Effect of Cation Structure in Trifluoromethanesulfonate-Based Ionic

Article pubs.acs.org/jced

Effect of Cation Structure in Trifluoromethanesulfonate-Based Ionic Liquids: Density, Viscosity, and Aqueous Biphasic Systems Involving Carbohydrates as “Salting-Out” Agents Marcin Okuniewski,† Kamil Paduszyński,*,† and Urszula Domańska†,‡ †

Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ‡ Thermodynamic Research Unit, School of Chemical Engineering University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4001, South Africa S Supporting Information *

ABSTRACT: This work reports new experimental data on density and viscosity of pure ionic liquids (ILs) based on trifluoromethanesulfonate anion, [CF3SO3]. In particular, a cation-core effect on these basic solvent characteristics is demonstrated and investigated. The studied structures comprise representative 1-butyl-1-methylpyrrolidinium, 1-butyl-1-methylpiperidinium, and 1-butyl-3-methylpyridinium cations. In the main part of this work, the possibility to form aqueous biphasic systems (ABS) based on the studied ILs and four carbohydrates, namely, D(+)-glucose, D-(−)-fructose, D-sorbitol, and xylitol, was tested. Liquid−liquid equilibrium phase diagrams at T = 298.15 K, including binodal curves and tie lines, were determined for 12 distinct ABS, and then, an impact of the type of cation of IL (pyrrolidinium, piperidinium, and pyridinium) on these properties was elucidated and discussed in terms of the molecular structure of both ILs and carbohydrates and molecular interactions between them. Moreover, an effect of temperature on ABS formation is also presented for a selected representative system.

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INTRODUCTION Aqueous biphasic systems (ABS) have been widely accepted as alternative extraction media, mainly for partitioning, separation, and purification of diverse biological materials (e.g., drugs, proteins, enzymes, and nucleic acids). Conventional ABS is formed by two aqueous solutions of two polymers or a polymer and an inorganic salt. In their pioneering work, Gutkowski et al.1 showed that liquid-phase split required to form an ABS can be induced by ionic liquids (ILs), when one adds them to aqueous solution of an inorganic salt. Since then, a vast amount of work regarding experimental studies on phase diagrams of such {IL + salt + water} systems has been reported. Thus, polymers constituting ABS can be replaced by ILs, what make certain extra benefits and open new perspectives for ABS due to unique physical and thermodynamic properties of ILs (e.g., their thermal stability and selectivity). Furthermore, basically an uncountable number of possible cation−anion combinations create a lot of opportunities to investigate various structural effects on ABS formation and phase behavior.2,3 In the past few years, some efforts have been put toward searching for new salting-out agents being alternative for traditional salts (henceforth, the phrase ”salting-out” refers to ability of a compound to promote ABS formation when added to a single-phase aqueous solution of IL). In particular, this becomes an urgent scientific activity because of the fact that the © XXXX American Chemical Society

presence of some inorganic salts in ABS may complicate their handling and performance, including processes of recycling or partitioning of biomolecules sensitive toward acid−base chemical equilibria and pH.4 Zhang et al.5 proposed carbohydrates as environmentally benign replacements for salts in IL-based ABS and investigated liquid−liquid equilibrium phase diagrams (both binodal curves and tie lines) for ternary mixtures of common IL 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1Im][BF4]) with fructose and water at three temperatures. This idea has turned out to be very pertinent, as it was established before the publication6 and confirmed thereafter7−12 that ILs can act as suitable solvents for the dissolution and processing of renewable feedstock (including carbohydrates). Following the work of Zhang et al.,5 Wu et al.13 investigated an impact of IL structure (alkyl chain and anion) on liquid−liquid equilibrium for ABS based on sucrose. They found that formation of ABS could be promoted by the increase of the side-chain length of ILs, but the effect was substantially weaker compared with that observed of the anion. The same group investigated an influence of the structure of various carbohydrates (xylose, sucrose, glucose, and Received: November 3, 2015 Accepted: February 18, 2016

A

DOI: 10.1021/acs.jced.5b00931 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Chemical structures of the ions forming the ionic liquids under study and carbohydrates: (1) 1-butyl-1-methylpyrrolidinium cation, [C4C1Pyr]; (2) 1-butyl-1-methylpiperidinium cation, [C4C1Pip]; (3) 1-butyl-3-methylpyridinium cation, [C4C1(3)Py]; (4) trifluoromethanesulfonate (triflate) anion, [CF3SO3]; (5) D-(+)-glucose; (6) D-(−)-fructose; (7) D-sorbitol; (8) xylitol.

structure on the solubility of selected compounds in pure ILs was established and discussed. Motivated by the promising results obtained previously by Freire et al.,4 we decided to focus on ABS formed by [CF3SO3]-based ILs and carbohydrates. Binodal curves and tie lines for 12 distinct ABS were measured and analyzed. In particular, an effect of cation type (pyrrolidinium vs piperidinium vs pyridinium) on the phase equilibrium diagram is demonstrated. Additionally, pure ILs were characterized in terms of basic solvent properties, namely, density and dynamic viscosity, which are essential data from the point of view of any applications. We believe that the data presented herein will encourage further investigation of potential applications of ILs in ABS extraction-based processes and techniques.

fructose) and temperature on the formation of ABS based on [C4C1Im][BF4].14,15 The results were nicely discussed in terms of stereochemistry of carbohydrates and available hydration models proposed for sugars. Chen et al.16 presented a more comprehensive study on ABS formation induced by glucose when added to aqueous solutions of imidazolium ILs based on three hydrophilic anions: [BF4], [Cl], and [Br]. It was shown that only in the case of [CnC1Im][BF4], where n = 3−10, can the formation of ABS be observed in the temperature range from 242 to 373 K. More recently, a detailed study on application of these ABS in phenol extraction was also carried out.17 Finally, the most recent contribution on {IL + carbohydrate + water} systems was published by Freire et al.4 This time, 1-butyl-3-methylimidazolium trifluoromethylsulfonate ([C4C 1 Im][CF3 SO3 ]) combined with a series of carbohydrates (glucose, fructose, galactose, mannose, arabinose, xylose, maltose, sucrose, maltitol, sorbitol, and xylitol) was investigated. The authors clearly justified and disscussed the selection of triflate-based IL in terms of enhanced H-bond acceptor capabilities of this anion according to an empirical scale of basicity. Besides, as pointed out by the authors an additional advantage of ILs based on [CF3SO3] anion is their chemical stability in aqueous solutions, compared to other ILs forming ABS such as those based on [BF4].4 Binodal curves and tie lines as well as densities and viscosities of both coexisting were measured for each ABS at T = 298.15 K. Besides, the extractive potential of the studied ABS in partitioning of simple biomolecules was tested. This work presents another contribution to the field of thermodynamic properties of ILs and their mixtures with sugar or sugar alcohols. Since 2013, we have published several works on solid−liquid equilibrium data in binary systems {IL + sugar, or sugar alcohol}.9−12 An impact of both cation and anion

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EXPERIMENTAL PROCEDURES Materials. The ILs under study, namely, 1-butyl-1methylpyrrolidinium trifluoromethanesulfonate ([C4C1Pyr][CF3SO3]; CAS Registry No. 367522-96-1), 1-butyl-1-methylpiperidinium trifluoromethanesulfonate ([C4C1Pip][CF3SO3]; CAS Registry No. 1357500-93-6), and 1-butyl-3-methylpyridinium trifluoromethanesulfonate ([C4C1(3)Py][CF3SO3]; CAS Registry No. 857841-32-8) were purchased from IoLiTec. The ILs were dried and degassed prior to any measurements by applying a moderate vacuum (≈1 mbar) at moderate temperature (≈323 K) for a minimum of 24 h. After that, the samples were stored under vacuum in a desiccator over freshly activated molecular sieves and phosphorus pentoxide used as drying agents. Sugars and sugar alcohols (henceforth referred to as carbohydrates), namely, D-(+)-glucose (CAS Registry No. 50-99-7), D-(−)-fructose (CAS Registry No. 5748-7), D-sorbitol (CAS Registry No. 50-70-4), and xylitol (CAS B



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Table 1. Summary of Chemical Sample Information for the Compounds Investigated in This Study chemical name

source a

[C4C1Pyr][CF3SO3] [C4C1Pip][CF3SO3]b [C4C1(3)Py][CF3SO3]c D-(+)-glucose D-(−)-fructose D-sorbitol xylitol water

IoLiTec IoLiTec IoLiTec Aldrich Aldrich Aldrich Aldrich our laboratory

initial mass fraction purity

purification method

0.99 0.99 0.99 0.995 0.99 0.99 0.99

a

final mole fraction purityd

vacuum drying vacuum drying vacuum drying none none none none none

0.992 0.993 0.993

analysis method KF KF KF

e

106wH2O 470 414 447

conductivityf

b

Full name: 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate. Full name: 1-butyl-1-methylpiperidinium trifluoromethanesulfonate. cFull name: 1-butyl-3-methylpyridinium trifluoromethanesulfonate. dCalculated based on wH2O. eKarl Fischer method. fPURE LAB Option Q Elga Water System; specific conductivity, κ = 8 μS at T = 293.15 K.

(water), whereas the second one was the external probe (Pt resistance thermometer having resolution 0.01 K and standard uncertainty 0.05 K) immersed in the smaller batch in which the sample test tube was placed. This allowed one to maintain constant temperature of the studied system within 0.05 K. Then, the amount of m12 by mass of a carbohydrate aqueous solution of weight fraction w02 ≈ 0.4 (almost saturated solution) was added dropwise to the cell until phase separation was unequivocally observed. After each addition of carbohydrate solution, vigorous stirring (with magnetic stirrer) of the resulting ternary mixture was applied for several minutes. Finally, a mass m03 of pure water was carefully added dropwise to the two-phase system until turbidity of the solution disappeared. The compositions (given as weight fractions) of the final mixture corresponding to the ith (i = 1, 2, ...) repetition of the procedure (and hence ith data point on the binodal curve) were obtained by using the following massbalance formulas:

Registry No. 87-99-0) were supplied by Sigma-Aldrich and used without further purification. Water content for pure ILs was determined by the Karl Fischer method by using Schott Instruments Titro-Line KF apparatus. The one-component reagent for volumetric Karl Fischer titration CombiTitrant 2 supplied by Merck was used. The samples were dissolved in methanol and titrated with steps of 2.5 × 10−3 cm3. The chemical structures of all of the ions forming the studied ILs as well as carbohydrates are shown in Figure 1. Complete information regarding all of the compounds considered in this work, including full name, supplier, initial and final mass fraction purities, analysis methods, and water content, are summarized in Table 1. Each measurement was repeated three times. Standard uncertainty of the water content determination was 10 × 10−6 of weight fraction. Density and Viscosity Measurements. Densities of pure ILs at ambient pressure p = 0.1 MPa were determined over temperature range from T = 293.15 to 363.15 K by using an Anton Paar 4500 M vibrating tube densimeter. Two integrated Pt 100 platinum thermometers provided control of temperature within 0.05 K. Prior to measurements, the densimeter was calibrated with doubly distilled and degassed water, benzene, and dried air. Resolution of the apparatus was 0.01 kg·m−3. Taking into account the sample impurities (see Table 1), standard uncertainty of the measurements was estimated to be 1 kg·m−3 (based on the assumptions proposed by NIST18). The Anton Paar AMVn rheometer based on “falling ball” principle was used to measure the dynamic viscosity of pure ILs over the temperature range from T = 293.15 to 363.15 K. The apparatus allows one to control the temperature within 0.05 K. The standard uncertainty and reproducibility of the measured viscosity was estimated to be 5% and 0.05%, respectively. For each IL under study, the tubes of both densimeter and viscosimeter were filled once and the temperature was changed with a step of +5 K. Temperature and density/viscosity were recorded automatically by the instrument. Binodal Curves Determinations. The binodal curves for the investigated {IL (1) + sugar, or sugar alcohol (2) + water (3)} ternary systems were determined at constant temperature T = 298.15 K and at ambient pressure p = 0.1 MPa with the “cloud point” method. First, m013 ≈ 1.5 g of an aqueous solution of IL with known weight fraction w01 ≈ 0.4 was prepared in a glass sealed test tube glass and placed in a jacketed glass cell connected to a thermostatic water bath (Julabo MA). During the experiments the temperature of the system was controlled by using two probes. The first one was the instrument internal probe measuring the temperature of the internal circulator fluid

0 m1, i = w10m13

(1a)

m2, i = m2, i − 1 + w20m12, i

(m2,0 = 0) i

0 m3, i = (1 − w10)m13 + (1 − w20) ∑ m12, k + k=1

wj , i =

mj , i m1, i + m2, i + m3, i

(1b) i

(j = 1, 2, 3)

∑ m3,0k

(1c)

k=1

(1d)

where mj,i denotes mass of component j after the ith titration step. It is important to note that the masses of added amounts of sugar solution and pure water were obtained by weighing the changes of the mass of the sample test tube. Weighing was performed with a Mettler Toledo XA105 DualRange analytical balance with a resolution of 0.0001 g and standard uncertainty of the direct mass determination of 0.0005 g. Combined standard uncertainties for the masses of all three components after the ith titration step can be obtained from eqs 1 following error propagation formula, taking into account weighing errors only. In particular, it can be shown that the combined uncertainty of the mass of sugar and water increases systematically during titration. However, the combined uncertainty of weight fractions still remains at the level of 0.001 during the entire process of binodal curve determination. The overall standard uncertainty of the binodal curve mass fraction compositions was 0.005. The experimental procedure described was checked and validated by some preliminary measurements for system {[C4C1Im][CF3SO3] + D-(+) -glucose + water} and comparing C



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the results with reference data taken from literature.4 Very good agreement between the measurments carried out in this work and the literature data was observed, what can be seen in Figure S1 in the Supporting Information. Tie-Lines Determinations. Tie lines of each ternary system were determined by using the gravimetric method proposed by Merchuck and co-workers.19 It is important to note that this method was successfully applied for tie-lines determinations in a great variety of ABS including those with ILs and carbohydrates.4,13 Two-phase systems were prepared in thermostated jacketed cells by means of weighing. The initial compositions of the mixtures (i.e., “feed” compositions wF1 and wF2 ) were established based on the binodal curves. The heterogeneous mixtures were stirred for 1 h and allowed to reach phase separation for 12 h. After equilibration, the top and bottom phases were carefully separated and individually weighed. Binodal curve data, given as the relationship between IL and sugar or sugar alcohol weight fraction, were regressed by using the following equation:19 w1 = f (w2) = A exp(Bw21/2 − Cw2 3)

Figure 2. Density, ρ, of the ionic liquids under study as a function of temperature, T, at pressure p = 0.1 MPa: circles, [C4C1Pyr][CF3SO3] (filled circles, literature data20−26); squares, [C4C1Pip][CF3SO3]; triangles, [C4C1(3)Py][CF3SO3] (filled triangles, literature data24,27). Solid lines designated by eq 4 with the parameters listed in Table 2.

(2)

Finally, determination of IL and sugar or sugar alcohol concentrations in top (“T”) and bottom (“B”) phases was performed by solving the following system of equations: w1T − f (w2T) = 0

(3a)

w1B − f (w2B) = 0

(3b)

w1Tβ + w1B(1 − β) = w1F

(3c)

w2Tβ + w2B(1 − β) = w2F

(3d)

where β denotes the ratio between the mass of the top phase and the total mass of the ABS. The first two equations correspond to the condition that both end points of each tie line are situated on the binodal curve, whereas the remaining equations represent so-called “lever rule” (material balance) applied for both top and bottom phases.

Figure 3. Viscosity, η, of the ionic liquids under study as a function of T at p = 0.1 MPa: circles, [C4C1Pyr][CF3SO3] (filled circles, literature data21,23,28); squares, [C4C1Pip][CF3SO3]; triangles, [C4C1(3)Py][CF3SO3] (filled triangles, literature data27). Solid lines designated by eq 5 with the parameters listed in Table 3.

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RESULTS AND DISCUSSION Density and Viscosity of Pure ILs. The obtained density (ρ) and dynamic viscosity (η) data for the investigated ILs are listed in Tables S1 and S2 of the Supporting Information. The data are shown graphically in Figure 2 and Figure 3, respectively, where they are additionally compared with available literature data.20−24,26−28 As can be noticed from Figure 2 density data measured in this work are in good agreement with the data reported by other authors. In particular, for [C4C1Pyr][CF3SO3] IL the overall average absolute relative deviation (AARD) between the literature and our measurements is below 0.05%, including the data sets of Vercher et al.,20,22 Gaciño et al.,21 and Seoane et al.23 as well as single data points reported by Domı ́nguez et al.,24 Gonzalez and Corderi,́ 25 and Requejo et al.26 These results confirmed a good quality of the sample as well as the measuring principle used. For [C4C1Pip][CF3SO3], no reference data were found, and thus, to our best knowledge, this is the very first time when density of this IL is reported. In the case of [C4C1(3)Py][CF3SO3] IL the agreement with literature is much worse compared to [C4C1Pyr][CF3SO3]. In particular, our data agree with the data published by Bittner et al.27 within AARD of 1.3%, whereas density measured at T =

298.15 K is in much better agreement (0.3%) with the data point reported by Domı ́nguez et al.24 As can be seen in Figure 2, the former data set exhibits a significant scatter and unusually higher slope for temperature dependence. Therefore, one may speculate that these data are somewhat corrupted, what is probably related to the measurement principle (pycnometer). Furthermore, an impact of the cation structure on density is evidenced in Figure 2. However, it is convinient to discuss this impact based on molar volume (Vm = M/ρ) which is closely related to the sizes of molecular species forming IL. It has turned out that the following ranking in Vm with respect to the cation was observed: [C4C1Pip][CF3SO3] > [C4C1(3)Py][CF3SO3] > [C4C1Pyr][CF3SO3]. The highest values observed for [C4C1Pip][CF3SO3] can be attributed to the geometry of piperidinium cation and its different possible conformations resulting in a lower degree of molecular packing. In turn, [C4C1(3)Py][CF3SO3] is based on planar pyridinium cations. The difference in Vm corresponding to switching from [C4C1(3)Py][CF3SO3] to [C4C1Pip][CF3SO3] is around 11.2 D



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Table 2. Coefficients a and b of Equation 4 Describing Densities of the Pure ILs under Study as a Funtion of Temperature, Their 95% Confidence Interval Half-Widths, 2u(a) and 2u(b), and Root-Mean-Square Error, RMSE IL

a ± 2u(a)

104 [b ± 2u(b)]/K−1

RMSE(ρ)/(kg·m−3)

[C4C1Pyr][CF3SO3] [C4C1Pip][CF3SO3] [C4C1(3)Py][CF3SO3]

7.1324 ± 0.00003 7.1241 ± 0.00003 7.1509 ± 0.00002

−5.8176 ± 0.007 −5.7437 ± 0.006 −6.0001 ± 0.005

0.035 0.009 0.025

Table 3. Coefficients A, B, and T0 of Equation 5 Describing Viscosities of the Pure ILs under Study as a Funtion of Temperature, Their 95% Confidence Interval Half-Widths, 2u(A), 2u(B), and 2u(T0), and Root-Mean-Square Error, RMSE IL

A ± 2u(A)

B ± 2u(B)/K

T0 ± 2u(T0)/K

RMSE(η)/(mPa·s)

[C4C1Pyr][CF3SO3] [C4C1Pip][CF3SO3] [C4C1(3)Py][CF3SO3]

−3.02 ± 0.77 −2.18 ± 0.28 −2.96 ± 1.12

1253.2 ± 256 963.0 ± 85 1106.9 ± 345

143.9 ± 16.9 192.7 ± 6.7 156.0 ± 23.8

0.82 0.05 1.01

Table 4. Coefficients A, B, and C of Equation 2 Describing Binodal Curves of ABS under Study, Their 95% Confidence Interval Half-Widths, 2u(A), 2u(B), and 2u(C), and Root-Mean-Square Error, RMSE, between Calculated and Experimental Weight Fractions of IL IL [C4C1Pyr][CF3SO3]

carbohydrate D-(+)-glucose

a

D-(+)-glucose

b

D-(−)-fructose D-sorbitol

xylitol [C4C1Pip][CF3SO3]

D-(+)-glucose D-(−)-fructose D-sorbitol

xylitol [C4C1(3)Py][CF3SO3]

D-(+)-glucose D-(−)-fructose D-sorbitol

xylitol a

A ± 2u(A) 1.245 1.280 1.334 1.302 1.287 1.416 1.498 1.402 1.572 1.674 1.963 1.692 1.857

± ± ± ± ± ± ± ± ± ± ± ± ±

B ± 2u(B)

0.053 0.020 0.015 0.024 0.063 0.061 0.073 0.093 0.076 0.060 0.110 0.093 0.109

−2.612 −2.771 −2.649 −3.076 −2.507 −3.886 −3.782 −4.219 −4.143 −4.868 −5.181 −5.441 −5.267

± ± ± ± ± ± ± ± ± ± ± ± ±

0.157 0.056 0.040 0.078 0.185 0.194 0.192 0.288 0.200 0.167 0.233 0.261 0.259

C ± 2u(C)

RMSE(w1)

± ± ± ± ± ± ± ± ± ± ± ± ±

0.0120 0.0027 0.0032 0.0065 0.0152 0.0093 0.0054 0.0111 0.0057 0.0056 0.0080 0.0089 0.0075

31.345 34.471 27.436 31.804 33.818 48.832 52.481 42.636 50.698 60.860 23.840 24.185 36.446

3.68 1.43 0.77 2.96 4.94 9.52 8.39 14.41 9.76 13.00 10.82 18.12 17.34

T = 298.15 K. bT = 288.15 K.

Figure 4. Binodal curves (expressed on weight fraction basis) for ABS {IL (1) + D-(+)-glucose (2) + water (3)} at T = 298.15 K and p = 0.1 MPa: circles, [C4C1Pyr][CF3SO3]; squares, [C4C1Pip][CF3SO3]; triangles, [C4C1(3)Py][CF3SO3]; filled circles, [C4C1Im][CF3SO3].4 Solid lines designated by eq 2 with the parameters listed in Table 4.

Figure 5. Binodal curves (expressed on weight fraction basis) for ABS {[C4C1Pyr][CF3SO3] (1) + carbohydrate (2) + water (3)} at T = 298.15 K and p = 0.1 MPa: circles, D-(+)-glucose; squares, D(−)-fructose; triangles, D-sorbitol; diamonds, xylitol. Solid lines designated by eq 2 with the parameters listed in Table 4.

cm−3·mol−1 regardless of temperature. This effect is slightly lower compared to switching from five-member pyrroldinium to six-member piperidinium cation (approximately 13.5 cm−3· mol−1). Viscosity data for all three ILs considered in this work are also consistent with literature values. In the case of [C4C1Pyr]-

[CF3SO3], AARD between our measurements and those published by Seoane et al.,23 Biso et al.,28 and Gaciño et al.21 is at the level of 4.5%. A similar value of AARD was observed for [C4C1(3)Py][CF3SO3], when compared to the values extracted from the study of Bittner et al.27 E



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Both density and viscosity experimental data reduction was performed by using simple equations in order to allow interpolation and extrapolation to temperatures different than those considered in our study. Temperature dependence of density was assumed to obey the following equation, resulting from a simplification that isobaric thermal expansion coefficient (α) is constant. Then ln[ρ /(kg· m−3)] = a + b(T − (298.15 K))

(4)

where a and b are the coefficients fitted to experimental data (then α = −b). In turn, the well-known Vogel−Tamman− Fulcher equation was adopted to correlate the measured viscosities: ln[η /(mPa· s)] = A + Figure 6. Binodal curves (expressed on weight fraction basis) for ternary system {[C4C1Pyr][CF3SO3] (1) + D-(+)-glucose (2) + water (3)} at p = 0.1 MPa: circles, T = 298.15 K; filled circles, T = 288.15 K. Solid lines designated by eq 2 with the parameters listed in Table 4.

B T − T0

(5)

where A, B, and T0 are the VTF equation coefficients adjusted to the experimental data. Accuracy of eqs 4 and 5 was estimated based on root-mean-square error (RMSE) defined as RMSE =

A significant influence of the cation’s structure on viscosity was also observed. In particular, the trends of variation of both molar volume and viscosity correspond, so that one may presume that viscosity is also governed by molecular packing, in particular conformations of cation.

1 N−k

N

∑ (Xicalcd − Xiexptl)2 (6)

i=1

where N stands for the number of data points, k denotes the number of fitted parameters, and X is the modeled property. The resulting values of all of the parameters (along with their confidence intervals) and RMSE are listed in Table 2 and Table

Table 5. Experimental Weight Fraction Compositions of Both Top and Bottom Phases for the Tie-Lines Obtained for the Studied ABS {IL (1) + Carbohydrate (2) + Water (3)} at T = 298. 15 K and p = 0. 1 MPaa feed composition ionic liquid [C4C1Pyr][CF3SO3]

carbohydrate

wT1

wT2

wB1

wB2

βb

TLSc

D-(+)-glucose

0.4404 0.4570 0.4574 0.4725 0.4297 0.4106 0.4241 0.3896 0.4658 0.4769 0.3198 0.3645 0.3761 0.3767 0.3926 0.3367 0.3590 0.3665 0.3811 0.3314 0.3410 0.3422 0.3679 0.3144 0.3276 0.3338 0.3488

0.1477 0.1525 0.1520 0.1565 0.1840 0.1374 0.1451 0.1700 0.1546 0.1575 0.2302 0.1217 0.1356 0.1276 0.1356 0.1139 0.1202 0.1211 0.1261 0.1148 0.1240 0.1389 0.1213 0.1015 0.1074 0.1131 0.1171

0.1861 0.1574 0.2034 0.1642 0.1146 0.2466 0.1858 0.1539 0.1834 0.1285 0.0997 0.2560 0.1696 0.2466 0.1773 0.2670 0.2085 0.2597 0.2034 0.2291 0.1780 0.1810 0.2198 0.2571 0.2189 0.2437 0.2103

0.2620 0.2798 0.2658 0.2899 0.3253 0.2046 0.2400 0.2608 0.2679 0.3015 0.3224 0.1549 0.1994 0.1686 0.2021 0.1354 0.1650 0.1528 0.1791 0.1400 0.1641 0.1855 0.1623 0.1153 0.1334 0.1356 0.1516

0.6049 0.7062 0.6090 0.6985 0.7216 0.5844 0.6750 0.7464 0.7194 0.7439 0.7700 0.6120 0.7156 0.6007 0.7032 0.5447 0.6524 0.5876 0.6561 0.6396 0.7253 0.6774 0.6358 0.5723 0.6449 0.6076 0.6683

0.0737 0.0466 0.0841 0.0587 0.0531 0.0662 0.0452 0.0325 0.0528 0.0471 0.0415 0.0461 0.0307 0.0571 0.0396 0.0497 0.0328 0.0554 0.0440 0.0388 0.0294 0.0420 0.0471 0.0396 0.0314 0.0447 0.0375

0.3929 0.4541 0.3737 0.4230 0.4809 0.5144 0.5129 0.6022 0.4731 0.4339 0.6717 0.6952 0.6218 0.6325 0.5906 0.7490 0.6610 0.6744 0.6075 0.7509 0.7022 0.6752 0.6440 0.8183 0.7449 0.7524 0.6976

2.225 2.354 2.232 2.310 2.231 2.441 2.511 2.596 2.492 2.419 2.386 3.273 3.238 3.177 3.236 3.238 3.355 3.367 3.351 4.055 4.063 3.460 3.612 4.166 4.174 4.003 4.013

D-sorbitol

xylitol

D-(+)-glucose

D-(−)-fructose

D-sorbitol

xylitol [C4C1(3)Py][CF3SO3]

bottom phase

wF2

D-(−)-fructose

[C4C1Pip][CF3SO3]

top phase

wF1

D-(+)-glucose

D-(−)-fructose

D-sorbitol

xylitol

a

Standard uncertainties of the measured properties are u(wFi ) = 0.005, u(T) = 0.05 K, and u(p) = 10 kPa. bRatio between the mass of the top phase and the total mass of the ABS; see eqs 3. cTie-line slope; see eq 7. F



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Figure 5 demonstrates the respective data for systems with fixed IL, namely, [C4C1Pyr][CF3SO3]. Additionally, the data reported by Freire et al.4 for [C4C1Im][CF3SO3] were presented in Figure 4 to complement analysis and discussion of the cation’s impact. As can be seen, the data have been measured in the mass fraction of carbohydrate varying between about 0.05 and 0.2−0.3, depending on a particular IL. Tendency of ABS formation for [CF3SO3]-based ILs decreases as follows: [C4C1(3)Py][CF3SO3] > [C4C1Pip][CF3SO3] > [C4C1Im][CF3SO3] > [C4C1Pyr][CF3SO3]. It is noteworthy that this trend is the same regardless of the kind of carbohydrate. Therefore, it is evidenced that six-member ringbased cations are more effective in promoting formation of ABS than five-member ring cations. The higher number of carbon atoms forming cyclic cations in piperidinium and pyridinium ILs lowers hydrophilicities of ILs (weaker IL−water, thus stronger carbohydrate−water interactions). Besides, ILs based on aromatic cations form ABS much easier compared to ILs having in their structure cations based on aliphatic rings. In fact, aromatic structure of the cation makes it possible to enhance an overall basicity of the IL. This is due to an extra possibility of acting in H-bond formation between IL and carbohydrate via delocalized π electrons. Figure 5 presents an influence of carbohydrate structure on ABS binodal curve in systems with [C4C1Pyr][CF3SO3]. As can be noticed the following trend of decreasing salting-out capacity of carbohydrates was observed (regardless of IL): D-sorbitol > D-(+)-glucose ≈ xylitol > D-(−)-fructose. This order is in agreement with the data published for [C4C1Im][CF3SO3].4 It can be explained in terms of the number of OH groups of carbohydrates. Higher number of possible H-bonds with water turns these compounds into stronger salting-out agents. In the case of sugars, not only a number of OH groups but also the molecule’s conformation plays an important role as it determines “accessibility” of the OH groups for intermolecular interactions defined, for example, by the number of axial/ equatorial OH groups. In particular, it has been established that D-(+)-glucose has more favorable conformation for H-bonding with water compared to D-(−)-fructose.29 Finally, an effect of temperature on the range of biphasic area was checked. The results of measurements performed for system {[C4C1Pyr][CF3SO3] + D-(+)-glucose + water} are presented in Figure 6. A full list of data points measured at T = 288.15 K and T = 298.15 K is given in Table S6 in the Supporting Information. As can be easily noticed from Figure 6, a decrease in temperature by 10 K makes the formation of ABS slightly more favorable. Wu et al.14 observed exactly the same for the {[C4C1Im][BF4] + sucrose + water} system. The authors attempted to explain this phenomenon in terms of the decyclization process of carbohydrate induced by the IL’s cation. This process is based on binding of cations by sugar’s molecules and then breaking its cyclic isomers into linear ones. This is favorable to form ABS, since decyclization deteriorates compatibility between the OH groups on cyclic molecules and the three-dimensional H-bonded structure of water. An increase in temperature inhibits the decyclization process and thus shifts an equilibrium state toward more stable cyclic isomers. Considering ABS in the state of liquid−liquid phase equilibrium, one needs to define which phase is rich in which component. In our study, due to a relatively high density of ILs, the top phase was the carbohydrate-rich phase, whereas the bottom phase was the IL-rich phase. Table 5 shows all of the experimental data collected during tie-lines determinations.

Figure 7. Tie-lines data for selected ABS {[C4C1Pyr][CF3SO3] (1) + xylitol (2) + water (3)} (a) and {[C4C1Pyr][CF3SO3] (1) + D(−)-fructose (2) + water (3)} (b): circles, binodal curve data; filled circles, tie-lines compositions; squares, overall (feed) compositions. Solid line designated by eq 2 with the parameters listed in Table 4.

3. As seen, the proposed equations are suitable for representing the experimental data with an excellent accuracy. Aqueous Biphasic Systems. In the framework of current study, we decided to check an impact of the cation’s structure of [CF3SO3]-based ILs on ABS behavior. It should be stressed that we also attempted to perform a more comprehensive study by checking some other hydrophilic ILs, namely, 1-ethyl-3methylimidazolium tricyanomethanide, 1-butyl-3-methylimidazolium tricyanomethanide, 1-ethyl-3-methylimidazolium trifluoroacetate, 1-butyl-3-methylimidazolium trifluoroacetate, 1butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium tetracyanoborate, 1-ethyl-3-methylimidazolium ethylsulfate, and tributylethylphosphonium diethylphosphate. Unfortunately, none of these ILs formed ABS with the carbohydrates studied. This confirmed that [CF3SO3] anion is very interesting and worth being investigated and hence eventually convinced us to perform a study of the cation’s impact. All of the experimental data points are listed in Tables S3−S5 in the Supporting Information. The data were correlated by using the simple expression proposed by Merchuk et al.;19 see eq 2. The equation’s coefficients along with the values of RMSE for each ABS are listed in Table 4. As can be easily seen, the proposed correlations are capable of reproducing the experimental data within RMSE at the level below 0.01 of weight fraction. Figure 4 shows the binodal curves for ternary systems {IL + D-(+)-glucose + water} for all three ILs under study, whereas G



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Systems: A Boost Brought about by Using Ionic Liquids. Chem. Soc. Rev. 2012, 41, 4966−4995. (4) Freire, M. G.; Louros, C. L. S.; Rebelo, L. P. N.; Coutinho, J. A. P. Aqueous Biphasic Systems Composed of a Water-stable Ionic Liquid + Carbohydrates and Their Applications. Green Chem. 2011, 13, 1536− 1545. (5) Zhang, Y.; Zhang, S.; Chen, Y.; Zhang, J. Aqueous Biphasic Systems Composed of Ionic Liquid and Fructose. Fluid Phase Equilib. 2007, 257, 173−176. (6) Liu, Q.; Janssen, M. H. A.; van Rantwijk, F.; Sheldon, R. A. Room-Temperature Ionic Liquids That Dissolve Carbohydrates in High Concentrations. Green Chem. 2005, 7, 39−42. (7) Rosatella, A. A.; Branco, L. C.; Afonso, C. A. M. Studies on Dissolution of Carbohydrates in Ionic Liquids and Extraction from Aqueous Phase. Green Chem. 2009, 11, 1406−1413. (8) Zakrzewska, M. E.; Bogel-Łukasik, E.; Bogel-Łukasik, R. Solubility of Carbohydrates in Ionic Liquids. Energy Fuels 2010, 24, 737−745. (9) Paduszyński, K.; Okuniewski, M.; Domańska, U. Renewable Feedstocks in Green Solvents: Thermodynamic Study on Phase Diagrams of d-Sorbitol and Xylitol with Dicyanamide Based Ionic Liquids. J. Phys. Chem. B 2013, 117, 7034−7046. (10) Paduszyński, K.; Okuniewski, M.; Domańska, U. Sweet-in-Green Systems Based on Sugars and Ionic Liquids: New Solubility Data and Thermodynamic Analysis. Ind. Eng. Chem. Res. 2013, 52, 18482− 18491. (11) Paduszyński, K.; Okuniewski, M.; Domańska, U. Solid-Liquid Phase Equilibria in Binary Mixtures of Functionalized Ionic Liquids with Sugar Alcohols: New Experimental Data and Modelling. Fluid Phase Equilib. 2015, 403, 167−175. (12) Paduszyński, K.; Okuniewski, M.; Domańska, U. An Effect of Cation Functionalization on Thermophysical Properties of Ionic Liquids and Solubility of Glucose in Them - Measurements and PCSAFT Calculations. J. Chem. Thermodyn. 2016, 92, 81−90. (13) Wu, B.; Zhang, Y. M.; Wang, H. P. Aqueous Biphasic Systems of Hydrophilic Ionic Liquids + Sucrose for Separation. J. Chem. Eng. Data 2008, 53, 983−985. (14) Wu, B.; Zhang, Y.; Wang, H. Phase Behavior for Ternary Systems Composed of Ionic Liquid + Saccharides + Water. J. Phys. Chem. B 2008, 112, 6426−6429. (15) Wu, B.; Zhang, Y.; Wang, H.; Yang, L. Temperature Dependence of Phase Behavior for Ternary Systems Composed of Ionic Liquid + Sucrose + Water. J. Phys. Chem. B 2008, 112, 13163− 13165. (16) Chen, Y.; Meng, Y.; Zhang, S.; Zhang, Y.; Liu, X.; Yang, J. Liquid-Liquid Equilibria of Aqueous Biphasic Systems Composed of 1Butyl-3-methyl Imidazolium Tetrafluoroborate + Sucrose/Maltose + Water. J. Chem. Eng. Data 2010, 55, 3612−3616. (17) Chen, Y.; Meng, Y.; Yang, J.; Li, H.; Liu, X. Phenol Distribution Behavior in Aqueous Biphasic Systems Composed of Ionic LiquidsCarbohydrate-Water. J. Chem. Eng. Data 2012, 57, 1910−1914. (18) Chirico, R. D.; Frenkel, M.; Magee, J. W.; Diky, V.; Muzny, C. D.; Kazakov, A. F.; Kroenlein, K.; Abdulagatov, I.; Hardin, G. R.; Acree, W. E., Jr.; et al. Improvement of Quality in Publication of Experimental Thermophysical Property Data: Challenges, Assessment Tools, Global Implementation, and Online Support. J. Chem. Eng. Data 2013, 58, 2699−2716. (19) Merchuk, J. C.; Andrews, B. A.; Asenjo, J. A. Aqueous TwoPhase Systems for Protein Separation. J. Chromatogr., Biomed. Appl. 1998, 711, 285−293. (20) Vercher, E.; Miguel, P. J.; Llopis, F. J.; Orchillés, A. V.; MartínezAndreu, A. Volumetric and Acoustic Properties of Aqueous Solutions of Trifluoromethanesulfonate-Based Ionic Liquids at Several Temperatures. J. Chem. Eng. Data 2012, 57, 1953−1963. (21) Gaciño, F. M.; Regueira, T.; Lugo, L.; Comuñas, M. J. P.; Fernández, J. Influence of Molecular Structure on Densities and Viscosities of Several Ionic Liquids. J. Chem. Eng. Data 2011, 56, 4984−4999. (22) Vercher, E.; Llopis, F. J.; González-Alfaro, V.; Miguel, P. J.; Martínez-Andreu, A. Refractive Indices and Deviations in Refractive

These data can be found very useful when considering the proposed ABS as separations media. As seen, two or three tie lines for each ABS were determined, whereas exemplary tie lines for two representative systems are depicted in Figure 7. As seen from Figure 7a, the tie lines for some systems have an approximately equal slope, defined as TLS =

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w1B − w1T w2B − w2T

(7)

CONCLUDING REMARKS An impact of the cation’s structure on several basic physical properties of pure ILs based on [CF3SO3] anion was established experimentally and discussed. Density and viscosity are strongly affected by an appropriate selection of the cation core, what is closely related to its geometry and the number conformational degrees of freedom. The studied ILs have been shown to form ABS when added to carbohydrates aqueous solutions. It was established that capacity of formation of ABS depends on the number of members of the cation’s rings. Besides this the salting-out effect was stronger for the ILs based on aromatic cations, when compared to their aliphatic counterparts. We believe that not only will this study provide a significant amount of useful experimental material for novel separation processes based on ABS but also that it will help in understanding more deeply how the molecular interactions and structures govern the phase behavior in such complex Hbonded systems studied.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00931. Figure S1 showing the results of the validation of the experimental method used to determine binodal curves for ABS, Table S1 showing detailed results of density measurements, Table S2 showing detailed results of viscosity measurements, and Tables S3−S6 showing detailed results of binodal curves determinations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +48 22 234 5640. Funding

Funding for this research was provided by the National Science Centre in years 2012−2015 (Grant 2011/03/N/ST5/04781). Notes

The authors declare no competing financial interest.

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REFERENCES

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Effect of Cation Structure in Trifluoromethanesulfonate-Based Ionic

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