Liquid–Liquid Equilibria of the Aqueous Two-Phase Systems

Article pubs.acs.org/jced

Liquid−Liquid Equilibria of the Aqueous Two-Phase Systems Composed of the N‑Ethylpyridinium Tetrafluoroborate Ionic Liquid and Ammonium Sulfate/Anhydrous Sodium Carbonate/Sodium Dihydrogen Phosphate and Water at 298.15 K Yuliang Li,* Xiao Shu, Xuewen Zhang, and Weisheng Guan School of Environmental Science and Engineering, Chang’an University, Xi’an, China ABSTRACT: In this work, the phase diagrams and the liquid−liquid equilibrium (LLE) data at 298.15 K of the aqueous [EPy]BF4 + ammonium sulfate + H2O, [EPy]BF4 + sodium dihydrogen phosphate + H2O, and [EPy]BF4 + sodium carbonate + H2O twophase systems were experimentally determined. The Merchuk equation was used to correlate the binodal data of the investigated systems. The tie-line data were correlated according to the Othmer−Tobias and Bancroft equations, the Setschenow equation, and a two-parameter equation. The results indicate that all of these equations can be successfully used to correlate the experimental data of the investigated systems at 298.15 K. Both the calculated effective excluded volume (EEV) and the binodal curves, which were plotted using the mass fractions, indicate the phase-separation abilities of the investigated salts, which follow the following order: Na2CO3 > (NH4)2SO4 > NaH2PO4.

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INTRODUCTION Aqueous two-phase systems (ATPSs) are formed by mixing two incompatible polymers or one polymer and one salt in water above a certain critical concentration.1 In the decades after ATPSs were first introduced by Beijernik in 1896,2 researchers found that ATPSs exhibit negligible volatility and nonflammability under ambient conditions and can thus be widely used in recycling and separation processes.3 The aqueous two-phase extraction technology for the separation of two or more components was first developed by Albertsson.4 Compared with traditional technology, ATPSs have several advantages, such as low cost, nontoxicity, and the possibility of large-scale applications, particularly for the separation and purification of amino acids,5 nucleic acids, viruses,6 cell membranes, proteins,7,8 and the effective components of natural medicines.9 The most commonly used ATPS is the PEG-dextran polymer−polymer system. However, due to the high cost of polymers,10 the industrial usage of the PEG−dextran system is limited. Therefore, the development of inexpensive ATPSs could yield great economic benefits. Due to the increasing interest in ATPSs, a number of new ATPSs have been found, such as the surfactant−surfactant water system, the aqueous two-phase micelle system, and the polymer−salt water system. In 2003, Rogers and colleagues first combined ionic liquids (ILs) with ATPSs,11 forming a new type of ATPS based on ILs. The ionic liquid-salt aqueous two-phase systems (ILATPSs) exhibit the advantages of both ILs and ATPSs, including low viscosity, thermal stability,12,13 nonreactivity with water,14 rapid phase separation, effective extraction, and a gentle, biocompatible environment, among others. However, their thermal stability is important issue that should be clarified before © 2014 American Chemical Society

industrial applications, such as the tetrafluoroborate-based ILs are not water-stable compounds since they hydrolyze under specific conditions dependent on the pH and temperature.15 Reliable liquid−liquid equilibrium data are necessary for the design of extraction processes and the development of models that can be used to predict phase partitioning. To design and optimize the aqueous two-phase extraction technique, many authors have developed thermodynamic models for the prediction of the liquid−liquid equilibrium (LLE) data of polymers. To date, many ILATPSs have been reported, such as PEG 400 + sodium sulfate/magnesium sulfate ATPSs,16 [Bmim]BF4 + sucrose/maltose ATPSs,17 F68 + Na3C6H5O7 ATPSs, F68 + Na 2 C 4 H 4 O 6 ATPSs, F68 + Na 2 CO 3 / Na2C4H4O4/(NH4)3C6H5O7 ATPSs,18 [C4mim]Br + K3PO4/ K2HPO4 ATPSs,19 PVP + K2HPO4/KH2PO4 ATPSs,20 and [C 4 mim]Cl/[C 6 mim]Cl/[C 8 mim]Cl + K 3 PO 4 /K 2 CO 3 ATPSs.21 Studies on the LLE data of ATPSs for imidazoliumbased ILs have attracted increasing interest in recent years. The phase diagrams and the tie-line data of the [Bmim]BF4 + salts (Na2CO3, (NH4)2SO4) have been reported.22 Li and his colleagues have studied the binodal curves and the tie-line data for [Bmim]BF4 + salts (Na2CO3, NaH2PO4) at T = 298.15 K, 313.15 K, and 333.15 K.23 Furthermore, ATPSs formed by Na2CO3, NaH2PO4, and (NH4)2SO4 with different types of imidazolium-based ILs have already been used in several separation processes.24−26 However, as an alternative, low-cost, stable ionic liquid, pyridium-based ILs have favorable properties for applications in separation and purification processes. ATPSs Received: December 2, 2012 Accepted: January 6, 2014 Published: January 13, 2014 176

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composed of [EPy]Br (N-ethylpyridinium bromide) and salt (K2HPO4) were developed for extracting and separating chloramphenicol (CAP) in chicken egg.27 The combination of [BPy]BF4 (nitrogen-butyl pyridine tetrafluoroborate) and an (NH4)2SO4 phase-forming salt was studied for the extraction and separation of rutin.28 The results of these studies showed that the ILATPSs had very high extraction efficiencies (E % > 90). Separation and process design often rely on thermodynamic data and phase diagrams of the mixtures. Reports of the equilibrium phase behaviors of ATPS composed of pyridiumbased ionic liquids are scarce. Therefore, additional studies on the phase behavior of systems that contain pyridium-based ILs and salts are necessary to improve our knowledge of these types of ATPSs. In this paper, to the best of our knowledge, LLE data for the new aqueous [EPy]BF4 + salt (Na2CO3, NaH2PO4, and (NH4)2SO4) + H2O system are reported for the first time. The obtained results are useful for designing an extraction experiment, which will provide additional information on [EPy]BF4 + salts + H2O ATPSs. The binodal data of the studied systems were correlated using the Merchuk equation.29 In addition, the tie-lines were correlated through the Setschenow-type equation,30 the Othmer−Tobias equation, and the Bancroft equation.31 These data can then be used for further exploration of the phase compositions of these systems.

Figure 1. Tie-lines for the [EPy]BF4 + salt (Na2CO3, NaH2PO4, and (NH4)2SO4) + H2O ATPS at 298.15 K. ●, NaH2PO4; ▲, Na2CO3; ■, (NH4)2SO4. The y axis represents the weight percent of the IL, whereas the x axis represents weight percent of the salts.

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The tie-lines (TLs) were determined through the addition of the appropriate mass of IL (w1), salts (w2), and water (w3) to a centrifuge tube; this mixture was then stirred and allowed to reach equilibrium. After 24 h, the mixture was separated into two clear phases at a temperature of 298.15K, using small ampules. The uncertainty in the mass fraction of [EPy]BF4 was determined to be ± 0.075. The concentrations of sodium dihydrogen phosphate and sodium carbonate in the top and bottom phases were determined by flame photometry. The uncertainty in the measurement of the mass fraction of the salts was estimated to be ± 0.001. The concentrations of ammonium sulfate in the top and bottom phases were determined by spectrophotography, and the uncertainty in the measurement of the mass fraction was estimated to be ± 0.01. Both the top (wt) and bottom phases (wb) were weighed using an analytical balance. The maximum uncertainty in the measurement of the mass fraction of water was estimated to be ± 0.01.The lever rule was applied to obtain the relationship between the top phase composition and the total system composition, which were then used to obtain the diagrams of each TL. The tie-line lengths (TLL) and the slope of the tie-line lines (S) for different compositions at 298.15 K were also calculated through eqs 1 and 2:

EXPERIMENTAL SECTION Materials and Samples. N-Ethylpyridinium tetrafluoroborate (CASRN = 350-48-1) was purchased from Chengjie Chemical Co., Ltd. (Shanghai, China) with a quoted mass fraction purity of higher than 0.99. Anhydrous sodium carbonate was obtained from Tianjin Baishi Chemical Co., Ltd. (Tianjin, China) with a minimum mass fraction purity of 0.99. Ammonium sulfate and sodium dihydrogen phosphate were supplied by Guangdong Guanghua Technology Co., Ltd. with a minimum mass fraction purity of 0.99. All of the chemicals were of analytical grade. The ionic liquids and salts were used without further purification. Double-distilled water was used for the preparation of the solutions. Apparatus and Procedure. The experimental results were used to obtain the tie-line phase diagrams and the binodal curves, which are shown in Figures 1 and 3, respectively. A combination of titration and turbidimetric methods was used to determine the binodal curves at a temperature of 298.15 K. A glass vessel with a volume of 50 cm3 was used for the determination of the phase equilibrium. The glass vessel was surrounded with an external jacket in which water at a constant temperature was magnetically stirred (Gongyi Yuhua Instrument Co., Ltd., China). Several grams of an IL solution with a known mass fraction were introduced into the glass vessel. A salt solution with a defined mass fraction was then added to the glass vessel until the mixture became turbid or cloudy. The composition of the resultant mixture was noted. The mass fraction of water in the equilibrium system includes the water content of ionic liquids. Water was then added until the turbidity disappeared. The procedure was repeated. The binodal curves were plotted using the mass fraction composition of the mixtures. The temperature was controlled to within ± 0.05 K throughout the experiment. The mass composition of the mixture at each point on the binodal curve was calculated using an analytical balance (BS 124S, Beijing Sartorius Instrument Co., Ltd., China) with an uncertainty of ± 1.0 × 10−7 kg.

TLL = [(w1t − w1b)2 + (w2t − w2b)2 ]0.5 S=

(1)

w1t − w1b w2t − w2b

(2)

In these two equations, wt1 and wt2 represent the equilibrium mass fraction of [EPy]BF4 and salts in the top phase, respectively. The variables wb1 and wb2 represent the equilibrium mass fraction of [EPy]BF4 and salts in the bottom phase, respectively. The superscripts “t” and “b” indicate the top phase and the bottom phase, respectively.

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RESULTS AND DISCUSSION Binodal Data and Correlations. Table 1 presents the binodal curve data of the [EPy]BF4 + salt (Na2CO3, NaH2PO4 177

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Table 1. Binodal Data as Mass Fraction for [EPy]BF4 (w2) + ((NH4)2SO4/Na2CO3/NaH2PO4) (w1) + H2O ATPS Pressure p = 0.1 MPaa 100w1

a

100w2

100w1

3.54 3.67 3.78 3.95 4.06 4.18 4.31 4.43 4.59 4.72 4.84

56.51 55.99 55.50 54.83 54.34 53.88 53.35 52.86 52.25 51.74 51.27

4.97 5.12 5.30 5.49 5.70 5.96 6.12 6.41 6.65 6.84 7.04

0.19 0.31 0.46 0.86 0.99 1.12 1.23 1.34 1.46 1.57 1.73 1.85

77.46 65.92 62.86 58.66 57.19 56.22 55.36 54.07 53.23 52.39 51.44 50.64

1.95 2.05 2.15 2.36 2.46 2.64 2.83 3.01 3.17 3.34 3.56 3.79

1.43 1.84 2.35 3.38 4.29

77.06 66.19 63.21 60.55 57.25

4.71 5.54 6.33 7.05 7.75

100w2

100w1

100w2

100w2

100w1

100w2

28.28 27.34 25.89 25.14 24.33 23.38 21.99 20.66 19.86 18.72 17.65

18.37 18.81 19.26 19.76 20.63 21.73 22.44 23.40 24.51 25.81 26.67

17.12 16.60 16.06 15.42 14.93 13.42 12.78 11.95 11.22 10.53 9.44

26.00 25.25 24.23 23.06 21.98 20.99 20.71 20.44 19.38 18.91 18.59 17.58

11.17 11.39 11.63 11.95 12.22 12.75 13.05 13.37 13.70 14.09 14.68

17.16 16.79 16.39 15.57 15.10 14.42 13.84 13.26 12.64 12.01 11.47

35.49 33.53 31.99 30.36 28.65

19.42 20.91

26.82 24.67

w1, mass fraction of the salts; w2, mass fraction of [EPy]BF4. The standard uncertainties are w = 0.001 and T = 0.05 K.

and (NH4)2SO4) + H2O ATPS. These binodal data were fitted using the Merchuk30 equation: w1 = a exp(bw2

0.5

3

− cw2 )

w1 = exp(a + bw20.5 + cw2 + dw22)

(3)

Table 2. Parameter Values of eq 3 for Each ATPS at 298.15 K system

a

b

10−5c

R2

sda

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

119.2833

−0.3828

4.0379

0.9968

0.84

86.4833

−0.3981

0.9959

3.34

96.4454

−0.2467

0.9878

0.59

19.252 2.4889

⎛ w ⎞ ⎛ w ⎞ w1 = a1 exp⎜ − 2 ⎟ + a 2 exp⎜ − 2 ⎟ + c ⎝ b2 ⎠ ⎝ b1 ⎠

2 0.5 100wexp 1 ) /n}

(5)

where w1 and w2 represent the mass fractions of the IL and the salts, respectively. The values of the coefficients a1, a2, b1, b2, c, and sd for the investigated systems are presented in Table 4. As shown in Table 4, the values of the standard deviations imply

sd = − where is the experimental mass fraction of [EPy]BF4 in Table 1, wexp 1 is the corresponding data calculated using eq 3, and n is the number of binodal data points. {(∑in= 1100wcal 1

(4)

where w1 is the mass fraction of IL, w2 is the mass fraction of the salts, and the coefficients are the fitting parameters. The correlation coefficients (R2) and the standard deviations (sd) of the investigated systems are shown in Table 3. In previous studies, this equation was successfully used to correlate the binodal data of ([C2mim]BF4, [C3mim]BF4, and [C4mim]BF4) ILs + salt ((NH4)2C4H4O6) by Han et al.32 This equation was also used with alcohol + salt systems.34,35 Based on the obtained values of R2 and sd, which are shown in Tables 2 and 3, it can be concluded that the four parameters of eq 4 allow this equation to more accurately fit the binodal data of the studied systems. Another equation has also been previously used to correlate the binodal data of ATPSs. This expression has been applied to the data of the aqueous PPG400 + biodegradable salts biphasic systems36 and hydrophilic organic solvents.37 The form of the equation is as follows:

where w1 and w2 are the mass factions of IL and salts, respectively, and the coefficients a, b, and c are adjustable parameters, which were determined through a least-squares analysis. The correlation coefficient (R2) and the standard deviations (sd) are given in Table 2. However, a more accurate fit was obtained through the following nonlinear empirical expression33 which has four fitting parameters:

a

100w1

[EPy]BF4 + (NH4)2SO4 + H2O System at 298.15 K 50.83 7.29 42.66 12.33 50.23 7.51 42.01 12.72 49.53 7.86 40.95 13.36 48.83 8.14 40.11 13.73 48.02 8.42 39.22 14.11 47.01 8.77 38.13 14.55 46.46 9.24 36.78 15.23 45.49 9.83 34.99 15.97 44.70 10.38 33.46 16.59 44.11 10.99 31.78 17.17 43.45 11.67 30.01 17.93 [EPy]BF4 + Na2CO3 + H2O System at 298.15 K 49.84 4.01 39.16 7.48 49.13 4.23 38.21 7.69 48.43 4.75 36.36 8.00 47.33 5.03 34.97 8.36 46.63 5.34 33.80 8.71 45.70 5.61 32.77 9.58 44.72 5.92 31.65 9.76 43.81 6.24 30.48 9.93 42.98 6.45 29.69 10.23 42.13 6.56 29.16 10.44 41.13 6.92 27.95 10.63 40.12 7.09 27.28 10.94 [EPy]BF4 + NaH2PO4 + H2O System at 298.15 K 55.74 8.75 46.01 14.31 53.65 9.97 43.64 15.43 51.66 11.06 41.51 16.27 49.82 12.08 39.57 17.22 48.13 13.26 37.44 18.25

wcal 1

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Table 3. Parameters and R of eq 4 for Determined Systems at 298.15 K system

a

b

c

d

R2

sda

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

3.9817 4.4745 4.8692

0.3008 −0.4494 −0.6050

−0.1501 0.0379 0.1103

0.0010 −0.0043 −0.0028

0.9998 0.9966 0.9907

0.05 0.91 1.84

a exp 2 cal exp 0.5 sd = {(∑in= 1100wcal 1,i − 100w1,i ) /n} where w1 is the experimental mass fraction of [EPy]BF4 in Table 1, w1 is the corresponding data calculated using eq 4, and n is the number of binodal data points.

Table 4. Parameters and R of eq 5 for Determined Systems at 298.15 K

a

system

a1

a2

b1

b2

c

R2

sda

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

15.3866 7.7761 21.5288

75.5322 60.3642 62.3301

−4.7869E28 −7.0899E20 −3.0961E18

12.7626 6.1747 12.7501

−15.7269 −0.4696 −7.3063

0.9994 0.9868 0.9759

0.13 3.55 4.79

exp 2 0.5 sd = (∑i n= 1((100wcal where N is the number of binodal data points. 1,j − 100w1,i ) )/N)

total compositions of the studied systems have no significant influence on the slope of the tie-lines. The Othmer−Tobias (eq 6) and Bancroft (eq 7) equations were used to assess and correlate the tie-lines obtained for the IL-salt ATPSs.

that eq 5 can be successfully used to correlate the binodal curves of the investigated systems. A comparison of the R2 values obtained with the three different equations shows that eq 4 provides the best results. Correlation of the Tie-Lines. The tie-line compositions of the [EPy]BF4 + salt (Na2CO3, NaH2PO4 and (NH4)2SO4) + H2O system at 298.15 K are given in Table 5 and illustrated in

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

Table 5. Tie-Line Data for the Determined Systems at 298.15 K and Pressure p = 0.1 MPaa total system 100w1 17.18 20.10 12.46 3.02 5.07 7.01 5.00 7.42 10.17

top phase

100w2

100w1

100w2

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

bottom phase 100w1

100w2

TLL

[EPy]BF4 + (NH4)2SO4 + H2O System at 298.15 K 25.34 2.46 65.42 21.54 13.48 55.33 20.16 1.83 71.00 23.29 11.30 63.45 35.21 2.90 62.04 18.84 17.28 47.51 [EPy]BF4 + Na2CO3 + H2O System at 298.15 K 49.99 1.10 56.92 13.29 12.89 45.68 45.06 0.84 59.98 15.19 9.33 52.65 35.99 1.09 57.11 13.75 11.98 46.87 [EPy]BF4 + NaH2PO4 + H2O System at 298.15 K 60.55 2.26 66.56 22.16 23.03 46.41 55.87 2.28 66.43 26.03 17.65 49.29 51.01 2.09 67.49 27.38 15.91 43.18

(6)

S

(7)

wt1

wt3

where is the mass fraction of the IL in the top phase, is the mass fraction of water in the top phase, wb2 represents the mass fraction of salt in the bottom phase, wb3 represents the mass fraction of water in the bottom phase, and k1, n, k2, and r are adjustable parameters. The plots of log[(1 − wt1)/wt1] as a function of log[(1 − wb2)/wb2], log[wb3/wb2], and log[wt3/wt1] indicate a linear dependency and acceptable consistency of the results. In addition, the values of the parameters, the coefficients (R2) and the standard deviations (sd) are shown in Table 6. A lower standard deviation indicates a more accurate result. As shown in Table 6, one result is not as accurate as would be expected; the other two results, however, show improved reliability in the calculation method and the corresponding tie-line data. Another equation was also used to correlate the tie-line data. This equation, which has two parameters that are derived from the binodal theory,38 has the following form:

−2.72 −2.78 −2.81 −3.61 −3.53 −3.56 −1.80 −1.81 −1.83

a

w1, mass fraction of the salts; w2, mass fraction of [EPy]BF4. The standard uncertainties, u, are u(w) = 0.01 and u(t) = 0.05 K.

Figure 1. In most systems, the IL concentration in the bottom phase is smaller than in the top phase. In some cases, the IL is almost completely excluded from the bottom phase. The opposite behavior is observed in the top phase. As shown in Table 5 and Figure 1, a greater difference in the mass fraction between the top and the bottom phases results in a higher TLL. In addition, the tie-lines are parallel to one another because the

⎛ wt ⎞ ln⎜⎜ 2b ⎟⎟ = β + k(w1b − w1t) ⎝ w2 ⎠

(8)

In eq 8, w1 and w2 represent the mass fraction of the IL and the salts, respectively, k is the salting-out coefficient, and β is the

Table 6. Parameters Values and Standard Deviation of eqs 6 and 7 for the Investigated Systems at 298.15 K Othmer−Tobias equation

Bancroft equation

system

k1

n

R2

sd1a

k2

r

R2

sd2a

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

0.1179 0.1546 0.3963

1.0656 0.8527 0.6308

0.8689 0.9269 0.7619

1.20 0.22 0.27

6.5326 7.1028 3.1688

0.4583 0.8409 1.0130

0.9003 0.9232 0.7603

0.19 0.01 0.00

top 2 bot bot 2 0.5 sd = {∑in= 1[(100wtop where the number of tie lines and j = 1 and j = 2, and sd1 and sd2 represent i,j,cal − 100wi,j,exp) + (100wi,j,cal − 100wi,i,exp) ]/2n} the mass percent standard deviations for the IL and salts, respectively.

a

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constant related to the activity coefficient. The superscripts “t” and “b” indicate the IL-rich phase and the salt-rich phase, respectively. This equation has been successfully applied to many systems, including the PEG + salts ATPS, which was studied by Zafarani-Moattar et al.39 and Huddlston et al.40 The calculated parameters, as well as the corresponding standard deviations, for the systems investigated in this study are presented in Table 7. The reported standard deviations indicate that eq 8 can be satisfactorily used to correlate the tieline data of the investigated systems. Table 7. Parameters Values of eq 8 for the Investigated Systems at 298.15 K system

k

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

0.04499 0.06146 0.04336

β

R2

sda

0.1520

0.9975

2.88 × 10−4

0.2242

0.9966

1.64 × 10−4

−0.0397

0.9890

1.46 × 10

Figure 2. Experimental and calculated tie-lines for the Setschenowtype plots of the [EPy]BF4 + salt (Na2CO3, NaH2PO4, and (NH 4 ) 2 SO 4 ) + H 2 O ATPS at 298.15 K. ● , Na 2 CO 3 ; ▲ , (NH4)2SO4; ■, NaH2PO4.

−4

top top bot sd = (∑j =3 1∑i =n 1[(100wi,j,cal − 100wi,j,exp )2 − (100wi,j,cal − bot 2 0.5 100wi,i,exp) ]/6n) where n is the number of tie lines, and j is the number of components in each phase. a

The Setschenow-type equation41 was also used to correlate the tie-line compositions of the studied systems: ⎛ ct ⎞ ln⎜⎜ 1b ⎟⎟ = kIL(c1b − c1t) + ks(c 2b − c 2t) ⎝ c1 ⎠

⎞ ⎛ w w * 2 + f ⎟ + V 213 * 1 =0 ln⎜V 213 213 M2 M1 ⎠ ⎝

(10)

⎛ w ⎞ w * 2 ⎟ + V 213 * 1 =0 ln⎜V 213 M2 ⎠ M1 ⎝

(11)

where V213 * is the scaled EEV of the salt, f 213 is the volume fraction of the unfilled effective available volume after the tight packing of the salt into the polymer network, and M1 and M2 are the molar masses of the polymer and the salt, respectively. * and f 213 were obtained In the studied systems, the values of V213 from the correlation of the binodal data and the corresponding standard deviations; these values are shown in Table 9.

(9)

In this equation, ks represents the salting-out coefficient, c1 and c2 are the molarities of the IL and salts, respectively, and kl is a parameter that relates the activity coefficient of IL to its concentration. The superscripts “t” and “b” indicate the [EPy]BF4-rich top phase and the salt-rich bottom phase. The fitting parameters, the corresponding intercepts, the correlation coefficients (R2) and the standard deviations (sd) are shown in Table 8. To examine the relationship between the Setschenowtype behavior and the phase diagrams, the plots of the tie-line data are shown in Figure 2. According to the values of the standard deviations, which are shown in Table 8 and Figure 2, the tie-line compositions of the [EPy]BF4 + salt (Na2CO3, NaH2PO4, and (NH4)2SO4) + H2O systems at 298.15 K can be satisfactorily characterized through Setschenow-type behavior. Effective Excluded Volume (EEV) and the Salting-Out Effect of the Salts. The EEV of the studied systems was calculated using the binodal model developed by Guan et al.,38 which was used for polymer−polymer systems in a previous study. The binodal equation for the systems analyzed in this study can be written using the following form:

Table 9. Parameters of eqs 10 and 11 for the Investigated Systems at 298.15 K system

V*213 (g·mol−1)

f 213

sda

[EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + NaH2PO4 + H2O

4.4933 2.9906 1.5707

0.2221 0.3491 0.5651

6.16 3.53 8.02

exp 2 0.5 sd = {(∑in= 1100wcal where wexp 1 − 100w1 ) /n} 1 is the experimental mass fraction of [EPy]BF4 in Table 1, wcal 1 is the corresponding data calculated using eq 10 or 11, and n is the number of binodal data points. a

According to the results shown in Table 9, we can determine that the EEV values follow the following order: Na2CO3 > (NH4)2SO4 > NaH2PO4. To more closely examine the relationship between the EEV values and the salting-out

Table 8. Parameters of eq 9 for the Investigated Systems at 298.15 K system

10−3ks (g·mol−1)

intercept

R2

δ(ks)b

δ(intercept)c

sda

[EPy]BF4 + (NH4)2SO4 + H2O [EPy]BF4 + Na2CO3 + H2O [EPy]BF4 + NaH2PO4 + H2O

1337.9649 1854.5119 885.4757

−0.3406 −0.6502 −0.4138

0.9972 0.9994 0.9999

50.1553 32.1495 1.9424

0.0719 0.0397 0.0036

2.21 × 10−4 2.38 × 10−5 1.25 × 10−7

a top 2 bot bot 2 0.5 sd = {∑j3= 1∑in= 1[(100wtop where n is the number of tie-lines and j is the number of components i,j,cal − 100wi,j,exp) − (100wi,j,cal − 100wi,i,exp) ]/6n) in each phase. bδ(k) is the standard deviation for the “ks” fitting parameter. cδ(intercept) is the standard deviation for the “intercept” fitting parameter.

180

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201310710057). Project supported by the Natural Science Foundation of ShaanXi Province (Grant No.2013JQ2019 ).

strength, the binodal data of the investigated systems were plotted in Figure 3. As shown, the increase in the EEV value is

Notes

The authors declare no competing financial interest.

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Figure 3. Binodal curve represented as the mass fractions of the [EPy]BF4 + salt (Na2CO3, NaH2PO4, and (NH4)2SO4) + H2O ATPS at 298.15 K. ●, Na2CO3; ■, (NH4)2SO4; ○, NaH2PO4. The y axis represents the weight percent of the IL, whereas the x axis represents weight percent of the salts.

reflected in the phase diagram through a shift in the position of the binodal curve toward the left, which corresponds to a decrease in the area of the single-phase mixture. Thus, this decrease in the concentration of the salt that is required to form a two-phase system indicates the salt’s higher salting-out strength.

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CONCLUSIONS In this study, the [EPy]BF4 + salts (Na2CO3, NaH2PO4, and (NH4)2SO4) + H2O two-phase systems were investigated. The binodal curves and the tie-lines of these systems at 298.15 K were obtained. The Merchuk equation, as well as two additional equations, was used to correlate the binodal data, whereas the tie-line data were correlated using both the Othmer−Tobias and the Bancroft equations. The results show that the majority of these equations satisfactorily correlate the experimental data of the investigated systems at 298.15 K, but it should be noted that the correlation is considerably lower for the Othmer− Tobias and Bancroft equations, especially for the [Epy]BF4 + NaH2PO4 + H2O system. The phase-forming ability of the salts (Na2CO3, NaH2PO4, and (NH4)2SO4) was clearly indicated by the calculated effective excluded volume (EEV), which follows the following order: Na2CO3 > (NH4)2SO4 > NaH2PO4.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-029-82334853. Fax: 86-029-82334566. E-mail: [email protected]; [email protected]; yulianglee175@ chd.edu.cn. Funding

This work was sponsored by the Ph.D. Programs Foundation of the Ministry of Education of China (No. 20110205110014), the Fundamental Research Funds for the Central Universities (No. 2013G2291015, 2013G1291071, and 2013G1502038), and National Training Projects of the University Students’ Inno vation and E ntrepreneurship p rogram (No. 181

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