Elsevier

Fluid Phase Equilibria

Volume 360, 25 December 2013, Pages 357-366
Fluid Phase Equilibria

Salting-out effect of sodium, potassium, carbonate, sulfite, tartrate and thiosulfate ions on aqueous mixtures of acetonitrile or 1-methyl-2-pyrrolidone: A liquid–liquid equilibrium study

https://doi.org/10.1016/j.fluid.2013.09.028Get rights and content

Highlights

  • Phase diagrams of {NMP or ACN} + {sodium or potassium carbonate} ATPSs were reported.

  • Two-phase formation ability of different salts and {NMP or ACN} was discussed.

  • Generalized Wilson and e-NRTL models were used to correlate the tie-line data.

  • Binary interaction parameters of Wilson and e-NRTL models were calculated.

  • The thermodynamics quantities of clouding point (CP) estimated using a simple method.

Abstract

The binodal curves, tie-line compositions and cloud point data as a function of temperature and concentration were measured for aqueous two phase systems composed of acetonitrile (ACN) + K2CO3 + H2O, ACN + Na2CO3 + H2O, 1-methyl-2-pyrrolidone (NMP) + K2CO3 + H2O and NMP + Na2CO3 + H2O. Additionally, salting-out ability of sodium sulfite, sodium thiosulfate and sodium tartrate was studied. The free energy, enthalpy and entropy of clouding point (CP) estimated using a simple method, and the driving force of the two-phase formation process was discussed on the base of the estimated free energy values. Also, an empirical equation was modified as a function of organic solvents density and dielectric constants and used for the simultaneous correlation of all experimental binodal data. Furthermore, e-NRTL and e-Wilson models were used for the correlation of tie-line compositions. The obtained results confirm the high performance of these models in the correlation of binodal and tie-line data.

Introduction

The liquid–liquid extraction technique is a powerful method for extraction and purification of significant chemical or biological substances such as proteins, enzymes, nucleic acids or even cell particles [1], [2]. In this method a biphasic system was used to extract the desired substance form one phase to the other one. Organic-water biphasic system is one of the favorite systems utilizing in these extraction technique. The time, cost, scale of the processing, physiochemical properties of the media and substance and also the bioenvironmental factors are some of the factors that affecting the selection of the biphasic system to extract the substance from the initial liquid phase [3], [4].

Most of the organic solvents denature proteins and therefore biochemists prefer to use the aqueous-organic mixed solvent rather than organic solvents in treatment of the proteins [5]. However, organic-aqueous systems in some cases are not the proper systems for extraction of biomolecules due to their unfavorable effects on them [6]. Therefore, the influence of the organic-water biphasic system on the extracted substrate should be considered in the selection of the extraction media.

Extraction efficiency can be improved by selection of different solvents, solvent volume, pH and by using the “salting” effect. The latest is more important, and is often referred to the “salting-out” or “salting-in” effects [7], [8], [9], [10]. The salting-out effect, in fact, involves the addition of a “kosmotropic” (i.e. water structure making) salt to a miscible aqueous-organic mixture. In this respect, when the concentration of the salt exceeds from a threshold concentration, the system becomes an immiscible biphasic one, in which, one phase is “water-rich phase” and the other phase is “organic-rich” [8]. Grover and Ryall [11] discussed different theories concern with the salting effect. However, a simple explanation of this effect can be made based on the “hydration” nature of the ions. In this way, when a kosmotropic salt was added to the aqueous solution of a soluble organic solvent, the intermolecular interaction was affected by ionization of the salt, whereas the stronger affinity between the ions and the water molecules reflected by decreasing the available water molecules for the third component (i.e. organic solvent), and therefore organic solvent enforced to increase its intermolecular interactions, and subsequently, at threshold concentration of the ionized species, the organic solvent was excluded from the rest of the solution as a separated phase [12].

Nowadays, salting effect found several applications in the numerous separation processes, such as in rectification, to shift favorably azeotropic conditions, in extraction to alter miscibility gaps and also in absorption and fractional crystallization to alter the distribution coefficients [13]. In this respect, several groups was studied the salting effect of different salts on different organic-aqueous systems based on the liquid–liquid equilibrium (LLE) measurements [14], [15], [16], [17], [18], [19].

Acetonitrile (ACN) and 1-methyl-2-pyrrolidone (NMP) are well-known medium-polarity solvents that are miscible in all proportions in water and widely used in organic synthesis. Water mixtures of both solvents are also used for separation and purification of organic compounds in organic synthesis. Both solvents belong to the class of dipolar aprotic solvents [20], [21], [22]. ACN is also one of the most preferred organic solvent or mobile phase in various separation techniques because of its proper physicochemical properties like low viscosity, high resolution and low boiling point [23]. However, ACN in acidic pH can yield hydrogen cyanide via hydrolysis in water [24]. Also, in the Federal Republic of Germany, the maximum concentration value at the workplace (MAK value) for ACN was suggested to be MAK  34 mg m−3 to prevent the skin damages [18]. Whereas, the toxicity of NMP is lower than ACN, and it is suggested that small values of NMP can easily be used as solubilizer cosolvent for medicinal agents at lower quantities compared to other common organic cosolvents (ethanol, isopropanol, propylene glycol and so on) [25]. Also, the value of MAK  80 mg m−3 was suggested for NMP [18]. Similar properties of these two solvents and the lower toxicity of NMP suggest that it may be a suitable alternative for ACN.

Also, sodium and potassium carbonate salts are nontoxic kosmotropic salts which are strongly hydrated with water molecules [26], [27], [28], and it is expected that have considerable effect on the aqueous mixture of ACN or NMP. However, there is no report on the study of the LLE of ternary water + salt + organic solvent systems involving sodium carbonate or potassium carbonate salts and ACN or NMP organic solvents. Therefore, the study these systems can give reliable information about physicochemical properties of the consequent biphasic systems.

It is also valuable to note that the thermodynamic investigation of the different organic + water + salt systems using reasonable models is an important part of the aforementioned studies. The local composition based models such as the NRTL [29] and electrolyte-NRTL [30] models and the group contribution based models such as UNIQUAC [31] or UNIFAC [32] models have been extensively used for the correlation of the LLE data in such mixed solvent-electrolyte systems. Also, in previous works we extended the e-NRTL [33] and e-Wilson [12], [19] models to represent the LLE of the systems composed of alcohol + salt + water systems and successfully used these models to describe such systems.

In this paper the LLE of the aqueous solutions of ACN and NMP in the presence of sodium carbonate and potassium carbonate were studied. In this respect, binodal curves and tie-lines at T = 298.15 K were measured. The effect of ACN, NMP, sodium and potassium cations and also carbonate, sulfite, tartrate and thiosulfate anions on the phase diagrams was discussed. The effect of temperature on the phase-separation ability of these systems was studied using the experimental cloud point (CP) data as a function of organic solvent mole fractions at T = 293.15–328.15 K with 5 K intervals. The experimental CP data was used to estimate the Gibbs free energy, enthalpy and entropy of clouding point (CP). The e-NRTL [33] and e-Wilson [12], [19] models were also used for the correlation of the tie-line compositions, and the binary interaction parameters were also calculated.

Section snippets

Materials

The physicochemical properties of the used chemicals were described in Table 1. These chemicals were used without further purification, and doubly distilled deionized water with specific conductance of about 0.70 μS cm−1 was used in all experiments.

Apparatus and procedure

The cloud point titration method was performed to collect the binodal curve data. In this method, an appropriate amount of aqueous solution of the salt solution or the organic solvent was placed in a double-wall glass cell, and the solution was stirred

Phase diagrams

The experimental binodal curves and tie-line compositions for acetonitrile (ACN) + K2CO3 + H2O, ACN + Na2CO3 + H2O, 1-methyl-2-pyrrolidone (NMP) + K2CO3 + H2O and NMP + Na2CO3 + H2O ternary systems at T = 298.15 K are reported in Table 3, Table 4, respectively. The uncertainties of data were evaluated using the recommended method of NIST [34]. Furthermore, as examples, the phase diagrams of ACN + K2CO3 + H2O and NMP + Na2CO3 + H2O systems are shown in Fig. 1, Fig. 2, respectively.

Effect of different solvents, anions and cations on the phase diagrams

Because the binodal curve represents the

Conclusions

Liquid–liquid equilibrium of NMP + Na2CO3 + H2O, NMP + K2CO3 + H2O, ACN + Na2CO3 + H2O and ACN + K2CO3 + H2O ATPSs were studied. The binodal curve data and tie-line compositions for the investigated systems were reported at T = 298.15 K. Also, the effect of temperature on the ATPSs was studied by measuring the cloud point data as a function of organic solvent compositions and temperature.

Also, binodal curves at T = 298.15 K for sodium sulfite, sodium thiosulfate and sodium tartrate + ACN + water systems were measured.

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