ionic liquid aqueous two-phase systems

Fluid Phase Equilibria 506 (2020) 112387

Contents lists available at ScienceDirect

Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d

Phase equilibrium for polymer/ionic liquid aqueous two-phase systems
nez a, b, *, C. Roma
n Freijeiro a, A. Soto a, O. Rodríguez a Y.P. Jime a b

Department of Chemical Engineering, Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain Departamento de Ingeniería Química y Procesos de Minerales, Facultad de Ingeniería, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2019 Received in revised form 23 October 2019 Accepted 4 November 2019 Available online 15 November 2019

In this work, the use of ionic liquids as phase-forming components of Aqueous Two-Phase Systems (ATPS) has been evaluated. All ionic liquids used are based on the imidazolium cation, with different alkyl side chains and anions (chloride, bromide, acetate or dicyanamide). As counterpart on the ATPS, a random co-polymer of ethylene oxide and propylene oxide monomers (named EOPO or UCON) was used. Phase diagrams (tie-lines and binodal curves) for a set of UCON/ionic liquid ATPS were measured experimentally at T ¼ (288.15, 298.15 and 308.15 K), using the density and refractive index of the mixtures to obtain the composition of equilibrium phases. The extension of the heterogeneous region increases but the slope of the tie-lines did not change significantly with temperature. Anions had a dramatic effect on the ATPS immiscibility, where acetate produced the largest heterogeneous region and thiocyanate the smallest, following the series OAc > Cl > Br > SCN. The Chen-NRTL and modified Wilson models were used to correlate the liquideliquid equilibrium data, but better results were obtained with the Chen-NRTL model. © 2019 Elsevier B.V. All rights reserved.

Keywords: Ionic liquids EOPO Aqueous two-phase systems Chen-NRTL Wilson

1. Introduction ATPS, made of two immiscible liquid phases whose solvent in both phases is water, allow the use of liquid extraction on biological mixtures [1]. ATPS formulation combines two chemicals whose aqueous solutions are immiscible above certain conditions (composition and temperature). While typical formulations use two polymers or a polymer and a salt [2], alternative components such as surfactants [3], ionic liquids (IL) [4], organic compounds (acetonitrile and alcohols) [5] or saccharides [6e8] are under investigation. ATPSs are used as separation media, mostly in biotechnology for the recovery of target biomolecules from cell debris and fermentation broths, and in hydrometallurgy to separate target ions [9e11]. Usually, proteins will partition to the more hydrophobic and less polar phase, while particulate and soluble matter will partition to the more polar phase, but different factors affect the partitioning and separation of either proteins and ions in ATPS,

* Corresponding author. Departamento de Ingeniería Química y Procesos de Minerales, Facultad de Ingeniería, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile.
nez). E-mail address: [email protected] (Y.P. Jime https://doi.org/10.1016/j.fluid.2019.112387 0378-3812/© 2019 Elsevier B.V. All rights reserved.

namely the temperature, pH, molar mass of the polymers, the ionic strength of the phase, the type of ions in the system or the addition of a supplementary salt. In the ionic liquid-based aqueous two-phase systems (IL-ATPSs), the chaotropic ionic liquids will act as the salting-in species and the anti-chaotropic ionic liquids will have the contrary role [12,13.] Unlike traditional ATPSs formed by salt and polymer, the use of ionic liquids instead of salts will adjust the phase polarities more adequately [12]. The last years, these interesting and advantageous properties of IL-ATPS motivated numerous studies of thermodynamic data, modeling, extraction, among others [14e17]. In this work, the use of ionic liquids as phase-forming components is evaluated experimentally. All ionic liquids used are based on the imidazolium cation, with different alkyl side chains and anions (chloride, bromide, acetate or dicyanamide). As counterpart on the ATPS, the random co-polymer EOPO or UCON was used. This is a thermoseparating polymer - its aqueous solubility decreases drastically with temperature. The polymer then splits from the aqueous solution by just changing the temperature and so it can be easily recovered and recycled to the extraction process where the ATPS is used [18]. Besides, ionic liquids have attracted scientists and technologists for the last 20 years due to some special features. Among them, their very low vapor pressure that make them an

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

2

alternative to VOCs, or their physical properties that can be tuned by choice of the ions used and the functional groups included in their chemical structure. Indeed, they have been considered green and designer solvents [19]. The ATPS liquid-liquid equilibrium (LLE) data have been obtained experimentally at different temperatures and correlated with Chen-NRTL https://www.sciencedirect.com/ science/article/pii/S0021961411001534 - b0120 [20,21] and modified Wilson [22,23] models. 2. Material and methods 2.1. Chemical reagents Poly(ethylene glycol-ran-propylene glycol) monobutyl ether, a random copolymer (commercially known as UCON) with 50% ethylene oxide and 50% propylene oxide (average molar mass M ¼ 3900), was obtained from Sigma-Aldrich. Ionic liquids 1-ethyl3-methylimidazolium chloride [C2mim]Cl, 1-ethyl-3methylimidazolium thiocyanate [C2mim][SCN], 1-ethyl-3methylimidazolium acetate [C2mim][OAc], all with purities  0.98 (mass fraction), and 1-ethyl-3-methylimidazolium bromide [C2mim]Br with purity  0.99 (mass fraction), were purchased from Iolitec GmbH. Water content was in all cases <2000 ppm, measured by Karl Fischer titration (Metrohm 737 KF coulometer). Distilled water was used for ATPS formulation and for dilution purposes in all experiments. Table 1 shows the chemical specifications and Scheme 1 the structure of ILs. 2.2. Determination of liquid-liquid equilibrium The liquid-liquid equilibrium experiments were carried out by using a microcentrifuge tube (Eppendorf®) of about 2 ml, where appropriate masses of the phase-forming substances (UCON, IL and water) were added. All solutions were prepared by mass in a Mettler Toledo balance model XPE205, with precision ±0.01 mg. After that, the systems were vigorously vortex-mixed in intervals of 5 min and then centrifuged by another 5 min at constant temperature and 14500 rpm; this procedure was carried out three times to finally let them settle for 24 h at the desired temperature. A thermostatic bath (Julabo F12-ED) was used to maintain the constant temperature. No observable change in the phase compositions was noticed for longer periods of stirring and settling. Plastic syringes were used to withdraw samples of top and bottom phases. Needles were used to pierce the tube and withdrawn the bottom phase sample without disturbing the top phase and interphase. These samples were then conveniently diluted with water for analysis by physical properties. Ionic liquid and polymer concentrations were determined from measurements of the sample density and refractive index at T ¼ 298.15 K. Similar procedures have been reported previously by Rico-Castro et al. [18] and Claros et al. [23]. Density and refractive index were measured in triplicate, obtaining deviations in agreement with the estimated uncertainties. Calibration curves relating each physical property to the concentration of ionic liquid and

Table 1 Chemical reagents used in LLE experiments. Chemical

CAS #

Source

Purity

Purification method

Water UCON [C2mim]Cl [C2mim]Br [C2mim][SCN] [C2mim][OAc]

7732-18-5 9038-95-3 65039-09-0 65039-08-9 331717-63-6 143314-17-4

e Sigma-Aldrich Iolitec Iolitec Iolitec Iolitec

e e 0.98 0.99 0.98 0.98

Distillation None None None None None

polymer were previously obtained. For this purpose, solutions with up to a total solute content of 10 %wt were prepared. Then, a densimeter Anton Paar DMA-5000 and a refractometer Anton Paar Abbemat 500 were used to measure density and refractive index, respectively. These experimental data of density and refractive index were correlated with polynomial expressions up to order 2 by least-squares regression to determine the best set of equations for composition analysis. For each system densities and refractive indices of a set of 8 mixtures within the composition range were prepared by weight and measured; subsequently, different sets of calibration equations were used to determine the compositions. The application of order 2 polynomials is unjustified, because the errors obtained were very similar between polynomials of order 1 and 2. In all cases, average errors of the calculated compositions for polymer and ionic liquid with these equations were below 0.006 and 0.004 (in mass fraction), respectively. The experimental data of density and refractive index and the set of equations used for composition analysis of each ATPS are reported as supplementary material (Eq. S(1) and Tables S1eS5).

3. Thermodynamic framework The Thermodynamic modeling of ATPS phase diagrams is complex due to the nature of the chemicals involved: polymers, electrolytes, water. The types of interactions among such chemicals are quite different. Traditionally, expansions of the osmotic second virial coefficient and excess Gibbs energy (GE) models have been used by different authors [24]. Recently, Sadowski and co-workers have proven the ability of the ePC-SAFT equation of state to predict the LLE of polymer/salt ATPS [25]. It is important to note that this model combines the suitability of the PC-SAFT equation for polymers with the Debye-Hückel theory for electrolytes, thus dealing with the complexity of chemicals on ATPS. In this work, the conventional GE approach has been chosen, also with an adequate selection of models to consider the macromolecular character of the UCON co-polymer and the electrolyte character of the ionic liquids. The Chen-NRTL www.sciencedirect.com/science/article/pii/ S0021961411001534 - b0120 [20,21] and the modified Wilson [22,23] models where used for data correlation. GE is calculated as a sum of three contributions [20,22,26]:

GE ¼ GE;Comb þ GE;LR þ GE;SR

(1)

GE,Comb, GE,LR and GE,SR are the combinatorial contribution, the longrange interaction contribution and the short-range interaction contribution, respectively. The activity coefficient of the component I (Polymer, ions, water), is therefore also expressed as the sum of three contributions: SR lngI ¼ ln gComb þ ln gLR I I þ ln gI :

(2)

The GE,Comb contribution takes into account the effect of macromolecules and is calculated using the Flory-Huggins expression [27]. GE,LR contribution takes into account the ionic nature of electrolytes and is calculated through the Pitzer-Debye-Hückel equation (PDH) [28]. Finally, short-range interactions are accounted into the GE,SR contribution using the local composition models ChenNRTL [20] or modified Wilson [22]. In this way, both approaches consider the complexity of the mixtures used in ATPS formulation.

3.1. Excess Gibbs energy for the combinatorial contributions GE,Comb expressed by the Flory-Huggins equation can be written as:

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

3

Scheme 1. Chemical structure of the studied ILs: (a) [C2mim]Cl, (b) [C2mim]Br, (c) [C2mim][SCN], (d) [C2mim][OAc].

GE;Comb RT

  X 4 ¼ nI ln I ; xI I

(3)

and

ln gComb ¼ ln I

  X4J  4I þ 1  rI ; xI rJ J

(4)

Vs ¼ x2 V2 þ x3 V3

(9)

X 4j εT;j

(10)

εT ¼

j

and

where

rx 4I ¼ P I I J r J xJ

(5)

Also, xI, nI, and rI are the mole fraction, the number of moles and the number of segments of the component I, respectively. Polymer molecules, ions or water are represented by I and J species. The value of r for the polymer is the ratio of the molar volume of polymer with respect to water. The molar volume of UCON has been calculated with V ¼ (5neopoþ1)Vw, where neopo is the number of monomers in the polymer (neopo ¼ 76.041) and Vw is the molar volume of water [29]. The specific volume of saturated liquid water at each temperature was used to calculate the molar volume of liquid water, Vw [30]. For all other species (water and ionic liquid's ions) r ¼ 1 was used, meaning that only one segment was considered for these species. 3.2. Excess Gibbs free energy for the long-range interaction contributions GE,LR was calculated using the PDH equation [28] normalized to the pure liquid electrolyte reference state. Thus, the activity coefficient equation for an ionic species i is as follows:

" ln

P fraction basis, Ix ¼ 0:5 xi Z 2i . In this work, UCON is considered as a pseudo-solvent; thus Vis and εT are the molar volume and dielectric constant of the mixed solvent, respectively. These are calculated by:

gPDH i

¼  Ax

! # 1=2 1=2 3=2 1 þ rI x Z 2i I x  2I x .   þ ln ; 1=2 r 1 þ rI x 1 þ rZi 21=2

2Z 2i

(6) where



1 2pNA Ax ¼ 3 Vs

1=2 

e2 4pεεT KT

4j ¼

(7)

(11)

where, the molar volume of the polymer and water are represented by V2 and V3, respectively. The dielectric constant of component j is εT,j and the salt-free volume fraction for polymer and water is fj. For UCON a constant value of dielectric constant, εT,2, of 2.12 at different temperatures was used. In the case of water, the dielectric constant values used at T ¼ (288.15, 298.15 and 308.15) K were εT,3 ¼ (82.22, 78.30 and 74.82), respectively [31,32]. In this work, the value of r ¼ 14.9 was used, since this value is frequently considered for electrolyte solutions [33]. 3.3. Excess Gibbs free energy for the short-range interaction contributions 3.3.1. The Chen-NRTL model The NRTL model was used to express the short-range interaction for polymer-salt-water systems, which was developed incorporating the assumptions of like-ion repulsion and local electroneutrality, initially proposed by Chen [34] and Chen and Evans [35]. Also, the model was extended by applying the local composition concept to solvent, segments and ions, rather than solvents, polymer and salt. The activity coefficient equations have been previously presented by Zafarani-Moattar and Sadeghi [20]. The interaction parameters as a function of temperature, tIJ are given by:

tIJ ¼

3=2

nj Vj n2 V2 þ n3 V3

AIJ ; RT

(12)

where AIJ is the interaction parameter for components I and J.

The equation for a neutral molecule is as follows:

ln

gPDH j

3=2

¼

2Ax I x

1=2

1 þ rI x

:

(8)

In the previous equations, Zi is charge number of ion I and r is the closest distance parameter. K, ε, NA and e are Boltzmann's constant, the permittivity of vacuum, Avogadro's number and electronic charge, respectively. Ix is the ionic strength in mole

3.3.2. The modified Wilson model The modification proposed of the Wilson model by Sadeghi [22] for aqueous polymer-salt system considers the local composition concept to the individual segments, ion and solvent molecule and incorporates the two assumptions of like-ion repulsion and local electroneutrality proposed by Chen [34] and Chen and Evans [35]. The equations for the activity coefficients have been presented before by Sadeghi [22].

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

4

The interaction parameters as a function of temperature, HIJ are given by:

  EIJ ; HIJ ¼ exp  10RT

(13)

where EIJ is the interaction parameter for components I and J and the effective coordination number in the system, C, is set to 10.

4. Results and discussion 4.1. Experimental results The LLE data for the UCON þ ionic liquid þ water systems at T ¼ (288.15, 298.15 and 308.15) K and atmospheric pressure are presented in Tables 2e5. Phase diagram for ionic liquid [C2mim][OAc] could not be obtained at 288.15 K due to the high viscosity of the phases. The slopes of the tie-lines (STL) were determined by linear regression of the feed, top and bottom compositions of each tieline, with coefficients of determination r2 > 0.993 in all cases. The tie-line length (TLL) is an empirical measure of the difference of compositions between the two equilibrium phases, defined by:

1=2  TLL ¼ DC 2p þ DC 2ca

(14)

where DCp and DCca are the difference of polymer and ionic liquid concentration, respectively, in the top and bottom phases. Both STL and TLL are also reported in Tables 2e5 Fig. 1 shows the phase diagrams of the ATPSs formed by UCON þ IL þ Water. The STL obtained (see Tables 2e5) cannot be considered constant within each ATPS and temperature. The experimental errors in phase compositions are far below the variations of the different STL. This is a typical finding of polymer/salt ATPS, and different from polymer/polymer ATPS that tend to have almost constant slopes within a given temperature. The absolute value of the STL decreases as the TLL increases (Tables 2e5) in all the four ATPSs studied. Another important issue is the aqueous nature of the equilibrium phases. In all systems water overall composition is < 0.5 in mass fraction, while in conventional polymer/salt ATPS is usually in the range 0.6e0.8. In polymer/polymer ATPS the water content may be even higher, up to 0.9. A in ATPS stands for “Aqueous”, meaning

that water is the solvent in the mixture. While that is evident on typical polymer/polymer and polymer/salt ATPS, here it may be dubious. On a molar scale there are far more water molecules than ionic liquid ions or polymer macromolecules on the equilibrium phases of any of the ATPS phase diagrams evaluated. That can be checked on the Supplementary Information (See Fig. S1) where phase diagrams are plotted in molality scale. The reason is that water molecular weight is one or two orders of magnitude lower than those of the ionic liquids and UCON. It is not the scope of this work to define the minimum amount of water needed to use the term “aqueous” or ATPS, but it is clear that the liquid character of the phase-forming components reduces the need for water as solvent in these systems. Of course, a reduced water content may have consequences on the application of such ATPS for biomolecules recovery. The binodal curves constituted by the ends of the tie-lines were fitted to the following empirical non-linear expression:

wp ¼ a  expðb  wca Þ

(15)

where a and b are fitting parameters, and wp and wca are the polymer and ionic liquid mass fractions, respectively. Least squares regression was used to obtain fitting parameters. The coefficient of determination, r2, and the standard error of the estimate, s, were used to evaluate the quality of the correlation. Standard error was obtained by equation (16).

s¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP  2 u N exp t i¼1 wp  wcal p

(16)

N

Superscripts exp and calc refer to experimental and calculated values respectively, while the number of experimental data points is denoted by N. The results of the correlation are presented in Table 6. According to the statistic parameters obtained, equation (15) can be used satisfactorily to reproduce the binodal curves of the present ATPS. That is also clear from inspection of Figs. 1e3 where the calculated binodal curves at the range of temperatures studied are shown.

4.2. Effect of temperature Fig. 1 shows the effect of temperature for the four different

Table 2 Feed and equilibrium compositions (mass fraction), tie-line length (TLL) and slope (STL) for the UCON (p) þ [C2mim]Cl (ca) þ Water (w) system at T ¼ (288.15, 298.15 and 308.15) K and 0.1 MPa.a Feed wp T ¼ 288.15 K 0.439 0.428 0.421 0.410 T ¼ 298.15 K 0.480 0.470 0.460 0.449 T ¼ 308.15 K 0.517 0.502 0.500 0.493 a

Top phase

Bottom phase

wca

wp

wca

wp

wca

TLL

STL

0.139 0.128 0.121 0.111

0.196 0.265 0.295 0.340

0.215 0.169 0.147 0.131

0.724 0.702 0.658 0.601

0.057 0.061 0.066 0.073

0.55 0.45 0.37 0.27

3.34 4.07 4.49 4.56

0.120 0.110 0.100 0.090

0.185 0.202 0.258 0.332

0.208 0.190 0.155 0.117

0.821 0.756 0.719 0.646

0.026 0.030 0.037 0.046

0.66 0.58 0.48 0.32

3.49 3.48 3.91 4.46

0.125 0.118 0.099 0.090

0.037 0.069 0.145 0.193

0.302 0.262 0.200 0.165

0.872 0.842 0.815 0.786

0.004 0.011 0.012 0.018

0.89 0.81 0.70 0.61

2.80 3.07 3.57 4.02

Uncertainties are u(T) ¼ 0.1 K, u(P) ¼ 5 kPa, u(wp) ¼ 0.006 and u(wca) ¼ 0.004.

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

5

Table 3 Feed and equilibrium compositions (mass fraction), tie-line length (TLL) and slope (STL) for the UCON (p) þ [C2mim]Br (ca) þ Water (w) system at T ¼ (288.15, 298.15 and 308.15) K and 0.1 MPa.a Feed wp T ¼ 288.15 K 0.342 0.329 0.301 T ¼ 298.15 K 0.231 0.206 0.203 T ¼ 308.15 K 0.339 0.320 0.310 0.300 a

Top phase

Bottom phase

wca

wp

wca

wp

wca

TLL

STL

0.489 0.464 0.450

0.971 0.970 0.969

0.012 0.011 0.005

0.000 0.000 0.000

0.763 0.706 0.657

1.23 1.19 1.17

1.30 1.41 1.49

0.559 0.542 0.528

0.992 0.976 0.970

0.000 0.004 0.002

0.000 0.000 0.000

0.741 0.694 0.673

1.24 1.19 1.18

1.34 1.42 1.45

0.491 0.470 0.460 0.451

0.987 0.974 0.976 0.974

0.003 0.006 0.004 0.005

0.000 0.000 0.000 0.000

0.761 0.708 0.683 0.658

1.24 1.20 1.19 1.17

1.31 1.39 1.45 1.49

Uncertainties are u(T) ¼ 0.1 K, u(P) ¼ 5 kPa, u(wp) ¼ 0.006 and u(wca) ¼ 0.004.

Table 4 Feed and equilibrium compositions (mass fraction), tie-line length (TLL) and slope (STL) for the UCON (p) þ [C2mim][OAc] (ca) þ Water (w) system at T ¼ (298.15 and 308.15) K and 0.1 MPa.a Feed wp T ¼ 298.15 K 0.495 0.479 0.457 0.439 0.434 0.421 T ¼ 308.15 K 0.486 0.478 0.470 0.460 0.441 0.420 0.406 a

Top Phase

Bottom Phase

wca

wp

wca

wp

wca

TLL

STL

0.181 0.161 0.140 0.131 0.121 0.099

0.000 0.000 0.000 0.020 0.033 0.140

0.376 0.338 0.305 0.280 0.257 0.171

0.906 0.880 0.866 0.820 0.795 0.705

0.000 0.000 0.001 0.006 0.010 0.026

0.98 0.94 0.92 0.85 0.80 0.58

2.41 2.61 2.85 2.93 3.07 3.91

0.171 0.158 0.149 0.138 0.119 0.099 0.080

0.004 0.006 0.006 0.000 0.000 0.038 0.105

0.344 0.323 0.306 0.292 0.261 0.203 0.143

0.899 0.878 0.866 0.859 0.830 0.790 0.724

0.005 0.008 0.008 0.011 0.007 0.009 0.015

0.96 0.93 0.91 0.90 0.87 0.78 0.63

2.64 2.77 2.89 3.05 3.27 3.87 4.81

Uncertainties are u(T) ¼ 0.1 K, u(P) ¼ 5 kPa, u(wp) ¼ 0.006 and u(wca) ¼ 0.004.

Table 5 Feed and equilibrium compositions (mass fraction), tie-line length (TLL) and slope (STL) for the UCON (p) þ [C2mim][SCN] (ca) þ Water (w) system at T ¼ (288.15, 298.15 and 308.15) K and 0.1 MPa.a Feed wp T ¼ 288.15 K 0.701 0.713 0.730 0.742 0.750 T ¼ 298.15 K 0.690 0.699 0.705 0.718 0.720 0.750 T ¼ 308.15 K 0.689 0.701 0.714 0.730 0.744 a

Top Phase

Bottom Phase

wca

wp

wca

wp

wca

TLL

STL

0.201 0.226 0.230 0.239 0.250

0.882 0.942 0.972 0.981 1.000

0.051 0.030 0.017 0.014 0.000

0.636 0.483 0.398 0.301 0.261

0.244 0.417 0.514 0.639 0.731

0.31 0.60 0.76 0.92 1.04

1.26 1.19 1.16 1.09 1.01

0.190 0.201 0.214 0.221 0.230 0.250

0.880 0.906 0.943 0.955 0.960 0.998

0.046 0.041 0.025 0.021 0.017 0.000

0.614 0.578 0.523 0.420 0.331 0.263

0.240 0.286 0.357 0.466 0.557 0.731

0.33 0.41 0.54 0.70 0.83 1.04

1.36 1.33 1.26 1.20 1.17 1.01

0.191 0.196 0.225 0.230 0.237

0.895 0.931 0.969 0.995 0.999

0.039 0.025 0.014 0.001 0.000

0.585 0.539 0.363 0.248 0.191

0.257 0.323 0.512 0.631 0.750

0.38 0.49 0.78 0.98 1.10

1.41 1.31 1.22 1.19 1.08

Uncertainties are u(T) ¼ 0.1 K, u(P) ¼ 5 kPa, u(wp) ¼ 0.006 and u(wca) ¼ 0.004.

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

6

Fig. 1. Experimental phase diagrams for UCON-IL ATPS: (a) [C2mim]Cl, (b) [C2mim]Br, (c) [C2mim][OAc] and (d) [C2mim][SCN]. Tie lines: ( ( ) 308.15 K; dashed lines: binodal curves correlated with equation (15).

) 288.15 K, (

) 298.15 K and

Table 6 Fitting parameters for equation (15), coefficients of determination (r2), and standard errors of the estimates (s) at different temperatures. T/K

a

UCONþ[C2mim]Cl þ water 288.15 1.2147 298.15 0.9943 308.15 0.9225 UCONþ[C2mim]Br þ water 288.15 1.0374 298.15 0.9969 308.15 1.0144 UCONþ[C2mim][OAc]þwater 298.15 0.8926 308.15 0.9655 UCONþ[C2mim][SCN]þwater 288.15 0.9952 298.15 0.9850 308.15 0.9915

b

r2

s

9.2856 8.7503 9.5222

0.993 0.995 0.999

0.018 0.018 0.010

6.493 7.9126 8.204

0.999 1.000 1.000

0.021 0.007 0.006

11.3300 16.4076

0.998 0.997

0.020 0.025

1.8149 1.8667 2.0585

0.999 0.998 0.997

0.012 0.013 0.018

ATPSs. For all the ionic liquid studied, the biphasic zone increases with temperature, thus reducing the mass of phase-forming components required to split the phases. Usually, for a polymer/salt ATPS, top and bottom phases are enriched in polymer and salt respectively. Unlike those systems, in

Fig. 2. Effect of the anion on the phase diagram at 298.15 K for UCON (p) þ IL (ca) þ H2O (w): ( ) Br, ( ) Cl, ( ) OAc, (d,d) SCN

the present work the ATPS formulated with UCON and ionic liquids have variable behaviors, since the bottom phases of the ATPSs

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

Fob ¼

XXn

top

7

top

wI;l;exp  wI;l;calc

2

2 o  bot þ wbot I;l;exp  wI;l;calc

(17)

I¼1 l¼1

wI,l being the mass fraction of the component I for the lth tie-line and the superscripts “exp” and “calc” referring to the experimental and calculated values, respectively. The LLE data of Tables 2e5 were correlated using equation (17) and the equilibrium condition:

ðxI gI Þtop ¼ ðxI gI Þbot ;

Fig. 3. Relation between binodal curve of the UCON (p) þ IL (ca) þ H2O (w) systems and Gibbs energy of hydration of the anion at 298.15 K [36].

formed by [C2mim]Cl and [C2mim][OAc] are enriched in UCON and those formulated with [C2mim]Br and [C2mim][SCN] are enriched in ionic liquid. This behavior remains for all three temperatures under study, thus it is possible to infer that the density of the IL-rich phase for [C2mim]Cl and [C2mim][OAc] ATPS is smaller than for the polymer-rich phase, contrary to what happens with the two other systems. This behavior can be observed in Tables 2e5

4.3. Effect of anion Anions have a dramatic effect on ATPS immiscibility. Acetate produces the largest heterogeneous region and thiocyanate the smallest, following the series OAc > Cl > Br > SCN. Fig. 2 shows the effect of the anion. At the same time, this effect on the position of the binodal curve follows the order of increasing (more negative) Gibbs energy of hydration of the anion, DGhyd, at 298.15 K [36]. Such behavior is shown in Fig. 3 and is typically found with conventional salts in polymer/salt ATPS.

(18)

where superscripts “top” and “bot” represent the top and bottom phases. An optimization procedure was used to obtain the wI;l;calc values in the top phase from wI;l;exp data in the bottom phase and vice versa. Despite, the polymer/water pair is present in all ATPS studied, their interaction parameters were correlated for each system, since better results were obtained with this procedure. The results of the correlation are shown in Table 7. The non-randomness factor in the Chen-NRTL model was considered as fitting parameter since it has a significant impact on the behavior of the segment-based model. It is important to note that rather high values of the non-randomness factor were obtained in the correlation. Indeed, a value aij > 1 provided the best result for [C2mim]Br ATPS, despite the physical meaning of aij is lost. In the case of the Wilson model, the effective coordination number (parameter C in equation (13)) was set to 10, since only a slightly better correlation was achieved considering it as a fitting parameter. The results in Table 7 show that Chen-NRTL model provides a better fit, but at the expense of using one more fitting parameter. Fig. 4 shows a comparison between the experimental tie-lines and those correlated with the Chen-NRTL model at 298.15 K. It demonstrates an excellent agreement between experimental and calculated results. In both models, considering the excess Gibbs energy contributions from the PDH and the FloryeHuggins equations allows for greater accuracy. Additionally, it is important to highlight that these parameters satisfactorily capture the temperature dependency of the liquid-liquid equilibrium of all systems studied. The parameters obtained are valid for the whole temperature range and may be used to interpolate phase diagrams at other temperatures. 5. Conclusions

4.4. Tie-line data correlation The Chen-NRTL [20] and modified Wilson [22] parameters were estimated by minimizing the following objective function:

Phase diagrams for four different polymer-ionic liquids ATPS were determined combining the thermo-separating polymer UCON with [C2mim]Cl, [C2mim]Br, [C2mim][SCN] or [C2mim][OAc]. These phase diagrams were obtained at T ¼ (288.15, 298.15 and 308.15) K.

Table 7 Interaction parameters (J/mol) for the Chen-NRTL and modified Wilson models, for all UCON (p)/IL (ca) ATPS at T ¼ (288.15, 298.15 and 308.15) K. Chen-NRTL model Ionic Liquid

Aca,p

Ap,ca

Aca,w

Aw,ca

Ap,w

Aw,p

aij

S.D.a

[C2mim]Cl [C2mim]Br [C2mim][OAc] [C2mim][SCN]

4442.45 15525.29 3639.54 28629.95

3929.68 7112.14 10371.95 6207.60

3312.55 2870.00 69287.46 6631.61

11494.91 914343.31 16993.26 3193.79

5033.67 11880.76 7072.88 5033.67

1661.68 1713.37 1242.60 1661.68

0.612 1.025 0.502 0.789

3.3$103 6.5$102 3.3$102 9.8$102

Ionic Liquid

Modified Wilson model Eca,p Ep,ca

Eca,w

Ew,ca

Ep,w

Ew,p

S.D.a

[C2mim]Cl [C2mim]Br [C2mim][OAc] [C2mim][SCN]

44942.15 44883.06 2269109.42 13361.49

465064.10 94228.94 239521.32 702804.16

307113.13 71630.83 389971.62 32431.28

21422.40 21248.90 14166.96 21422.40

12693.39 12595.57 8608.89 12693.40

8.7$104 5.9$102 2.1$102 2.0$101

a

297459.19 66106.96 388418.56 18775.71

S:D: ¼ ðFob =6NÞ, where Fob and N are the function objective and number of tie-lines, respectively.

8

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387

References

Fig. 4. Equilibrium phase diagram for the UCON(p) þ [C2mim][OAc] þ H2O(w) system ) tie-lines and ( ) tie-lines calculated by Chen-NRTL at T ¼ 298.15 K: ( model.

The extension of the heterogeneous region increases with temperature, similar to the behavior found with conventional salts, but the slopes of the tie-lines do not change significantly with temperature. This behavior is different to the conventional ATPSs formed by Polymer/Salt. Anions have a dramatic effect on the ATPS immiscibility, e.g. acetate produces the largest heterogeneous region and thiocyanate the smallest, following the series OAc > Cl > Br > SCN. At the same time, this effect on the position of the binodal curve follows the series of the Gibbs energy of hydration of the anion, a behavior typically observed with conventional salts. The amount of water in the biphasic system varies enormously with the anion and can be very low, with systems formulated with [C2mim][SCN] presenting overall water compositions 10 %wt. Liquideliquid equilibrium data were correlated by the ChenNRTL and modified Wilson models. According to the fitting quality, the interaction parameters accurately represent the phase diagram at the three temperatures studied, meaning that the models capture the temperature dependency of the systems. Better results were obtained with the Chen-NRTL model.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments The authors are grateful to Xunta de Galicia for financial support through Project ED431F 2017/05. YJ thanks to the Santander Foundation and to the Universidad de Antofagasta for their support to carry out the research stay at Universidade de Santiago de Compostela.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.fluid.2019.112387.

[1] P.-Å. Albertsson, Partition of Cell Particles and Macromolecules: Separation and Purification of Biomolecules, Cell Organelles, Membranes, and Cells in Aqueous Polymer Two-Phase Systems and Their Use in Biochemical Analysis and Biotechnology, Wiley, New York, 1986. [2] B.Y. Zaslavsky, Aqueous Two-phase Partitioning: Physical Chemistry and Bioanalytical Applications, CRC Press, Marcel Dekker, Inc, New York, 1995. [3] T. Lu, Z.H. Li, J.B. Huang, H.L. Fu, Aqueous surfactant two-phase systems in a mixture of cationic gemini and anionic surfactants, Langmuir 24 (2008) 10723e10728. [4] M.G. Freire, Ionic-Liquid-Based Aqueous Biphasic Systems: Fundamentals and Applications, Springer, Berlin Heidelberg, 2016. [5] I.N. Souza, C.M.F. Soares, R.L. Souza, M.G. Freire, A.S. Lima, Aqueous two-phase systems formed by maltodextrin and acetonitrile: phase diagrams and partitioning studies, Fluid Phase Equilib. 476 (2018) 179e185.
, J.M.M. Araújo, L.P.N. Rebelo, J.A.P. Coutinho, [6] S. Shahriari, L.C. Tome I.M. Marrucho, M.G. Freire, Aqueous biphasic systems: a benign route using cholinium-based ionic liquids, RSC Adv. 3 (2013) 1835e1843. ~o, F.M. Pereira, M.G. Freire, A.T. Fricks, [7] G. de Brito Cardoso, T. Moura
Lima, Aqueous two-phase systems based on acetonitrile and C.M.F. Soares, A.S. carbohydrates and their application to the extraction of vanillin, Separ. Purif. Technol. 104 (2013) 106e113. [8] K.M. Sousa, G.E.L.O. Maciel, F.S. Buarque, A.J. Santos, M.N. Marques,
Lima, Novel phase diagrams of aqueous twoE.B. Cavalcanti, C.M.F. Soares, A.S. phase systems based on tetrahydrofuran þ carbohydrates þ water: equilibrium data and partitioning experiments, Fluid Phase Equilib. 433 (2017) 1e9. [9] J.A. Asenjo, B.A. Andrews, Aqueous two-phase systems for protein separation: a perspective, J. Chromatogr. A 1218 (2011) 8826e8835. [10] A. Dur
an, M. Claros, Y.P. Jimenez, Molybdate ion partition in the aqueous twophase system formed by CuSO4þPEG 4000þH2O at different pH and temperatures, J. Mol. Liq. 249 (2018) 562e572. [11] L. Muruchi, Y.P. Jimenez, Partitioning of perrhenate anion by aqueous twophase systems using design of experiments methodology, J. Mol. Liq. 248 (2017) 479e489. [12] M.G. Freire, A.F.M. Claudio, J.M. Araujo, J.A. Coutinho, I.M. Marrucho, J.N.C. Lopes, L.P.N. Rebelo, Aqueous biphasic systems: a boost brought about by using ionic liquids, Chem. Soc. Rev. 41 (2012) 4966e4995. [13] M.G. Freire, J.F. Pereira, M. Francisco, H. Rodríguez, L.P.N. Rebelo, R.D. Rogers, J.A. Coutinho, Insight into the interactions that control the phase behaviour of new aqueous biphasic systems composed of polyethylene glycol polymers and ionic liquids, Chem. Eur J. 18 (2012) 1831e1839. [14] J. Han, C. Yu, Y. Wang, X. Xie, Y. Yan, G. Yin, W. Guan, Liquideliquid equilibria of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate and sodium citrate/tartrate/acetate aqueous two-phase systems at 298.15 K: experiment and correlation, Fluid Phase Equilib. 295 (2010) 98e103. [15] S. Shahriari, C.M. Neves, M.G. Freire, J.o.A. Coutinho, Role of the Hofmeister series in the formation of ionic-liquid-based aqueous biphasic systems, J. Phys. Chem. B 116 (2012) 7252e7258. [16] X. Tang, J. Han, Y. Hu, Y. Wang, Y. Lu, T. Chen, L. Ni, The study of phase behavior of aqueous two-phase system containing [Cnmim] BF4 (n¼ 2, 3, 4)þ(NH4) 2SO4þ H2O at different temperatures, Fluid Phase Equilib. 383 (2014) 100e107. [17] Z. Li, Y. Pei, H. Wang, J. Fan, J. Wang, Ionic liquid-based aqueous two-phase systems and their applications in green separation processes, Trac. Trends Anal. Chem. 29 (2010) 1336e1346.
lez-Amado, A. Soto, O. Rodríguez, Aqueous two-phase [18] X. Rico-Castro, M. Gonza systems with thermo-sensitive EOPO co-polymer (UCON) and sulfate salts: effect of temperature and cation, J. Chem. Thermodyn. 108 (2017) 136e142. [19] M. Freemantle, Designer solvents, Chem. Eng. News Arch. 76 (1998) 32e37. [20] M.T. Zafarani-Moattar, R. Sadeghi, Measurement and correlation of liquidliquid equilibria of the aqueous two-phase system polyvinylpyrrolidonesodium dihydrogen phosphate, Fluid Phase Equilib. 203 (2002) 177e191. [21] Y.P. Jimenez, H.R. Galleguillos, (Liquid þ liquid) equilibrium of (NaNO3 þ PEG 4000 þ H2O) ternary system at different temperatures, J. Chem. Thermodyn. 43 (2011) 1573e1578. [22] R. Sadeghi, A modified Wilson model for the calculation of vapourþliquid equilibrium of aqueous polymerþsalt solutions, J. Chem. Thermodyn. 37 (2005) 323e329. [23] M. Claros, M.E. Taboada, H.R. Galleguillos, Y.P. Jimenez, Liquideliquid equilibrium of the CuSO4 þ PEG 4000þ H2O system at different temperatures, Fluid Phase Equilib. 363 (2014) 199e206. [24] M.T. Zafarani-Moattar, R. Sadeghi, A.A. Hamidi, Liquid-liquid equilibria of an aqueous two-phase system containing polyethylene glycol and sodium citrate: experiment and correlation, Fluid Phase Equilib. 219 (2004) 149e155. [25] T. Reschke, C. Brandenbusch, G. Sadowski, Modeling aqueous two-phase systems: I. Polyethylene glycol and inorganic salts as ATPS former, Fluid Phase Equilib. 368 (2014) 91e103. [26] Y.P. Jimenez, H.R. Galleguillos, (Liquid þ liquid) equilibrium of (NaClO4 þ PEG 4000þH2O) ternary system at different temperatures, J. Chem. Thermodyn. 42 (2010) 419e424. [27] P.J. Flory, Thermodynamics of high polymer solutions, J. Chem. Phys. 10 (1942) 51e61. [28] K.S. Pitzer, Electrolytes. From dilute solutions to fused salts, J. Am. Chem. Soc.

Y.P. Jim
enez et al. / Fluid Phase Equilibria 506 (2020) 112387 102 (1980) 2902e2906. [29] P.P. Madeira, X. Xu, Y.-T. Wu, J.A. Teixeira, E.A. Macedo, LiquidLiquid equilibrium of aqueous polymer two-phase systems using the modified Wilson equation, Ind. Eng. Chem. Res. 44 (2005) 2328e2332. [30] M.J. Moran, H.N. Shapiro, Fundamentals of Engineering Thermodynamics, Wiley, 2006. [31] L. Muruchi, H.R. Galleguillos, Y.P. Jimenez, Aqueous two-phase system of poly(ethylene glycol) 4000 and ferrous sulfate at different temperatures, Fluid Phase Equilib. 412 (2016) 29e38. [32] W. Li, D.Q. Lin, Z.Q. Zhu, Measurement of water activities and prediction of liquid-liquid equilibria for water plus ethylene oxide-propylene oxide

[33]

[34] [35] [36]

9

random copolymer plus ammonium sulfate systems, Fluid Phase Equilib. 175 (2000) 7e18. J.M. Simonson, K.S. Pitzer, Thermodynamics of multicomponent, miscible ionic systems: the system lithium nitrate-potassium nitrate-water, J. Phys. Chem. US 90 (1986) 3009e3013. C.-C. Chen, A segment-based local composition model for the gibbs energy of polymer solutions, Fluid Phase Equilib. 83 (1993) 301e312. C.-C. Chen, L.B. Evans, A local composition model for the excess Gibbs energy of aqueous electrolyte systems, AIChE J. 32 (1986) 444e454. Y. Marcus, Thermodynamics of solvation of ions. Part 5.dgibbs free energy of hydration at 298.15 K, J. Chem. Soc., Faraday Trans. 87 (1991) 2995e2999.