Elsevier

Thermochimica Acta

Volume 516, Issues 1–2, 20 March 2011, Pages 46-51
Thermochimica Acta

Liquid–liquid equilibrium of aqueous two-phase systems containing poly(propylene glycol) and salt ((NH4)2SO4, MgSO4, KCl, and KAc): experiment and correlation

https://doi.org/10.1016/j.tca.2011.01.010Get rights and content

Abstract

Binodal data of poly(propylene glycol) PPG400 + salt ((NH4)2SO4, MgSO4, and KCl) + H2O systems were determined at T = 298.15 K, and binodal data of PPG400 + KAc + H2O system was determined at T = (298.15, 308.15, and 318.15) K. Moreover, the binodal curves were fitted to Merchuk equation, and the tie-lines were fitted to both the Othmer–Tobias and Bancroft equations. The effects of temperature and salts on the binodals and tie-lines of the investigated ATPSs were also studied. Finally, the salting-out ability of salts on the polymer was obtained by fitting the Setschenow-type equation to the tie-line data to derive a salting-out coefficient (kS) for each salt. For potassium salts, the relative effectiveness of the anions in inducing the phase segregation in ATPSs follows the Hofmeister series. The relative salting-out effectiveness of the sulfate salts is influenced by complexation of the cations by PPG.

Research highlights

► We determine the binodal/liquid–liquid equilibrium data of PPG400-salt ATPSs. ► We correlate the binodal curves and the tie-lines using some appropriate equations. ► The salting-out ability follows the order: K3PO4 > K2HPO4 > K2CO3 > KAc > KCl. ► The two-phase area increases with increasing temperature. The absolute values of slopes of the tie-lines are similar.

Introduction

Liquid–liquid extraction has often been a favored choice in separation and purification processes. However, it faced some challenge such as toxicity, volatility, and flammability. Aqueous two-phase systems (ATPSs) have been of increasing importance as a method for the separation and purification of biological material [1], [2], [3] in biochemistry and biotechnology. It has been established that ATPSs can be formed by combining hydrophilic solutes (polymer–polymer or polymer–salt) in aqueous solution above critical concentrations. Low molecular weights of polypropylene glycol (PPG) are completely soluble in water, while high molecular weights are only partially soluble. This polymer can also be used for the separation of biomolecules, since its aqueous solutions form a two-phase system. A few years ago, Salabat et al. used ATPS of PPG425 + MgSO4 + H2O at 298.15 K to separate amino acids [4]. Now they investigated the applicability of the systems composed of PPG and some inorganic salts to proteomic analysis [5]. Reliable liquid–liquid equilibrium (LLE) data are necessary for the design of a separation process, understanding of general factors determining partition of solutes and particles in such ATPSs, and development and testing of both thermodynamic and mass transfer models of ATPSs. However, LLE data of PPG-based ATPSs are relatively scarce. The LLE data of PPG-based ATPSs containing phosphates, sulphates, carbonates, and citrates have been studied [6], [7], [8], [9], [10].

Ananthapadmanabhan et al. [11], [12], [13] systematically investigated the phenomenon of aqueous two-phase formation in mixtures of PEO and electrolytes in water. They found that two-phase formation occurs with anions such as sulfate which have a marked salting-out effect on non-electrolytes but for singly charged ions such as iodide only one phase exists. The formation of the two phases in polymer–salt system is due to the “salting-out” effect of salt, in other words, polymer and salt are strongly associated with the water molecules but exclude each other by separating into two phases. In many previous studies, the salting-out ability of salts or ions is always compared by binodal curves plotted in mass fraction [14], [15], [16]. In fact, it cannot exactly reflect the nature of interaction between molecules in the system. Wang et al. investigated the salting-out abilities of organic salts in several hydrophilic alcohol + citrate ATPSs at 298.15 K by the binodal curves plotted in molality [17]. Many ionic liquid + salt ATPSs also used binodal curves plotted in molality to discuss salting-out abilities of salts [18], [19]. Zafarani-Moattar et al. used Setschenow-type equation to describe the salting-out effects of the tri-potassium citrate salt on 1-butyl-3-methylimidazolium bromide in the IL + salt ATPS at different temperatures [19]. Therefore, we decided to use the binodal curves plotted in molality and salting-out coefficient (ks) of Setschenow-type equation to evaluate the salting-out ability of different salts in the investigated PPG400 + salt ATPS.

In this research, LLE binodal curves and tie-lines for the aqueous PPG400 + salt ((NH4)2SO4, MgSO4, KCl, and KAc) + H2O ATPSs have been studied. The obtained binodal data and tie-lines were correlated using empirical equations. Moreover, the effect of temperature on the binodal curves and tie-lines were studied. Finally, the influence of salts on the phase-forming ability has also been studied.

Section snippets

Chemicals

PPG with a quoted molar mass of 400 g mol−1 was obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Potassium acetate (KAc), (NH4)2SO4, MgSO4, and KCl were purchased from Sinopharm Chemical Reagent Co., Ltd. (Nanjing, China) with a minimum mass fraction purity of 0.985, 0.990, 0.990, 0.990, and 0.995, respectively. The polymer and salts were used without further purification. Double distilled deionized water was used in the experiments.

Apparatus and procedure

The phase diagram includes the binodal curve and

Binodal data and correlation

For PPG400 + salt ((NH4)2SO4, MgSO4, KCl, and KAc) + H2O systems, the binodal data determined at T = 298.15 K, and in part at T = (308.15–318.15) K are shown in Table 1. For correlation of binodal data for the studied systems, the following non-linear expression developed by Merchuk [21] has been successfully used by Huddleston et al. [22]:w1=aexp(bw20.5cw23)where w1 and w2 are the mass fractions of PPG400 and salt, respectively. Recently, this equation has been successfully used for the correlation of

Conclusions

Experimental binodal data and liquid–liquid equilibrium data have been determined for the PPG400 + salt ((NH4)2SO4, MgSO4, KCl, and KAc) + H2O aqueous two-phase systems at T = 298.15 K, and in part at T = (308.15–318.15) K. The binodal curves were fitted to Merchuk equation relating the concentrations of PPG and salt. Moreover, the effect of temperature on phase-forming ability has been studied, and it was observed that the two-phase area increased with increase in temperature. The influence of salts on

Acknowledgement

This work was supported by the National Natural Science Foundation of China (no. 21076098), the Natural Science Foundation of Jiangsu Province (No. BK2010349), and the Ph.D. Programs Foundation of Ministry of Education of China (No. 200807100004).

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