Journal of Chemistry

Volume 2013 (2013), Article ID 857272, 11 pages

http://dx.doi.org/10.1155/2013/857272

Research Article

Liquid-Liquid Equilibrium Data for the Ionic Liquid N-Ethyl-Pyridinium Bromide with Several Sodium Salts and Potassium Salts

Yuliang Li, Xiaojia Lu, Jing Hao, and Changquan Chen

School of Environmental Science and Engineering, Chang'an University, Xi'an 710064, China

Received 7 June 2013; Revised 1 September 2013; Accepted 13 September 2013

Academic Editor: Elena Gomez

Copyright © 2013 Yuliang Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The liquid-liquid equilibrium (LLE) data for systems containing N-ethyl-pyridinium bromide ([EPy]Br), salt (, , , ), and water have been measured experimentally at  K and the formations of these four aqueous two-phase systems (ATPSs) have been discussed. Also, the effective excluded volume (EEV) values obtained from the binodal models for the four systems were determined and the salting-out abilities of different salts follow the order of . The solubility data were correlated by the Merchuk and other equations while the tie-line data by the Othmer-Tobias, Bancroft, two-parameter, and Setschenow-type equations. The correlation coefficients evidenced that experimental data fitted well to all these equations. These four salts were proved successfully to form ATPSs with N-ethyl-pyridinium bromide, making a significant contribution to the further study of this kind of ATPS.

1. Introduction

Aqueous two-phase systems (ATPSs) [1] extraction, an economical and efficient technique for separation, extraction, and purification, have found wide application in comprehensive separation, concentration, and fractionation of biological solutes and particles such as cells and proteins [2, 3] for the past few years. An ATPS is essentially a mixture that exists in two phases and is usually formed by two high polymers, a high polymer and a salt, or a hydrophilic organic solvent and a salt [4]. Compared with traditional organic solvent extractions, ATPSs are more effective and can be carried out under mild processing conditions [5].

In recent years, a new type of ATPS based on ionic liquids (ILs) has been investigated, the ILATPS. The ILs are entirely composed of organic cation and organic or inorganic anion, which hold attention because of their chemical and physical properties such as a good thermal, chemical, and electrochemical stability, negligible volatility, a high ionic conductivity, and tenability [6, 7]. Thus, ILATPSs, combining both the advantages of traditional ATPSs and the aforementioned benefits of ILs, have been successfully used in the separation, concentration, and purification of proteins [8], heavy metal ions [9], and small organic molecules [10]. ILATPSs have also been applied to extract antibiotics, such as penicillin G [11] and roxithromycin [12].

Consequently, further work is necessary for the development, optimization, and scale-up of extraction process using ILATPSs. Recently, an aqueous biphasic system (ABS) containing 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim]BF₄) + saccharides + water [13, 14] has been reported in the literature. In addition, ATPSs composed of [Cnmim]X (X = Br and Cl) + salts (KOH, K₂HPO₄, KH₂PO₄, K₂CO₃, K₃PO₄, K₃C₆H₅O₇ and sucrose) [13, 15–18] and [Bmim]BF₄ + salts (Na₃PO₄, Na₂CO₃, Na₂SO₄, Na₂SO₃, (NH₄)₂SO₄, NaH₂PO₄, NaCl, Na₃C₆H₅O₇, Na₂C₄H₄O₆, NaC₂H₃O₂, and (NH₄)₃C₆H₅O₇) [19–22] also have been reported. For ATPSs containing pyridinium-based ionic liquids, there are 4-methyl-N-butyl pyridinium tetrafluoroborate + cyclohexane [23]. Furthermore, Bridges et al. described the phase diagrams for imidazolium-, pyridium-, and quaternary ammonium- and phosphonium-based chloride salts (all chaotropic salts) salted out by K₃PO₄, K₂HPO₄, K₂CO₃, KOH, and (NH₄)₂SO₄ (all kosmotropic salts) [24]. Phase diagrams of ATPS composed of Mpy CH₃SO₄ and Na₂CO₃ have also been experimentally ascertained at .15 K [25] and the effects of the cation core such as methylimidazolium, methylpyridinium, methylpyrrolidinium and methylpiperidinium, the length of the alkyl side chain, and the positional isomerism on ATPS formation ability of chloride-based ionic liquids such as [1-C₄-2-C₁py]Cl, [1-C₄-3-C₁py]Cl, [1-C₄-4-C₁py]Cl have been investigated [26].

However, to our knowledge, the experimental work devoted to ILATPSs is far from enough. Compared to the existing imidazolium-based ionic liquids, pyridine-based ionic liquids provide lower cost, lower vapor pressure, better thermal, and chemical stability and are less polluting. An ATPS formed from an ionic liquid which is water soluble at room temperature, [BPy]BF₄ (nitrogen-butyl pyridine tetrafluoroborate) and a phase-forming salt (NH₄)₂SO₄ was studied for the extraction and separation of rutin [27]. The ATPS formed by the [EPy]Br and K₂HPO₄ for extracting and separating chloramphenicol (CAP) in eggs has also been reported [28]. However, studies regarding this type of ILATPSs are insufficient and require further development.

In this work, phase diagrams and liquid-liquid equilibrium (LLE) data for the [EPy]Br and four kosmotropic salts (Na₂HPO₄, K₂HPO₄, K₂SO₄, C₄O₆H₄KNa) have been investigated. The solubility curves were fitted to three nonlinear equations, and the tie-lines were described using the Othmer-Tobias, Bancroft, and Setschenow-type equations and a two-parameter equation. Moreover, the effective excluded volume (EEV) values obtained from the binodal models for the four systems were determined, and the effect of salts on solubility curves and tie-lines were discussed. The as-obtained results are necessary for the design and optimization of extraction processes as well as the development of both thermodynamic and mass transfer models of ILATPSs.

2. Experimental Section

2.1. Materials

The [EPy]Br was purchased from Chengjie Chemical Co., Ltd. (Shanghai, China) with a quoted purity of a mass fraction above 0.99 and was dried under high vacuum at 343.15 K using a semi-microdistiller with slow heating to remove impurities, mainly water. To prevent contact with the ambient air, the IL was kept in a sealed vessel and placed in refrigerator. The Na₂HPO₄·12H₂O, K₂HPO₄·3H₂O, K₂SO₄ and C₄O₆H₄KNa·4H₂O were analytical grade reagents which were purchased from the Guanghua Chemical Factory Co., Ltd. (Guangdong, China). Double-distilled deionized water was also used in the experiments.

2.2. Experimental Procedure

The solubility curves were determined using the cloud point method. First, an IL solution of known mass fraction was added into a vessel, and the salt solution of known mass fraction was added dropwise until the mixture became cloudy. The composition of the mixture was calculated and the water content of the ionic liquid was calculated into the water mass fraction in the experiment mass balance. Then water was added dropwise until the mixture became clear. The procedure was repeated to obtain all the points on the solubility curves until there was little precipitation at the bottom of the vessel. The vessel was placed in a DC-2008 water thermostat throughout the process, so that the temperature of the system could be kept constant (at  K). The temperature was controlled to within ±0.05 K. An analytical balance (model BS 124S, Beijing Sartorius Instrument Co., China) with a precision of  kg was used to measure the composition of the mixture at each point.

To determine the tie-lines, a series of ATPSs formed from three known compositions (including salt and water) were placed in a temperature-controlled bath. The system was held for at least 24 h to allow the formation of two phases. Both the upper and the lower phases were sampled for analysis. The concentrations of the salts in the two phases were determined via flame photometry. The uncertainty in the mass fractions of the salts was estimated to be ±0.001. The mass fraction of [EPy]Br in both the top phase and bottom phase was determined using a UV-vis spectrophotometer (model UV-2450, Shimadzu Corporation, Japan) and the uncertainty was determined to be less than 7.5%. A suitable sample of the top phase or bottom phase was removed and placed into a vessel to be mixed with an appropriate quantity of water. The absorbance of the solution was then measured at a wavelength of 211 nm to obtain the mass fraction of the ionic liquid according to a standard curve.

The tie-line length (TLL), which reflects the differences between composition of top and bottom phase and the slope of the tie-line , which reflects the ability of phase formation to ATPS, were also calculated at different compositions, using the following two equations, respectively, [29]:

where , , , and represent the equilibrium mass fraction of the [EPy]Br and salt in the top and bottom phases, respectively. The tie-line data are provided in Table 1.

Table 1: Ti1e-line data for the [EPy]Br (1) + salt (2) + water (3) ATPS at 298.15 K.

System
Total system
IL-rich phase
Salt-rich phase
TLL

100
100
100
100
100
100

[Epy]Br + Na₂HPO₄
30.98
20.46
1.75
59.45
45.09
1.63
72.26

20.29
30.19
2.74
53.24
41.17
2.77
63.44

10.09
41.67
3.16
51.07
37.73
4.17
58.05

[Epy]Br + K₂HPO₄
3.00
44.00
0.42
60.27
3.90
38.33
22.21

4.44
38.00
0.20
64.80
4.74
36.11
29.05

1.77
50.00
0.65
57.07
3.32
40.23
17.05

[Epy]Br + K₂SO₄
6.15
17.00
0.24
67.93
6.45
14.45
53.84

4.83
24.00
0.42
62.20
5.36
19.42
43.06

3.63
30.00
0.79
54.37
4.11
25.86
28.70

[EPy]Br + C₄O₆H₄KNa
9.77
62.95
0.30
81.03
31.62
21.24
67.50

6.99
57.11
1.80
67.59
19.21
32.48
39.19

8.32
62.16
0.52
77.83
26.59
25.45
58.51

3. Results and Discussion

3.1. Solubility Data and Correlation

The solubility data determined at  K are listed in Table 2. The data were fitted using the empirical nonlinear expression developed by Merchuk et al. [30] as follows:

where and are the mass fractions of the [EPy]Br and the salt, respectively. This expression has been used for the correlation of IL + salt ATPSs [31, 32] and IL + sugars ATPSs [14]. The parameters for this equation were determined from the experimental data obtained by the cloud point method. Coefficients , , and obtained from the correlation of the experimental solubility data with the corresponding standard deviations (sd) and correlation coefficient are given in Table 3.

Table 2: Solubility Data for [EPy]Br + salt + water ATPS at = 298.15 K.

Salt
100
100
100
100
100
100
100
100

Na₂HPO₄
0.39
75.96
2.65
54.63
8.37
34.17
15.96
23.49

0.53
72.39
2.67
54.30
8.50
33.98
16.05
23.39

0.63
70.30
2.68
54.47
8.61
33.74
16.20
23.13

0.68
68.79
2.71
54.15
8.71
33.60
16.30
23.00

0.69
68.13
2.72
53.74
8.84
33.38
16.70
22.53

0.78
66.87
2.73
54.33
8.95
33.21
16.84
22.25

0.80
66.11
2.74
53.91
9.05
33.03
16.95
22.13

0.95
65.96
2.75
53.59
9.17
32.83
17.05
22.01

0.96
66.23
2.76
53.76
9.28
32.66
17.15
21.91

0.99
65.67
2.79
53.44
9.41
32.39
17.36
21.71

1.02
65.43
2.80
53.61
9.53
32.17
17.56
21.41

1.05
65.18
2.82
53.36
9.65
31.96
17.78
21.15

1.09
64.76
2.83
53.54
9.77
31.75
18.02
20.90

1.14
64.30
2.86
53.16
9.90
31.54
18.19
20.62

1.17
64.06
2.87
53.35
10.03
31.32
18.31
20.47

1.20
63.74
2.90
53.02
10.16
31.11
18.49
20.15

1.23
63.50
2.91
53.20
10.29
30.87
18.62
19.99

1.27
63.28
2.93
52.88
10.47
30.44
18.74
19.86

1.30
62.99
2.94
53.06
10.59
30.30
18.87
19.73

1.35
62.71
3.00
52.62
10.71
30.14
19.00
19.59

1.39
62.67
3.01
52.81
10.83
30.14
19.13
19.45

1.45
62.13
3.04
52.44
10.97
29.99
19.27
19.31

1.52
61.54
3.05
52.63
11.08
29.77
19.42
19.17

1.56
61.25
3.08
52.26
11.17
29.67
19.62
18.82

1.63
60.68
3.10
52.45
11.28
29.45
19.75
18.66

1.68
60.32
3.12
51.50
11.40
29.23
19.89
18.50

1.74
59.92
3.17
52.27
11.52
29.01
20.02
18.34

1.79
59.51
3.41
49.90
11.62
28.90
20.16
18.17

1.84
59.14
3.99
47.46
11.77
28.73
20.30
18.00

1.89
58.56
4.61
45.13
11.89
28.50
20.45
17.85

1.90
58.69
5.04
40.83
12.00
28.38
20.59
17.67

1.92
58.35
5.15
40.62
12.11
28.20
20.89
17.32

1.93
58.48
5.16
41.91
12.24
27.96
21.20
16.98

1.96
58.12
5.29
40.81
12.35
27.83
21.52
16.62

1.97
58.25
5.37
40.83
12.49
27.59
21.85
16.22

2.00
57.95
5.45
40.51
12.63
27.35
22.19
15.84

2.01
58.09
5.48
40.33
12.74
27.23
22.54
15.43

2.04
57.71
5.52
40.41
12.88
26.96
22.90
14.99

2.05
57.85
5.60
40.15
13.00
26.82
23.25
14.56

2.09
57.46
5.75
39.45
13.13
26.69
23.62
14.10

2.10
57.60
5.77
39.01
13.28
26.40
24.00
13.67

2.14
57.20
5.85
39.26
13.41
26.25
24.46
13.23

2.15
57.35
5.95
38.89
13.57
25.97
24.88
12.78

2.19
56.87
6.10
38.85
13.73
25.72
25.29
12.31

2.20
57.02
6.22
38.67
13.83
25.70
25.75
11.83

2.24
56.69
6.32
38.40
13.93
25.68
26.49
11.01

2.25
56.84
6.43
38.18
14.04
25.62
26.85
10.76

2.28
56.26
6.56
37.72
14.21
25.61
27.10
10.48

2.29
56.41
6.66
37.41
14.35
25.40
27.46
10.20

2.33
55.80
6.77
37.22
14.42
25.31
27.99
9.64

2.34
55.95
6.87
37.19
14.49
25.26
28.62
9.06

2.36
55.84
6.99
37.00
14.60
25.01
29.31
8.52

2.37
56.00
7.10
36.69
14.72
24.97
30.02
7.93

2.40
55.89
7.21
36.50
14.80
24.88
30.97
7.34

2.41
56.05
7.33
36.15
14.87
24.78
31.78
6.69

2.45
55.62
7.43
35.91
14.95
24.69
32.63
6.01

2.46
55.78
7.54
35.81
15.08
24.45
33.14
5.68

2.51
55.36
7.63
35.74
15.20
24.40
33.66
5.35

2.52
55.53
7.73
35.55
15.29
24.34
34.39
5.05

2.56
54.95
7.82
35.45
15.38
24.25
35.15
4.71

2.57
55.12
7.94
35.10
15.50
24.01
35.90
4.39

2.61
54.59
8.03
34.87
15.58
23.90
36.81
4.02

2.62
54.76
8.14
34.65
15.68
23.80
37.75
3.57

2.64
54.46
8.27
34.41
15.77
23.69
39.04
3.17

K₂HPO₄
1.24
51.48
1.72
47.66
2.53
43.64
3.70
38.59

1.26
51.29
1.77
47.21
2.62
43.11
3.80
38.28

1.30
50.97
1.82
46.95
2.76
42.32
3.89
38.05

1.34
50.68
1.88
46.75
2.86
42.07
3.94
38.02

1.38
50.28
1.93
46.41
3.02
41.38
4.04
37.73

1.41
50.02
2.00
46.07
3.19
40.64
4.18
37.35

1.44
49.83
2.06
45.66
3.24
40.42
4.33
36.85

1.47
49.54
2.12
45.22
3.30
40.17
4.44
37.02

1.51
49.28
2.20
44.97
3.36
39.94
4.55
36.49

1.56
48.82
2.28
44.71
3.43
39.72
4.70
36.10

1.60
48.63
2.34
44.43
3.48
39.45
4.88
35.93

1.64
48.25
2.38
44.18
3.55
39.37
5.00
35.30

1.68
47.99
2.42
43.91
3.63
39.11
5.17
34.74

K₂SO₄
7.36
10.51
4.56
23.33
3.33
30.28
2.46
36.40

6.74
13.54
4.32
24.69
3.18
31.45
2.34
37.26

6.15
15.80
4.14
25.71
3.03
32.45
2.24
37.88

5.70
17.96
3.94
26.70
2.92
33.20
2.14
38.80

5.41
19.19
3.77
27.79
2.80
33.79

5.18
20.37
3.63
28.73
2.68
34.89

4.84
21.91
3.49
29.41
2.57
35.51

C₄O₆H₄KNa
0.48
78.95
3.40
59.50
7.67
48.26
14.41
37.88

0.70
77.03
3.55
58.99
8.06
47.57
15.45
36.29

0.80
75.16
3.75
58.28
8.23
47.19
16.20
34.94

0.82
75.03
4.10
57.11
8.72
46.26
16.75
34.77

0.98
73.40
5.19
54.71
9.49
44.88
17.19
34.17

1.17
71.64
5.45
53.74
10.39
43.43
17.34
33.96

1.34
70.30
5.49
53.55
11.09
42.35
17.79
33.54

1.98
66.65
5.81
52.73
12.12
40.77
18.26
33.07

2.19
64.97
6.04
52.08
12.68
39.86
18.78
32.53

2.38
64.09
6.23
51.73
13.32
39.11
19.22
32.18

2.67
62.83
6.64
50.68
13.68
38.94
19.31
31.97

2.82
61.98
6.82
50.35
13.95
38.49
19.38
31.81

2.99
61.25
7.08
49.62
14.18
38.20

3.21
60.31
7.28
49.23
14.27
38.27

Table 3: Parameters of (2) for the [EPy]Br + salt + water ATPS at = 298.15 K.

System
a
b
c
R 2
sda

EPyBr + Na₂HPO₄
92.2445

0.9978
0.84

EPyBr + K₂HPO₄
75.9749

0.9991
0.88

EPyBr + K₂SO₄
89.4813

0.9997
0.12

EPyBr + C₄O₆H₄KNa
91.7650

0.9994
0.17

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (2). The term “N” represents the number of solubility data.

To obtain a more accurate fit, a nonlinear empirical expression [33] of the following form was proposed to correlate the solubility data

where is the mass fraction of the IL, is the mass fraction of the salts, and the coefficients , , , and are fitting parameters. These parameters, along with the correlation coefficient and standard deviations (sd), are given in Table 4.

Table 4: Parameters of (3) for the [EPy]Br + salt + water ATPS at  K.

System

R 2
sda

EPyBr + Na₂HPO₄
4.5736

0.0271

0.9972
0.96

EPyBr + K₂HPO₄
4.7432

0.3074

0.9992
0.14

EPy + K₂SO₄
4.9165

0.3488

0.9996
0.15

EPyBr + C₄O₆H₄KNa
4.5676

0.0246

0.9995
0.30

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (3). The term “N” represents the number of solubility data.

The following equation was also successfully used in this work to correlate the data:

where and are the mass fractions of the IL and salts, respectively. This equation has been extended to fit the results of the ATPSs based on hydrophilic organic solvents [34, 35]. The coefficients , , , , and along with the correlation coefficient and the standard deviations (sd) are listed in Table 5.

Table 5: Parameters of (4) for the [EPy]Br (1) + salt (2) + water (3) ATPS at  K.

System
a 1
b 1
a 2
b 2

R 2
sda

EPyBr + Na₂HPO₄
23.3946

61.9954
11.0376

0.9859
2.14

EPyBr + K₂HPO₄
9.4472

32.6463
2.7530
20.9157
0.9968
0.27

EPy + K₂SO₄
9.0913

73.7805
7.5619

0.9996
0.49

EPyBr + C₄O₆H₄KNa
9.8253

49.0484
8.0236
19.4759
0.9911
1.28

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (4). The term “N” represents the number of solubility data.

The solubility curves determined at  K for the [EPy]Br salt (Na₂HPO₄, K₂HPO₄, K₂SO₄, C₄O₆H₄KNa) + water systems, which provided the minimum concentration required for the formation of these four ATPSs, are plotted in Figure 1, from which it can be observed that an ATPS can be produced by adding an appropriate amount of one salt to an aqueous solution of [EPy]Br. Moreover, one can easily conclude that the phase-separation abilities of the salts follow the order of by comparing different curves in the phase diagram.

Figure 1: Solubility curves of water ATPS at  K: ⋆, Na₂HPO₄; □, K₂HPO₄; △, K₂SO₄; ▼, C₄O₆H₄KNa.

Based on the as-obtained and standard deviation values in Tables 3–5, it can be concluded that (2)–(4) are satisfactory for correlating the solubility curves of the investigated systems. The same levels of satisfactory results were obtained in other ILATPS as well [36]. Furthermore, the Merchuk equation performs best of the three while it possesses only three adjustable parameters.

3.2. Effective Excluded Volume (EEV) and Salting-out Ability

A binodal model based on the statistical geometry method developed by Guan et al. [37] for aqueous polymer systems was applied to correlate the experimental solubility data of ATPSs containing Na₂HPO₄, K₂HPO₄, K₂SO₄, and C₄O₆H₄KNa in this paper. The binodal equation can be written as follows:

where , , , and are the scaled EEV of the salts, the volume fraction of the unfilled effective available volume after tightly packing the salt molecules into the network of ionic liquid molecules in the ionic liquid aqueous solutions, and the molar masses of the ionic liquid and salt, respectively. The values of and derived from the correlation of the experimental solubility data and the corresponding correlation coefficients () and standard deviations (sd) are given in Tables 6 and 7.

Table 6: Parameters of (5a) for the [EPy]Br (1) + salt (2) + water (3) ATPS at = 298.15 K.

System

f 213
R 2
sda

EPyBr + Na₂HPO₄
5.2183
0.2018
0.9828
0.64

EPyBr + K₂HPO₄
5.6397
0.1259
0.9824
2.39

EPyBr + K₂SO₄
11.4851

0.9814
1.10

EPyBr + C₄O₆H₄KNa
4.7262
0.1524
0.9730
2.26

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (6). The term “N” represents the number of solubility data.

Table 7: Parameters of (5b) for the [EPy]Br (1) + salt (2) + water (3) ATPS at =298.15 K.

System

R 2
sda

EPyBr + Na₂HPO₄
8.4568
0.0758
21.75

EPyBr + K₂HPO₄
8.5867
0.8820
38.25

EPyBr + K₂SO₄
10.2939
0.9601
2.57

EPyBr + C₄O₆H₄KNa
7.4313
0.1268
160.50

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (7). The term “N” represents the number of solubility data.

The salting-out ability of the salt could be related to the EEV [38–40]. The salt with higher salting-out ability has a larger EEV value at the same temperature. This is because with a increase in EEV, the solubility line moves to the left of the phase diagram and the single-phase area decreases correspondingly, resulting in the decline of salts content forming ILATPS, which means that salting-out ability becomes stronger. The EEV represents the smallest spacing of the individual ionic liquid that will adopt an individual salt, reflecting the compatibility of both components in a system. In this study, the EEVs have been calculated using the binodal model developed by Guan et al. [37]. In the original application, (7) was used to correlate the solubility data of polymer-polymer systems because the two components significantly vary in size. The value is so small that it can be neglected without obvious influence. From Table 7, it can be found that the parameter was small enough to be neglected for the investigated systems, and the standard deviation in (7) differed significantly from that in (6) for the solubility data fitting, rendering (7) unusable.

According to Table 6, the salting-out ability of the salts at a constant temperature follows the order: K₂SO4 > K₂HPO4 > Na₂HPO4 > C₄O₆H₄KNa, which is in agreement with the phase-separation abilities determined from Figure 1. Apparently, (6) satisfactorily reproduce the solubility curves of the investigated systems. However, when compared to the Merchuk and two other equations, there see a big gap.

Some also insist that salting-out abilities of different salts have something to do with ion hydration free energy (ΔG hyd) [41], the more negative the ions, the stronger their salting-out abilities. K₂HPO₄ and Na₂HPO₄ share the same anion, the cation radius of cation K⁺ exceeds that of Na⁺, and ΔG hyd of K⁺ and Na⁺ are −295 and −365 kJ·mol⁻¹, respectively. In accordance with the ion hydration free energy theory, the salting-out ability of Na₂HPO₄ should be stronger than K₂HPO₄. However, we found that in our study that although ΔG hyd of K⁺ is greater than Na⁺, the salting-out ability is much higher. Still, the anions promote the formation of ATPSs in the following order: (ΔG hyd = −1080 kJ·mol⁻¹) > (ΔG hyd = −1789 kJ·mol⁻¹), which also deviates from the ion hydration free energy theory.

3.3. Tie-Line Data and Correlation

The tie-line compositions, tie-line length (TLL), and average slopes determined at  K are listed in Table 1 and Figures 2 and 3. In this work, the tie-line compositions are closely connected by the Othmer-Tobias equation (6) and Bancroft equation (7) [42, 43] as follows:

Figure 2: Tie-lines of [EPy]Br ATPS at  K: ●, Na₂HPO₄; △, K₂SO₄.

Figure 3: Tie-lines of [EPy]Br ATPS at  K: □, K₂HPO₄; ▼, C₄O₆H₄KNa.

In these equations, is the mass fraction of the IL in the top phase; is the mass fraction of the salt in the bottom phase; and are the mass fractions of water in the bottom and top phases, respectively. Recently, (6) and (7) have been successfully verified by other ATPS researchers, indicating that the two equations can accurately correlate the tie-line data of the systems. The plots of log[] against log[/] and log against log shows linear relationships, indicating acceptable consistency of the results. The values of the fit parameters , , , and , the coefficient values (),and the standard deviations (sd) are listed in Table 8.

Table 8: Parameters of (6) and (7) for the [EPy]Br (1) + salt (2) + water (3) ATPS at = 298.15 K.

System

EPyBr + Na₂HPO₄
0.5605
1.1210
0.8815
2.24
1.6422
0.8044
0.8802
0.06

EPy]Br + K₂HPO₄
0.0309
0.8786
0.9946
0.08
22.7372
0.9823
0.9950
0.16

EPyBr + K₂SO₄
0.0190
1.2043
0.9984
0.07
19.0417
0.5867
0.9978
0.11

EPyBr + C₄O₆H₄KNa
0.0956
1.1154
0.9828
0.85
4.5475
0.7426
0.9868
0.06

sd = . Where N is the number of tie-lines and j = 1 and j = 2, sd₁ and sd₂ represent the mass percent standard deviations for [EPy]Br and the salt, respectively.

For further confirmation, the Setschenow-type equation [44] was used to correlate the tie-line compositions of the [EPy]Br + Na₂HPO₄/K₂HPO₄/K₂SO₄/C₄O₆H₄KNa ATPSs at  K:

In the above equation, , , , and represent the molality of the IL, the molality of the salt, a parameter relating the activity coefficient of the IL to its concentration, and the salting-out coefficient, respectively. The two superscripts “” and “” represent the IL-rich phase and the salt-rich phase, respectively. Assuming that the first term on the right side of this equation is negligible in comparison with the second term, a Setschenow-type equation can be obtained, implying that because the absolute values of exceed those of . This equation was successfully used for the correlation of tie-line data for the IL + salt ATPSs [42]. The salting-out coefficients, , with the corresponding intercepts, correlation coefficient () as well as standard deviations (sd) are all listed in Table 9.

Table 9: Parameters of (8) for the [EPy]Br (1) + salt (2) + water (3) ATPS at  K.

System

Intercept
R 2

100sda

EPyBr + Na₂HPO₄
2845.9

0.9981
88.60
0.1512
0.09

EPyBr + K₂HPO₄
1781.0
0.0146
0.9999
9.71
0.0025
0.60

EPyBr + K₂SO₄
4837.4

0.9963
208.70
0.0590
2.41

EPyBr + C₄O₆H₄KNa
1231.1

0.9996
18.37
0.0167
2.45

sd = . Where N is the number of tie-lines and j is the number of components in each phase. is the standard deviation for the fitting parameters of “”. is the standard deviation of the fitting parameters of the “intercept”.

In our study, a correlating equation with two parameters has also been used to determine the tie-line data which can be derived using the binodal theory [37]. The equation has the following form:

in which is the salting-out coefficient, and is the constant most closely related to the activity coefficient. The superscripts “” and “” represent the IL-rich phase and the salt-rich phase, respectively. Recently, (9) was successfully used to correlate the tie-line data for a polymer-salt ATPS [45, 46]. The fitting parameters of this equation, the correlation coefficient values (), and the standard deviations (sd) are provided in Table 10.

Table 10: Parameters of (9) for the [EPy]Br (1) + salt (2) + water (3) ATPS at = 298.15 K.

System

β
R 2
sda

EPyBr + Na₂HPO₄
0.0709
0.8535
0.9981
0.02

EPyBr + K₂HPO₄
0.1300
0.5822
0.9956
0.05

EPyBr + K₂SO₄
0.0656
0.2329
0.9984
0.03

EPyBr + C₄O₆H₄KNa
0.0926
0.8957
0.9991
0.04

sd = . Where N is the number of tie-lines and j is the number of components in each phase.

The standard deviations in Tables 8–10 are small, indicating that (6)–(9) are appropriate for correlating the tie-line data of the ATPSs especially the Setschenow-type equation. For more details about the relationship between the Setschenow-type behavior and the phase diagrams, the Setschenow-type plots of the tie-line data for the investigated systems are also shown in Figure 4. The Setschenow-type plots with data from Table 9 indicate that the larger the slope, the larger the . When combining with Figure 1, the larger the , the higher salting-out ability. This behavior is in agreement with the reported results for other ATPSs [47].

Figure 4: Tie-lines of [EPy]Br ATPS fitted by Setschenow-type equation at  K: ■, Na₂HPO₄; ●, K₂HPO₄; ⋆, K₂SO₄; ▼, C₄O₆H₄KNa.

3.4. Calculation of the Plait Point

Plait points are points on the solubility curves where the length of tie-lines shrank to almost zero, indicating that the two liquid phases become identical [48]. The plait points data for the investigated ATPSs in this study were calculated via the following linear equation:

in which and are the fitting parameters. For the four systems, the estimated values of the plait points along with the fitting parameters obtained from (10) and the corresponding correlation coefficients are given in Table 11.

Table 11: Parameters of (10) for the EPyBr (1) + salt (2) + water (3) ATPS at = 298.15 K.

System

R 2
100
100
sd

EPyBr + Na₂HPO₄
56.8201

0.8777
23.77
15.06
39.64

EPyBr + K₂HPO₄
54.9576

0.9644
46.67
1.89
0.84

EPyBr + K₂SO₄
49.0957

0.9904
40.03
1.94
0.62

EPyBr + C₄O₆H₄KNa
69.0866

0.9203
47.75
8.15
14.64

sd = . Where is the experimental mass fraction of IL and is the corresponding data calculated using (10). The term “N” represents the number of solubility data.

3.5. Effect of Salts on Solubility Curves and Tie-Lines

Figure 1 presents the effect of different salts on the solubility curves. It is clear that different salts possess two-phase areas of different sizes. In this study, K₂SO₄ owns the largest area of two phase; therefore, the phase-separation ability of K₂SO₄ precedes the other three.

Considering that the salts Na₂HPO₄ and K₂HPO₄ share a common anion but contain different cations, one can conclude that the salting-out ability of K⁺ is higher than that of Na⁺ at a constant temperature, which is contrary to what Chen and Wang [49] obtained from their research. This may be due to the relatively large radius of . Also, the K₂HPO₄ and K₂SO₄ share a common cation K⁺, but the EEV for K₂SO₄ is higher than that of K₂HPO₄; therefore, the salting-out ability of is higher than that of at the same temperature. When it comes to C₄O₆H₄KNa, which owns both K+ and Na⁺, it might be the organic ion C₄O₆ that brings about the weakest salting-out ability.

Figures 2 and 3 present the effect of different salts on the phase compositions, from which it can be informed that the slopes of tie-lines differ greatly towards different salt types. The slopes of tie-lines for ATPS containing K₂SO₄ are of the largest value, illustrating that K₂SO₄ hydrates the most water, thus decreasing the amount of water available to hydrate [EPy]Br.

4. Conclusions

The liquid-liquid equilibrium data have been determined for [EPy]Br + salt (Na₂HPO₄, K₂HPO₄, K₂SO₄ and C₄O₆H₄KNa) ATPSs at  K. The effect of salt on solubility curves as well as tie-lines was studied. The volume (EEV) values obtained from the binodal model together with the phase diagram indicated the order of phase separation abilities of the four salts. The Merchuk equation and two other equations were used to correlate the solubility data and the Othmer-Tobias, Bancroft, and Setschenow-type equations and a two-parameter equation were used to correlate the tie-line data. The results indicate that the calculation method and the corresponding tie-line data are reliable. At fixed temperatures, the salting-out ability of K⁺ is higher than that of Na⁺, and that of anions follows .

List of Symbols

: | Number of solubility data
: | Number of tie-lines
: | Temperature (K)
TLL: | Tie-line length
: | Slope of the tie-line
: | Mass fraction
, : | Salting-out coefficient
: | Interaction parameter
: | Scaled EEV of salt
: | Volume fraction
: | Molar mass
: | Fitting parameters
: | Correlation coefficient
sd: | Standard deviation.

Super/Subscript

: | Bottom phase
: | Top phase
cal: | Calculated value
exp: | Experimental value
1: | [EPy]Br
2: | Salt
3: | Water.

Conflict of Interests

None of the authors for this paper have a direct financial relation with the commercial identity mentioned in this paper.

Acknowledgments

This work was sponsored by the Research and Development Project of Science and Technology of Shaanxi Province (no. 2013JQ2019), the Fundamental Research Funds for the Central Universities (no. 2013G2291015, 2013G1291071, and 2013G1502038) and the National Training Projects of the University Students’ Innovation and Entrepreneurship program (no. 201310710057).

References

P. A. Albertsson, Partition of Cell Particles and Macromolec, John Wiley & Sons, 3rd edition, 1986.

W. Riedl and T. Raiser, “Membrane-supported extraction of biomolecules with aqueous two-phase systems,” Desalination, vol. 224, no. 1–3, pp. 160–167, 2008.

F. Luechau, T. C. Ling, and A. Lyddiatt, “Partition of plasmid DNA in polymer-salt aqueous two-phase systems,” Separation and Purification Technology, vol. 66, no. 2, pp. 397–404, 2009.

S. M. Baxter, P. R. Sperry, and Z. Fu, “Partitioning of polymer and inorganic colloids in two-phase aqueous polymer systems,” Langmuir, vol. 13, no. 15, pp. 3948–3952, 1997.

A. Boaglio, G. Bassani, G. Picó, and B. Nerli, “Features of the milk whey protein partitioning in polyethyleneglycol-sodium citrate aqueous two-phase systems with the goal of isolating human alpha-1 antitrypsin expressed in bovine milk,” Journal of Chromatography B, vol. 837, no. 1-2, pp. 18–23, 2006.

P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, 2nd edition, 2008.

D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, and M. Forsyth, “Pyrrolidinium imides: a new family of molten salts and conductive plastic crystal phases,” Journal of Physical Chemistry B, vol. 103, no. 20, pp. 4164–4170, 1999.

C. Y. He, S. H. Li, H. W. Liu, K. Li, and F. Liu, “Extraction of testosterone and epitestosterone in human urine using aqueous two-phase systems of ionic liquid and salt,” Journal of Chromatography A, vol. 1082, no. 2, pp. 143–149, 2005.

R. Lertlapwasin, N. Bhawawet, A. Imyim, and S. Fuangswasdi, “Ionic liquid extraction of heavy metal ions by 2-aminothiophenol in 1-butyl-3-methylimidazolium hexafluorophosphate and their association constants,” Separation and Purification Technology, vol. 72, no. 1, pp. 70–76, 2010.

B. Jiang, Z.-G. Li, J.-Y. Dai, D.-J. Zhang, and Z.-L. Xiu, “Aqueous two-phase extraction of 2, 3-butanediol from fermentation broths using an ethanol/phosphate system,” Process Biochemistry, vol. 44, no. 1, pp. 112–117, 2009.

Y. Y. Jiang, H. S. Xia, C. Guo, I. Mahmood, and H. Liu, “Enzymatic hydrolysis of penicillin in mixed ionic liquids/water two-phase system,” Biotechnology Progress, vol. 23, no. 4, pp. 829–835, 2007.

C. X. Li, J. Han, Y. Wang, Y. S. Yan, X. H. Xu, and J. M. Pan, “Extraction and mechanism investigation of trace roxithromycin in real water samples by use of ionic liquid-salt aqueous two-phase system,” Analytica Chimica Acta, vol. 653, no. 2, pp. 178–183, 2009.

B. Wu, Y. M. Zhang, and H. P. Wang, “Aqueous biphasic systems of hydrophilic ionic liquids + sucrose for separation,” Journal of Chemical & Engineering Data, vol. 53, no. 4, pp. 983–985, 2008.

B. Wu, Y. M. Zhang, and H. P. Wang, “Phase behavior for ternary systems composed of ionic liquid + saccharides + water,” Journal of Physical Chemistry B, vol. 112, no. 20, pp. 6426–6429, 2008.

Y. C. Pei, J. J. Wang, L. Liu, K. Wu, and Y. Zhao, “Liquid-liquid equilibria of aqueous biphasic systems containing selected imidazolium ionic liquids and salts,” Journal of Chemical & Engineering Data, vol. 52, no. 5, pp. 2026–2031, 2007.

M. T. Zafarani-Moattar and S. Hamzehzadeh, “Liquid-liquid equilibria of aqueous two-phase systems containing 1-butyl-3-methylimidazolium bromide and potassium phosphate or dipotassium hydrogen phosphate at 298.15 K,” Journal of Chemical & Engineering Data, vol. 52, no. 5, pp. 1686–1692, 2007.

M. T. Zafarani-Moattar and S. Hamzehzadeh, “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,” Journal of Chemical & Engineering Data, vol. 54, no. 3, pp. 833–841, 2009.

T. Mourao, A. F. M. Claudio, I. Boal-Palheiros, M. G. Freire, and J. A. P. Coutinho, “Evaluation of the impact of phosphate salts on the formation of ionic-liquid-based aqueous biphasic systems,” The Journal of Chemical Thermodynamics, vol. 54, pp. 398–405, 2012.

C. X. Li, J. Han, Y. Wang et al., “Phase behavior for the aqueous two-phase systems containing the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate and kosmotropic salts,” Journal of Chemical & Engineering Data, vol. 55, no. 3, pp. 1087–1092, 2010.

J. Han, C. L. Yu, Y. Wang et al., “Liquid-liquid equilibria of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate and sodium citrate/tartrate/acetate aqueous two-phase systems at 298.15 K: experiment and correlation,” Fluid Phase Equilibria, vol. 295, no. 1, pp. 98–103, 2010.

Y. Wang, X. H. Xu, Y. S. Yan, J. Han, and Z. L. Zhang, “Phase behavior for the [Bmim]BF4 aqueous two-phase systems containing ammonium sulfate/sodium carbonate salts at different temperatures: experimental and correlation,” Thermochimica Acta, vol. 501, no. 1-2, pp. 112–118, 2010.

J. Han, R. Pan, X. Q. Xie et al., “Liquid-liquid equilibria of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate + sodium and ammonium citrate aqueous two-phase systems at (298.15, 308.15, and 323.15) K,” Journal of Chemical & Engineering Data, vol. 55, no. 9, pp. 3749–3754, 2010.

S. I. Abu-Eishah and A. M. Dowaidar, “Liquid-liquid equilibrium of ternary systems of cyclohexane + (benzene, + toluene, + ethylbenzene, or + o-xylene) + 4-methyl-N-butyl pyridinium tetrafluoroborate ionic liquid at 303.15 K,” Journal of Chemical & Engineering Data, vol. 53, no. 8, pp. 1708–1712, 2008.

N. J. Bridges, K. E. Gutowski, and R. D. Rogers, “Investigation of aqueous biphasic systems formed from solutions of chaotropic salts with kosmotropic salts (salt-salt ABS),” Green Chemistry, vol. 9, no. 2, pp. 177–183, 2007.

F. J. Deive, M. A. Rivas, and A. Rodriguez, “Sodium carbonate as phase promoter in aqueous solutions of imidazolium and pyridinium ionic liquids,” The Journal of Chemical Thermodynamics, vol. 43, no. 8, pp. 1153–1158, 2011.

S. P. M. Ventura, S. G. Sousa, L. S. Serafim, Á. S. Lima, M. G. Freire, and J. A. P. Coutinho, “Ionic liquid based aqueous biphasic systems with controlled pH: the ionic liquid cation effect,” Journal of Chemical & Engineering Data, vol. 56, no. 11, pp. 4253–4260, 2011.

J. Na, Q. H. Yang, X. C. Dong, and S. N. Zhao, “Extraction and separation of rutin in ionic liquid aqueous two-phase system,” Yunnan Chemical Technology, vol. 35, no. 3, pp. 36–41, 2008.

D.-H. Zhao, Y.-B. Zeng, L. Li et al., “Determination of chloramphenicol in eggs using an aqueous two phase systems of pyridine ionic liquid and salt,” Chinese Journal of Analytical Chemistry, vol. 37, no. 3, pp. 445–448, 2009.

J. Chen, S. K. Spear, J. G. Huddleston, J. D. Holbrey, R. P. Swatloski, and R. D. Rogers, “Application of poly(ethylene glycol)-based aqueous biphasic systems as reaction and reactive extraction media,” Industrial and Engineering Chemistry Research, vol. 43, no. 17, pp. 5358–5364, 2004.

J. C. Merchuk, B. A. Andrews, and J. A. Asenjo, “Aqueous two-phase systems for protein separation studies on phase inversion,” Journal of Chromatography B, vol. 711, no. 1-2, pp. 285–293, 1998.

L. A. Ferreira and J. A. Teixeira, “Salt effect on the aqueous two-phase system PEG 8000-sodium sulfate,” Journal of Chemical & Engineering Data, vol. 56, no. 1, pp. 133–137, 2011.

M. Foroutan, “Liquid-liquid equilibria of aqueous two-phase poly(vinylpyrrolidone) and K₂HPO₄/KH₂PO₄ buffer: effects of pH and temperature,” Journal of Chemical & Engineering Data, vol. 52, no. 3, pp. 859–862, 2007.

M. Hu, Q. Zhai, Z. Liu, and S. Xia, “Liquid-liquid and solid-liquid equilibrium of the ternary system ethanol + cesium sulfate + water at (10, 30, and 50)°C,” Journal of Chemical & Engineering Data, vol. 48, no. 6, pp. 1561–1564, 2003.

H. Shekaari, R. Sadeghi, and S. A. Jafari, “Liquid-liquid equilibria for aliphatic alcohols + dipotassium oxalate + water,” Journal of Chemical & Engineering Data, vol. 55, no. 11, pp. 4586–4591, 2010.

Y. Wang, Y. S. Yan, and S. P. Hu, “Phase diagrams of ammonium sulfate + ethanol/1-propanol/2-propanol + water aqueous two-phase systems at 298.15 K and correlation,” Journal of Chemical & Engineering Data, vol. 55, no. 2, pp. 876–881, 2010.

X. Q. Xie, J. Han, Y. Wang, Y. S. Yan, G. W. Yin, and W. X. Guan, “Measurement and correlation of the phase diagram data for PPG₄₀₀ + (K₃PO₄, K₂CO₃, and K₂HPO₄) + H₂O aqueous two-phase systems at T = 298.15 K,” Journal of Chemical & Engineering Data, vol. 55, no. 11, pp. 4741–4745, 2010.

Y. Guan, T. H. Lilley, and T. E. Treffry, “A new excluded volume theory and its application to the coexistence curves of aqueous polymer two-phase systems,” Macromolecules, vol. 26, no. 15, pp. 3971–3979, 1993.

I. Regupathi, S. Murugesan, R. Govindarajan, S. P. Amaresh, and M. Thanapalan, “Liquid-liquid equilibrium of poly(ethylene glycol) 6000 + triammonium citrate + water systems at different temperatures,” Journal of Chemical & Engineering Data, vol. 54, no. 3, pp. 1094–1097, 2009.

J. G. Huddleston, H. D. Willauer, and R. D. Rogers, “Phase diagram data for several PEG + salt aqueous biphasic systems at 25°C,” Journal of Chemical & Engineering Data, vol. 48, no. 5, pp. 1230–1236, 2003.

M. T. Zafarani-Moattar and S. Hamzehzadeh, “Liquid-liquid equilibria of aqueous two-phase systems containing polyethylene glycol and sodium succinate or sodium formate,” Calphad, vol. 29, no. 1, pp. 1–6, 2005.

R. D. Rogers, A. H. Bond, C. B. Bauer, J. Zhang, and S. T. Griffin, “Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data,” Journal of Chromatography B, vol. 680, no. 1-2, pp. 221–229, 1996.

D. F. Othmer and P. E. Tobias, “Liquid-liquid extraction data—toluene and acetaldehyde systems,” Industrial & Engineering Chemistry Research, vol. 34, no. 6, pp. 690–692, 1942.

P. G. González-Tello, F. Camacho, G. Blázquez, and F. J. Alarc, “Liquid-liquid equilibrium in the system poly(ethylene glycol) + MgSO₄ + H₂O at 298 K,” Journal of Chemical & Engineering Data, vol. 41, no. 6, pp. 1333–1336, 1996.

M. J. Hey, D. P. Jackson, and H. Yan, “The salting-out effect and phase separation in aqueous solutions of electrolytes and poly(ethylene glycol),” Polymer, vol. 46, no. 8, pp. 2567–2572, 2005.

M. T. Zafarani-Moattar and S. Hamzehzadeh, “Liquid-liquid equilibria of aqueous two-phase systems containing polyethylene glycol and sodium succinate or sodium formate,” Calphad, vol. 29, no. 1, pp. 1–6, 2005.

M. T. Zafarani-Moattar and P. Seifi-Aghjekohal, “Liquid-liquid equilibria of aqueous two-phase systems containing polyvinylpyrrolidone and tripotassium phosphate or dipotassium hydrogen phosphate: experiment and correlation,” Calphad, vol. 31, no. 4, pp. 553–559, 2007.

X. Q. Xie, Y. S. Yan, J. Han, Y. Wang, G. W. Yin, and W. S. Guan, “Liquid-liquid equilibrium of aqueous two-phase systems of PPG₄₀₀ and biodegradable salts at temperatures of (298.15, 308.15, and 318.15) K,” Journal of Chemical & Engineering Data, vol. 55, no. 8, pp. 2857–2861, 2010.

R. M. de Oliveira, J. S. dos Reis Coimbra, K. R. Francisco, L. A. Minim, L. H. M. Da Silva, and E. E. G. Rojas, “Equilibrium data of the biphasic system poly(ethylene oxide) 4000 + copper sulfate + water at (5, 10, 35, and 45)°C,” Journal of Chemical & Engineering Data, vol. 53, no. 7, pp. 1571–1573, 2008.

J. Chen and Q. Wang, “Combustion characteristics and dynamic modeling of micro engine,” Journal of Jiangsu University, vol. 31, no. 3, pp. 304–308, 2010.