Equilibrium data and thermodynamic studies of L-tryptophan partition in alcohol/phosphate potassium salt-based aqueous two phase systems
Graphical abstract
Introduction
The knowledge of the thermodynamic behaviour of electrolyte solutions associated with mixed solvent especially alcohol-salt ATPSs (Aqueous two-phase systems) is of interest in many biological and environmental processes such as enzymes separation [1], [2], enantiomer separation [3], human antibodies' purification [4] and bio-product treatment [5]. Prediction of mean ionic activity in a mixed solvent electrolyte solutions of the biphasic system is required to have a reliable model in the wide range of solvent properties, salt concentrations, and temperatures. Recently, several pieces of researches have been done to model the ionic activity of mixed solvent electrolyte solutions [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24].Most of them separate the excess Gibbs energy into three terms: the long-range interaction of species in response of electrostatic attractions, the middle range solute-solute or solute-solvent interactions, and the short-range solvent-solvent interaction including a thermal and residual part.
The Pitzer-Debye-Huckle (PDH) model included Debye-Huckle electrostatic interaction term and ion-ion middle range interaction was used successfully to predict the ionic activity of salt in mixed solvent solution in a wide range of ionic strength and mixed solvent properties [12], [19], [22]. The Coupled Pitzer-Debye-Huckle contribution of long-range ion-ion interaction and short-range ion-solvent interaction of the solvation effect were implemented as a three-characteristic model to electrolyte solutions with a versatile mixed solvent physical property [18], [25]. In these researches, the ion-ion and ion-solvent interaction account in the function of a physical property of water-co-solvent mixture like density, viscosity, dielectric constant. On the other side, the local composition model concurrently equipped with electrostatic contribution and ion-interaction models. Li et al. [7] calculated the excess Gibbs energy as the sum of Debye-Huckle theory, UNIQUAC short-range effects, and a contribution include the indirect impact of the charge interactions, e.g., the charge-dipole interactions and the charge-induced dipole interactions. Later, many studies performed using this model as modified LIQUAC [14], generalized LIQUAC [16], and revised LIQUAC [20]. Recently, solvent composition dependence and born contribution were considered an aid to improve the newest form of UNIQUAC models [10], [17].
The knowledge about the impact of mixed solvent property on the ion-ion, ion-solvent, and solvent-solvent interaction in ATPS of K2HPO4 + straight and branch light alcohols is vital for much biological and environmental process design. Although, as a complete survey in the literature, Katayama et al. presented LLE data for ATPS of K2HPO4 with methanol, ethanol, 1-propanol, and 2-propanol in three different temperatures [26], [27], until now, the phase equilibrium data of straight and branch butanol in the aqueous solution of K2HPO4 has not reported.
Consequently, in the present study, the experimental phase equilibrium of (water + 1-butanol/2-butanol/t-butyl alcohol + K2HPO4) in 298.15 K was investigated. Also, separation of L-Tryptophan was done in these systems, and the final pH and partition factor are reported. The proposed excess Gibbs energy model (HIS-UNIQUAC) included in an original UNIQUAC term, the solvent property dependent Pitzer-Debye-Huckle contribution, born term, and an Ion-solvation short-range term. It was applied to describe the effect of mixed solvent characteristics of {water + light alcohol (methanol to butyl alcohol)} system. Finally, the impact of long-range, middle-range, and short-range of the excess Gibbs energy model on the partitioning coefficient of amino acid in all three orders is discussed.
Section snippets
Materials
To prepare the materials, we used 1-butanol, 2-butanol, tert-butanol, and potassium hydrogen phosphate (K2HPO4) with no further purification. The amino acid studied, L-tryptophan (Trp), was used without further purification. Distilled deionized water (conductivity = 0.056 μS∙cm−1) was used for the preparation of solutions. All the other materials had an analytical grade. The chemical name, CAS number (CAS), supplier, purification method, and purity are shown in Table 1.
Determination of phase diagrams
The cloud-point titration
Mixed solvent reference state
The main role of most ATPS's determined by manipulating the top and bottom phases ensure to achieve the partitioning ability. Hence, both phases have a different composition. In our study, salt-alcohol systems, two distinct aqueous-organic phases are observable. While the top phase, (aq + org)1, is enriched in an organic solvent and bottom phase, (aq + org)2, mostly composed of water as a solvent. So the molar chemical potential of aqueous-organic solution (μ±) definite as:
Phases diagrams and correlations
The experimental binodal data of the systems composed of alcohol (1-butanol/2-butanol/tert-butanol), K2HPO4, and water at 298.15 K are shown in Table 5. Due to mutual solubility, it can be seen that the biphasic area of 1-butanol and 2-butanol ATPS systems are segregated. The binodal results were fitted using Eq. (1), (2), the adjusted parameters and degrees of closure (R2 and SD) are present in Table S2. For all of the systems, the coefficient of determination is sufficiently approached to
Conclusion
In this work, the binodal data and the liquid-liquid equilibrium (LLE) tie-line for (water + 1-butanol/2-butanol/t-butyl alcohol + K2HPO4) systems were experimentally obtained at 298.15 K. The binodal data were fitted using Merchuk Equation and a two-Gaussian correlation with satisfactory correlation coefficients, R2 > 0.992. The partition coefficient of L-Tryptophan (Trp) in these systems was measured at a constant initial alcohol content of the feed, and the final pH of both phases was
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to express their special thanks of gratitude to John M. Prausnitz; Professor of Chemical Engineering at the University of California, Berkeley who gave us the golden opportunity to do this wonderful project on the equilibrium thermodynamics by his important considerations and suggestions in order to improve the thermodynamics model, and for always pointing out the relevance of the results. We are really thankful to him.
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