LCST-Type Phase Behavior of Aqueous Biphasic Systems Composed

Article pubs.acs.org/jced

LCST-Type Phase Behavior of Aqueous Biphasic Systems Composed of Phosphonium-Based Ionic Liquids and Potassium Phosphate Jing Gao,* Jieyi Guo, Fanghong Nie, Hongwu Ji, and Shucheng Liu College of Food Science and Technology, Guangdong Provincial Key Laboratory of Aquatic Products Processing and Safety, Key Laboratory of Advanced Processing of Aquatic Products of Guangdong Higher Education Institution, Guangdong Ocean University, Zhanjiang 524088, China S Supporting Information *

ABSTRACT: Although aqueous biphasic systems (ABS) composed of phosphonium-based ionic liquids (ILs) have demonstrated superior performance as viable media for biocompatible extraction processes, the formation of ABS with tetrabutylphosphonium trifluoroacetate ([P4444]CF3COO) and tributyloctylphosphonium bromide ([P4448]Br) were seldom investigated. To evaluate the hydrophilicity of [P4444]CF3COO and [P4448]Br, the solubility curves for the mixture of the ILs and water were determined. The influence of temperature on the phase behavior for the [P4448]Br + K3PO4 + H2O and [P4444] CF3COO + K3PO4 + H2O systems were also obtained. Results show that both [P4448]Br + H2O and [P4444]CF3COO + H2O binary systems undergo lower critical solution temperature-type phase transition, and the ABS composed of [P4444]CF3COO and [P4448]Br can be formed with K3PO4 at a wide range of temperatures. Moreover, the ABS are highly temperature dependent, and biphasic region expands with an increase in temperature. On the other hand, the capability of the ILs to induce ABS follows the tread [P4448]Br > [P4444]CF3COO. This suggests that the lower cation hydrophilicity of ILs results in greater biphasic area in the ABS formed by phosphonium-based ILs.

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solvents, through temperature or pH changes,25 a large set of literature data has focused on the effects of ILs,7,26−29 salts,1,30 organic solvents,31,32 pH,7,33,34 and temperatures33,35−37 on the phase behavior and extraction efficiency of IL-based ABS. Recently, Passos et al.38 studied ABS composed of protic ILs and polymers, which were highly temperature dependent. It is interesting to note that an increase in temperature mostly decreases the two-phase area of the binodal curve in IL + salt ABS but expands the two-phase area of the IL + polymer binodal curve.37,39 The ILs used in most studies exhibit an upper critical solution temperature (UCST)-type phase change in water.37,40 However, only a few examples of lower critical solution temperature (LCST)-type phase change in IL/water mixtureshave been studied,40−42 and no report on the design of IL-based ABS that undergo LCST-type behavior upon mixing with salts is yet available. In the current work, we present a novel class of LCST-type ABS formed using phosphonium-based ILs and K3PO4. Phosphonium-based ILs were selected based on their temperature-sensitive feature in water.40 The inorganic salt used in this study induced the strongest salting-out effect.30 To confirm the effect of temperature on the hydrophilicity of phosphoniumbased ILs, the solubility curves for the binary systems of

INTRODUCTION Aqueous biphasic systems (ABS) are typically formed as a result of mutual incompatibility in aqueous solutions of polymer/ polymer, polymer/salt, or salt/salt above a certain concentration.1 These environmentally benign systems mainly composed of water can provide an efficient and green approach to the extraction, separation, and purification of (bio)molecules.2,3 Although polymer-based ABS have received considerable attention as environmentally friendly techniques and are applied to a huge number of extraction processes with immense success, most polymer-based ABS present high viscosities, form opaque systems, and exhibit limited polarity differences between phases, thereby resulting in low extraction yields and narrow purification conditions. 1,4 These issues have prompted researchers to investigate alternative separation strategies. ABS based on ionic liquids (ILs) have been extensively explored and proposed as promising alternatives to polymerbased ABS because of their low viscosity, rapid two-phase formation, high extraction efficiency, and the prospect of controlling the polarities of the phases by selecting IL-constituting ions.5−8 Rogers et al.9 first introduced IL-based ABS in 2003. To date, a series of ABS composed of ILs + inorganic salts,10−12 ILs + organic salts,2,13−15 ILs + saccharides,16 ILs + polymers,17,18 ILs + amino acids,19 and ILs + Good’s buffers20,21 have been reported and used to extract enzymes and proteins.22−25 Because of the low chemical stability and thermal stability of enzymes and proteins in the presence of salts or orgainc © XXXX American Chemical Society

Received: October 16, 2016 Accepted: March 3, 2017

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DOI: 10.1021/acs.jced.6b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

water in [P4444]CF3COO, [P4448]Br and K3PO4, w, was 0.0015, 0.0042 and 0.0005, respectively. The water contents in the ILs and K3PO4 were taken into account during preparation of the aqueous solutions. Solubility Curves for Binary Systems. The IL + water binary phase diagrams were determined using the method presented by Christensen et al.43 A series of mixed solutions of different ILs and water were prepared at a mass ratio of 1:1 in separating funnels. The solutions were incubated in a thermotank at the desired temperature (within an uncertainty of 0.1 K) for a minimum of 12 h. After this period, both phases were carefully separated and individually weighed and the amount of ILs in each phase was quantified via high-performance liquid chromatography (HPLC) (LC-20A, Prominence, Japan). Each phase was diluted at varying ratios in water before injection. The HPLC equipped with SPD-20A/20AV ultraviolet−visible detector was used. The wavelength was set to 198 nm. The quantification of the IL in each phase was carried out by external standard calibration method ([P4444]CF3COO, R2 = 0.9995; [P4448]Br, R2 = 0.9985); compare the Supporting Information

tetrabutylphosphonium trifluoroacetate ([P4444]CF3COO) + water and tributyloctylphosphonium bromide ([P4448]Br) + water were determined, and the binodal curves for ABS composed of [P4448]Br + K3PO4 + H2O and [P4444]CF3COO + K3PO4 + H2O were obtained at different temperatures under atmospheric pressure.

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EXPERIMENTAL SECTION Materials. The ILs used in this work were [P4444]CF3COO (>98%) and [P4448]Br (>98%). All the ILs were supplied by Lanzhou Institute of Chemical Physics (China), and their mass fraction purities were further confirmed by 1H NMR and 13C NMR (Supporting Information Figure S1). Their ionic structures are presented in Figure 1. K3PO4 was obtained from Sigma (>98%). The basic properties of above materials are presented in Table 1. The water content in [P4444]CF3COO, [P4448]Br and K3PO4 was determined using an automatic moisture analyzer (HX204, Mettler-Toledo, Switzerland). The mass fraction of

Figure 1. Chemical structures of the studied ILs: (a) [P4444]CF3COO and (b) [P4448]Br.

Table 1. Provenance and Mass Fraction Purity of the Compounds Studied chemical name

mass fraction purity

source

[P4444] CF3COO

Lanzhou Institute of Chemical Physics, China

≥0.98

[P4448]Br

Lanzhou Institute of Chemical Physics, China

≥0.98

K3PO4·3H2O Sigma

≥0.98

analysis method 1

H NMR and 13 C NMR 1 H NMR and 13 C NMR None

Figure 2. Mutual solubility for [P4444]CF3COO + H2O: ●, bottom phase; ■, top phase. The IL solution was colored with oil red (10 × 10−3mg/100 mL).

Table 2. Mass Fractions of the ILs in the Top and Bottom Phases (wT and wB, respectively) of the Liquid−Liquid Equilibria of IL + Water Systems at the Corresponding Temperatures T and Pressure p = 0.1 MPaa T/K 273.15 293.15 303.15 313.15 323.15 333.15 343.15 273.15 293.15 303.15 313.15 323.15 333.15

wT/% [P4444]CF3COO + H2O one phase one phase 23.52 15.61 13.15 11.06 10.38 [P4448]Br + H2O one phase 11.54 8.99 4.63 4.23 3.95

wB/% one phase one phase 62.13 67.05 70.41 70.80 71.27 one phase 70.30 73.77 74.96 75.21 76.49

Figure 3. Mutual solubility for [P4448]Br + H2O: ●, bottom phase; ■, top phase. The IL solution was colored with oil red (10 × 10−3mg/ 100 mL).

a

Standard uncertainties u are u(w) = 0.1%, u(T) = 0.1 K and u(p) = 10 kPa. B



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Table 3. Correlation Parameters (A, B, and C) and Least Squares Regression (R2) Obtained from the Regression of eq 1 for the ABS Composed of IL + Water + K3PO4 at the Corresponding Temperatures T and Pressure p = 0.1 MPaa T/K 293.15 298.15 303.15 308.15 323.15 333.15 343.15 293.15 298.15 303.15 308.15 323.15 333.15 343.15

Figure 4. Ternary phase diagrams for ABS composed of [P4444]CF3COO + K3PO4 + H2O at ■ 293.15 K and adjusted binodal data through eq 1 (−). a

A

B

C

[P4444]CF3COO + K3PO4 + H2O × 10−3 −48.76 −41.19 × 10−2 −23.05 −14.48 × 10−2 −27.81 −19.58 × 10−2 −24.40 −16.47 × 10−2 −14.30 −7.47 × 10−2 −20.78 −13.50 × 10−2 −15.15 −8.11 [P4448]Br + K3PO4 + H2O 5.68 × 10−1 −8.68 −0.97 1.59 × 10−1 −15.27 −5.50 7.21 × 10−2 −16.81 −8.14 3.77 × 10−6 −64.27 −58.40 9.43 × 10−5 −49.62 −41.79 9.51 × 10−4 −37.55 −29.39 1.24 × 10−4 −48.42 −40.25 2.15 3.73 1.26 1.62 5.10 1.72 3.91

R2 0.9991 0.9981 0.9968 0.9969 0.9912 0.9976 0.9964 0.9977 0.9996 0.9988 0.9995 0.9986 0.9992 0.9991

Standard uncertainties u are u(T) = 0.1 K and u(p) = 10 kPa.

Figure 5. Ternary phase diagrams for ABS composed of [P4448]Br + K3PO4 + H2O at ■ 333.15 K and adjusted binodal data through eq 1 (−). Figure 6. Ternary phase diagrams for ABS composed of [P4444]CF3COO + K3PO4 + H2O at: ■, 293.15 K; □, 298.15 K; ▲, 303.15 K; ▽, 308.15 K; ◆, 323.15 K; ◊, 333.15 K; ●, 343.15 K.

with the established calibration curve and associated standard deviations. Binodal Curves of Ternary Systems. The binodal curves for IL + K3PO4 + water ABS were determined using the cloud point titration method at different temperatures. The experimental procedure adopted in this work follows an already validated method for ABS constituted by other ILs and salts.2,32,44,45 The experimental binodal curves were fitted via least-squares regression according to eq 1 [IL] = A exp[(B × [Salt]0.5 ) − (C × [Salt]0.3 )]

water and [P4448]Br + water exhibited one-phase miscibility at both low and high concentrations of ILs and twophase separation in the middle at all temperatures between 303.15 and 343.15 K. Phase separation occurred for both binary systems when temperature increased to a critical value. For example, [P4444]CF3COO solution was a homogeneous mixture at 293.15 K but was clearly separated into an aqueous phase and an IL phase above 303.15 K. These separated phases mixed homogeneously again upon cooling below 293.15 K. The solubility of water in [P4444]CF3COO increased upon cooling. A similar phase behavior with water was observed in [P4448]Br. These results indicate that [P4444]CF3COO and [P4448]Br exhibit LCST-type phase separation with water, which is in good agreement with the study of Kohno et al.41 The phase behavior of ABS composed of phosphoniumbased ILs has been assessed by several authors. Tetrabutylphosphonium bromide ([P4444]Br) and tetrabutylphosphonium

(1)

where A, B, and C are fitted constants obtained through the regression of the experimental data.46

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RESULTS AND DISCUSSION Mutual Solubility for IL + Water. Table 2 presents the measured equilibrium compositions of the aqueous mixtures with [P4444]CF3COO (w = 50%) and [P4448]Br (w = 50%). These data are plotted as solubility curves for the binary systems in Figures 2 and 3. The mixtures of [P4444]CF3COO + C



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the systems, thereby indicating that the fitting could be used to predict the data in a given region of the phase diagram where no experimental result was available. The corresponding binodal curves for [P4444]CF3COO + K3PO4 + H2O and [P4448]Br + K3PO4 + H2O ABS are shown in Figures 6 and 7, respectively. In both cases, the binodal curves shift toward the left with an increase in temperature. This result implies that less mass of salt is required to salt-out IL as temperature increases. Conversely, a higher mass of salt is required to form ABS when temperature decreases. This observed deviation is in contrast to that observed in previously reported IL + salt ABS37,39 but is similar to trends obtained in IL + polymer systems.39 This result is explained based on the critical solution temperature as follows. Interestingly, although salt + salt ABS will follow the UCST-type phenomenon, phase separation in [P4444]CF3COO + K3PO4 and [P4448]Br + K3PO4 ABS will adhere to an LCST-type phenomenon. The effect of ILs on ABS formation was evaluated at 303.15 and 333.15 K. The results are reported in Figure 8. An analysis showed that the capacity of the two ILs was confirmed to be in the following order: [P4444]CF3COO < [P4448]Br. This order is consistent with the results of temperature at phase separation (Tc) for the IL + water mixtures. As shown in Figures 2 and 3, the Tc values of [P4444]CF3COO and [P4448]Br are 303.15 and 293.15 K, respectively. Kohno and Ohno proposed a “hydrophilicity index” (HI) which was the number of water molecules per ion pair in the IL phase at 333 K.40 The HI value highly depends on both cation and anion species, and less hydrophilic ions lead to a lower HI value. For ILs containing the same cation, the HI value of Br− is larger than the CF3COO− anion, and the hydrophilicity of the cations is believed to be in the following order: [P4444]+ > [P4448]+. By contrast, the HI values of [P4444]CF3COO and [P4448]Br are 9.0 and 6.7, respectively. These results imply that the hydrophilicity of [P4444]CF3COO and [P4448]Br is mainly determined by cation hydrophilicity, and [P4444]CF3COO is more hydrophilic than [P4448]Br.

Figure 7. Ternary phase diagrams for ABS composed of [P4448]Br + K3PO4 + H2O at: ■, 293.15 K; □, 298.15 K; ▲, 303.15 K; ▽, 308.15 K; ◆, 323.15 K; ◊, 333.15 K; ●, 343.15 K.

chloride ([P4444]Cl) were used to form ABS as a platform for extraction processes.1,25 Most studies were conducted at a fixed temperature because only ABS formed by the ILs combined with salts was evaluated, and these ABS displayed weak dependence on temperature.38 [P4444]Cl, [P4444]Br, [P4444]NO3, [P4448]Cl, and [P4448]CH3SO3 were freely miscible with water. However, [P4444]CF3COO and [P4448]Br formed homogeneous mixtures with water at low temperatures. A clear solution became turbid upon gentle heating, and phase separation occurred. These data suggest that the hydrophilicity of [P4444]CF3COO and [P4448]Br are temperature dependent. Phase Diagrams of ABS Composed of IL + Water + K3PO4. To study the effect of temperature on the phase behavior of the ABS composed of [P4444]CF3COO + water + K3PO4, and [P4448]Br + water + K3PO4, respective phase diagrams at T = 293.15, 298.15, 303.15, 308.15, 323.15, 333.15, and 343.15 K were drawn. The experimental curves for all the systems were correlated using eq 1. Figures 4 and 5 show the adjusted binodal data for [P4444]CF3COO + K3PO4 + H2O at 293.15 K and [P4448]Br + K3PO4 + H2O at 333.15 K. The fitted parameters were estimated via least-squares regression (R2). The regression parameters are presented in Table 3. Generally, excellent correlation coefficients were generally obtained for all

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CONCLUSION Novel LCST-type phase behavior of ABS with a controlled temperature was introduced. The phase separation of [P4444]CF3COO + water + K3PO4 and [P4448]Br + water + K3PO4 systems increases as temperature increases. Moreover, the hydrophilicity of [P4444]CF3COO and [P4448]Br is mainly determined

Figure 8. Ternary phase diagrams for ABS composed of IL + K3PO4 + H2O at (a) 303.15 K and (b) 333.15 K: ■, [P4444]CF3COO; □, [P4448]Br. D



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by cation, and the phase-forming capability of the ILs is in the following order: [P4448]Br > [P4444]CF3COO. Such an LCSTtype ABS is a novel system that due to its thermoreversible properties may open interesting paths for the separation of temperature-sensitive biomolecule.

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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.6b00884. Experimental binodal mass fraction data for the ABS composed by each system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 86 759 2396029. Fax: + 86 759 2396029. ORCID

Jing Gao: 0000-0003-1264-9416 Funding

The project was supported by the Fund for Guangdong Province Innovation School Project (2015KQNCX061, GDOU2015050240), and Fund of Key Laboratory of Aquatic Product Processing, Ministry of Agriculture, China (B16387). Notes

The authors declare no competing financial interest.

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LCST-Type Phase Behavior of Aqueous Biphasic Systems Composed

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