equation. The temperature dependence on the binodal curves and the tie lines have discussed. The results indicate that, with the higher temperatures, the higher concentration of the salt and IL is required for phase separation. The relationship between the fitting parameter of the Merchuk equation and the molecule weight M of the salts or salt mixtures was described by a linear equation. The tie-line data were successfully correlated by Othmer−Tobias and Bancraft equations.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-371-67781267; fax: +86-371-67781267. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Chen, J.; Spear, S. K.; Huddleston, J. G.; Rogers, R. D. Polyethylene glycol and solutions of polyethylene glycol as green reaction media. Green Chem. 2005, 7 (2), 64−82. (2) Han, J.; Wang, Y.; Kang, W.; Li, C.; Yan, Y.; Pan, J.; Xie, X. Phase equilibrium and macrolide antibiotics partitioning in real water samples using a two-phase system composed of the ionic liquid 1butyl-3-methylimidazolium tetrafluoroborate and an aqueous solution of an inorganic salt. Microchim. Acta 2010, 169 (1), 15−22. (3) Deive, F. J.; Rodriguez, A.; Pereiro, A. B.; Araujo, J. M. M.; Longo, M. A.; Coelho, M. A. Z.; Lopes, J. N. C.; Esperanca, J. M. S. S.; Rebelo, L. P. N.; Marrucho, I. M. Ionic liquid-based aqueous biphasic system for lipase extraction. Green Chem. 2011, 13 (2), 390−396. (4) Bridges, N. J.; Gutowski, K. E.; Rogers, R. D. Investigation of aqueous biphasic systems formed from solutions of chaotropic salts with kosmotropic salts (salt-salt ABS). Green Chem. 2007, 9 (2), 177− 183. (5) Rogers, R. D.; Seddon, K. R. Ionic LiquidsSolvents of the Future? Science 2003, 302 (5646), 792−793. (6) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071−2083. (7) Freire, M. G.; Neves, C. M. S. S.; Marrucho, I. M.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Coutinho, J. A. P. High-performance extraction of alkaloids using aqueous two-phase systems with ionic liquids. Green Chem. 2010, 12 (10), 1715−1718. (8) He, C. Y.; Li, S. H.; Liu, H. W.; Li, K.; Liu, F. Extraction of testosterone and epitestosterone in human urine using aqueous twophase systems of ionic liquid and salt. J. Chromatogr., A 2005, 1082 (2), 143−149. (9) Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Controlling the Aqueous Miscibility of Ionic Liquids: Aqueous Biphasic Systems of Water-Miscible Ionic Liquids and Water-Structuring Salts for Recycle, Metathesis, and Separations. J. Am. Chem. Soc. 2003, 125 (22), 6632− 6633. (10) Ventura, S. P. M.; Neves, C. M. S. S.; Freire, M. G.; Marrucho, I. M.; Oliveira, J.; Coutinho, J. A. P. Evaluation of Anion Influence on the Formation and Extraction Capacity of Ionic-Liquid-Based Aqueous Biphasic Systems. J. Phys. Chem. B 2009, 113 (27), 9304−9310. (11) Rantwijk, F. v.; Sheldon, R. A. Biocatalysis in Ionic Liquids. Chem. Rev. 2007, 107, 2757−2785. (12) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green processing using ionic liquids and CO2. Nature 1999, 399 (6731), 28−29. (13) Du, Z.; Yu, Y. L.; Wang, J. H. Extraction of proteins from biological fluids by use of an ionic liquid/aqueous two-phase system. Chem.Eur. J. 2007, 13 (7), 2130−2137. (14) Pereira, J. F. B.; Lima, A. S.; Freire, M. G.; Coutinho, J. A. P. Ionic liquids as adjuvants for the tailored extraction of biomolecules in aqueous biphasic systems. Green Chem. 2010, 12 (9), 1661−1669. 2385

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386

Journal of Chemical & Engineering Data

Article

Containing the Ionic Liquid 1-Butyl-3-methylimidazolium Tetrafluoroborate and Kosmotropic Salts. J. Chem. Eng. Data 2010, 55 (3), 1087−1092. (34) Zafarani-Moattar, M. T.; Hamzehzadeh, S. Phase Diagrams for the Aqueous Two-Phase Ternary System Containing the Ionic Liquid 1-Butyl-3-methylimidazolium Bromide and Tri-potassium Citrate at T = (278.15, 298.15, and 318.15) K. J. Chem. Eng. Data 2009, 54 (3), 833−841. (35) Marcus, Y. Thermodynamics of solvation of ions. Part 5. Gibbs free energy of hydration at 298.15 K. J. Chem. Soc., Faraday Trans. 1991, 87 (18), 2995−2999. (36) Claudio, A. F. M.; Ferreira, A. M.; Shahriari, S.; Freire, M. G.; Coutinho, J. A. P. Critical Assessment of the Formation of IonicLiquid-Based Aqueous Two-Phase Systems in Acidic Media. J. Phys. Chem. B 2011, 115 (38), 11145−11153. (37) Othmer, D. F.; Tobias, P. E. Liquid-Liquid Extraction Data -Toluene and Acetaldehyde Systems. Ind. Eng. Chem. 1942, 34 (6), 690−692. (38) Pei, Y.; Wang, J.; Liu, L.; Wu, K.; Zhao, Y. Liquid−Liquid Equilibria of Aqueous Biphasic Systems Containing Selected Imidazolium Ionic Liquids and Salts. J. Chem. Eng. Data 2007, 52 (5), 2026−2031.

2386

dx.doi.org/10.1021/je300195c | J. Chem. Eng. Data 2012, 57, 2379−2386