Phase Behavior in Aqueous Two-Phase Systems Based-Ionic Liquid

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Phase Behavior in Aqueous Two-Phase Systems Based-Ionic Liquid Composed of 1‑Butyl-3-methylimidazolium Tetrafluoroborate and Copper Sulfate in Different Temperatures Pedro Lúcio Bonifácio, Cínthia das Dores Aguiar, Bruno Giordano Alvarenga, Nelson Henrique Teixeira Lemes, and Luciano Sindra Virtuoso* Colloid Chemistry Group, Chemistry Institute, Federal University of Alfenas, Rua Gabriel Monteiro da Silva 700, 37130-000 Alfenas-MG, Brazil J. Chem. Eng. Data Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/25/19. For personal use only.

S Supporting Information *

ABSTRACT: Phase behavior of new aqueous two-phase systems (ATPSs) composed by 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) + copper sulfate (CuSO4) + water systems were determined experimentally at T = (283.15, 298.15, and 313.15) K. The phase diagrams obtained at the different study temperatures describe the liquid− liquid equilibrium (LLE) and, in some cases, the liquid−liquid−solid equilibrium for different mixture compositions. The effects of the temperature, composition, and ion exchange in the formation of this ATPS were available. The temperature had a remarkable effect on the position of phase diagrams. The decrease in temperature promoted phase separation indicating the exothermic character of formation of these ATPSs, and further, at temperatures of 283.15 and 298.15 K it was observed that there was phase inversion for some mixture compositions that occurred. The extent of the ionic exchange of the original ionic pairs between the phases in equilibrium was evaluated considering the electroneutrality of the phases. It was observed experimentally that, in the LLE condition established, there was no significant exchange of the ionic pairs. The ability of different cations, from different sulfate salts, to induce the formation of ATPSs in mixtures involving [Bmim]BF4 was evaluated. For this, thermodynamic data of hydration of different cations reported in the literature were used together with experimental data of saturation solubility to establish a scale. Thermodynamic parameters of transfer of components (cations, anions, and water) between the phases were also calculated from the experimental data and indicated that the material transfer of the bottom phase to the top is not spontaneous and tends to be less spontaneous as the length of the tie line value increases. Additionally, the equilibrium data and binodal curves were fitted to an empirical nonlinear expression (Merchuk equation), and the salting out effect was explored using the type-Setschenow equation.

1. INTRODUCTION Aqueous two-phase systems (ATPS) are clean alternatives for traditional organic−water solvent extraction systems, which consist of two macroscopic aqueous-rich phases formed by mixing in water two incompatible polymers, one polymer and one salt, or by the combination of two different salts. Above a critical concentration of those components, spontaneous phase separation takes place, and the extraction of biomolecules can be achieved by the manipulation of their affinity for each of the aqueous-rich phases. The two phases are mostly composed of water and nonvolatile components, thus eliminating volatile organic compounds. They have been used for many years in biotechnological applications as nondenaturing and benign separation media.1 The ATPS has been widely used for the extraction and purification of analytes such as ions,2 small molecules,3 and biomaterials such as proteins,4 cellular organelles,5 and microorganisms.6 These systems are a variant of conventional liquid−liquid extraction with the advantages of favoring the stability of molecules with biological origin, besides the low © XXXX American Chemical Society

cost to implement the technique and ease of application in large scale with less environmental impact. From the pioneering work of Albertson,7 in which ATPS formed by aqueous mixtures of salts and polymers or even mixtures of aqueous solutions of two polymers have been studied, until the present day, new types of ATPS, among them those based on ionic liquids, have been reported in the literature.8 Ionic liquids (ILs) have been classified as green solvents due to their properties such as negligible vapor pressure, nonflammability, high thermal and chemical stabilities, and their strong solubilizing power of various types of organic and metallic analytes.1 Initially, biphasic systems consisting of hydrophobic IL and water were implemented as extraction methodologies;9 however, undesired aspects such as high ILphase viscosity, which may lead to the denaturation of Special Issue: Latin America Received: November 14, 2018 Accepted: April 19, 2019

A

DOI: 10.1021/acs.jced.8b01076 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. List of Chemicalsa chemical name/molecular formula tetrafluoroborate 1-butyl-3-methylimidazolium/[Bmim]BF4 orC8H15BF4N2 copper sulfate pentahydrate/CuSO4·5H2O water/H2O

purity (mass fraction)

water contentsb (mass fraction)

water content analysis method

purification method

source

0.987

0.012

TGA

noa

Aldrich

0.99

0.359

TGA

noa Milli-Q (Millipore)

Merck

Material was used without further purification as stated by the supplier. bWater contents in the [Bmim]BF4 and CuSO4·5H2O used in this work were determinate by TGA (Figures 1S and 2S of the Supporting Information). The water content present in both commercial products, ionic liquid and copper sulfate, was considered in all the compositions reported in the present work. a

Li2SO4, MgSO4,27 ZnSO4,27,28 NiSO4,28 and (NH4)2SO4.25,29 However, a significant part of these publications has been limited to a description of the LLE at a single temperature and, furthermore, considered the equilibrium analysis in terms of a limited amount of experimental information, for example, assuming that there are no significant exchanges of the ionics pairs between the phases. Although this premise has proven to be true, it has not been sufficiently verified experimentally.25−29 In this work we present and describe in detail the liquid− liquid equilibrium for a new ILATPS formed by [Bmim]BF4 + CuSO4 + H2O at temperatures of 283.15, 298.15, and 313.15K. The extent of the exchange of ionic pairs in the liquid−liquid equilibrium and some details about the occurrence of a liquid−liquid−solid equilibrium (LLSE) along some tie lines are discussed. The consistency of the equilibrium data was evaluated through the equations of Othmer-Tobias and Bancroft,30,31 and the binodal curves of the ILATPS were adjusted by nonlinear equations.32 The effect of salting out was also evaluated by linearization of the equilibrium experimental data using a Setschenow equation.33

biomacromolecules, and high cost of hydrophobic ILs eventually directed the studies to obtain new biphasic aqueous systems based on ionic liquids (ILATPS).10 These new ATPS are obtained by mixing aqueous solutions of hydrophilic IL and salts (organic or inorganic) under certain conditions of mix composition, temperature, and pressure. After the report, in 2003, of the first work involving IL-based ATPS in the literature, significant advances have been made in the discovery, characterization, and application of several new ILbased ATPSs.11,12 In fact, the importance of understanding the phase behavior of ATPSs, which are obtained through the construction of complete phase diagrams, with data adjustment and associated thermodynamic modeling, is decisive for the strict control of extraction, purification, and/or preconcentration of analytes involving the application these systems. Because of the distinct characteristics of IL-based ATPSs compared to traditional polymer-based ATPSs, such as relatively lower viscosity, rapid phase separation, better polarity tunability, excellent ILs’ solvability, among other interesting physicochemical properties, these systems have been used in the green and efficient extraction of a large variety of solutes, as simple alcohols, pharmaceuticals, amino acids, proteins, complex enzymes, and other biomasses.13−17 Although several studies on the liquid−liquid equilibrium (LLE) related to different IL-based ATPS containing imidazole ring have been reported since 2003,11 there are still many aspects of the behavior of these systems that need to be better understood. Particularly, the characterization of new ILATPS presents an extra challenge compared to the traditional ATPS formed by polymer + polymer or by polymer + salt, which is to describe the intricate ionic equilibrium between the phases generated by the mixture of the two salts (hydrophilic IL + inorganic or organic salt). In this sense, in the last years we have accompanied advances and contributed with the description of ATPS in which hydrophilic IL of lower costs, such as [Bmim]BF4,18 have been used together with sulfate salts to obtain a new ILATPS. According to a previously published study, it is important to point out that the high cost of producing ILs is still an obstacle for their use in large-scale engineering processes.19 To make the application of IL-based ATPS possible on an industrial scale, much scientific effort in the development of methods to recover ILs and to provide their reuse with minimization of costs in extraction processes has been proposed in the literature.20−24 Because of the high kosmotropocity of sulfate salts, these are efficient in inducing phase separation25 and therefore have been widely used in obtaining ATPS. Our interest has been understanding how different factors such as the nature of cation and anion, temperature, composition, among others, influence this balance. Recent works have described new ILATPS formed by [Bmim]BF4 and salt sulfates like MnSO4,26

2. EXPERIMENTAL SECTION 2.1. Materials. The materials used in this work, as well as their origin and purity specifications, are presented in Table 1. Water contents in the [Bmim]BF4 and CuSO4 in mass fraction were determined using a thermogravimetric balance SDTQ600 (TA Instruments) with a precision of 1 × 10−7 g operating with a constant nitrogen gas flow rate of 100 mL/min and using an open alumina pan with 10 mm diameter and 2 mm height. The thermogravimetric analysis (TGA) method was in good agreement with the value quoted by the suppliers (Figures 1S and 2S of the Supporting Information). In all studies deionized water Milli-Q (Millipore) was used to prepare the solutions. 2.2. Construction of Binodal Curves. For determination of the spinodal curves the turbidimetric titration method was used,10 and for this, ∼2.0 g of a stock solution of [Bmim]BF4 (∼100w1 = 90.00) was titrated with aliquots of 10 μL of a stock solution of the CuSO4 (∼100w2 = 10.00), or vice versa, until the occurrence of turbidity in the mixture. All solutions and the solvent were kept in a thermostatic bath (SOLAB, SL 152/10) long enough to ensure strict control of temperature. The composition of the mixture was registered, and then aliquots of water of 100 μL were added to the mixture, until the complete disappearance of turbidity. The procedure was repeated several times to obtain a set of points in each of the working temperatures (283.15, 298.15, and 313.15)K. Finally, the positioning of each of the turbidimetric curves (quasiequilibrium curves or spinodal curves) obtained was compared with the binodal points obtained by analyzing the equilibrium phases at a given temperature. In this way the position of the B



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binodal curve was obtained. The binodal data were then fitted with the well-known Merchuk equation34

directions, and thus the transmittance decreases with increasing concentration of sulfate ions. The transmittance of the BaSO4 formed was determined by UV−vis spectroscopy measured at 420 nm (Biochrom Libra 522), and the sulfate ion concentration was determined by comparison of the reading with a standard curve. All measurements were performed in triplicate. The content of the tetrafluoroborate anion was determined by the difference in percentual of mass fraction in relation to the other constituents of the mixture: 100wBF−4 = 100 − 100wBmim+ − 100wCu2+ − 100wSO2− − 100ww. After determination of four of 4 2+ the five species (Bmim+, SO2− 4 , Cu , and water) present in each phase of the assembled ATPSs, it was possible to describe the liquid−liquid equilibrium from the tie lines obtained. The length of the tie line (TLL) and the slope of the tie line (STL), in the different mixture compositions and temperatures, were calculated using Equations 2 and 3, respectively.

i w y w1 = [α0 + α1(T − T0)] + [β0 + β1(T − T0)]lnjjj 2 zzz k 100 { + [γ0 + γ1(T − T0)](w2/100)

(1)

in which the parameters α0, α1, β0, β1, γ0, and γ1 are now independent of temperature. In Equation 1, T is the absolute temperature, and T0 is assumed to be the reference temperature, T0 = 273.15 K. The Simplex optimization algorithm was used to find the parameters that provided the best fits of the experimental binodal curves to the empirical equations. The least-squares of differences between the experimental data and calculated values were used as a criterion of the best fit. 2.3. Construction of Phase Diagrams. To study the liquid−liquid equilibrium, five different ATPSs of ∼8.0 g each, representing five different tie lines, were assembled from the overall composition points, chosen symmetrically above the binodal curve (biphasic region). The ATPSs were mounted in falcon tubes of 15 mL and kept at controlled temperature for at least 24 h or until the phases became perfectly transparent. The top and bottom phases of the systems were then collected, and their mixture compositions were analyzed. After the collection of the phases, the top and bottom phases were diluted with fresh water, and then the mass fraction of the Cu2+ in each of the phases was determined using atomic absorption spectroscopy (Thermo Scientific, ICE 3000) at 324.8 nm. The concentration of the imidazolium cation was determined by UV−vis spectroscopy (Shimadzu UV-2401 PC) at 211 nm, which corresponds to the typical absorption of the imidazole ring. For the determination of the water content in the phases of the ATPS aliquots, the top and bottom phases were dried to constant mass in an oven at ∼383 K. The difference in mass before and after the drying process was used to determine the dry matter and the water content. Sulfate content was determined in both phases by turbidimetric method that consisted in the addition of a concentrated solution of barium salt (BaCl2) and some additives, to help keep colloidal suspension stable, to the solution containing sulfate ions to obtain a barium sulfate suspension, which can be quantified by UV−vis. Aliquots collected from the salt enriched phases (∼0.02 g) and enriched in the ionic liquid (∼0.05 g) were diluted by a factor of ∼1 × 104 times. Then was added 0.15 g of a conditioning solution and 0.03 g of BaCl2 (0.144 mmol) over an aliquot of (∼3 g) of the diluted phase. The conditioning solution consisted of a mixture containing 0.085 71 g of H2O, 0.0180 g of glycerol, 0.0225 g of ethanol, 0.0214 g of NaCl, and 0.0019 g of HCl. The amount of BaCl2 added in the diluted sample of the phase guarantees a large excess of Ba2+ cations in relation to the content of SO2− 4 anions present in the phase. After it was mixed, all the sulfate present in the sample precipitated as insoluble BaSO4 in the form of a uniform colloidal suspension. These additives increase the stability of the colloidal BaSO4 and enable reproducibility in the quantification of the sulfate ion in the various phases of the ATPSs. Still, in turbidimetry, light scattering is the phenomenon responsible for establishing a linear mathematical relationship between analyte concentration and transmittance; that is, the presence of insoluble BaSO4 in suspension causes the incident light to spread and be reflected in several

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

(2)

w1t − w1b w2t − w2b

(3)

where wti and wbi represent the compositions at equilibrium expressed as percentual of mass fraction (100w) for [Bmim]BF4 (1) and CuSO4 (2) at the top (t) or bottom (b) phases, respectively. To determine the TLL and STL parameters and construction of all phase diagram graphics in the present work, the [Bmim]BF4 and CuSO4 concentrations were regarded as the sum of concentrations of cation and anion of the ion pair, respectively, expressed in mass fraction. 2.4. Treatment of Experimental Data. 2.4.1. Ion Exchange Evaluated. The electroneutrality in the bulk phases was available by α α α α wSO − 2− wBF wCu 2+ + wBmim 4 4 ε = +2 − −2 MBmim+ MCu2+ MBF−4 MSO24− α

(4)

in which, again, α corresponds to the top or the bottom phase. Because of the electroneutrality condition the expected value of ε is zero, which one expects to happen within uncertainty given by 2 ij 2 α α jjij ur(wBmim+)wBmim +y jij ur(wCu2+)wCu2+ zyz zz j j zz uε ≈ jjjj zz + jjj2 z z jjj MBmim+ MCu2+ jk { k { k 0.5 2 α α y2 i ur(wSO24−)wSO y yzzz ij ur(wBF−)wBF −z 2− z j 4 4 z 4 z zz zzz zz + jjjj2 + jjjj zz zz zz jj j MBF−4 MSO24− zz k { k {{ α

(5)

2.4.2. Thermodynamics Parameters of Transfers. Also, the partitioning behavior of species involved in the phase equilibrium in the assembled ATPSs were evaluated by calculating the distribution ratio (Di) of each of the five constituents between the phases, defined as Di =

Cit Cib

(6) +

BF−4 ,

2+

SO2− 4 ,

where D and i correspond to Bmim , Cu , or water. The values of the calculated distribution ratios were used in the determination of variation of the free energy of C



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Figure 1. Effect of temperature on the equilibrium phase compositions for the ATPSs formed by [Bmim]BF4 (1) + CuSO4 (2) + H2O (3) in at temperatures (●) 283.15, (□) 298.15, and (▲) 313.15 K. (a) Representation in ternary diagram of binodal curves (only the region 0−50 or 50− 100 expressed as percentage of mass fraction of each axis is presented to expand the region containing the binodal curves and so put it highlighted) and (b) representation in rectangular diagram of the LLE data. The five tie lines that appear dashed at 283.15 K (in blue) and the first three dashed lines at 298.15 K (in black) refer to the occurrence of phase inversion. In both diagrams the concentrations are expressed as percentage of mass fraction. b ji c zyz z = kIL + ks(cst − csb) lnjjjj IL t z z c k IL {

transfer between the phases of the components (ΔtrGoi ), defined by Equation 7, for each liquid−liquid equilibrium analyzed. Δtr Gio = − niRT ln Di

where kIL is the constant that correlates the activity coefficient of [Bmim]BF4 with its concentration; ks represents the copper sulfate salting out coefficient; cIL and cS correspond to the concentrations of [Bmim]BF4 and copper sulfate expressed in molality, and the superscripts refer to the top (t) and bottom (b) phases, respectively.

(7)

where ni corresponds to the amount of total matter of the species i involved in the transfer, R is the gas constant, and T is the temperature. In this case the activity coefficient factor −RT ln(γti /γbi ) is considered close to zero and is neglected. This equation is widely used in solubility study.35 Then we apply the above concept when more than one species is transferred assuming that the Gibbs energy change of total transfer of a multicomponent system, ΔtrGototal, can be calculated as the summation of the independent contributions from each component i, which is given by o Δtr Gtotal =

= m∑ i

3. RESULTS AND DISCUSSION 3.1. Phase Diagrams and Liquid−Liquid Equilibrium. The experimental data of the liquid−liquid equilibrium and binodal curves for ATPSs formed by [Bmim]BF4 (1) + CuSO4 (2) + water (3) at temperatures of T = (283.15, 298.15, and 313.15) K are shown in Figure 1 and Tables 2 and 3, respectively, where TL refers to the tie line, TLL refers to the tie line length, and the compositional data presented are expressed in percentage of mass fraction (100w). In Figure 1a is observed that the decrease in temperature leads to a significant increase in the biphasic area of the diagram. Because of this increase in the biphasic area of the diagram, the absolute value of STL and TLL increases for each tie line as the temperature decreases, as can be seen in Table 2. This trend is directly related to the decrease of the solubility of [Bmim]BF48 and CuSO4 in water with decreasing temperature and indicates that the phase separation for such systems is exothermic. Similar phenomena have been reported for ATPSs formed by aqueous PEG + electrolyte mixtures12,13,25−31 and in previous work published by our research group, where the behavior of the phase of ATPSs formed by [Bmim]BF4 + sulfates of manganese26 or nickel or zinc28 + water at different temperatures was evaluated. Another characteristic observed in the ATPSs formed by copper sulfate that is like the phase behavior already reported for other cations with the same oxidation number (Ni2+, Zn2+ and Mn2+) is the occurrence of phase inversion at lower temperatures. In these cases, the densest phase is that enriched in the ionic liquid. Furthermore, in the ATPS formed with

∑ niΔtr Gio = n ∑ xiΔtr Gio i

[wi /100Mi ]Δtr Gio

(9)

i

(8)

As the amount of component transferred is different for each component, the Equation 8 included the contribution weights for each component to total transfer Gibbs energy. The calculated values of ΔtrGototal were to 100 g of total mass. 2.4.3. Cation Effect. A thermodynamic approach, utilizing thermodynamics parameters of hydration to quantify the ability of different cations in to induce the formation of ATPSs in mixtures involving [Bmim]BF4, was applied in the present work. For this, thermodynamic data of hydration (ΔhydG,33 ΔhydH,35 TΔhydS,33,36 and Vom 37) of different cations reported in the literature were used together with experimental data of saturation solubility to establish a scale. 2.4.4. Salting Out Coefficients. The ability of copper sulfate to induce phase separation of [Bmim]BF4 in the ATPS obtained in this work, at different temperatures, was quantitatively expressed by the so-called salting out coefficient (ks) calculated by Equation 9, known as Setschenow equation, according to Hey.31 D


50.71 52.76 55.14 58.17 60.22

46.08 49.51 53.98 56.83 59.10

44.42 48.39 53.38 57.70 62.40

1 2 3 4c 5c

1 2 3c 4c 5c

1 2 3 4 5

100w[Bmim]BF4

20.14 23.09 25.94 29.07 31.86

28.03 32.00 36.34 39.97 44.01

22.94 26.06 28.34 31.98 35.01

STL

−10.70 −9.49 −8.08 −7.53 −7.11

−7.71 −7.05 −5.98 −5.48 −5.72

−5.59 −5.23 −4.85 −4.43 −4.21 7.02 7.33 7.45 8.01 8.28

4.25 4.57 4.82 5.17 5.47

4.09 4.50 4.81 5.20 5.64

100wCuSO4

70.04 66.61 64.21 60.01 56.71

67.72 63.43 58.84 54.86 50.52

75.77 72.42 69.25 65.73 62.50

100ww

36.93 37.40 39.63 40.99 41.88

8.68 7.63 6.72 40.84 41.11

5.59 5.28 4.73 4.25 3.96

100wBmim+

100wCu2+

283.15 K 3.64 1.90 3.32 2.23 2.88 2.80 2.60 3.19 2.63 3.37 298.15 K 5.33 2.45 4.78 2.90 4.28 3.44 25.54 0.05 27.09 0.03 313.15 K 22.78 0.15 23.98 0.13 24.70 0.07 25.43 0.06 27.41 0.02

100wBF−4

top phaseb

0.26 0.12 0.11 0.07 0.09

3.78 4.30 5.65 0.08 0.10

3.02 3.47 4.13 4.60 5.13

100wSO2− 4

39.88 38.37 35.49 33.45 30.60

79.76 80.39 78.85 33.49 31.67

85.54 85.90 85.20 84.62 85.41

100ww

9.92 8.53 7.34 6.27 5.33

36.72 37.67 39.64 6.37 6.21

36.70 37.65 38.25 39.64 40.63

100wBmim+

6.06 5.32 4.71 3.87 3.25

22.99 23.76 24.60 4.10 3.77

23.02 23.42 24.08 24.87 25.59

100wBF−4

3.27 3.89 4.42 5.11 5.78

0.12 0.09 0.08 4.01 4.00

0.10 0.07 0.06 0.06 0.04

100wCu2+

bottom phaseb

4.96 5.44 6.54 7.73 8.74

0.18 0.16 0.10 6.32 6.31

0.10 0.10 0.10 0.07 0.07

100wSO2− 4

75.78 76.36 76.99 77.01 77.01

39.99 38.32 35.58 79.21 80.18

39.96 38.68 37.52 35.38 33.67

100ww

Standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. b100wBmim+ , 100wCu2+ , 100wSO2− , and 100ww represented mass fractions percent of constituents 4 determined experimentally with relative standard uncertainties of ur(wBmim+) = u(wBmim+)/wBmim+≤ 0.012, ur(wCu2 +) = u(wCu2 +)/wCu2 +≤ 0.009, ur(wSO24−) = u(wSO24−)/wSO24−≤0.031, and ur(ww) = u(ww)/ ww≤ 0.0012. The uncertainty for 100wBF−4 was obtained through the propagation of uncertainty, since the composition of BF−4 was obtained by mass balance. Therefore, the relative standard uncertainty is ur(wBF−4 ) = u(wBF−4 )/wBF−4 ≤ 0.058. The TLL and STL values were calculated from eqs 2 and (3), respectively. cPrecipitation of a crystalline solid occurs at the fourth and fifth tie lines at 283.15 K and at 3a, 4a, and 5a tie lines at 298.15 K. The composition of the crystalline precipitates in all these situations was 100w[Bmim]BF4 = 0.00; 100wCuSO4 = 63.92, and 100ww = 36.08, consistent with the formation of CuSO4·5H2O crystals. The compositions were determined by TGA (Figure S5 in the Supporting Information).

a

TLL

TL

overall

Table 2. Equilibrium Data for the [Bmim]BF4 (1) + CuSO4 (2) + H2O (3) System at T = (283.15, 298.15, and 313.15) K and Atmospheric Pressure (∼94 kPa)a

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Table 3. Binodal Data for the [Bmim]BF4(1) + CuSO4 (2) + H2O (3) System at T = (283.15, 298.15, and 313.15)K and Atmospheric Pressure (∼94 kPa)a 283.15 K

298.15 K

313.15 K

100w1b

100w2b

100w1b

100w2b

100w1b

100w2b

69.66 60.12 52.87 49.83 47.13 44.64 42.45 38.62 34.7 31.98 29.63 25.7 23.44 20.53 17.47 15.01 13.3 11.78 10.48 9.67 8.62 8.28 7.65 7.02 6.84 6.47 5.98 5.46 4.94 4.71 4.48 4.26 4.24 3.99 3.72

0.06 0.08 0.1 0.12 0.13 0.17 0.18 0.21 0.26 0.34 0.43 0.64 0.93 1.33 2.1 2.71 3.26 3.76 4.28 4.83 5.49 5.97 6.65 7.58 8.14 8.79 9.28 10.13 10.79 11.15 11.87 12.17 12.44 12.83 13.22

69.66 66.81 64.18 61.75 59.30 56.84 54.76 52.82 51.02 47.49 44.42 41.63 39.08 36.98 34.02 32.18 29.76 27.77 24.96 22.02 19.46 17.06 14.78 13.72 12.82 11.95 11.15 10.91 10.63 10.37 10.04 9.79 9.36 9.17 8.89 8.53 8.25 8.16 8.02 7.71

0.06 0.12 0.18 0.23 0.32 0.47 0.55 0.63 0.70 0.91 1.09 1.29 1.50 1.62 1.86 2.06 2.31 2.48 2.96 3.59 4.16 4.86 5.56 6.24 6.54 7.32 7.97 8.39 8.87 9.13 9.69 10.25 10.60 10.97 11.70 12.15 12.64 13.17 13.69 14.38

68.41 58.15 55.6 53.27 51.06 48.78 46.7 44.58 42.43 40.48 38.53 36.61 35.01 32.07 30.73 28.08 26.71 25.1 23.27 21.39 18.49 16.89 16.26 15.82 15.75 15.26 14.73 13.69 13.06 12.37 11.62 11.37 10.87 10.49 10.03 9.42 8.79

0.21 0.44 0.61 0.78 1.02 1.3 1.54 1.8 2.1 2.38 2.69 3.02 3.28 3.77 4 4.53 4.85 5.3 5.87 6.3 6.96 7.68 8.15 8.42 8.66 8.96 9.29 9.72 10.11 10.54 11 11.4 11.89 12.38 12.94 13.54 14.23

Figure 2. Electroneutrality in the top and bottom phases was plotted against the tie lines length in the different temperatures. The continuum line and the dashed short line represents the electroneutrality in top and the bottom phase, respectively. The behavior of electroneutrality of the phases at (●) 283.15, (■) 298.15, and (▲) 313.15 K, respectively.

Table 4. Distribution Ratio of the Constituents (Di) between the Phases of ATPSs Formed by [Bmim]BF4 + CuSO4 + Water at Different Temperatures

Standard uncertainties σ for temperature and pressure are u(T) = 0.05 K and u(p) = 0.5 kPa, respectively. b100w1 and 100w2 represented mass fractions percent of [Bmim]BF4 and CuSO4, respectively, and the relative uncertainty associated was 0.058 and u r ( w[Bmim]BF4) = u(w[Bmim]BF4)/w[Bmim]BF4 = ur(wCuSO4) = u(wCuSO4)/wCuSO4=0.031. a

TL

DCu2+

DSO42−

1 2 3 4 5

19.00 31.86 46.67 53.17 84.25

30.20 34.70 41.30 65.71 73.29

1 2 3 4 5

20.42 32.22 43.00 0.012 0.008

21.00 26.87 56.50 0.013 0.016

1 2 3 4 5

0.046 0.033 0.016 0.012 0.003

0.052 0.022 0.017 0.009 0.010

DBmim+

DBF4−

283.15 K 0.152 0.158 0.140 0.142 0.124 0.120 0.107 0.105 0.097 0.103 298.15 K 0.236 0.232 0.203 0.201 0.170 0.174 6.410 6.229 6.620 7.186 313.15 K 3.720 3.760 4.380 4.510 5.400 5.240 6.540 6.570 7.860 8.430

Dw

ΔtrGototal, kJ mol−1

2.140 2.220 2.270 2.390 2.540

−7.34 −7.31 −7.12 −7.13 −7.20

1.994 2.098 2.216 0.423 0.395

−6.19 −6.14 −6.03 6.04 5.91

0.526 0.502 0.461 0.434 0.397

6.49 6.61 7.14 7.14 7.40

indicates that the presence of the ionic liquid further decreases the solubility of the copper sulfate in the phase and that the saturation concentration of the CuSO4 in the phases is reached in smaller values. The excess salt crystallizes during the system to remain in the bath as a blue crystal of CuSO4·5H2O, as verified by TGA and infrared spectroscopy. The ionic equilibrium established between the phases in the ATPS formed by CuSO4 + [Bmim]BF4 + H2O in different temperatures was evaluated. The extent of the ion exchange, between the phases in equilibrium, of the cations and anions present in the original salts ([Bmim]BF4 and CuSO4) was quantified and shown in Table 2 and Figure 2. In Table 2 the concentrations in percentage of mass fraction for each ion present in the upper and lower phases, and of water in the respective phases, are presented. Note that, of the five constituents present in the phase, only the BF−4 content was determined considering the difference in relation to the total

CuSO4, an LLSE was established with the occurrence of copper sulfate precipitation (formation and growth of crystals of CuSO4·5H2O) in the phases rich in this salt at temperatures of 283.15 and 298.15 K (Figures S3−S6 in the Supporting Information). Again, this behavior can be explained in terms of the decrease in solubility of CuSO4 with the decrease in temperature. It is observed in Figure 1b that the concentrations of CuSO4 in the salt-rich phase for tie lines 4 and 5 (283.15 K) and 5 (298.15 K) are below the maximum solubility observed for that salt in water for these same temperatures. This F



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Figure 3. Plot of the natural log of the distribution ratio (D) as a function of TLL for the ATPS formed by [Bmim]BF4 + CuSO4 + H2O at: 283.15 (●), 298.15 (■), and 313.15 K (▲). Dashed lines refer to the occurrence of phase inversion. The calculations were normalized considering an LLE involving equal masses of the top and bottom phases.

Figure 4. Salting-out effect of the different cations on the aqueous two-phase systems formed for [Bmim]BF4 + salt sulfate + H2O at 298.15 K: (■) MnSO4;26 (▼) Li2SO4;27 (▲) MgSO4;27 (Δ) ZnSO4;28 (○) NiSO4;28 (◀) (NH4)2SO4;29 (∇) Na2SO4;32 and (□) CuSO4 (present work).

(Di) and the free energy of transfer of each of the five constituents between the phases (ΔtrGoi ), respectively. The results are shown in Table 4 and Figure 3. By observing the data presented in Table 4 it is possible to verify that, when the TLL increases at 283.15 K, the distribution ratio of ions Cu2+ and SO2− 4 increase, which is consistent with the observation that the top phase is rich in copper sulfate. The water distribution ratio is ∼2, which indicates the existence of approximately twice as much water in the top phase at 283.15 K in relation to the bottom phase. The opposite occurs at the temperature of 313.15 K. At 298.15 K it is possible to observe the gradual change of phase density by varying the distribution of the components between the phases and the calculated ΔtrGototal values (Figure 3). Note that, in the first three tie lines, the top phase of the assembled ATPS is rich in copper sulfate; that is, the concentration of ions Cu2+ and SO2− 4 and water is much higher in this phase. Although there is a much larger number of constituents in the top phase, the density of this phase is lower. Therefore, the temperature has a great influence on the phase behavior of the ATPS formed by [Bmim]BF4 + CuSO4 + water. 3.2. Adjustment of Binodal Curve. The binodal curves obtained for the ATPS formed by [Bmim]BF4 + CuSO4 + H2O were adjusted with the Equation 1 (Merchuk equation). The Simplex optimization algorithm was used to find the parameters and the least-squares of differences between the experimental data, and calculated values were used as a criterion of the best fit. The adjusted parameters αi, βi, and γi, with i = 0, 1, and 2, are given in Table 5.

α α α − = 100 − 100w phase content (100wBF Bmim+ − 100wCu 2 + − 4 α α 100wSO 2 − − 100ww ), where α refers to the top or bottom 4

phases. The uncertainty for 100wBF−4 was obtained through the propagation of uncertainty, since the composition of BF−4 was obtained by mass balance. Therefore, the relative standard uncertainty is ur(wBF−4 ) = u(wBF−4 )/wBF−4 = 0.058. The electroneutrality condition of phases was used to find available the ion exchange. The electroneutrality in the bulk phases was available by Equation 4, and its uncertainty was available by Equation 5, respectively. The Figure 2 shows the values of ε in the top and bottom phases against the tie lines length in the different temperatures, where it can be observed that, in all cases the values, in the different phases and temperatures, these values are very close to zero. In general, the values of ε are approximately equal or minor that 3.4uε, except for a few points, where the value suffered deviations of up to 12. However, even in these cases, it is observed that the electroneutrality is very close to zero, which allows to affirm that the extent of the ion exchange of the salts [Bmim]BF4 and CuSO4 between the phases is very small. Therefore, these results show that the mass and the electroneutrality conservation are experimentally verified within experimental errors. The partitioning behavior of the ATPS components formed by [Bmim]BF4 + CuSO4 + water along the different tie lines and control temperatures was evaluated. For this purpose, Equations 6 and 7 were used to calculate the distribution ratio

Table 5. Values of the Parameters of the Equation 1, for the ATPS Formed by [Bmim]BF4 + CuSO4 + H2O at T = (283.15, 298.15 and 313.15)K T, K

α0

α1

β0

β1

γ0

γ1

sda

283.15 298.15 313.15 average

−0.369

0.006 17

−0.116

−0.000 511

1.23

−0.0446

0.03 0.04 0.02 0.03

N

0,5

exp sd = (∑i = 1 (100w1teo − 100w1exp)2 /N ) where N is the number of points in the binodal curves, and 100wteo 1 and 100w1 correspond to the percentage in theoretical and experimental mass fraction, respectively. a

G



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Figure 5. Relationship between the molality of saturation solubility and energetic terms (a) ΔhydG,33 (b) ΔhydH,36 (c) TΔhydS,33,36 and (d) Vom.37

(ΔhydS),33,36 and partial molar volume of ions (Vom)37 also were used to generate a rank of the cation’s ability to induce phase separation (Figure 5). These parameters were plotted against the saturation solubility point, a parameter obtained from the binodal curves obtained experimentally, defined as the equality condition between the lowest concentrations of IL and the sulfate salt ([IL] = [Salt]), expressed in molality, capable of inducing phase separation. Figure 4 shows the salting-out effect of the different cations on the aqueous two-phase systems formed for [Bmim]BF4 + salt sulfate + H2O at 298.15 K. In this figure it can be observed that the order for the strength of salting out of cations is Zn2+ > Ni2+ ≈ Cu2+ ≈ Mn2+ > Mg2+ ≫ Li+ > Na+ ≫ NH+4 . Otherwise, Figure 5 allows to evaluate how the different thermodynamic parameters of hydration seem to contribute to the general behavior of ATPS formation at 298.15 K. If we consider as a reference for comparing the saturation solubility point, defined above, and thermodynamic parameters of ΔhydG and ΔhydH (Figure 5a,b) is observed that the order for the strength of salting out of cations is Zn2+ ≈ Ni2+ ≈ Cu2+ > Mg2+ > Mn2+ ≫ Li+ > Na+ > NH+4 . In relation to the entropic term TΔhydS the behavior is somewhat different from the previous series, Ni2+ > Mg2+ > Zn2+ ≈ Cu2+ ≈ Mn2+ ≫ Li+ > Na+ > NH+4 (Figure 5c). However, the magnitude of the ΔhydH values dominate in relation to the entropic term TΔhydS, making ΔhydH and ΔhydG present the same standard of series. Finally, in Figure 5d the behavior of the standard molar partial volume of hydration (Vom) of the various cations was plotted against the saturation solubility point. In terms of this hydration parameter the observed capacity for the different cations in inducing

Table 6. Adjustment Parameters for the Setschenow Equation, Linear Regression (R2) for the Tie Lines for the [Bmim]BF4 + CuSO4 + H2O Systems Are Given in (283.15; 298.15 and 313.15)K T, K

ks, kg mol−1

kIL, kg mol−1

R2

283.15 298.15a 313.15

2.1959 1.5502 1.9087

1.8974 1.4892 0.7903

0.9935 0.9822 0.9985

Data from the fifth tie line at 298.15 K was not considered in the adjustment because of high precipitation of copper sulfate.

a

The average standard deviations obtained (sd = 0.03) show that Equation 1 can be used to reproduce the data of the binodal curves and accurately predict the characteristics of the binodal curve for the systems studied at temperatures where no experimental data are available. 3.3. Cation Effect. The ability of different cations, from different sulfate salts, to induce the formation of ATPSs in mixtures involving [Bmim]BF4 was evaluated in the present work. In previous work our group presented an evaluation on the influence of mono- and bivalent cations from sulfate salts and their ability to induce the formation of ATPS, at 298.15 K, when present in mixtures containing [Bmim]BF4 and water.28 Now we present in Figures 4 and 5 an extension of this analysis considering the effect produced by Cu2+ cation. The binodal curves for various ATPS at 298 K are shown in Figure 4 in molality units to facilitate the comparison of the effect of monovalent and divalent cations on the phaseseparation process. Hydration values for the Gibbs free energy (ΔhydG),33 hydration enthalpy (ΔhydH),36 entropy of hydration H



Article

ATPS formation at 298 K is Ni2+ ≈ Cu2+ > Zn2+ ≈ Mg2+ > Mn2+ ≫ Li+ ≈ Na+ > NH+4 . The standard molar partial volume of hydration of ions is directly related to the ability of different ions to coordinate in its surroundings water molecules forming clathrates. In general, the molar partial quantities of mixtures allow to infer about the behavior of a mixture in terms of the intermolecular forces acting on it. However, the extent of the action of these forces and their influence on the phase separation process is beyond the scope of this work. This is a task of great complexity due to quinary composition of the phases. In this work the greater interest is to provide a theoretical correlation with the saturation solubility data. 3.4. Salting-Out Coefficients. Equation 9 correlates the logarithm of the molality ratio of the ionic liquid (cbIL/ctIL) as a linear function between the difference in salt concentrations (cts−cbs ) present in the top and bottom phases, respectively. The phase separation process by salting-out effect refers to the ability of the salt, present at high concentrations at one phase, to induce the separation of other components from the mixture to the opposite phase. In Table 6 are shown the adjustment parameters for the Seteschenow equation, where it can be observed that the coefficient of salting out (ks) is higher at 283.15 K. This shows that Cu2+ ions become more kosmotropic; that is, they contribute to the stability and structure of water−water interactions, at lower temperatures favoring phase separation. The salting-out effect may be related to the phase-inversion behavior observed experimentally in the evaluated systems, since the lower the temperature, the greater is the ks and the greater the salting-out effect of the sulfate salts over the ionic liquid, forcing the migration of water molecules from ionic liquid rich phase (increasing its concentration) to the rich salt phase (reducing salt concentration), which leads to change in the density of the phases and the inversion phenomenon.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +55 35 3701-9710. E-mail: [email protected]. ORCID

Luciano Sindra Virtuoso: 0000-0001-9814-1394 Funding

We gratefully acknowledge the “Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais” (FAPEMIG, Belo Horizonte, Brazil) and the “Coordenaçaõ de Aperfeiçoamento ́ Superior” (CAPES) for their financial de Pessoal de Nivel support. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION The formation of a new aqueous two-phase systems based in ionic liquid formed by the mixture of [Bmim]BF4 + CuSO4 + H2O at T = (283.15, 298.15 and 313.15)K was reported in the present work. The experimental data obtained allowed a detailed description of the phase behavior of these mixtures under different control conditions. The liquid−liquid equilibrium and, in some cases, the liquid−liquid−solid equilibrium were analyzed from the thermodynamic point of view. The phase-inversion process was observed to occur at temperatures of 283.15 and 298.15 K. The formation of these new ATPSs is strongly temperature-dependent and, being the exothermic phase-separation process, low temperatures lead to a significant increase of the biphasic area in the phase diagram. The ability of several mono- and bivalent cations from sulfate salts to induce the formation of ATPS with the ionic liquid [Bmim]BF 4 was also evaluated, and it was verified experimentally that copper sulfate is a salt very capable of inducing the formation of ATPS with this ionic liquid. Some models such as Seteschenow and Merchuk were used to theoretically correlate the experimental data. It was observed that good adjustments of the experimental data were obtained for the binodal curves and the liquid−liquid equilibrium data.

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Phase behavior in aqueous two-phase systems basedionic liquid composed of 1-butyl-3-methylimidazolium tetrafluoroborate and copper sulfate in different temperatures (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01076. I



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Phase Behavior in Aqueous Two-Phase Systems Based-Ionic Liquid

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