German Edition: DOI: 10.1002/ange.201705704Ionic LiquidsInternational Edition: DOI: 10.1002/anie.201705704

A Triple Salting-Out Effect is Required for the Formation of Ionic-Liquid-Based Aqueous Multiphase SystemsHelena Passos, Sara H. Costa, Ana M. Fernandes, Mara G. Freire, Robin D. Rogers, andJo¼o A. P. Coutinho*

Abstract: Novel aqueous multiphase systems (MuPSs) formedby quaternary mixtures composed of cholinium-based ionicliquids (ILs), polymers, inorganic salts, and water are reportedherein. The influence of several ILs, polymers, and salts wasstudied, demonstrating that a triple salting-out is a requiredphenomenon to prepare MuPSs. The respective phase dia-grams and “tie-surfaces” were determined, followed by theevaluation of the effect of temperature. Finally, the remarkableability of IL-based MuPSs to selectively separate mixtures oftextile dyes is shown.

Separation processes based on three-liquid-phase systemsare promising approaches for the isolation of differentcompounds present in complex mixtures, allowing theirsimultaneous separation amongst the different phases ina single step.[1] These systems are usually prepared by theaddition of an organic solvent to a polymer-salt-basedaqueous biphasic system (ABS), resulting in the formationof an organic-solvent-rich top phase, a polymer-rich middlephase and a salt-rich bottom phase. Furthermore, because oftheir composition and the chemical nature of each phase, thecoexisting phases present a wide range of polarities and highlydistinct chemical properties. This is one of the main reasonsbehind the considerably higher selectivities displayed bythree-liquid phase systems when compared to conventionalwater–oil liquid–liquid systems and ABSs.[2]

In 2012, Mace et al.[3] introduced the concept of aqueousmultiphase systems (MuPSs), that is, systems composed ofthree or more aqueous phases, without volatile organicsolvents employed. The authors[3] reported the formation ofmore than 300 MuPSs by mixing different polymers and

surfactants in aqueous solutions. Following their approach,Liang et al.[4] presented an in-depth study on an aqueous four-phase system constituted by sodium dodecyl sulphate (SDS),dodecyltrimethylammonium bromide (DTAB), polyethyleneglycol (PEG) with a molecular weight of 6000 gmol@1, andNaBr. In this work, the composition of the phases and theirproperties, as well as the partition of xylenol orange betweenthe coexisting phases, were studied and reported.[4] Otherauthors have demonstrated that MuPSs present character-istics useful for specific applications, such as the separation ofnanoparticles by rate-zonal centrifugation[5] or even in thetreatment of cellular components of human blood for medicalpurposes.[6] Nevertheless, the molecular-level knowledgebehind the formation of MuPSs is still very limited, and themechanisms associated with the multiple phasesQ separationwere not fully elucidated in these pioneering works.[3–6]

Furthermore, the phase-forming components proposed forthe creation of MuPSs were always polymers, surfactants, andpolysaccharides of high molecular weights, resulting in highlyviscous aqueous phases and further difficulties in the phasesQseparation and mass transfer phenomena.

The application of ionic liquids (ILs) as phase-formingcomponents of ABSs has been a hot area of research.[7, 8] Theirunique properties, such as their high solvation ability fora large range of compounds and the possibility of tuning theirproperties by the correct choice of both the cation and anion,makes IL-based ABSs valuable in the extraction and sepa-ration of a wide range of compounds.[9] Furthermore, thesetypes of systems present additional outstanding advantageswhen compared to the more traditional polymer-basedsystems, such as low viscosity, quick phase separation, andhigh extraction efficiencies for the most diverse biomole-cules,[7] contributing to the development of more cost-effective processes. Although not investigated to date, theseadvantages could be transposed to MuPSs if ILs could be usedas phase-forming components. Thus, in this work, we evaluatethe possibility of using ILs for the formation of MuPSs,address their phase diagrams and the molecular-level mech-anisms behind the observed phase transitions, and evaluatetheir efficacy in separation processes.

Quaternary mixtures of cholinium ([N111(2OH)])-based ILs(cholinium butanoate, [N111(2OH)][But]; cholinium propanoate,[N111(2OH)][Pro]; cholinium lactate, [N111(2OH)][Lac]; choliniumacetate, [N111(2OH)][Ace]; cholinium glycolate, [N111(2OH)][Gly];cholinium dihydrogenphosphate, [N111(2OH)][DHP]; and choli-nium chloride, [N111(2OH)]Cl), PEG with a molecular weight of600 gmol@1 (PEG 600), potassium phosphate (K3PO4), andwater were used to prepare MuPSs. The mixtures able to formthree-phase systems (or not) are identified in Table 1,

[*] Dr. H. Passos, S. H. Costa, Dr. M. G. Freire, Prof. Dr. J. A. P. CoutinhoCICECO—Aveiro Institute of Materials, Department of Chemistry,University of Aveiro3810-193 Aveiro (Portugal)E-mail: jcoutinho@ua.pt

Dr. A. M. FernandesQOPNA Unit, Department of Chemistry, University of Aveiro3810-193 Aveiro (Portugal)

Prof. Dr. R. D. RogersDepartment of Chemistry, Otto Maass Chemistry Building, McGillUniversity801 Sherbrooke St. West, Montreal, QC, H3A 0B8 (Canada),and525 Solutions, Inc.P.O. Box 2206, Tuscaloosa, AL 35403 (USA)

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201705704.

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complemented with the information on the ability of the sameILs to form ABS (two-phase systems) with PEG 600 orK3PO4. Further details on the experimental procedure aregiven in the Supporting Information. [N111(2OH)]Cl and[N111(2OH)][DHP] do not fall within the IL category if theirmelting temperatures are considered as a threshold(> 100 88C). However, when dealing with IL-based ABSs andrelated aqueous systems, the phenomenon is more complexand does not depend only on the melting temperature of eachsalt.[10, 11] Therefore, for simplicity, all the investigated choli-nium-based salts will be described as ILs and K3PO4 as the(inorganic) salt.

With the exception of [N111(2OH)][Pro] and [N111(2OH)][But],all the studied [N111(2OH)]-based ILs are able to form three-phase systems when mixed (in correct proportions) withaqueous solutions of PEG 600 and K3PO4, see Table 1. Thesedata reveal that the ability of an IL to form a three-phasesystem is related with its ability to form ABSs with the othertwo solutes, that is, with the salt or the polymer. These resultsare in good agreement with the criteria used by previousauthors,[3–6] who suggested that if the aqueous mixturescomposed of the solutes A/B, B/C, and A/C are able toform ABSs, aqueous mixtures composed of A/B/C will resultin the formation of three-phase systems. However, anddespite the apparent validity of this criterion, its applicationrequires previous knowledge on the ability of the pairs ofsolutes to form ABSs, resulting in a difficult and time-consuming procedure. To overcome this drawback, it iscrucial to understand the molecular-level mechanismsbehind the formation of MuPSs to be able to predict whichmixtures allow their formation.

The demixing of two aqueous phases, and consequentformation of an ABS, occurs when two water-soluble solutesare mixed above certain concentrations and/or temperature inan aqueous solution. These solutes compete for the formationof hydration complexes and the phase separation occurs.[7, 10,14]

Several pairs of solutes, such as polymers, salts, ILs, carbohy-drates, and amino acids, among others, can be used in ABSformation and, depending on the selected pair, the samesolute can act as the salting-out or the salted-out species. Forinstance, in systems constituted by [N111(2OH)]-based ILs andK3PO4, the liquid–liquid demixing results from the salting-outeffect of the salt over the IL, that is, high-charge-density saltions are preferentially hydrated leading to the exclusion of theless hydrophilic IL to a second liquid phase.[12, 13] Similarly, for

systems composed of polymers and salts, such as PEG 600/K3PO4/H2O and PEG 600/[N111(2OH)]-based ILs/H2O, the salt(or IL) are the preferentially hydrated species, thus inducingthe exclusion of the polymer to a second aqueous phase—salting-out effect of the salt (or IL) over the polymer.[10] Whena quaternary mixture is prepared using three solutes that areable to induce ABS formation when mixed as pairs in aqueousmedia, all these salting-out phenomena will occur, resulting inthe formation of three distinct aqueous phases: An IL-richphase, a salt-rich phase, and a polymer-rich phase.

A schematic representation of the salting-out effectsidentified in the quaternary mixtures under study is presentedin Figure 1. The vertices of the triangles represent the three

solutes that compose the aqueous quaternary mixture,whereas the edges represent the ternary mixtures (two solutesand water) that are, or are not, able to form an ABS. In thecases in which phase separation occurs, the arrows indicatethe direction of the salting-out effect. As discussed above, andas represented in Figure 1A, when all the solutes thatcompose the quaternary system are able to induce a two-phase separation when mixed in pairs in aqueous media, threesalting-out effects occur simultaneously in the mixture—theK3PO4 salting-out over [N111(2OH)]Cl and PEG 600 and the[N111(2OH)]Cl salting-out over PEG 600—and a MuPS isobtained. This behavior is common to the quaternarymixtures composed of all the [N111(2OH)]-based ILs able toform MuPSs (see Table 1). However, for the cases in whichthere are only two salting-out effects occurring—the K3PO4

salting-out over both PEG 600 and [N111(2OH)][Pro] and nosalting-out of the IL over the polymer—it is impossible toform MuPSs (Figure 1B). This last example occurs for the ILs[N111(2OH)][Pro] and [N111(2OH)][But], which present favorableIL-PEG interactions,[10] avoiding the salting-out of the IL overthe polymer and, consequently, preventing their exclusion todifferent phases and further formation of MuPSs. This resultsuggests that a triple salting-out effect is required for theformation of triphasic systems.

This hypothesis was additionally confirmed with otherpolymers and salts. It was previously reported that [N111(2OH)]-[Pro] and [N111(2OH)][But] are able to salt-out polypropyleneglycol (PPG) with an average molecular weight of 400 gmol@1

and consequently induce the formation of an ABS.[15] In fact,by replacing PEG 600 by PPG 400, the formation of a MuPS([N111(2OH)][Pro] or [N111(2OH)][But]/PPG 400/K3PO4/H2O, 23/

Table 1: Identification of mixtures able (3) or not able (7) to form ABSs orMuPSs with aqueous solutions of PEG 600 and/or K3PO4.

[N111(2OH)]-based IL ABS MuPS

PEG 600[10] K3PO4 PEG 600/K3PO4

[N111(2OH)]Cl 3 3[12] 3

[N111(2OH)][Ace] 3 3[12] 3

[N111(2OH)][Lac] 3 3[a] 3

[N111(2OH)][Gly] 3 3[a] 3

[N111(2OH)][DHP] 3 3[a] 3

[N111(2OH)][Pro] 7 3[13] 7

[N111(2OH)][But] 7 3[13] 7

[a] Phase diagrams determined in this work (experimental data areprovided in the Supporting Information).

Figure 1. Schematic of the salting-out effect in quaternary mixturescomposed of K3PO4, PEG 600, water, and A) [N111(2OH)]Cl orB) [N111(2OH)][Pro].

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23/23/31 wt%) was observed. A weaker salting-out salt wasalso investigated, namely Na2SO4 that is not able to formABSs with the investigated ILs and for which the formation ofa MuPS was not observed. All of these results support theidea that a simultaneous triple-salting out effect is required tocreate triphasic systems and that all the three water-solublesolutes are relevant for MuPS formation.

To achieve a better understanding of the three-phasecomposition of IL-based MuPSs, the system composed of[N111(2OH)]Cl/PEG 600/K3PO4/H2O was studied in detail. Thesurfaces limiting the monophasic, biphasic, triphasic, andsolid–liquid regions were established (Figure 2). Details onthe experimental procedure and equilibrium surfaces dataobtained are given in the Supporting Information. In Fig-ure 2A it is possible to distinguish a surface connecting thebinodal curves of the ternary phase diagrams composed of[N111(2OH)]Cl/K3PO4/H2O (blue dots) and PEG 600/K3PO4/H2O (green dots), limited in the bottom by the [N111(2OH)]Cl/PEG 600/H2O system (red dots). All quaternary mixturesprepared at concentrations above this surface will result inhomogeneous solutions—monophasic region; whereas, belowthe surface, all mixtures are within the multiphasic region.This surface is the phase boundary, whereas the three-phaseregion is within the multiphase region delimited by the phaseboundary. In Figure 2B it is shown that the three-phase regionis considerably smaller than the biphasic region and is limitedin the bottom by the solid–liquid region, in which the solutesare not completely soluble and a solid phase appears (seeFigure 2B).

The composition of the coexisting phases of two quater-nary mixtures were also analytically determined for thefollowing mixtures: [N111(2OH)]Cl/PEG 600/K3PO4/H2O (30.70/29.64/9.86/29.81 wt %) and [N111(2OH)]Cl/PEG 600/K3PO4/H2O(22.61/21.36/22.63/33.40 wt %). Details on the experimentalprocedure and results are given in the Supporting Informa-tion. The obtained results are depicted in Figure 2 C as “tie

surfaces”. As expected, each phase is composed of water,[N111(2OH)]Cl, PEG 600, and K3PO4. However, each aqueousphase is richer in one of these three components. In the twomixture points studied, the top phases are rich in [N111(2OH)]Cl,the middle phases are mostly composed of PEG 600, and thebottom phases are mainly constituted by the inorganic salt.The quantification of each ion (of the salt and IL) in eachphase was also carried out, demonstrating that the ionexchange between the phases is negligible in these systems(detailed data are given in the Supporting Information).

The temperature effect on the formation of MuPSs wasfurther evaluated. The surface that limits the biphasic and thetriphasic regions was determined at 45 and 65 88C andcompared to the surface previously determined at 25 88C.The obtained results are presented in the Supporting Infor-mation. It is observed that an increase in the temperatureleads to an increase of the three-phase region, that is, isfavorable for the phase demixing. This behavior is observedover the entire range of the phase diagram. The reportedbehavior is similar to that observed in polymer-salt-basedABSs in which the systemQs temperature dependency isdominated by hydrogen-bonding interactions occurringbetween the polymer and water.[16]

To assess the application of the studied systems inseparation processes, their ability for the selective separationof a mixture of three textile dyes—sudan III, Prussian bluepigment (PB) 27, and chloranilic acid—was evaluated. Morethan 1 million tons of dyes are produced by year, and 10–15%of them are discharged by textile industries into aqueouseffluents leading to both environmental and economic con-cerns.[17] In this context, environmental regulations arebecoming increasingly stricter in regards to the removal ofdyes from aqueous effluents. Despite the large number ofbiological, physical, and chemical processes proposed for theremoval of dyes from water and wastewater, most of themdisplay major drawbacks, such as a high cost, low efficiency,

Figure 2. Phase diagram of a MuPS composed of [N111(2OH)]Cl/PEG 600/K3PO4/H2O. A) Phase boundary between the monophasic and multiphasicregions—ternary phase diagrams composed of [N111(2OH)]Cl/K3PO4/H2O (blue dots), PEG 600/K3PO4/H2O (green dots), and [N111(2OH)]Cl/PEG 600/H2O (red dots), and quaternary mixtures composed of [N111(2OH)]Cl/PEG 600/K3PO4/H2O (black dots). B) Limits between the biphasic andtriphasic regions (pink dots) and between the triphasic and solid–liquid regions (yellow dots), inside the phase boundary (grey surface). C) Tiesurfaces determined in the mixture points 30.70 wt% [N111(2OH)]Cl/29.64 wt% PEG 600/9.86 wt% K3PO4/29.81 wt % H2O (tie surface 1, orange)and 22.61 wt% [N111(2OH)]Cl/21.36 wt% PEG 600/22.63 wt % K3PO4/33.40 wt% H2O (tie surface 2, green), inside the phase boundary.MR= monophasic region, BR =biphasic region, TR = triphasic region, SLR= solid–liquid region, MP =mixture points. Different perspectives ofphase diagram are given in Supporting Information.

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and labor-intensive operations.[18] Therefore, the MuPSsstudied in this work may contribute to the development ofalternative and cost-effective removal strategies, which froma circular economy point of view may allow not only thetreatment of dye-containing wastewater but also the recoveryand reuse of these materials inside the industrial processesand the reuse of the final products. Details on the exper-imental procedure and results are given in the SupportingInformation. The macroscopic appearance, as well as theextraction efficiencies of the [N111(2OH)][Ace]-based MuPSsfor dyes, are presented in Figure 3A. Remarkably, the

investigated system allows the separation of the three dyesby the three phases, with an extraction efficiency of 65% forone of the dyes and the complete separation of the other two.Sudan III, which has a less polar character (log(Kow) =

7.47),[19] is completely extracted into the more hydrophobicPEG 600-rich phase (purple-dyed phase). On the other hand,charged species, such as PB27 and chloranilic acid (log(Kow)<0),[19] partition preferentially into the more charged and polarphases, the IL- and salt-rich phases (pink- and blue-dyedphases, respectively). A remarkable separation performancewas also obtained with the remaining MuPSs, with thecomplete extraction of sudan to the PEG-rich phase, thecomplete extraction of PB27 to the salt-rich phase, anda partial extraction of the chloranilic acid dye with anefficiency that depended on the IL chemical structure (Fig-ure 3B). The extraction of sudan III and chloranilic acidthrough the application of IL-based ABSs was previously

reported.[20] However, both dyes partitioned preferentiallyinto the same phase of the system, and consequently, theirsimultaneous extraction and separation is not possible. Thus,MuPSs appear as an alternative for the separation of textiledyes.

The use of MuPSs for the separation of other types of(bio)molecules, namely amino acids, phenolic compounds andalkaloids was additionally investigated (see the SupportingInformation). Not only molecules with very distinct size andchemical structure but also molecules of high chemicalstructure similarity can be separated with the investigatedsystems. Overall, these data support the versatility of IL-based MuPS applications in separation processes.

In summary, the formation of novel IL-based MuPSs wasdemonstrated in this work. A large number of novel MuPSscomposed of cholinium-based ILs, salts, polymers, and waterwere investigated, allowing the conclusion that the formationof three-aqueous-phase systems is ruled by a simultaneoustriple salting-out effect. Due to the use of ILs as phase-forming components, these systems are of low viscosity anddisplay an improved separation performance for a widevariety of compounds.

Acknowledgements

This work was developed in the scope of the projectCICECO-Aveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013), financed by national funds through theFCT/MEC and when applicable co-financed by FEDERunder the PT2020 Partnership Agreement. The authors alsoacknowledge FCT for the doctoral grant SRH/BD/85248/2012of H.P. M.G.F. acknowledges the European Research Councilunder the European UnionQs Seventh Framework Pro-gramme (FP7/2007–2013)/ ERC grant agreement n88 337753.This research was undertaken, in part, thanks to funding fromthe Canada Excellence Research Chairs Program.

Conflict of interest

The authors declare no conflict of interest.

Keywords: aqueous multiphase systems · dyes · ionic liquids ·salting-out effect · separations

How to cite: Angew. Chem. Int. Ed. 2017, 56, 15058–15062Angew. Chem. 2017, 129, 15254–15258

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Figure 3. Dyes mixture separation in MuPSs composed of [N111(2OH)]-based IL/PEG 600/K3PO4/H2O. A) Selective extraction of chloranilicacid, sudan III and PB 27 in [N111(2OH)][Ace]-based MuPS. B) IL anioneffect on the selective extraction of chloranilic acid dye between the IL-, polymer-, and salt-rich phases.

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Manuscript received: June 5, 2017Revised manuscript received: October 1, 2017Accepted manuscript online: October 2, 2017Version of record online: October 19, 2017

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