Catarina M. S. S.
Neves‡
a,
Christoph
Held‡
*b,
Sultan
Mohammad
b,
Miko
Schleinitz
b,
João A. P.
Coutinho
a and
Mara G.
Freire
*a
aCICECO – Aveiro Institute of Materials, Chemistry Department, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail: maragfreire@ua.pt
bLaboratory of Thermodynamics, Department of Biochemical and Chemical Engineering, Technische Universitaet Dortmund, Emil-Figge-Str. 70, 44227 Dortmund, Germany. E-mail: christoph.held@bci.tu-dortmund.de
First published on 9th November 2015
Due to scarce available experimental data, as well as due to the absence of predictive models, the influence of salts on the solubility of ionic liquids (ILs) in water is still poorly understood. To this end, this work addresses the solubility of the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]), at 298.15 K and 0.1 MPa, in aqueous salt solutions (from 0.1 to 1.5 mol kg−1). At salt molalities higher than 0.2 mol kg−1, all salts caused salting-out of [C4C1im][NTf2] from aqueous solution with their strength decreasing in the following order: Al2(SO4)3 > ZnSO4 > K3C6H5O7 > KNaC4H4O6 > K3PO4 > Mg(CH3CO2)2 > K2HPO4 > MgSO4 > KH2PO4 > KCH3CO2. Some of these salts lead however to the salting-in of [C4C1im][NTf2] in aqueous medium at salt molalities lower than 0.2 mol kg−1. To attempt the development of a model able to describe the salt effects, comprising both the salting-in and salting-out phenomena observed, the electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was applied using ion-specific parameters. The gathered experimental data was modelled using ePC-SAFT parameters complemented by fitting a single binary parameter between K+ and the IL-ions to the IL solubility in K3PO4 aqueous solutions. Based on this approach, the description of anion-specific salting-out effects of the remaining potassium salts was found to be in good agreement with experimental data. Remarkably, ePC-SAFT is even able to predict the salting-in effect induced by K2HPO4, based on the single K+/IL-ions binary parameter which was fitted to an exclusively salting-out effect promoted by K3PO4. Finally, ePC-SAFT was applied to predict the influence of other sodium salts on the [C4C1im][NTf2] solubility in water, with experimental data taken from literature, leading to an excellent description of the liquid–liquid phase behaviour.
In general, the availability of models for the description of the ILs solubility in aqueous saline solutions is very important for the design of separation and extraction processes. Currently, empirical or semi-empirical methods are used for the description of the liquid–liquid phase behaviour of systems involving ILs. When dealing with water-miscible ILs and the formation of ABS, the most common approach is the use of the correlation originally proposed by Merchuk et al.18 to describe the binodal/solubility curve of the ternary systems. This correlation was developed to describe experimental data of polymer-based systems, and it was successfully extended to ABS composed of ILs.2 This approach was latter modified by Zafarani-Moattar et al.19–21 in order to describe the temperature and pH effects on the binodal curve of each liquid–liquid system. More complex empirical correlations were also proposed in the past decade.22–30 To fully characterize these systems, tie-lines may be estimated based on the description of each binodal curve combined with a mass balance approach.18 The reliability of tie-lines is often assessed by the Othmer–Tobias31 and Bancroft25 equations. This mathematical procedure is appropriate for the accurate description of available experimental data, and thus valuable in engineering design for well-defined and known systems and conditions, but it has a limited extrapolation capability, i.e., it can hardly be extended towards other concentrations, temperatures or pH ranges, as well as to new systems composed of other ILs or salts.
A more general modelling with a sounder physical–chemical basis that explicitly accounts for molecular interactions between species is thus highly desirable, and where activity coefficient models fit within. Zafarani-Moattar and Hamzehzadeh20 applied modified NRTL and a modified Wilson model to describe the phase behaviour of ABS formed by ILs and an organic salt. With this approach, the authors20 could also describe the pH effects. The modified NRTL model required five pH-independent adjustable parameters and four pH-dependent parameters.20 By applying the Wilson model, the number of adjustable parameters was reduced to two pH-independent parameters and four pH-dependent parameters. Although explicitly accounting molecular interactions, these parameters cannot be transferred to other systems or other temperatures, thus making this approach as limited as the empirical approaches described above.
Equations of state (EoS) bear good prospects as predictive models.32 Their parameterization, unlike those of activity coefficient models, is based on pure-component parameters which are valid independently of the mixture being considered. In addition, binary parameters might be used whenever necessary. These are usually transferable to higher (e.g. ternary) systems and are often temperature independent. Thus, EoS combine the advantages of requiring a very low number of binary parameters with the use of a small number of ready available (or easily estimable) pure-component parameters.33 This opens the door for developing predictive models, which are becoming the preferred approaches in different research fields at both academia and industry.34 To the best of our knowledge, EoS have not been previously used for modelling complex water + salt + IL systems.
The use of ePC-SAFT (electrolyte Perturbed-Chain Statistical Associating Fluid Theory) to model phase equilibria of electrolyte systems can be considered highly promising.35–39 In order to apply ePC-SAFT to water + salt + IL systems both salts and ILs have to be characterized. For modelling inorganic salts with ePC-SAFT, ion-specific parameters are used. That is, modelling a salt solution requires pure-component parameters of the ions (e.g. Na+) that are independent of the salt which they are part of (e.g. NaCl, NaNO3). The newest strategy proposed35 allows describing also concentrated salt solutions by including dispersion effects between anions and cations. Different approaches have been proposed to characterize ILs using SAFT-based models. Among them, truncated PCP-SAFT (Perturbed-Chain Polar Statistical Associating Fluid Theory)40 was used assuming ILs as polar chain molecules. Using soft-SAFT,41 ILs were modelled as neutral chain molecules with association sites. Ji et al.42 applied hetero-segmented SAFT to model densities of imidazolium-based ILs by considering them as molecules comprising a chain-like cation and a spherical anion, each having association sites. Also classical PC-SAFT has been applied to model thermodynamic properties of ILs.43–46 All these works neglected the Coulombic interactions of ILs or replaced them by association forces. In other recent works,47,48 it was shown that by considering Coulombic forces allowed the modelling of IL densities and gas solubility in ILs with higher accuracy compared to classical non-electrolyte PC-SAFT versions.
COSMO-RS (COnductor like Screening MOdel for Real Solvents)49–51 already shown to be useful as a priori tool for the prediction of the phase equilibrium behaviour of systems comprising ILs.52–54 However, and although being of a predictive character, this model inherently requires previous quantum chemical calculations to obtain the sigma-profiles of the different solvents/solutes – a harder task when compared with the calculations needed to apply ePC-SAFT.
In this work, the solubility of [C4C1im][NTf2] in aqueous solutions of 10 different salts (not previously investigated) was measured at salt concentrations ranging from (0 to 1.5) mol kg−1, at 298.15 K and atmospheric pressure. The effects of the cation and anion of the added salt and respective salting-in/salting-out phenomena are discussed. The experimental data were finally modelled using ePC-SAFT aiming at evaluating the ability of this EoS to predict the salt influence on the solubility of ILs in aqueous media.
ares = ahc + adisp + aassoc + aion | (1) |
The thermodynamic description of one compound i requires three pure-compound parameters: segment number (msegi), segment diameter (σi) and dispersion-energy parameter (ui/kB). The compounds with association properties are described with two additional parameters: the association-energy parameter, εAiBi/kB, and the association-volume parameter, kAiBi.
The segment diameter and dispersion-energy parameter in the mixture of compounds i and j are modelled by combining rules of Lorentz and Berthelot, corrected with the binary parameters lij and kij:
![]() | (2) |
![]() | (3) |
![]() | (4) |
![]() | (5) |
Compound | σ i (Å) | m segi | u i/kB (k) | k ij with water |
---|---|---|---|---|
a For water, the 2B association approach with the parameters εAiBi/kB = 2425.67 K and kAiBi = 0.0451 was used. | ||||
Watera | 2.797161 | 1.204761 | 353.9561 | — |
[C4C1im]+ | 3.637147 | 2.480547 | 218.14447 | −0.016 |
[NTf2]− | 3.746947 | 6.010347 | 375.6547 | −0.016 |
K+ | 2.823235 | 1.00035 | 230.0035 | 0.2035 |
Na+ | 2.823235 | 1.00035 | 230.0035 | 4.77 × 10−4![]() |
NH4+ | 3.574035 | 1.00035 | 230.0035 | 0.06435 |
H3O+ | 3.465435 | 1.00035 | 500.0035 | 0.2535 |
Mg2+ | 3.132735 | 1.00035 | 1500.0035 | −0.2535 |
Zn2+ | 2.979835 | 1.00035 | 1250.0035 | −0.2535 |
Cl− | 2.756035 | 1.00035 | 170.0035 | −0.2535 |
NO3− | 3.298835 | 1.00035 | 130.0035 | 0.09835 |
C6H5O73− | 5.0991 | 1.000 | 258.99 | −0.25 |
HPO42− | 2.162135 | 1.00035 | 146.0235 | 0.2535 |
PO43− | 2.551635 | 1.00035 | 310.0035 | −0.2535 |
CH3CO2− | 3.932835 | 1.00035 | 150.0035 | −0.2335 |
SO42− | 3.298835 | 1.00035 | 130.0035 | 0.2535 |
CO32− | 2.442235 | 1.00035 | 249.2635 | −0.2535 |
For modelling mixtures with EoS, binary interaction parameters are needed. The kij between water and [C4C1im][NTf2] was fitted to LLE data of that binary system at 298.15 K. The kij values between IL-cation and water as well as between IL-anion and water were considered to be identical. The kij parameters between water and salt-ions as well as between these were taken from literature.35 All kij values used in this work are listed in Tables 1 and 2. For modelling the LLE of the mixture water + salt + IL no further binary parameters were applied. Based on these parameters, the IL solubility in the pseudo-ternary mixture containing five species (water, IL-ions, salt-ions) was then predicted.
Pair | k ij | Pair | l ij |
---|---|---|---|
Na+–Cl− | 0.31735 | K+–[NTf2]− | 0.55 |
Na+–NO3− | −0.30035 | K+–[C4C1im]+ | 0.55 |
Na+–CO32− | −1.00035 | Zn2+–[NTf2]− | 0.12 |
H3O+–Cl− | 0.65435 | Zn2+–[C4C1im]+ | 0.12 |
NH4+–Cl− | −0.56635 | Mg2+–[NTf2]− | — |
K+–CH3CO2− | 1.00035 | Mg2+–[C4C1im]+ | — |
K+–C6H5O73− | 1.000 | ||
K+–HPO42− | 1.00035 | ||
K+–PO43− | 1.00035 | ||
Zn2+–SO42− | −0.44635 | ||
Mg2+–SO42− | −1.00035 | ||
Mg2+–CH3CO2− | −0.99835 |
For the comparison between experimental and modelling results, ARD (average relative deviation) values were determined. The ARD between the experimental (exp.) and modelled (mod.) mass fraction of the IL, w1, was evaluated according to:
![]() | (6) |
The ARD for the solubility of the investigated IL in the aqueous phase of the salt-free solution was found to be 1.93%.
For some salts, namely KH2PO4, K2HPO4, Al2(SO4)3 and Mg(CH3CO2)2, a salting-in effect is observed for concentrations up to 0.20 mol kg−1. Above this concentration, the salting-out effect occurs for all salts studied. Furthermore, the solubility of the IL decreases up to one order of magnitude for the higher salt concentrations studied. Upon salt addition at 1.5 mol kg−1, the solubility of the IL is strongly decreased by ZnSO4 while KCH3CO2 has the lowest impact. In summary, at 1.5 mol kg−1 of salt, the IL solubility decreases in the following order of salts: ZnSO4 > K3C6H5O7 > KNaC4H4O6 > K3PO4 > Mg(CH3CO2)2 > K2HPO4 > MgSO4 > KH2PO4 > KCH3CO2. The salt Al2(SO4)3 is not in this sequence as its solubility in water is much lower than this sequence refers to. Nevertheless, at 0.75 mol kg−1, Al2(SO4)3 causes the strongest salting-out effect observed amongst all salts under investigation. In fact, the salting-out effect of this salt at 0.75 mol kg−1 is similar to that displayed by KNaC4H4O6 at 1.5 mol kg−1.
In addition to the salt-specific effects, also ion-specific effects can be observed from the experimental solubility data. In Fig. 1 and 2, the anion-specific (potassium salts) and cation-specific (acetates and sulphates) effects are illustrated. Fig. 1 shows that potassium-based salts decrease the solubility of [C4C1im][NTf2] in water with the exception of K2HPO4 and KH2PO4 at low concentrations (<0.2 mol kg−1), for which a salting-in phenomenon is observed. This is analogous to what is typically observed considering proteins solubility in salt aqueous solutions.62 For the salts shown in Fig. 1, the salting-out strength observed at 1.5 mol kg−1 is according to: KNaC4H4O6 ≈ K3C6H5O7 > K3PO4 ≈ K2HPO4 ≫ KH2PO4 > KCH3CO2. This order follows the Hofmeister series63 and can thus be considered an expected result; the same series was observed at the presence of sodium-based salts in a previous work.7
Fig. 2 presents the effect of sulphate- and acetate-based salts on the solubility of [C4C1im][NTf2] in water. These effects might again be assumed ion-specific. However, combined with the anion SO42−, the trivalent ion Al3+ induces a stronger salting-out than the divalent ions Mg2+ and Zn2+, followed by the monovalent K+. Therefore, the higher the cation's valence, the stronger the salting-out effect of the salt over the IL in aqueous media. This trend is in good agreement with previous results.7,14 Nevertheless, for these salts, the concentration-dependent speciation has a strong impact on their salting-out inducing abilities and should also be considered.64
The optimal strategy developed requires a maximum of one binary parameter between a salt-cation and the IL-ions. The results using this strategy are illustrated in Fig. 3 and 4. Fig. 3 shows that fitting the binary parameter lij between K+ and IL-ions to the data of IL solubility in K3PO4 solutions yields a quantitative description of the experimental salt-induced solubility of the IL. This is a promising result keeping in mind that usually several binary parameters are required to model the LLE of inorganic salt-containing or IL-containing mixtures. For instance, a recent work20 used up to 9 binary parameters to correlate the LLE behaviour of water/IL/salt systems. Even more promising, the availability of an ion-specific model, such as ePC-SAFT, allows the development of transferable binary parameters. In this line, the fitted lij parameter between K+ and IL-ions was further applied to predict the solubility of potassium-based salts other than the K3PO4 used for fitting the lij. The results are illustrated in Fig. 3 for systems with KCH3CO2, K3C6H5O7, and K2HPO4. Applying ePC-SAFT and the binary parameter lij for K+/IL-ions fitted to the solubility data of the water/IL/K3PO4 system allows predicting the salting-out of the IL from the aqueous phase with good agreement to the experimental observations. This is a quite good result, as these are modelling predictions; i.e., these data were neither used to fit the binary lij parameter nor any pure-component parameters. This statement might seemed weakened by the fact that the binary parameter was fitted to the system containing K3PO4, which induces salting-out in the whole salt-concentration range, thus showing the same influence upon the IL solubility as the salts KCH3CO2 and K3C6H5O7. Nevertheless, using the lij parameter, ePC-SAFT is also able to correctly predict the influence of K2HPO4 on the IL solubility, including the salting-in region and the solubility maximum at 0.1 mol kg−1 experimentally observed. This is one of the most remarkable achievements as the binary parameter was fitted to a system containing K3PO4, which induces only salting-out along all the concentrations studied. To the best of our knowledge, a thermodynamic model or an equation of state were never used and demonstrated to be able to describe these remarkable type of results (salting-in vs. salting-out).
![]() | ||
Fig. 3 Solubility of [C4C1im][NTf2] in water upon addition of potassium-based salts at 298.15 K: ![]() ![]() ![]() ![]() |
![]() | ||
Fig. 4 Solubility of [C4C1im][NTf2] in water upon addition of acetate-based salts and sulphate-based salts at 298.15 K: ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Fig. 4 shows the influence of sulphate-based salts and acetate-based salts on the solubility of [C4C1im][NTf2] in aqueous solutions. All these salts induce a salting-out of the IL from the aqueous (IL-poor) phase. As shown in Fig. 4, ePC-SAFT can reproduce adequately the IL solubility in the different salt solutions. ePC-SAFT correctly predicts the salting-out ability strength for all salts with the exception for ZnSO4, for which a binary parameter lij Zn2+/IL-ion had to be fitted to the experimental data. This limitation should be related with the ZnSO4 speciation64 that cannot be captured by the modelling approach used in this work. The species distribution of ZnSO4 in aqueous solution is highly complex and strongly depends on the salt concentration – cf. the ESI.† In order to describe this solution (and the salt-dependent IL solubility) adequately, the ePC-SAFT parameters for all species that will form (including the ion pairs) are required. However, this was outside the scope of this work as it would introduce additional binary parameters as well.
Encouraged by these promising results, ePC-SAFT was further applied as a fully predictive model to describe the salt influence on the [C4C1im][NTf2] solubility in presence of a broad plethora of salts. Using the pure-component parameters for water, IL-ions, and salt-ions, the effect of the salt concentration upon the [C4C1im][NTf2] solubility in aqueous solutions of sodium- and chloride-based salts was further predicted and compared to experimental literature data.7 The objective of this attempt was the validation of the predictive capability of ePC-SAFT to describe ion-specific salting-out/-in effects.
Fig. 5 shows the experimental solubilities of [C4C1im][NTf2] in aqueous salt solutions and the predictions achieved with ePC-SAFT. The impact of chloride-based salts on the solubility of [C4C1im][NTf2] is depicted in Fig. 5a. Below a salt molality of 0.75 mol kg−1, all salts induce a salting-in of the IL, while NaCl induces a salting-out at molalities higher than 1.5 mol kg−1.
![]() | ||
Fig. 5 Solubility of [C4C1im][NTf2] in water upon addition of (a) chloride and (b) sodium salts at 298.15 K. Symbols represent the experimental data taken from literature,7 and lines represent predictions with ePC-SAFT using parameters from Tables 1 and 2: ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Fig. 5b shows the influence of sodium-based salts on the solubility of [C4C1im][NTf2]. A salting-in phenomenon is observed for NaCl and Na2CO3 at low salt concentrations, followed by a salting-out at higher salt concentrations. In contrast, NaNO3 acts as a salting-in inducing salt in the entire salt concentration region. The strength of the salting-in of NaCl and Na2CO3 at low salt concentrations is very similar; however, Na2CO3 causes a stronger salting-out at higher salt concentrations. The molecular mechanism behind these salting-in/-out effects was established in a previous work.7
The experimental behaviour of the six systems described above is highly non-linear, unlike observed for many other molecules that follow a much simpler behaviour typically described using Setschenow coefficients.66 In Fig. 5, the predictive ePC-SAFT modelling results are compared with the experimental data for these systems. It must be again stressed out that neither pure-compound nor binary interaction parameters were fitted to data of the ternary water/[C4C1im][NTf2]/salt specific systems. The salting-in/-out effects of NaCl and Na2CO3 are predicted in good agreement with the experimental data. Also the stronger salting-out effect of Na2CO3 at higher salt concentrations is correctly predicted by ePC-SAFT. The model also correctly predicts the salting-out effect of the chloride salts upon the [C4C1im][NTf2] solubility. In summary, this prediction capacity of ePC-SAFT is remarkable given the complexity of the systems studied, with four ions in solution, and the highly non-linear behaviour of [C4C1im][NTf2] solubility in aqueous salt solutions.
In order to classify the results of the ePC-SAFT predictions, it should be noted that semi-empirical approaches using activity coefficient models, such as NRTL, require a large number of binary parameters. These allow correlating the behaviour of such complex systems. This work shows that the LLE behaviour of different [C4C1im][NTf2]–salt–water systems can be captured with ePC-SAFT in a predictive way, using only pure-compound parameters and ion-specific binary interaction parameters that are universally valid and ion-independent.
The measured experimental data were further described with ePC-SAFT, for which ion-specific ePC-SAFT parameters were used to describe the salts and the IL. A single binary parameter between K+ and the IL-ions was fitted to solubility data of the IL in K3PO4 aqueous solutions. Further, this binary parameter was applied to predict the influence of potassium-based salts on the solubility of the IL, and a good prediction was found. ePC-SAFT was even shown to be able to predict the salting-in induced by e.g., K2HPO4. This is a remarkable result as the binary parameter between K+/IL-ions was fitted to solubility data in K3PO4 solution, which only induces the IL salting-out.
Finally, ePC-SAFT was applied to fully predict the influence of sodium- and chloride-based salts on the IL solubility in aqueous media. A good accuracy was found when compared with the experimental data taken from literature. Remarkably, both salting-in/-out effects were predicted.
In contrast to ePC-SAFT, semi-empirical approaches require a much higher number of binary parameters in order to correlate the behaviour of complex systems, such as IL–salt–water systems. This work shows that the solubility behaviour of ILs in aqueous solutions could be captured with ePC-SAFT in a predictive way, using only pure-compound parameters and ion-specific binary interaction parameters that are universally valid and independent of the ions in solution. To the best of our knowledge, such type of results has never previously been achieved using a thermodynamic model, while using a single equation of state.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental solubility data in weight and molality units; ion speciation profile of ZnSO4 in aqueous solutions; ePC-SAFT parameters estimation for the citrate anion. See DOI: 10.1039/c5cp06166k |
‡ Equally contributing authors. |
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