Effect of Salt Type and Alkyl Chain Length on the Binodal Curve of an

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

pubs.acs.org/jced

Effect of Salt Type and Alkyl Chain Length on the Binodal Curve of an Aqueous Two-Phase System Composed of Imidazolium Ionic Liquids Xueyan Wu,† Yang Liu,*,† Yongjie Zhao,‡ and Kit-Leong Cheong† †

Guangdong Provincial Key Laboratory of Marine Biotechnology, STU-UNIVPM Joint Algal Research Center, Department of Biology, College of Science, Shantou University, Shantou, Guangdong 515063, PR China ‡ Department of Mechanical Engineering, College of Engineering, Shantou University, Shantou, Guangdong 515063, P.R. China J. Chem. Eng. Data Downloaded from pubs.acs.org by WASHINGTON UNIV on 08/08/18. For personal use only.

S Supporting Information *

ABSTRACT: The binodal data for an aqueous two-phase system (ATPS) containing 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim]BF4, n = 2, 3, 4) or 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl], n = 2, 4, 6, 8, 10, 12) and inorganic salts (K2HPO4, NaH2PO4, (NH4)2SO4, and NaCl) or organic salts (sodium citrate, sodium tartrate, and sodium acetate) were widely determined at T = 298.15 K. The various binodal curves could be fitted and summarized as two kinds of empirical nonlinear equations with the lowest standard deviation (SD). The effects of the salt type and cation alkyl chain length of the ionic liquid (IL) on ATPS binodal curves were further discussed. It was displayed that the salting-out ability of different salts was a major factor in these ATPS formations. For [Cnmim]BF4-based ATPS, the salting-out ability of various salts decreased in the following order: (NH4)2SO4 > Na3C6H5O7 > Na2C4H4O6 > NaH2PO4 > NaC2H3O2 > NaCl. For the [Cnmim]Cl aqueous solution, only K2HPO4 or (NH4)2SO4 could cause the phase separation and formed ATPS, and the salting-out ability of K2HPO4 was stronger than the effect of (NH4)2SO4. The phase separation abilities of the researched [Cnmim]BF4 in the corresponding ATPS increased with the increase of cation alkyl chain length, while [C8mim]Cl had the best phase separation ability of the researched [Cnmim]Clbased ATPS.

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INTRODUCTION In the recent years, a novel kind of green solvent−ionic liquid (IL) has attracted more and more attention due to its unique properties, such as negligible volatility, inflammability, excellent solvation of organic and inorganic compounds, and relative ease of structure modification to elicit the desired physical properties.1 These unique characteristics have made ILs an environmentally friendly and benign replacement for traditional volatile organic solvents. An ionic liquid-based aqueous two-phase system (ILATPS) has been investigated as a novel aqueous two-phase system (ATPS) in recent years.2,3 There are many advantages to the combined IL and ATPS, such as low viscosity, little emulsion formation, no need to use volatile organic solvent, quick phase separation, high extraction efficiency, and gentle biocompatible environment. ILATPSs have been successfully used to separate drugs,4,5 polysaccharides,6,7 proteins,8−11 enzymes12−17 and antibiotics,18 and food colorants.19 Precise liquid−liquid equilibrium (LLE) data is essential for a proper understanding of extraction processes. Reliable LLE data is beneficial to the design and process optimization of the ILATPS extraction technique. The analysis of the two phases’ composition in equilibrium supplies considerable information © XXXX American Chemical Society

for mass balance and mass transfer calculations in the design and optimization of separation processes. Various kinds of ILATPSs have been researched on LLE and application, which is described in the following sections. Several ILATPSs composed of IL and salt have been developed and correlated, including [Cnmim]X (X = Br and Cl) + salt (KOH, K2HPO4, K2CO3, K3PO4, K3C6H5O7, and sucrose) + H2O ATPSs,8,20−23 [Cnmim][CH3COO] (n = 4, 6, 8) + inorganic salts (K3PO4, K2HPO4, and K2CO3) + H2O ATPSs,24 [Cnmim][NO3] (n = 4, 6, 8) + Na2CO3 + H2O ATPSs,25 [Cnmim]BF4 (n = 2, 3, 4) + salts ((NH4)2SO4, Na3C6H5O7, Na2C4H4O6, NaC2H3O2) + H2O ATPSs,26−28 [Cnmim]DCA (n = 2, 3, 4, 6)/C4Mpyr DCA + organic salts (Na3C6H5O7, K3C6H5O7) + H2O ATPSs,29 as well as [C 4py]BF4 + salts (Na3C6H5O 7, (NH4)3C6H5O7, Na3C6H5O7, Na2C4H4O4, CH3COONa, Na2HPO4, Na2SO4, (NH4)2SO4, NaCl) + H2O ATPSs.30,31 Some of their extraction capacities were evaluated through their application on the extraction model protein. Phase diagrams of ILATPSs formed by IL and surfactant have been Received: March 9, 2018 Accepted: July 25, 2018

A

DOI: 10.1021/acs.jced.8b00188 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Interaction between ILs and Organic Salts or Inorganic Salts at T = 298.15 K and P = 0.1 MPaa organic salts

inorganic salts

ILb

Na3C6H5O7

Na2C4H4O6

NaC2H3O2

K2HPO4

NaH2PO4

(NH4)2SO4

NaCl

[C2mim]BF4 (L) [C3mim]BF4 (L) [C4mim]BF4 (L) [C2mim]Cl (S) [C4mim]Cl (S) [C6mim]Cl (L) [C8mim]Cl (L) [C10mim]Cl (S) [C12mim]Cl (S)

+c + + − − − − − −

+ + + − − − − − −

+ + + − − − − − −

Pc P P + + + + + P

+ + + − − − − − −

+ + + P P P + − −

−c − + − − − − − −

a

Standard uncertainties u are u(T) = 0.01 K and u(p) = 1 kPa. b(L) denotes IL is liquid state. (S) denotes IL is solid state. c(+) denotes ATPS can be formed by IL and salt. (−) denotes ATPS cannot be formed by IL and salt. (P) denotes IL and salt generate precipitation.

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reported in many literature works.32−34 Wu and his co-workers synthesized a kind of tetra-alkyl-ammonium glycine IL and utilized these glycine ILs and K2HPO4 to form an ILATPS. They further applied these glycine ILATPSs to extract dinitrophenylated (DNP) amino acids and cytochrome-c.35 IL was also used as a polymer assistant for the traditional ATPS to extract penicillin and probe dyes.36,37 Yu and his partner developed an ATPS consisting of [C4mim]BF4, SDS, and (NH4)2SO4 to extract Sudan I−IV.38 Taking into account the vast amount of literature reviewed, imidazolium-based ILATPSs have demonstrated a variety of favorable applications in biomolecule separation and purification. They are widely used for extraction of proteins,10−13,15,16 polysaccharides,6,7 antibodies,18 drugs,19,38 and enzymes.12−14 In general, the ATPS binodal curve with a larger two-phase area has better extraction ability potential because it can dissolve more solvent. Many literatures have revealed that some factors, such as salt type, alkyl chain length, and temperature, have a great impact on imidazolium-based ILATPS phase formation. However, most studies20,24−26,28,39 have usually focused on just one or another kind of imidazolium IL. Despite the broad researches on the calculation of binodal curve data of these ILATPSs, a comprehensive and in-depth investigation about imidazolium-based ILATPS is still at the infancy stage and so is their further actual application. Systematic binodal curve studies of imidazolium-based ILATPS composed of hydrophobic or hydrophilic ILs with different alkyl chain lengths and various inorganic or organic salts are rarely reported in the literature, while such data are very useful for the design of an extraction process, including forward and backward extraction using imidazolium-based ILATPS. Complete understanding of the solvent system is indispensable for the extraction process. In order to create this part of solvent data, in this article, the binodal curve data of imidazolium-based ILATPS containing the selected hydrophobic imidazolium ILs ([Cnmim]BF4 (n = 2, 3, 4) or hydrophilic imidazolium ILs [Cnmim]Cl (n = 2, 4, 6, 8, 10, 12) + inorganic salts (K2HPO4, NaH2PO4, (NH4)2SO4, NaCl) or organic salts (sodium citrate, sodium tartrate, sodium acetate) were systematically reported on, respectively. The binodal curves were experimentally determined, correlated, and summarized as two kinds of empirical equations. Besides, the salting-out ability of various salts and the phase-separation abilities of the investigated ILs were further discussed.

EXPERIMENTAL SECTION

Chemicals. Both [Cnmim]Cl (1-alkyl-3-methylimidazolium chloride, n = 2, 4, 6, 8, 10, 12) and [Cnmim]BF4 (1-alkyl-3methylimidazolium tetrafluoroborate, n = 2, 3, 4) were purchased from ChengJie Chemical Co., Ltd. (Shanghai, China) with a purity >99% and were used without further purification. K 2 HPO 4 , NaH 2 PO 4 , (NH 4 ) 2 SO 4 , NaCl, Na3C6H5O7 (sodium citrate), Na2C4H4O6 (sodium tartrate), and NaC2H3O2 (sodium acetate) were of analytical grade and purchased from Xilong Chemical Co., Ltd. (Guangdong, China). All other chemicals were of analytical grade and were provided from local sources. All of the data concerning chemical purities and provenance are shown in Table S1. Aqueous solutions were prepared with deionized and doubly distilled water. Experimental Procedure. The binodal data of [Cnmim]Cl-inorganic salt, [Cnmim]Cl-organic salt, [Cnmim]BF4inorganic salt, and [Cnmim]BF4-organic salt ATPSs with different alkyl chain lengths were measured by the cloud point titration method. The titrations were carried out in a DC-0506 constant low-temperature bath (Shanghai, China), and the experimental setting temperature was 298.15 K. Stock IL solutions of 50% (w/w) for the [Cnmim]Cl solution or 80% (w/w) for the [Cnmim]BF4 solution were prepared in advance and were carefully added dropwise into different salt solutions, including the 50% (w/w) K2HPO4, 50% (w/w) NaH2PO4, 40% (w/w) (NH4)2SO4, 20% (w/w) NaCl, 30% (w/w) Na3 C6 H5O 7, 25% (w/w) Na2C4H4O 6, or 40% (w/w) NaC2H3O2 solution, until the mixed solution became turbid (reached cloud point). The cloudy solution was weighed repeatedly by a BSM-120.4 analytical balance (Shanghai, China). The composition weight of this mixture was noted. Afterward, water was added dropwise to the mixed solution to get a clear one-phase solution, and the procedure was repeated to collect sufficient data for the construction of the ILATPS phase diagram. The composition concentration of the mixture could be calculated for each point on a binodal curve in various ILATPSs. On the basis of the standard deviation (SD) minimization between the experimental data and predicted data, the experimental titration data were fitted with various empirical equations of the binodal curve using ATPS-LLE software developed by our laboratory21,40,41 for the best-fitting result. B



Article

Table 2. Coefficients of Different Equations for Correlated Binodal Data of Investigated ATPS at T = 298.15 K and P = 0.1 MPaa coefficients ATPS

empirical equations

[C2mim]BF4-Na3C6H5O7 [C3mim]BF4-Na3C6H5O7 [C4mim]BF4-Na3C6H5O7 [C2mim]BF4-Na2C4H4O6 [C3mim]BF4-Na2C4H4O6 [C4mim]BF4-Na2C4H4O6 [C2mim]BF4-NaC2H3O2 [C3mim]BF4-NaC2H3O2 [C4mim]BF4-NaC2H3O2 [C2mim]BF4-NaH2PO4 [C3mim]BF4-NaH2PO4 [C4mim]BF4-NaH2PO4 [C2mim]BF4-(NH4)2SO4 [C3mim]BF4-(NH4)2SO4 [C4mim]BF4-(NH4)2SO4 [C4mim]BF4-NaCl [C2mim]Cl-K2HPO4 [C4mim]Cl-K2HPO4 [C6mim]Cl-K2HPO4 [C8mim]Cl-K2HPO4 [C10mim]Cl-K2HPO4

① ① ① ① ① ① ① ① ① ① ① ① ① ① ① ① ① ① ① ② ②

b

a

b

c

d

e

SDc

0.4084 0.2791 0.1297 0.1778 0.4702 0.1297 2.3342 0.2804 0.8719 0.5475 0.3037 0.2938 0.5147 0.2976 0.3281 0.1908 0.7067 0.1009 0.1916 −0.3226 0.9200

0.0493 0.2196 0.2774 0.3043 0.048 0.2774 0.0441 0.3744 0.0495 0.0498 0.6146 0.1222 1.5193 0.0503 0.0745 0.3569 0.7091 0.059 0.0274 2.0133 0.9085

0.3180 39.696 0.4216 0.1778 0.2286 0.4216 0.4506 0.8003 0.1233 0.5044 0.4313 0.3603 0.3073 0.1999 0.0734 6.2964 0.1396 0.5104 0.5977 −1.7937 −1.8111

0.4732 0.0156 0.0477 0.3043 0.6353 0.0477 0.5586 0.0508 0.2389 0.6364 0.0595 0.0334 0.0681 0.3764 1.2161 0.0391 0.0641 0.3255 0.2661 0.4483 0.5936

−0.0640 0.0021 −0.0032 −0.0235 −0.0700 −0.0032 −0.0874 −0.0134 0.0086 −0.1683 −0.0884 0.0159 −0.3307 −0.0245 −0.0367 0.00002 −0.3174 −0.0917 −0.0821

0.0021 0.0016 0.0014 0.0014 0.0015 0.0014 0.0014 0.0030 0.0033 0.0010 0.0003 0.0017 0.0004 0.0022 0.0013 0.0005 0.0007 0.0030 0.0019 0.0018 0.0009

(

Standard uncertainties u are u(T) = 0.01 K and u(p) = 1 kPa. b①: w1 = aexp −

a

[Σi n= 1(w1,cal

calculated by SD = fraction of salt, respectively.

W2 b

) + cexp(− Wd ) + e ; ②: w 2

1

= aw23 + bw22 + cw2 + d cSD was

− w1,exp) /n] , where n is the number of binodal data, w1,cal and w1,exp are the calculated and experimental mass 2

0.5

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RESULTS AND DISCUSSION Phase Separation Ability of ILATPS. The investigated imidazolium IL molecules with different anionic types and cation alkyl chain lengths would cause some IL and salt solutions to trigger phase separation based on the salting-out ability. As shown in Table 1, ATPSs could be formed by adding appropriate amounts of Na3C6H5O7, Na2C4H4O6, NaH2PO4, NaC2H3O2, and (NH4)2SO4 into [Cnmim]BF4; NaCl into [C4mim]BF4; K2HPO4 into [Cnmim]Cl; or adding (NH4)2SO4 into [C8mim]Cl solution, while other IL/salt combinations did not cause a phase separation. This indicated that, using the hydrophobic imidazolium IL [Cnmim]BF4, it was easier to form ATPSs than when the hydrophilic imidazolium IL [Cnmim]Cl was used. There were quite a few factors affecting the phase separation. Rogers et al.42 suggested that this was a solvophobic phenomenon. The kosmotropic ions (e.g., HPO42−, SO42−, OH−, CO32−, and PO43−)42−44 were beneficial to ATPS formation with [Cnmim] Cl because the interaction between these anions and water molecules was stronger than between the water molecules. Thus, in these investigated salts, K2HPO4 could cause the phase separation of the hydrophilic imidazolium IL [Cnmim]Cl, whereas when hydrophobic imidazolium IL [Cnmim]BF4 solution met K2HPO4, it may cause precipitation. However, as cation alkyl chain length increased, its hydrophobicity increased as well. Therefore, [C12mim]Cl solution also appeared to precipitate when K2HPO4 was added. Chaotropic ions (e.g., NH4+, K+, and H2PO4−)43,44 exhibit weaker interactions with water, so they were unable to form ATPSs with the hydrophilic imidazolium IL [Cnmim]Cl. Hydrophobic imidazolium IL [Cnmim]BF4 were easily salted-out by salts, so the ATPS could be formed by [Cnmim]BF4 using almost any

of the researched salts. Obviously, hydrophobicity of the IL was also an important factor. In addition, the concentration of inorganic salts was a significant factor affecting the two-phase formation. The solubility of NaCl (about 35 wt %) was lower than that of any other investigated salt (generally more than 50 wt %); therefore, only the addition of [C4mim]BF4 with strong hydrophobicity to the NaCl solution caused a two-phase separation. Phase Diagrams. For the ATPSs composed of an imidazolium IL and salt, the binodal data composition determined by the cloud-point measurement was correlated with different binodal empirical equations by ATPS-LLE software.21,40,41 To obtain the best-fitting result, the best-fitting empirical equation of the binodal curve with the lowest SD value was expressed as follows: i Wy i Wy w1 = aexpjjj− 2 zzz + c expjjj− 2 zzz + e k b { k d {

(1)

w1 = aw2 3 + bw2 2 + cw2 + d

(2)

where w1 and w2 were the mass fractions of composition 1 and composition 2, respectively. a, b, c, d, and e were fitting parameters. The fitting parameters obtained from the correlation of experimental binodal data along with the corresponding SD of eqs 1 and 2 are given in Table 2. There is 1-order of magnitude in these SD values, including the minimum of 0.0004 and the maximum of 0.0033, which is generally lower than the corresponding SD values of ILATPS in other reports.35,36,45 On the basis of the obtained standard deviations (all SD < 0.005), we concluded that these empirical equations could be well used to correlate the binodal curves of the investigated systems. Figures 1 and 2 show the phase C



Article

Figure 1. Effect of [Cnmim]BF4 with different carbon chain lengths on the phase formation of various imidazolium IL-organic salt ATPSs. The different symbols (Δ, ⧫, ○) denote the experimental binodal data, and the solid curve denotes the correlated binodal curve.

Figure 2. Effect of [Cnmim]BF4 with different carbon chain lengths on the phase formation of various imidazolium IL-inorganic salt ATPSs. The different symbols (▲, ●, □, ◊, Δ, ⧫, ○) denote the experimental binodal data, and the solid curve denotes the correlated binodal curve.

The binonal data of these ATPSs were compared with other reported data,20,26,28,46−50 which showed that these data of the same ATPS were very similar with very small deviations. This convinced the veracity of our research. The deviations were reasonable and in reasonable portion because the cloud point was not an exact point. Different researchers had their own estimation of it.

diagrams, including the experimental binodal data and the best fit binodal curves of IL-inorganic salt and IL-organic salt ATPSs, and the specific binodal data are displayed in Tables S2−S8 of the Supporting Information. The area above the binodal curve indicates the separated two-phase area, while the area below represents the homogeneous system. It is visible that the separated two-phase region was enlarged by an increase in the cation alkyl chain length of [Cnmim]BF4. D



Article

Figure 3. Total binodal curves of IL-salt ATPSs including the experimental data and best-fitting curve data at 298.15 K. The different symbols (▲, Δ, ■, □, ⧫, ◊, ●, ○, ×) denote the experimental binodal data, and the solid curve denotes the correlated binodal curve.

Phase Separation Ability of the Different ILs. The twophase area, which was salted out by the same salt, varies in different cation alkyl chain lengths of one kind of IL. As shown in Figures 1 and 2, the two-phase area expended with the cation alkyl chain length of hydrophobic imidazolium ILs, which indicates that the phase separation ability increases with the cation alkyl chain length. The phase-separation ability of investigated [Cnmim]BF4 was in the order of [C4mim]BF4 > [C3mim]BF4 > [C2mim]BF4, which presents the same result as the other researchers’ studies.27,28 It was common knowledge that an increase in cation alkyl chain length gave rise to an increase of the IL’s hydrophobic nature and thus, to an adverse affinity for water.28 Generally speaking, an IL with an unfavorable water affinity needs less salt to form a two-phase system. So, the binodal curve is closer to the origin point, and there is a larger two-phase region. The binodal curve obtained at 298.15 K for the studied [Cnmim]Cl (n = 2, 4, 6, 8, 10) + K2HPO4 + H2O ATPSs is given in Figure 2. It can be seen that the phase-separation abilities of the investigated [Cnmim]Cl are in the order of [C8mim]Cl > [C4mim]Cl > [C6mim]Cl > [C2mim]Cl > [C10mim]Cl. [C8mim]Cl has the best phase-separation ability. The too long or too short cation alkyl chain of [Cnmim]Cl is not beneficial to ATPS formation. When (NH4)2SO4 is added into the [Cnmim]Cl solvent, only [C8mim]Cl/salt ATPS was formed. This also could certify that [C8mim]Cl had the best phase-separation ability. These results confirm the findings of other researchers. Pei and his co-workers20 showed that in the presence of K2HPO4, the phase-forming ability of [Cnmim]Cl is in the order of [C6mim]Cl > [C4mim]Cl. The study by Cao17 stated that as the cation alkyl chain length of the [Cnmim]Cl increased from [C2mim]Cl to [C8mim]Cl, the two phase regions expanded. As the cation alkyl chain length of IL continued to increase, the two phase areas became smaller according to Najdanovic-Visak’s work.51 The reason for this

was not fully understood. It can be seen from Figures 1 and 2, in comparison with [Cnmim]Cl, the cation alkyl chain length had a greater influence on [Cnmim]BF4. Salting-Out Ability of Salts. The effect of the salts on the binodal curves is illustrated in Figure 3. For a specific IL, the leftward shift in the position of the binodal curve reflected a stronger salting-out ability of the salt, which meant less salt was required for the formation of ATPS. According to Figure 3, as for [Cnmim]BF4, the binodal curves became closer to the origin for the salts from (NH4)2SO4, Na3C6H5O7, Na2C4H4O6, NaH2PO4, NaC2H3O2 to NaCl, which indicated that the ATPS formation required lower concentrations of (NH4)2SO4 than NaCl. Therefore, the salting-out ability of these salts were in the following order: (NH4)2SO4 > Na3C6H5O7 > Na2C4H4O6 > NaH2PO4 > NaC2H3O2 > NaCl. For [C8mim]Cl-based ATPS, the salting-out ability followed the order of K2HPO4 > (NH4)2SO4, which is not identical in order to the other ILATPSs.52,53 The salting-out ability of the salts was related to quite a few factors. First, as stated in the Phase Separation Ability of ILATPS section, the difference in the kosmotropicity of the salts used was an influential element due to their saltingout ability.52 K2HPO4 was a very strong kosmotropic salt, and when it met the hydrophobic IL [Cnmim]BF4, precipitation occurred. When it came to the aqueous solutions of hydrophilic ILs, the competition for water between the two compositions (K2HPO4 and [Cnmim]Cl) only resulted in phase separation because the hydrophilic IL would not precipitate. If the IL binds strongly to water, the IL-rich phase in the ATPS can have a relatively large amount of water, which means that the salt-rich phase can become saturated in salt (depleted of water) and precipitation can occur.51 This phenomenon appeared in [Cnmim]Cl/(NH4)2SO4 ATPS. (NH4)2SO4 was another kosmotropic salt but weaker than K2HPO4, so it failed to create precipitation in the [Cnmim]BF4 solution, and it can only form ATPS with [C8mim]Cl among E



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the other five [Cnmim]Cls. However, [C2mim]Cl and [C4mim]Cl were bound more strongly with water than other [Cnmim]Cl, so in these two systems (NH4)2SO4 were salted out. Besides, the salting-out ability could also be related to the Gibbs free energy of hydration of the constituent ions. When the salts share the same cation, ions have large, negative Gibbs free energy values due to the formation of the structured water around them; and therefore, they are better as salting-out agents.20,26,27,30 In addition, from Figure 3, it can be seen that anions with a higher valence were better salting-out agents than those with a lower valence. This is because they could hydrate more water molecules than the lower valence anion, thus decreasing the amount of water available to hydrate the ILs. This was also confirmed by other researchers.26

CONCLUSION In this work, we determined LLE data for selected hydrophobic imidazolium IL [Cnmim]BF4/hydrophilic imidazolium IL [Cnmim]Cl + organic salts (Na3C6H5O7/Na2C4H4O6/ NaC 2 H 3 O 2 )/inorganic salts ((NH 4 ) 2 SO 4 /K 2 HPO 4 / NaH2PO4/NaCl) + H2O ATPSs at T = 298.15 K. It was demonstrated that ATPS can be formed by a hydrophobic imidazolium IL [Cnmim]BF4 and most investigated salts (Na 3 C 6 H 5 O 7 , Na 2 C 4 H 4 O 6 , NaH 2 PO 4 , NaC 2 H 3 O 2 , (NH4)2SO4, and NaCl), and the salting-out ability of these salts were ranked in the descending order of (NH4)2SO4 > Na3C6H5O7 > Na2C4H4O6 > NaH2PO4 > NaC2H3O2 > NaCl and were discussed on the basis of the hydration ability of salt constituent ions. The phase-separation ability of investigated [Cnmim]BF4 was in the order of [C4mim]BF4 > [C3mim]BF4 > [C2mim]BF4. Hydrophilic imidazolium IL [C8mim]Cl can form ATPS only with two investigated salts, (NH4)2SO4 and K2HPO4, and it had the best phase-separation ability. The salting-out ability of the two salts was in the order of K2HPO4 > (NH4)2SO 4. The experimental binodal curves were successfully correlated by the Merchuk equation with low SD values. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00188.

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REFERENCES

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Experimental binodal curve mass fraction data for all 22 ATPS systems (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 754 86503093; Fax: +86 754 86502726; E-mail: [email protected]. ORCID

Yang Liu: 0000-0002-5717-471X Funding

This work is supported by the National Natural Science Foundation of China (21476135), Educational Commission of Guangdong Province, China (2016KZDXM014), Natural Science Foundation of Guangdong Province, China (2017A030307014), and Ocean and Fisheries Administration Project of Guangdong Province, China (2017). Notes

The authors declare no competing financial interest. F



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