Phase Diagrams, Densities, and Refractive Indexes of Aqueous Two

Article pubs.acs.org/jced

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

Phase Diagrams, Densities, and Refractive Indexes of Aqueous TwoPhase Systems Comprising (F68, L64, or PEO1500) + (Ammonium, Sodium, or Potassium Thiocyanate Salts) + Water: Effect of Cation and Type of Macromolecule J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/16/19. For personal use only.

Maria C. Hespanhol,*,†,‡ Luis Henrique Mendes da Silva,† Beatriz M. Fontoura,† Vivianne M. de Andrade,† Aparecida B. Mageste,†,§ and Leandro R. de Lemos†,∥ †

Grupo de Química Verde Coloidal e Macromolecular, and ‡Group of Analysis and Education for Sustainability, Departamento de Química, Centro de Ciências Exatas e Tecnológicas, Universidade Federal de Viçosa, Viçosa, Minas Gerais 36570−900, Brazil § Departamento de Química, ICEB, Universidade Federal de Ouro Preto (UFOP), Ouro Preto, Minas Gerais 35400-000, Brazil ∥ Departamento de Química, FACET, Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM), Diamantina, Minas Gerais 39803-371, Brazil S Supporting Information *

ABSTRACT: In extraction procedures, the more commonly used aqueous two-phase systems (ATPS) comprise mainly water, salt, and macromolecule, particularly the macromolecule poly(ethylene oxide) (PEO). However, one limitation of such ATPS is their capacity to separate compounds that are more hydrophobic. One possible solution to overcome this restriction is the use of ATPS formed with triblock copolymers, which are more hydrophobic and therefore enable the extraction of hydrophobic solutes. In addition, the range of applications of ATPS formed with thiocyanate salts can be broader, mainly to extract metal ions. In view of this, equilibrium data were acquired in this work by constructing phase diagrams for ATPS comprising macromolecules [poly(ethylene oxide), PEO, or (poly(ethylene oxide))-(poly(propylene oxide))-(poly(ethylene oxide)) triblock copolymers, F68 or L64] + thiocyanate salts (ammonium, sodium, or potassium) + water at 25.0 °C. The influence of the nature of the cation on the formation of the ATPS was investigated and followed the order K+ > Na+ > NH4+. The capacity of different macromolecules to enable ATPS formation was also examined and followed the order L64 > F68 > PEO1500. Phase inversion occurred with the (L64 or F68) + NH4SCN + water ATPS, in that the top phase is rich in salt and the bottom phase is rich in macromolecule. This aspect is different in most ATPS that are typically described in the literature.

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(PEO)22−28 and salts such as sulfates,17 phosphates,29 citrates,30 and tartrates.28,30,31 ATPS containing PEO are mainly used for the partition of water-soluble compounds, since the selective distribution of substances between phases in equilibrium is a direct consequence of interactions that are established between solutes and the constituents that form the phases. As a result, applications of such ATPS are limited and involve the separation of more hydrophilic compounds. As an option to overcome such restriction, systems formed with triblock copolymers can be proposed as excellent candidates for the effective extraction of hydrophobic solutes. This type of macromolecule can easily self-assemble as micelles in an aqueous environment under specific a temperature and

INTRODUCTION Aqueous two-phase systems (ATPS) were reported for the first time by Beijernick1 in 1896, and in recent decades have become a very important technology in the separation and purification of several compounds, such as aminophenols,2 biomolecules,3−5 nanoparticles,6 dyes,7 and metallic ions.8−14 Their specific advantages involve high extraction efficiency, low energy consumption, the use of biocompatible compounds, and the absence of volatile organic solvents.15 ATPS can be formed by mixing different aqueous solutions which contain macromolecule−macromolecule,16 salt−macromolecule,17−19 macromolecule−ionic liquid20 and salt−ionic liquid,21 under specific conditions of temperature, pressure, and concentration. The final system shows two phases, each of which is enriched with one of the components and both being mostly formed by water. The more commonly reported systems are formed by macromolecules and salts, particularly by poly(ethylene oxide) © XXXX American Chemical Society

Special Issue: Latin America Received: October 31, 2018 Accepted: April 9, 2019

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DOI: 10.1021/acs.jced.8b01005 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

expansion on the number of analytes that can be separated or extracted from different matrices. This work was dedicated to the determination of liquid− liquid equilibrium data of ATPS formed with macromolecules (PEO, F68, or L64), thiocyanate salts (NaSCN, KSCN, or NH4SCN) and water at 25.0 °C. The densities and refractive indexes of both phases of the L64 and F68 systems were also measured. The influence of the hydrophobicity of the macromolecule and the type of cation (NH4+, Na+, or K+) on the formation of ATPS was also examined.

concentration. These micelles have a corona with PEO hydrophilic units and a core constituted of poly(propylene oxide) (PPO) hydrophobic units.8,32,33 Therefore, both hydrophilic and hydrophobic solutes could be extracted with ATPS formed with triblock copolymers. Nevertheless, liquid− liquid equilibria data for such ATPS are scarce in the literature (Table 1) when compared with ATPS containing PEO. Table 1. Aqueous Two-Phase Systems Formed with Triblock Copolymers + Salts + Water Reported in the Literature ATPS reported in the literature

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L64 + ZnSO4 + H2O L64 + MgSO4 + H2O L64 + Na2SO4 + H2O L64 + Li2SO4 + H2O L35 + Na2CO3 + H2O L35 + Na2C4H4O4 + H2O L35 + Na(CH3COO) + H2O L35 + (NH4)2C6H5O7 + H2O L35 + (NH4)2SO4 + H2O L35 + K2HPO4 + H2O L35 + K2HPO4 + KOH + H2O F68 + K2HPO4 + H2O F68 + ZnSO4 + H2O F68 + (NH4)2SO4 + H2O F68 + Na2SO4 + H2O F68 + Li2SO4 + H2O F68 + Na3C6H5O7 + H2O F68 + (NH4)3C6H5O7 + H2O F68 + Na2C4H4O6 + H2O F68 + Na2CO3 + H2O F68 + Na2C4H4O4 + H2O L35 + KOH + H2O L35 + NaOH + H2O L35 + Na3C6H5O7 + H2O L35 + Na2C4H4O6 + H2O L35 + NaHSO3 + H2O L64+(NH4)3C6H5O7+ H2O L64+ Na2C4H4O4 + H2O L64+ Na3C6H5O7 + H2O L64 + Na2C4H4O6 + H2O L35 + NH4NO3 + H2O L35 + (NH4)2C4H4O6 + H2O L35 + (NH4)2HPO4 + H2O, L35 + NH4C2H3O2 + H2O L31+ (NH4)2SO4 + H2O L31 + Na2SO4 + H2O L31 + Li2SO4 + H2O L31 + Na2C4H4O6 + H2O (L61 or L35) + K2C4H4O6 + H2O (L61 or L35) + K2CO3 + H2O

17 17 17 17 18 18 18 18 18 34 34 34 35 35 35 35 36 36 36 36 36 37 37 38 38 38 39 39 39 39 40 40 40 40 41 41 41 41 42 42

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EXPERIMENTAL SECTION

Materials. Poly(ethylene oxide) with molar mass of 1500 g· mol−1 [HO(EO)34H], named as PEO1500, was purchased from Synth (Brazil). The (poly(ethylene oxide))-(poly(propylene oxide))-(poly(ethylene oxide)) triblock copolymers (EO)76(PO)30(EO)76, named as F68, with molar mass of 8460 g·mol−1, and (EO)13(PO)30(EO)13, named as L64, with molar mass of 2900 g·mol−1, were supplied by Sigma-Aldrich (USA). Sodium thiocyanate (98.0 %), potassium thiocyanate (98.0 %), ammonium thiocyanate (97.5 %), ammonium iron(III) sulfate (100 %), and nitric acid (65.0 %) were provided by Vetec Quı ́mica Fina (Brazil). Silver nitrate (99.9 %) was purchased from Impex Quı ́mica (Spain). All chemicals were analytical grade. Deionized water (Millipore Corp., Massachusetts, USA) was used in the preparation of all solutions. Preparation of the ATPS. In test tubes, precise amounts of macromolecules, thiocyanate salt, and water were added, with the help of an analytical balance (model AY-220, Shimadzu, Brazil, uncertainty = ±0.0001 g), to form 40.00 g of ATPS with the global compositions shown in Tables S1 to S3. The contents of the tubes were stirred until they became turbid, and then the tubes were kept in a water bath (model MQBTC 99-20, Microquı ́mica, Brazil, with an uncertainty of ±0.1 °C) at 25.0 °C for 72 h. After that, aliquots of each phase were collected with a syringe to determine the concentration of all components. Quantitative Analysis of the Phases. The concentration of thiocyanate in each phase of the ATPS, prepared according to the methodology described above, was determined by the volumetric precipitation technique, as reported in the literature.43 To do this, 10.00 mL of a 0.100 mol L−1 silver nitrate solution (previously standardized), 2.5 mL of a 6.0 mol L−1 nitric acid solution, and 0.50 mL of the ammonium iron(III) sulfate indicator solution were added to an erlenmeyer flask. A buret was appropriately filled with the macromolecule-rich phase or with the salt-rich phase (SRP), each of which had been previously diluted 20 times. The mixture in the erlenmeyer was titrated with the diluted solution of one of the ATPS phases until the end point of the titration procedure (indicated by the red color of the solution). The determination of the amount of water was carried out by gravimetry. To do this, precisely 2.000 g of each ATPS phase were weighed in a tube which was kept in an oven (DeLeo, Brazil) at 120.0 °C until the mass was constant. Then, the weight percentage amount of water (ww) in the system was calculated with eq 1:

Moreover, to the best of our knowledge, there are no reports on the use of thiocyanate salts in the investigation of equilibrium data and phase diagrams of ATPS. Thiocyanate salts are strategic, because they are able to form complexes with metallic ions, thereby enabling the separation of such ions with the use of ATPS. The determination of equilibrium data of ATPS formed with macromolecules and different thiocyanate salts is therefore very important, because of the

Ww% = B

(mphase − mdry) mphase

100 (1) DOI: 10.1021/acs.jced.8b01005 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 2. Slope of the Tie-Lines (STL) for the Systems Macromolecule + Salt + Water (ww) at 25 °C L64 + salt + water

F68 + salt + water

PEO1500 + salt + water

TL

NH4SCN

KSCN

NaSCN

NH4SCN

KSCN

NaSCN

KSCN

1 2 3 4 5 6

−5.15 −5.23 −5.68 −5.50 −5.82 −5.51

−5.38 −4.36 −4.14 −3.96 −3.58 −3.52

−4.93 −4.39 −4.05 −3.74 −4.10

−3.07 −3.10 −3.12 −2.93 −2.83

−3.94 −3.57 −3.49 −3.14 −2.96

−3.89 −3.87 −3.67 −3.63 −3.44

−3.87 −3.36 −3.10 −2.93

Figure 1. Influence of the cation on the biphasic region of the ATPS formed with (A) L64 + thiocyanate salt + water, and (B) F68 + thiocyanate salt + water at 25 °C and pressure p = 0.09 MPa: (●) KSCN, (□) NaSCN, and (▲) NH4SCN.

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where mphase is the initial mass of the phase, and mdry is the mass of the phase after drying. The amount of macromolecule (wm) was determined with eq 2: wm = wt − ws − ww

Composition of the ATPS. Normally, the top phase of the ATPS is rich in macromolecule and poor in salt, while the bottom phase is salt rich and poor in macromolecule.19,22,36,37,39,40,45−50 However, one new ATPS reported in this work, formed with (F68 or L64) + NH4SCN + water, showed phase inversion. In the new ATPS the top phase is rich in salt and the bottom phase is rich in macromolecule. The liquid−liquid equilibrium data, in weight percentage (w), of ATPS formed with L64 + NaSCN + H2O, L64 + KSCN + H2O, L64 + NH4SCN + H2O, F68 + NaSCN + H2O, F68 + KSCN + H2O, F68 + NH4SCN + H2O, and PEO1500 + KSCN + H2O at 25.0 °C, and their corresponding TLL, are presented in Tables S4 to S10. These sets of data indicate the concentration of each component that is required to prepare an aqueous two-phase system, together with the composition of each phase for each condition of thermodynamic equilibrium. Phase separation occurs due to a segregation process between macromolecules and ions, the result of which is the preferential partition of salts to one phase and macromolecules to another phase. The polymer−salt segregation is driven by interactions that take place among similar components (salt−salt and macromolecule−macromolecule), which are more favorable than those among salts and macromolecules, thereby decreasing the Gibbs free energy of the system. In general, such a decrease in free energy observed in the formation of ATPS is associated with an increase in the system entropy during phase separation.44 An increase in the global composition also reduces the concentration of water in both ATPS phases.

(2)

where ww and ws denote the weight percentage of water and salt, respectively, and wt is the total weight percentage, which must be 100 %. All analyses were performed in triplicate. The tie-line length (TLL) is an important thermodynamic parameter, which expresses, at constant temperature and pressure, the difference between the intensive thermodynamic properties of the ATPS phases.44 Therefore, the TLL is normally a parameter that affects partition of a given solute in ATPS, and can be calculated with the concentrations of all components in each phase by using eq 3: SRP 2 TLL = [(CMMRP − CM ) + (CSMRP − CSSRP)2 ]1/2

RESULTS AND DISCUSSION

(3)

where: CMRP and CSRP are the concentrations of macroM M molecule, in % (w/w), in the MPR and SRP, respectively; whilst CMRP and CSRP are the concentrations of salt, in % (w/ S S w), in the MPR and SRP, respectively. Determination of Densities and Refractive Indexes. The density of each phase was measured with an Anton Par densimeter, model DMA 5000 M (Austria), with an uncertainty of ±0.001 kg·m−3. The densimeter was calibrated with air and deionized water at 25.0 °C. The refractive index of each solution was determined with an Analytic Jena AG Abbe refractometer, model 09-2001 (Germany), with an uncertainty of ±0.001). All measurements of density and refractive index were carried out in triplicate at 25.0 °C. C



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investigated in this study is the relatively high concentration of salt in both phases, which suggests that the macromolecule− salt interactions are strong. The strong L64−NH4SCN or F68−NH4SCN interactions promoted a high molecular packing in the MRP resulting in a large density. Effect of the ATPS-Forming Macromolecule. Figure 2 shows how the hydrophobicity of the macromolecule that

The composition of the phases can also be analyzed by considering the slope of the tie-lines (STL), which can be calculated with eq 4:49 STL =

SRP CMMRP − CM

CSMRP − CSSRP

(4)

where CMRP and CSRP are the concentrations of macroM M molecule, in % (w/w), in the MPR and SRP, respectively; whilst CMRP and CSRP are the concentrations of salt, in % (w/ S S w), in the MPR and SRP, respectively. The STL values for the ATPS investigated in the present work are listed in Table 2. These values decrease slightly with increasing TLL for all ATPS. This occurs because the concentration of salt in the SRP increases markedly with increasing TLL, thereby augmenting the denominator of eq 4.19 Furthermore, the term CSRP in the denominator is always S higher than CMRP , and therefore their difference is always S negative in the denominator. Effect of the ATPS-Forming Cation. Figure 1, panels A and B show the effect of the cations of the electrolytes that form the ATPS on the biphasic regions of the systems at 25.0 °C. Lower amounts of (potassium or sodium) thiocyanate are required to generate both phases than ammonium thiocyanate. This demonstrates that the cations K+ and Na+ are more capable of inducing phase separation than NH4+. This observation is explained by the model proposed by da Silva and Loh,46 who suggested that such behavior is a consequence of different energies of interaction between the cations and the EO group of the macromolecules, and also because of the increase in translational entropy of water molecules that are released during the occurrence of such interactions. According to their model, when samples of the salt and macromolecule aqueous solutions are mixed with compositions corresponding to points below the binodal curve of the ATPS phase diagram, the cations of the electrolyte interact with the macromolecules mainly with their EO group. In microcalorimetry studies conducted by da Silva and Loh,46 it was also verified that such interaction has an endothermic nature and the mixing is typically driven by the increase in system entropy, which in turn was attributed to the release of water molecules that are replaced with the salt cations on the hydration layer of the macromolecule. The mixture of both salt and macromolecule solutions will still induce the formation of a monophasic system until the macromolecule is energetically saturated by the cations, from which point there is no further increase in entropy as a result of any new cation−EO interactions. Moreover, an increase in salt concentration from this point promotes the separation into two phases. Therefore, more salt will be required to energetically saturate the macromolecule as the intensity of the cation−EO interaction increases, which ultimately causes phase separation.17,35,40,50 In this case, the cation NH4+ interacts more strongly with the EO monomers of the macromolecule. This justifies the fact that the phase diagrams of the ATPS (L64 or F68) + NH4SCN + H2O have a smaller biphasic region when compared to those of ATPS formed with potassium and sodium salts. Besides, the ATPS (L64 or F68) + NH4SCN + H2O are atypical, in that their top phases are more concentrated in salts, and their bottom phases are rich in macromolecules. The opposite occurs in most ATPS reported in the literature, containing macromolecule, salt, and water.17,18,30,34,37,40,47−50 Another interesting feature of the ATPS (L64 or F68) + NH4SCN + H2O and the others

Figure 2. Influence of the hydrophobicity of the macromolecule on the phase diagram of the ATPS formed with macromolecule + KSCN + water at 25 °C and pressure p = 0.09 MPa: (○) L64, (■) F68, and (Δ) PEO1500.

forms the ATPS affects the corresponding phase diagrams at 25.0 °C. The biphasic region in the phase diagram of the ATPS containing PEO1500 was smaller than those of the L64 or F68 systems. An analysis of the chemical structures of PEO (Figure 3A) and the triblock copolymers L64 or F68 (Figure 3B),

Figure 3. Chemical structures of (A) PEO and (B) L64 or F68.

shows that PEO1500 has only ethylene oxide (EO) groups, and therefore is more hydrophilic. The molecules of L64 and F68 comprise two EO blocks and one more hydrophobic propylene oxide (PO) block, which render their ATPS also more hydrophobic. The effect of the hydrophobicity of the macromolecule on the formation of ATPS can be explained by considering the solubility of the macromolecule. It is known that the solubility of (L64 or F68) in water is lower than that of PEO1500. The intensities of the macromolecule−water and macromolecule− salt interactions decrease when the macromolecule is less soluble. In such a case, more water molecules will be released to the bulk solution when copolymers are used to form the ATPS, thereby enhancing the water−salt interactions. These water molecules will assemble in such a way as to solvate the salt, which reduces the system entropy and therefore increases the system Gibbs free energy (ΔmixG > 0). To minimize the Gibbs free energy, the system separates in two phases. Therefore, phase separation will be favored with lower amounts of the system components when the hydrophobicity of the macromolecule increases.19 Besides, the increase in the molar mass of the macromolecule (the molar masses of L64 D



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Figure 4. Refractive index (RI) of the phases of the ATPS: (A) L64 + thiocyanate salt + water, and (B) F68 + thiocyanate salt + water, as a function of TLL. (●,○) KSCN; (■,□) NaSCN; (▲,△) NH4SCN. Black symbols, RI of the MRP; white symbols, RI of the SRP.

Figure 5. Density of the phases of the ATPS: (A) L64 + thiocyanate salt + water, and (B) F68 + thiocyanate salts + water, as a function of TLL. (●,○) KSCN; (■,□) NaSCN; (▲,△) NH4SCN. Full symbols, densities of the MRP; open symbols, densities of the SRP.

or F68) + thiocyanate salt + H2O. The refractive index increases with increasing TLL, showing that the propagation of light is more difficult as the concentrations of ATPS components increase.30 Besides, the refractive index of the MRP is higher than that of the SRP, because higher concentrations of macromolecules will force the light to be more refracted when compared to the effect of salt concentration. These results agree with reports by Rengifo et al.30 and Santos et al.49 Figure 5 panels A and B depict the relationship between density and TLL for both phases of the ATPS formed with (L64 or F68) + thiocyanate salt + H2O. The density also increases with increasing TLL for all ATPS. However, for ATPS containing NH4SCN, the density of the MRP was higher and, consequently, the MRP was the bottom phase. This behavior characterizes the phase inversion phenomenon. For most of the ATPS, this phenomenon is observed only for a narrow range of global compositions of the system and it occurs due to the relative change of the densities of the both phases.51 In the systems formed by copolymer and NH4SCN, the phase inversion was observed for all compositions studied. This result was observed due to the high concentrations of salt in the MRP, which were close of that in the SRP.

and F68 are higher than that of PEO1500) implies a lower mixing entropy due to the lower number of possible spacial distributions of macromolecules in the solution,19 thereby favoring phase separation. Densities and Refractive Indexes of the ATPS Phases. As discussed above, it is surprising that ATPS formed with (L64 or F68) + NH4SCN + H2O show phase inversion, that is, the MRP is the bottom phase and the SRP is the top phase. This phenomenon is not common in ATPS formed with salts and macromolecules. Because of this finding, the densities and refractive indexes of both phases of all ATPS formed with L64 or F68 were measured. However, it was not possible to determine the density of the fourth and fifth tie-lines for the (F68 + sodium thiocyanate + water) system, due to the formation of a gel on the macromolecule-rich phase. The corresponding values of density and refractive index are reported in Tables S11 to S16. The magnitude of these intensive thermodynamic properties is different between the phases in equilibrium, and increases for both phases as the TLL increases.30 For all ATPS, an increase in the TLL also increases the values of densities and refractive indexes, because these parameters depend on the total concentrations of all components in the phases, which increase as the TLL increases.22 Figure 4 panels A and B show the relationship between the refractive index and the TLL for both phases of the ATPS (L64 E



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CONCLUSIONS In this work, new equilibrium data were acquired for aqueous two-phase systems comprising L64 + NaSCN + H2O, L64 + KSCN + H2O, L64 + NH4SCN + H2O, F68 + NaSCN + H2O, F68 + KSCN + H2O, F68 + NH4SCN + H2O, and PEO1500 + KSCN + H2O, at 25.0 °C and for different tie-line length values. The cation and the macromolecule that form the ATPS can markedly affect the extent of the biphasic region in phase diagrams. Phase separation can be promoted more easily depending on the cation, in the following order: K+ > Na+ > NH4+. This behavior is related to the interactions that are established between cations and macromolecules. The hydrophobicity of the macromolecule also affects the extent of the biphasic region, generating larger regions in the following order: L64 > F68 > PEO1500. Phase separation is enhanced for lower concentrations of the ATPS components as the hydrophobicity of the macromolecule increases. For all ATPS, an increase in TLL increased both the refractive index and the density of the phases, since these properties depend on the total amount of components (concentrations of salt, macromolecule, and water). These results are very important, increasing the number of reports in the literature on phase diagrams of systems formed with copolymer, salts, and water, besides demonstrating the possibility of preparing ATPS with salts that are typically used in the complexation of metals, such as the thiocyanates.

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ASSOCIATED CONTENT

* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01005. [Table S1 to S15] Masses of components needed to prepare the studied systems; equilibrium data; densities and refractive indexes (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 31 38992175. Fax: +55 31 38993065. E-mail: [email protected]; [email protected]. Address: Departamento de Quı ́mica, Centro de Ciências Exatas e Tecnológicas, Universidade Federal de Viçosa, 36570-900, Viçosa, MG, Brasil. ORCID

Maria C. Hespanhol: 0000-0003-2296-4516 Luis Henrique Mendes da Silva: 0000-0002-8262-1091 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Cientı ́fico e Tecnológico (CNPq), Instituto Nacional de Ciências e Tecnologias Analı ́ticas Avançadas (INCTAA), and Coordenaçaõ de Aperfeiçoamento de Pessoal de Nı ́vel Superior (CAPES) for financial support of this research. B.M.F., A.B.M., and L.R.L. acknowledge CAPES, and M.C.H. and L.H.M.S. acknowledge CNPq for a research studentship. F



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