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Phase Diagrams for Liquid−Liquid Equilibrium of Aqueous Two-Phase System Containing Poly(ethylene glycol) (4000, 6000, or 10 000 g mol−1 + Sodium Hydrogen Sulfite + Water) at Different Temperatures Pedro L. Bonifácio,† Cínthia D. Aguiar,† Bruno G. Alvarenga,† Nelson H. T. Lemes,† Eduardo C. Figueiredo,† Anderson F. Mesquita,‡ and Luciano S. Virtuoso*,† †

Colloid Chemistry Group, Chemistry Institute, Universidade Federal de Alfenas (UNIFAL-MG), Rua Gabriel Monteiro da Silva, 700, 37130-000 Alfenas, Minas Gerais Brazil ‡ Macromolecules and Surfactantes Group, Chemistry Department, Universidade Federal do Espírito Santo (UFES), 29075-910, Vitória, Espírito Santo Brazil S Supporting Information *

ABSTRACT: The liquid−liquid equilibrium for the mixtures of poly(ethylene glycol) (PEG) with mass-average molar of 4000, 6000, or 10 000 g mol−1 + sodium hydrogen sulfite (NaHSO3) + water systems has been determined experimentally at T = 288.15, 298.15, 308.15, and 318.15 K. The temperature, PEG mass molar, mixture composition, and nature of anion effects on phase equilibrium were studied. The binodal curves at different temperatures were fitted to an empirical equation that correlates the concentrations of polymers and salt. The consistence of experimental data of equilibrium was available through of the Othmer−Tobias and Bancroft equations, and the parameters were reported.

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INTRODUCTION The advances in studies and applications of the so-called aqueous two-phase systems (ATPS), involving liquid−liquid equilibrium between two phases rich in water, began in the 1950s when Albertsson1 employed ATPS for to purification process of biological materials. These studies have boosted the search area involving ATPS.1 ATPS has been extensively used in separation processes involving ions,2 small molecules,3 macromolecules,4 and a variety of biomaterials, including bacteria,5 viruses,6 and cellular organelles.7 ATPS applications, involving both the clarification processes and partial purification in a single step, have been reported, enabling scientists to obtain high yields of recovery and purity. Another important feature of these systems is the possibility of scaling up, which is of industrial interest. In general, ATPS are formed by mixtures of aqueous solutions of two polymers, or two salts, or even of a polymer and a salt in certain controlled conditions of temperature, pressure, and component composition.8−10 Knowledge about the phase behavior of these mixtures, expressed as phase diagrams, has fundamental importance to the applications of the ATPS, as separation technique, purification, and preconcentration of analytes. It is a widely known fact that the large proportion of water, present in the phases of the ATPS, provides a biocompatible environment for materials of biological origin, © XXXX American Chemical Society

as well as the lower environmental impact compared to other separation/extraction processes that employ organic solvents. In this sense, various studies cited in specialized literature correlate the use of this system as being of great interest to biomaterial recovery maintaining their activity besides mentioning that the processes involving the use of ATPS follow the principles of Green Chemistry.11−15 The most studied ATPS are those formed by mixing a polymer and salt, in particular polyethylene glycol, due to the advantageous characteristics these systems have as low cost of used materials, low viscosity phase, low toxicity, biodegradability, ability to reuse components of the mixture, and high ratio of water phase in formed ATPS.16−18 In recent years, experimental data of phase equilibrium involving ATPS formed by polymer19 or copolymer20 + sodium sulfite or sodium hydrogen sulfite have been reported. The study of these new systems has been explored because of the particular properties of sulfites that incorporate desirable features to the ATPS with application prospects in separation processes in food industry.21 The sodium hydrogen sulfite, for example, is a widely used salt in food industry as a preservative due to its antimicrobial action and its effectiveness in Received: December 6, 2015 Accepted: April 27, 2016

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DOI: 10.1021/acs.jced.5b01038 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(Millipore) was used in the experiments (with the thermal conductivity coefficient κ = 0.054 μS·cm−1). Obtaining the Binodal Curves. The binodal curves were determined using the cloud point method.8 In the present work, the binodal curves were determined using several starting binary solutions. In general, a salt solution of known concentration was titrated with the polymer solution or vice versa until the solution turned turbid in controlled temperature. The composition of this mixture was noted for each cloud point. Then, water was added dropwise to the vessel to yield a clear one-phase system and this procedure was repeated. The vessel was immersed in a jacketed glass vessel, and the temperature of the system was determined at T = 288.15, 293.15, 298.15, 303.15, and 308.15 K within 0.05 K using a water thermostat (TC 184 Tecnal, Brazil). The composition of the mixture by mass at each point on the binodal curve was determined using an analytical balance (AG 220, Shimadzu, U.S.A.) with an uncertainty of 1.0 × 10−7 kg. Construction of Phase Diagrams. Previously, PEG and salt stock solutions were prepared by weighing appropriate amounts of reagents on an analytical balance (Shimadzu, AG 220 with an uncertainty of ±0.0001 g). ATPS were prepared by mixing appropriate quantities of the stock solutions of PEG, NaHSO3, and water in glass vessels, according to the global compositions estimated above the binodal curve previously obtained. In a typical experiment, 10 g of each system was prepared, and two phases with approximately equal volumes were obtained. After being vigorously stirred, the system became turbid and was allowed to rest for 24−72 h at the operation temperature of

preventing undesirable oxidation processes preserving food flavor.22 In this sense, this work aimed to study the phase behavior at different temperatures of the new ATPS formed by mixtures of sodium hydrogen sulfite + PEG (4000, 6000, or 10 000 g mol−1) + water. The influences of temperature, molar mass, and composition of the mixture on the phase diagram were evaluated.

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MATERIALS AND METHODS Materials. Poly(ethylene glycol) (PEG) with average molar masses of 4000, 6000, and 10 000 g·mol−1 were purchased from Sigma-Aldrich (U.S.A.) without further purification. Analytical grade sodium hydrogen sulfite (NaHSO3) was obtained from Synth (Brazil) (Table 1). Double distilled deionized water Table 1. Provenance and Purities of the Used Chemicals chemical name

supplier

polyethylene glycol (Mwa = 4000 g mol−1) polyethylene glycol (Mwa = 6000 g mol−1) polyethylene glycol (Mwa = 10 000 g mol−1) sodium hydrogen sulfite

Sigma-Aldrich, St. Louis, MO, United States

Synth, SP, Brazil

a

initial mole fraction purity

purification method

0.99

nob

0.99

nob

0.99

nob

0.982

nob

b

Mw is average molecular weight. Material was used without further purification.

Table 2. Equilibrium Data for the PEG 4000 g mol−1 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa overallb tie line

TLL

STL

w1

w2

top phaseb w3

1 2 3 4 5

27.87 31.87 35.29 39.56 43.86

−2.08 −1.90 −1.78 −1.66 −1.68

17.80 18.67 19.95 20.86 21.61

20.07 20.66 22.01 23.07 23.56

62.13 60.66 58.03 56.07 54.83

1 2 3 4 5

21.92 26.72 30.91 33.50 37.45

−3.60 −2.96 −2.58 −2.44 −2.31

19.08 20.04 21.03 22.00 23.08

14.95 15.98 17.02 17.99 18.99

65.97 63.98 61.95 60.01 57.93

1 2 3 4 5

18.74 23.27 26.90 30.35 34.42

−3.21 −2.89 −2.50 −2.33 −2.27

17.46 17.97 18.50 19.02 19.53

15.04 16.04 16.97 17.98 19.01

67.50 65.99 64.53 63.00 61.46

1 2 3 4 5

20.98 25.94 29.73 33.68 38.01

−3.37 −2.87 −2.44 −2.16 −2.00

18.00 19.10 19.84 21.00 22.00

14.00 14.94 16.18 17.00 17.96

68.00 65.96 63.98 62.00 60.04

w1 288.15 K 36.52 38.25 40.04 41.46 44.00 298.15 K 35.52 36.95 38.85 40.76 43.27 308.15 K 30.23 32.22 34.14 36.86 39.65 318.15 K 30.12 32.79 34.52 36.00 38.00

bottom phaseb

w2

w3

w1

w2

w3

Ks

10.83 10.58 10.29 10.09 10.00

52.65 51.17 49.67 48.46 46.00

11.40 10.05 9.28 7.55 6.29

22.90 25.42 27.58 30.47 32.40

65.69 64.54 63.14 61.97 61.32

2.11 2.40 2.68 3.02 3.24

10.63 10.42 10.18 9.99 9.69

53.85 52.62 50.97 49.25 47.04

14.40 11.64 10.03 9.76 8.90

16.50 18.99 21.34 22.67 24.57

69.09 69.37 68.62 67.57 66.54

1.55 1.82 2.10 2.27 2.54

10.86 10.57 10.32 10.16 10.00

58.91 57.20 55.53 52.98 50.34

12.33 10.23 9.17 8.97 8.15

16.43 18.18 20.31 22.13 23.88

71.23 71.59 70.52 68.90 67.96

1.51 1.72 1.97 2.18 2.39

10.03 9.90 9.67 9.40 9.17

59.84 57.31 55.80 54.60 52.83

10.01 8.29 7.00 5.45 4.00

16.00 18.42 20.93 23.57 26.17

73.99 73.29 72.06 70.97 69.83

1.60 1.86 2.16 2.51 2.85

The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1, w2, and w3 represented mass N fractions percent of PEG 4000 g mol−1, NaHSO3, and water, respectively and the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 = 4.1%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.062. The TLL, Ks, and STL values were calculated from eqs 2, 3, and 4, respectively. a

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DOI: 10.1021/acs.jced.5b01038 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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288.15, 298.15, 308.15, and 318.15 K in a temperature-controlled bath (MQBTC 99-20 with an uncertainty of ±0.05 K). The equilibrium state was characterized by the absence of turbidity in both top and bottom phases. Aliquots of the top and bottom phases were collected with a syringe for analysis. Then, the concentrations of sodium hydrogen sulfite in the top and bottom phases were determined by flame photometry. The uncertainty in the measurement of the NaHSO3 mass fraction is ±0.002. The polymer was quantified at T = 298.15 K using a refractometer (Analytik Jena AG Abbe refractometer 09-2001, Germany). The uncertainty in the refractive index measurement is ±0.0002. Because the refractive index of the phase samples depends on the polymer and salt concentrations and is an additive property, we obtained the PEG concentration by subtracting the NaHSO3 concentration (obtained by flame photometry) from the total solution composition (obtained by the refraction index). For dilute aqueous solutions containing a polymer and a salt, the relationship between the refractive index, nD, and the mass fractions of PEG, w1, and salt, w2, is given by nD = a0 + a1w1 + a 2w2

Table 3. Binodal Data for the PEG 4000 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa 288.15 K

(1)

This equation has previously been used in various studies. The estimated values for the coefficients were a0 = 1.331, a1 = 1.458 × 10−3 to 1.478 × 10−3, and a2 = 1.469 × 10−3. The standard deviation of the mass percentage of the polymer was on the order of 0.05%. The water concentration was obtained through subtraction. All of the analytical measurements were performed in triplicate. The tie line lengths (TLL) for the different compositions were calculated according to

(2)

The TLLs are expressed in mass fractions. The partition coefficient of the salt (Ks) was calculated as the fraction of salt retained in the bottom phase divided by the salt in the top phase according to

Ks =

w2b w2t

(3)

Finally, the slopes of tie-lines (STL) were calculated as the ratio between the variation of the polymer and salt concentration in each phase of the ATPS according to STL =

w1t − w1b w2t − w2b

RESULTS AND DISCUSSION

Experimental Results. For the PEG (4000, 6000, or 10 000 g mol−1) + NaHSO3 + water systems the experimental binodal data and the tie-line compositions at T = (288.15, 298.15, 308.15, and 318.15) K are given in Tables 2 to 7. The subscripts 1, 2, and 3 in the tables represent the PEG, NaHSO3, and water components, respectively. All concentrations are expressed in mass fraction percentage, and for each polymer/salt combination five tie-lines were determined at each temperature. The slopes of the tie lines present in Tables 2, 4, and 6, were determined through the linear regression of the corresponding set of composition points (overall, top-phase, and bottom-phase compositions).

23,24

TLL = [(w1t − w1b)2 + (w2t − w2b)2 ]1/2

Article

(4)

where in eqs 2, 3, and 4 w1t, w1b, w2t, and w2b are the top (t) and bottom (b) equilibrium mass fractions of PEG (1) and sodium hydrogen sulfite (2), respectively. Several pH measurements of the top and bottom phases of the ATPS formed by PEG 6000 and PEG 10 000 + Na2SO3 + H2O were conducted in a pH meter (Model W3B, Italy, with precision of ±2 mV) at T = 298.15 K. The measurements were performed on the mixture of compositions of the first and fifth tie lines. The pH was measured every 30 min for the first 3 h, then every 60 min for an additional 4 h, and for every 24 h until completing 78 h. Concomitantly, the collected concentrations of the components in the top and bottom phases were determined according to the procedure previously described in order to check the time interval for the equilibrium being achieved.

298.15 K

308.15 K

318.15 K

w1b

w2b

w1

w2

w1

w2

w1

w2

47.34 44.46 39.80 38.57 34.74 33.54 37.10 35.88 31.72 29.78 27.97 25.30 22.97 21.25 3.55 3.83 5.22 5.76 6.59 7.03 7.14 7.82 8.18 8.77 9.57 10.07 10.54 10.97 12.00 13.64 15.15 16.99 18.78

8.70 10.05 10.41 10.36 10.75 10.90 11.05 11.38 11.43 11.99 12.65 13.05 13.64 14.56 38.01 36.43 34.36 32.88 31.35 30.11 29.11 27.91 26.92 25.89 24.83 23.95 23.13 22.36 21.41 19.79 18.80 17.74 16.75

46.59 44.24 40.17 38.74 37.41 36.18 35.02 34.06 33.03 32.06 31.15 30.28 29.47 25.51 23.27 20.21 18.69 3.13 3.43 3.70 4.32 4.89 5.41 5.58 6.04 6.47 6.86 7.23 7.84 7.90 9.21 10.29 11.41 12.68 14.35 15.76

9.55 10.32 9.93 10.13 10.31 10.48 10.64 10.59 10.74 10.87 11.00 11.13 11.24 11.54 12.00 12.56 13.08 38.26 36.65 35.18 33.63 32.21 30.90 29.85 28.72 27.68 26.71 25.80 24.85 24.17 21.90 20.03 18.79 17.24 15.75 14.51

46.11 44.46 39.98 38.74 37.41 36.32 34.19 32.30 31.37 30.49 29.67 27.52 25.00 22.43 20.39 18.57 0.92 1.33 2.10 2.43 3.46 3.71 3.93 4.47 4.97 5.13 5.57 5.71 6.10 6.22 8.36 10.29 12.75 14.56

10.10 10.05 10.17 10.13 10.31 10.27 10.39 10.50 10.64 10.77 10.90 11.08 11.48 11.89 12.10 12.60 39.54 37.83 36.04 34.61 32.93 31.73 30.62 29.44 28.34 27.45 26.50 25.72 24.88 24.19 21.68 18.35 16.54 14.83

38.62 37.02 35.73 33.25 31.09 30.18 28.27 26.59 24.50 22.01 20.50 19.13 17.57 2.26 3.01 3.69 4.30 4.87 5.39 6.18 6.60 7.00 7.37 7.71 9.29 10.21 11.83 13.29 15.28

9.17 9.42 9.39 9.58 9.75 9.72 10.07 10.37 10.59 11.20 11.47 11.84 12.22 27.40 26.06 24.84 23.73 22.71 21.78 20.81 20.03 19.30 18.63 17.99 16.57 15.87 14.64 13.90 13.05

The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1 and w2 represented mass fractions percent of PEG 4000 g mol−1 and NaHSO3, respectively and N the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 ≤ 3.9%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.058. a

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DOI: 10.1021/acs.jced.5b01038 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. Equilibrium Data for the PEG 6000 g mol−1 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa overallb tie line

TLL

STL

w1

top phaseb

w2

w3

w1

1 2 3 4 5

26.82 31.04 35.98 40.82 43.87

−2.72 −2.51 −2.49 −2.34 −2.33

15.41 17.77 19.60 20.74 22.52

11.45 11.97 12.22 12.72 13.03

73.14 70.26 68.18 66.54 64.45

1 2 3 4 5

33.22 34.46 37.15 40.70 44.90

−2.15 −2.01 −2.02 −2.03 −2.00

12.34 13.02 13.02 13.54 13.82

12.39 14.37 15.97 16.80 19.94

75.27 72.61 71.01 69.66 66.24

1 2 3 4 5

22.07 25.21 29.39 33.11 35.37

−2.45 −2.25 −2.33 −2.41 −2.34

12.03 13.11 14.23 15.23 16.02

9.35 10.02 10.57 11.47 12.02

78.62 76.87 75.20 73.30 71.96

1 2 3 4 5

27.15 30.37 33.57 38.79 43.74

−2.37 −2.34 −2.21 −2.21 −2.45

15.44 16.23 17.89 18.44 21.24

7.30 8.23 8.74 9.07 9.89

77.26 75.54 73.37 72.49 68.87

288.15 K 31.62 34.73 37.74 40.97 43.67 298.15 K 32.66 33.83 36.05 39.07 42.67 308.15 K 25.17 27.19 30.72 33.65 35.24 318.15 K 27.46 29.90 32.42 36.78 41.71

bottom phaseb

w2

w3

w1

w2

w3

Ks

5.27 5.02 4.71 4.01 3.78

63.11 60.25 57.55 55.02 52.55

6.45 5.90 4.36 3.43 3.36

14.52 16.51 18.14 20.04 21.10

79.03 77.59 77.50 76.53 75.54

2.75 3.28 3.85 5.84 6.27

3.15 2.82 2.56 2.15 1.98

64.19 63.35 61.39 58.78 55.35

2.73 2.65 2.42 2.12 2.02

17.04 18.34 19.23 20.34 22.34

81.93 80.97 80.73 80.11 79.84

5.40 6.50 7.51 9.46 11.28

4.99 3.98 3.63 3.33 3.13

69.84 68.83 65.65 63.02 61.63

4.74 4.15 3.72 3.06 2.72

13.34 14.21 15.23 16.01 17.04

81.92 81.64 81.05 80.93 80.24

2.67 3.57 4.19 4.80 5.44

2.54 2.38 2.07 1.68 1.69

69.99 67.72 65.51 61.54 56.61

2.45 1.97 1.88 1.45 1.22

13.11 14.32 16.01 17.68 18.23

84.44 83.71 82.11 80.87 80.55

5.16 6.01 7.73 10.52 10.78

The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1, w2, and w3 represented mass N fractions percent of PEG 6000 g mol−1, NaHSO3 and water, respectively and the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 = 3.7%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.098. The TLL, Ks, and STL values were calculated from eqs 2, 3, and 4, respectively. a

Table 5. Binodal Data (in Mass Fraction Percent) for the PEG 6000 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa 288.15 K w1

b

64.25 51.02 47.87 45.34 42.34 40.34 37.99 35.29 33.77 32.78 30.53 29.09 27.74 26.78 22.19 19.47 18.92 16.42 14.35 13.95 12.34 11.67 10.78

w2

298.15 K b

1.58 2.31 2.75 2.98 3.33 3.81 4.06 4.77 5.02 5.21 5.51 6.11 6.38 6.56 7.04 8.37 9.45 9.89 10.34 11.87 12.45 13.88 14.72

308.15 K

Table 5. continued 288.15 K

318.15 K

298.15 K

308.15 K

318.15 K

w1b

w2b

w1

w2

w1

w2

w1

w2

w1

w2

w1

w2

w1

w2

8.83

15.87

6.45

14.37

5.12

14.54

4.01

13.89

66.05 53.04 50.15 47.77 45.16 43.54 39.65 35.89 33.18 31.76 29.19 28.25 25.32 22.14 18.88 16.64 15.28 14.04 13.25 9.34 8.74 7.32 7.02

1.62 1.74 1.92 2.01 2.28 2.57 2.66 2.89 3.18 3.44 3.77 4.39 4.54 5.01 5.36 6.73 7.45 8.39 9.12 10.64 11.37 12.45 13.83

64.73 55.39 50.12 47.85 45.28 41.29 38.89 36.01 34.24 32.65 30.63 26.41 24.85 22.11 19.78 16.86 13.78 12.73 10.45 9.54 8.45 7.23 6.34

1.36 1.62 1.89 2.11 2.28 2.65 3.17 3.70 3.88 4.05 4.23 4.46 4.60 4.65 4.73 4.99 6.55 7.83 8.43 10.23 11.87 12.73 13.78

68.02 61.68 56.45 51.77 48.78 42.91 40.16 38.78 36.52 34.31 30.76 27.62 25.68 24.05 20.61 17.12 12.04 11.45 10.03 8.78 7.23 6.44 4.74

0.72 0.79 0.83 0.91 0.98 1.13 1.31 1.35 1.50 1.73 1.94 2.15 2.59 3.20 3.42 4.22 5.35 6.39 7.23 8.27 9.24 10.35 12.73

7.45

17.89

5.73

16.77

4.22

15.37

3.64

14.27

6.45

18.67

5.34

17.92

3.02

16.39

2.02

15.33

5.73

19.33

5.02

18.24

2.12

17.34

1.78

16.78

4.49

20.54

5.75

19.33

1.85

18.66

1.02

17.22

4.02

22.56

4.42

21.34

1.75

19.12

0.98

18.39

3.73

24.78

4.02

23.65

1.45

20.13

0.75

19.45

The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1 and w2 represented mass fractions percent of PEG 6000 g mol−1 and NaHSO3, respectively and N the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 ≤ 3.8%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.060. a

Tables 2, 4, and 6 also list the value of the partition coefficient (Ks) of the salt calculated according to eq 3. Temperature Effect. The binodal curve and tie lines for the systems formed by PEG (4000, 6000, or 10 000) g mol−1 + sodium hydrogen sulfite + water at 288.15 and 318.15 K are plotted in Figure 1. For all the studied ATPS, it was observed that the position of phase diagram shifts to regions of lower concentrations of salt and polymer, prominently, with the increase in D

DOI: 10.1021/acs.jced.5b01038 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 6. Equilibrium Data for the PEG 10 000 g mol−1 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa overallb tie line

TLL

STL

w1

w2

top phaseb w3

1 2 3 4 5

30.84 33.62 37.09 41.59 45.69

−2.27 −2.26 −2.32 −2.29 −2.29

12.71 13.03 13.56 13.77 14.03

13.55 14.73 15.56 17.45 19.34

73.74 72.24 70.88 68.78 66.63

1 2 3 4 5

31.17 32.74 35.53 38.95 42.89

−2.49 −2.37 −2.41 −2.44 −2.59

12.39 12.88 13.07 14.28 14.54

11.02 11.54 11.82 12.35 13.01

76.59 75.58 75.11 73.37 72.45

1 2 3 4 5

27.74 29.73 32.75 37.58 40.81

−2.68 −2.62 −2.62 −2.89 −2.97

12.03 12.51 13.03 13.43 14.02

9.05 10.02 10.57 11.07 11.52

78.92 77.47 76.40 75.50 74.46

1 2 3 4 5

26.88 29.48 32.42 37.16 42.35

−2.92 −2.99 −2.99 −3.19 −3.42

9.05 9.34 9.89 10.12 11.05

9.03 9.58 9.89 10.14 10.72

81.92 81.08 80.22 79.74 78.23

w1 288.15 K 33.27 35.65 38.43 41.54 45.22 298.15 K 32.66 33.83 36.05 39.07 42.67 308.15 K 29.56 31.22 33.72 38.58 41.22 318.15 K 27.46 29.90 32.42 36.78 41.71

bottom phaseb

w2

w3

w1

w2

w3

Ks

3.89 3.75 3.55 3.22 3.05

62.84 60.60 58.02 55.24 51.73

5.05 4.90 4.36 3.43 3.36

16.34 17.34 18.22 19.87 21.35

78.61 77.76 77.42 76.70 75.29

4.20 4.62 5.13 6.16 7.00

2.73 2.65 2.42 2.12 2.02

64.61 63.52 61.53 58.81 55.31

3.73 3.66 3.23 3.02 2.67

14.33 15.37 16.04 16.87 17.49

81.93 80.97 80.73 80.11 79.84

5.25 5.80 6.62 7.95 8.65

2.65 2.34 2.21 2.01 1.94

67.79 66.44 64.07 59.41 56.84

3.57 3.45 3.12 3.05 2.55

12.35 12.95 13.89 14.33 14.97

84.08 83.60 82.99 82.62 82.48

4.66 5.53 6.28 7.12 7.71

2.33 2.16 2.01 1.65 1.22

70.21 67.94 65.57 61.57 57.07

2.03 1.94 1.67 1.32 1.06

11.24 11.73 12.34 12.78 13.56

86.73 86.33 85.99 85.90 85.38

4.82 5.43 6.13 7.74 11.11

The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1, w2, and w3 represented mass N fractions percent of PEG 10 000 g mol−1, NaHSO3, and water, respectively and the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 = 5.0%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.061. The TLL, Ks, and STL values were calculated from eqs 2, 3, and 4, respectively. a

experimentally observed molar mass was rather sharp with changing PEG 4000−6000 g mol−1 and less pronounced with PEG 6000−10 000 g mol−1. With the increase in temperature, the effect of the molar mass becomes slightly more pronounced between PEG 6000 and 10 000 and the position of all diagrams move to the left (biphasic increase in area). In particular, the shift of the diagram referring to ATPS formed by PEG 10 000 is one that presents the more pronounced displacement. This behavior has been already related in literature for the various systems involving ATPS formed by PEG and inorganic salts31,32 and may be attributed by an increase in the hydrophobic character of PEG chains with the increase in their molar mass. It is a well-known fact that poly(ethylene glycol) (PEG) has good solubility in water at room temperature and undergoes phase separations at high temperatures. In this sense, the increase in PEG hydrophobicity is attributed to the increase in temperature. However, it is also known that at even higher temperatures PEG shows good solubility again. The phase separation mechanism in PEG solutions is attributed to the competition between the formation of PEG−water and water−water hydrogen bonds. Anion Effect. The ability of the acid sulfite anion to provide phase separation in aqueous mixtures containing PEG was also evaluated (Figure 4). Previously published results involving liquid−liquid equilibrium data for various systems formed by PEG 4000 + inorganic salts + water were used in this comparison.19,33,34 It was observed that the ability of anions to induce the formation of ATPS follows the following order 2− − − SO2− 4 ≈ SO3 > HSO3 > NO3 .

temperature. This is consistent with an endothermic process of phase separation. A similar behavior of the influence of temperature in the position of the phase diagram of other ATPSs formed by mixtures of the PEG + inorganic salts + water was reported.25−27 The influence of the temperature on phase equilibrium was also analyzed in terms of the concept of slope of the tie-line (STL) that was calculated by eq 4. Tables 2, 4, and 6 present the STL values for all the studied systems at different temperatures. For all ATPS, in general an increase in temperature promotes an increase in STL (Figure 1a−c). In previous studies, this experimental observation has related the increase in hydrophobicity of the PEG molecules to the increase in temperature. This leads to a conformational change in PEG chains resulting and spontaneous diffusion of water molecules from the top phase to the bottom phase providing a concomitant increase in the polymer concentration in the upper phase and a relative reduction in the salt composition in the lower phase due to the increase in the water molecules in the bottom phase.28−30 For all ATPS, studied in this work a linear and positive correlation was observed in each temperature between the TLL and the Ks parameters (Figure 3). PEG Molar Mass Effect. In Figures 1a−c and 2, the effect of PEG mass molar was also observed on the ATPS phase diagram. These figures show that when PEG with higher molar mass is added, the biphasic region becomes larger, requiring lower concentrations of PEG for the phase separation. As shown in Figure 2, the binodal curve for the PEG (with different molar mass) + NaHSO3 + water at different temperatures moves closer to the origin with increasing molar mass. The effect of the E

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Table 7. Binodal Data (in Mass Fraction Percent) for the PEG 10 000 (w1) + NaHSO3 (w2) + Water (w3) System at T = 288.15−318.15 K and Atmospheric Pressure ≈ 94 kPaa 288.15 K w1

b

67.03 57.87 51.34 48.26 46.23 44.37 41.27 38.56 35.34 33.36 31.24 28.37 25.33 23.49 20.22 18.34 17.33 15.34 13.83 12.67 10.87 9.02 7.34 6.45 5.84 5.01 4.34 4.07 3.89 3.45

w2

298.15 K b

1.48 1.85 2.03 2.45 2.98 3.02 3.44 3.78 4.21 4.88 5.04 5.99 6.22 7.32 8.04 8.55 9.02 10.22 10.92 11.02 12.23 13.01 14.05 15.23 16.77 17.78 18.34 19.03 20.93 22.34

308.15 K

318.15 K

w1

w2

w1

w2

w1

w2

68.04 61.34 55.34 51.97 48.54 45.12 41.33 38.23 35.02 32.11 30.12 29.01 27.03 25.23 22.14 21.89 18.23 17.02 15.23 11.9 10.24 9.03 8.23 7.78 7.01 6.22 4.23 3.78 3.11 2.95

1.37 1.45 1.67 2.05 2.12 2.29 2.54 2.89 3.01 3.11 3.27 3.52 3.88 4.11 4.49 4.67 5.87 6.02 7.32 8.34 9.15 10.77 11.05 12.45 13.22 14.78 15.22 16.28 17.21 19.25

68.78 62.73 55.27 48.33 46.02 42.84 41.42 38.03 36.02 34.18 32.34 29.33 26.15 24.89 22.03 19.22 14.28 12.39 11.03 10.79 9.39 8.25 7.74 6.2 5.37 4.67 4.03 3.55 3.01 2.76

1.12 1.22 1.34 1.57 1.78 2.03 2.12 2.34 2.23 2.57 2.72 2.98 3.12 3.25 3.87 4.05 5.12 6.27 7.71 8.24 9.21 10.02 10.42 11.24 12.57 12.89 13.45 14.78 16.34 18.93

68.02 61.68 56.45 51.77 48.78 42.91 40.16 38.78 36.52 34.31 30.76 27.62 25.68 24.05 20.61 17.12 12.04 11.45 10.03 8.78 7.23 6.44 4.74 4.01 3.64 2.02 1.78 1.02 0.98 0.75

0.89 0.94 1.05 1.12 1.21 1.34 1.42 1.57 1.69 1.79 1.89 1.98 2.14 2.32 2.57 2.98 3.57 4.05 4.67 5.67 6.98 7.34 8.67 9.78 10.27 11.35 12.89 13.55 14.29 16.37

a The standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. bw1 and w2 represented mass fractions percent of PEG 10 000 g mol−1 and NaHSO3, respectively N and the error (e) associated was e = (∑1 → 3 S ̅ )/Vm × 100 ≤ 4.5%, where S̅ is the standard deviation and Vm is the average value of the three measured systems. The standard uncertainties for mass fraction is u(w) ≤ 0.067.

Figure 1. Effect of temperature on the equilibrium phase compositions. Experimental tie lines at (□) 288.15 K and (○) 318.15 K and binodal data at (■) 288.15 K and (●) 318.15 K for the systems formed by (a) PEG 4000 g mol−1 + NaHSO3 + water, (b) PEG 6000 g mol−1 + NaHSO3 + water, and (c) PEG 10 000 g mol−1 + NaHSO3 + water. All concentrations are expressed in mass fraction percentage. The experiments were conducted at atmospheric pressure of 94 kPa.

Evaluating the Sulfite Species. pH measurements in the top and bottom phases of the first and fifth tie lines for the studied ATPSs formed by PEG 6000 or 10 000, at each temperature were conducted for all collected phases in order to check the stability of the HSO−3 species during 78 h. The pH values were within the range of 4.79−5.04 for the analyzed top phases and 3.95−4.68 for the bottom phases. It is a known fact that sulfite species may exist in different forms in aqueous solutions depending on the pH. The obtained results are within the pH range of 1.5 to 6.0, which is typically related to the prevalence of bisulfite isomers (HSO−3 and SO3H−).35 In a recent study, published by our group, involving the equilibrium phase of ATPS formed by PEG + Na2SO3 + water,19 for all phases the analyzed alkaline pH values were observed to be typical for the expected predominance of the sulfite species (SO2− 3 ) in the middle. These studies can be seen as complementary, as they make it possible for a specific interest in purification/extraction of an analyte any, choose to use ATPS containing alkaline (SO2− 3 ) or acidic phases (HSO−3 ), because the ability these anions have to

form ATPS is very similar. In the compositional analysis of the top and bottom phases, conducted after the pH measurements, it was found that soon after the first 3 h the equilibrium compositions have already been achieved within the considered error range ( HSO3 > NO3 . The pH analysis of the phases over time indicate that bisulfite species remained stable.

Table 9. Adjustment Parameters of the Binodal Curves Obtained for the Equation 7 T/K

a

b

c

sda

−1

288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 a

sd =

PEG 4000 g mol + NaHSO3 + water 4.6227 −7.8399 0.83710 7.2260 −9.6075 11.011 4.2549 −8.1451 −12.161 14.864 −12.586 13.521 PEG 6000 g mol−1 + NaHSO3 + water 1.5468 −6.9651 −26.317 1.7474 −9.0605 53.4237 1.6946 −8.9273 −92.999 1.0788 −8.4994 −168.7628 PEG 10 000 g mol−1 + NaHSO3 + water 1.7495 −7.8044 −38.946 1.7773 −9.3058 −8.4215 1.7661 −10.320 40.874 1.8726 −13.018 −77.920

■

0.0156 0.0319 0.0438 0.0166

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b01038. Figure S1: The effects of temperature on the experimental binodal curves for the ATPS formed by PEG (4000, 6000, or 10 000 g mol−1) + NaHSO3 + water. Table S1: Parameters of adjustment of refractive index data. Table S2: Values of pH for the phases of ATPS formed by PEG (6000 or 10 000 g mol−1) + NaHSO3 + water. Table S3: Compositions of the phases of ATPS formed by PEG (6000 or 10 000 g mol−1) + NaHSO3 + water. (PDF)

0.0107 0.0224 0.0277 0.0396 0.0173 0.0234 0.0253 0.0367

■

∑iN= 1(w1cal − w1exp)2 N−1

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +55-35-3299-1260. Fax: +55-35 3299 1384.

to the adjustments of the binodal curves performed by eq 7 are shown in Table 9. For all adjustments, the standard deviations were less than 0.045. These values are below those found to the adjustments made to the systems formed by PEG + MgSO4 + water, PEG + phosphates + water,40,41 therefore the Merchuk equation can correlate the binodal curves obtained in this work to an acceptable accuracy. Figure 5 shows the experimental curve adjusted to the system PEG 4000 + NaHSO3 + water at T = 298.15 K.

Notes

The authors declare no competing financial interest. Funding

We gratefully acknowledge the Fundaçaõ de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) for financially supporting this Project and the Secretaria Estadual de Educaçaõ -MG and the secretariat of Minas Gerais by the release of the master’s student PLB.

■ ■

ACKNOWLEDGMENTS The authors thank the MSc Raphael A. B. Gomes for the help with the abstract graphic. REFERENCES

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Figure 5. Experimental and calculated binodal curves for the PEG 4000 g mol−1 + sodium hydrogen sulfite + water system at 298.15 K: (□) experimental results and (solid line) predicted values with the Merchuk eq 7. All concentrations are expressed in mass fraction percentage.

■

CONCLUSION Liquid−liquid equilibrium data were obtained for the systems PEG 4000 g mol−1 + NaHSO3 + water, PEG 6000 g mol−1 + NaHSO3 + water, and PEG 10 000 g mol−1 + NaHSO3 + water at T = 288.15, 298.15, 308.15, and 318.15 K. The effect of temperature, mass molar, and anion in the phenomenon of phase separation were available. The increase in temperature promoted an increase in the biphasic area. This behavior is typical to the endothermic phase-separation process. It was also observed that an increase in the molar mass of the PEG moves the position of H

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