The new experimental data and a new thermodynamic model based on group contribution for correlation liquid–liquid equilibria in aqueous two-phase systems of PEG and (K2HPO4 or Na2SO4)

The new experimental data of liquid–liquid equilibria for aqueous two-phase systems PEG–K₂HPO₄–water and PEG–Na₂SO₄–water are presented. The effects of pH and molecular weight of polyethylene glycol were investigated and the tie lines with binodal curves for both systems are shown. A new thermodynamic model based on group contribution has been proposed for studying the phase behavior of aqueous two-phase polymer–salt systems. The assumptions of NRTL-NRF model and the activity coefficient equation of UNIQUAC-NRF model have been used for the groups. In this new model, UNIFAC-NRF, the nonrandom state of groups were selected as a reference state. The binary interaction parameters were adjusted using the data of binary salt–water systems and the ternary systems were correlated with only six binary adjustable parameters. The Debye–Huckel equation based on Fowller–Guggenheim equation was used to calculate the long range electrostatic interaction of the ions. The UNIFAC-NRF model was applied to correlate the experimental data of aqueous two-phase systems: PEG–K₂HPO₄–water and PEG–Na₂SO₄–water for two different molecular weight of PEG at different pH. The results of the new model showed that it can be used to correlate the LLE in aqueous solution of polymer–salt very well.     

Correlations between distribution coefficients of various biomolecules in different polymer/polymer aqueous two-phase systems

Distribution coefficients for a variety of proteins and certain other biomolecules (peptides, amino acids, and carbohydrates) (overall 27 different solutes) were measured in aqueous two-phase systems (ATPSs) dextran (Dex)–polyethylene glycol (PEG) and Dex–Ucon 50-HB-5100 (Ucon—a random copolymer of ethylene glycol and propylene glycol) both containing 0.15 M NaCl in 0.01 M phosphate buffer, pH 7.4, at 23 °C. Distribution coefficients of some selected solutes were also measured in the above two-phase systems at three different polymer concentrations for each system. It was established that the distribution coefficients for all the proteins examined in the ATPSs are correlated according to the so-called Collander linear equation.     

Biodegradable thermosensitive micelles of PEG-PLGA-PEG triblock copolymers

Micellization of poly(ethylene glycol-b-(DL-lactic acid-co-glycolic acid)-b-ethylene glycol), PEG-PLGA-PEG, triblock copolymers in water was studied by ¹³C-NMR (nuclear magnetic resonance), dye solubilization method, and light scattering. Critical micelle concentration was decreased by half by increasing temperature from 20 to 50°C. Enthalpy of micellization was calculated to be 10–20 KJ/mole. Apparent aggregation number, diameter of a micelle abruptly increased while second viral coefficient decreased at ≈30°C suggesting that micellar growth as well as an increase in polymer–polymer attraction occur at this temperature. These findings might suggest that the sol to gel transition at high concentration of aqueous solutions of this polymer is driven by conformational changes of micelles.     

New phase supports for partition chromatography of biopolymers

Liquid-liquid partition chromatography of bio-polymers requires aqueous two-phase systems for reasons of sample solubility and stability. Such aqueous two-phase systems form when thermodynamically incompatible polymers are co-dissolved in water. The most common polymer combination providing a two-phase system at reasonably low polymer concentrations is the combination of poly(ethylene glycol) (“PEG”) and dextran (“DX”), detected and introduced for the separation of biopolymers and cells by Albertsson about 30 years ago. The application of this powerful system for liquid-liquid partition chromatography requires support materials with surfaces able to immobilize selectively one of the two aqueous phases. This phase immobilisation may be achieved by exploiting incompatibilities between the polymers dominating in the phases and the hydrated support surface. Examples involving diol-modified silica and polyacrylamide coated diol-silica as support materials in aqueous PEG-DX and PEG-salt systems are presented. The application of such systems for the separation of biopolymers is demonstrated.     

A Modified Local Composition-Based Model for Correlating the Vapor-Liquid and Liquid-Liquid Phase Equilibria of Aqueous Polymer-Salt Systems

In this research, a new local composition model has been proposed to study the vapor-liquid and liquid-liquid phase equilibria of polyelectrolyte solutions. The proposed model has been used in order to obtain the activity of water in polyethylene glycol (PEG) and polypropylene glycol (PPG) solutions. The interaction parameters introduced into the proposed model have been reported. The interaction parameters between the salt and water molecule have been estimated using the experimental mean ionic activity coefficients of aqueous electrolytes studied in this work. Also, the interaction parameters between the polymer and salt molecule, and the polymer and water molecule have been computed using the experimental activity of water data in aqueous polymer solutions. The results showed that the proposed model, segment-based Wilson and segment-based NRTL models have good accuracy in correlating the vapor-liquid phase equilibria of the water-polymer and water-polymer-salt systems. Also, the liquid-liquid phase behavior of the polymer-salt aqueous two-phase systems has been correlated using the proposed model. The results show that the proposed model can more accurately correlate the phase behavior of aqueous two-phase systems than the UNIQUAC and the modified Wilson models.     

"On the Collander equation": protein partitioning in polymer/polymer aqueous two-phase systems

Distribution coefficients of randomly selected proteins were measured in aqueous two-phase systems (ATPSs) formed by different combinations of Dextran-75 (Dex), Ficoll-70, polyethylene glycol-8000 (PEG), hydroxypropyl starch-100 (PES), and Ucon50HB5100 (Ucon, a random copolymer of ethylene glycol and propylene glycol) at particular polymer concentrations, all containing 0.15M NaCl in 0.01 M phosphate buffer, pH 7.4. Most of the proteins in the PEG-Ucon system precipitated at the interface. In the other ATPSs, namely, PES-PEG, PES-Ucon, Ficoll-PEG, Ficoll-Ucon, and in Dex-PEG and Dex-Ucon described earlier the distribution coefficients for the proteins were correlated according to the solvent regression equation: lnKi=aiolnKo+bio, where Ki and Ko are the distribution coefficients for any protein in the ith and oth two-phase systems. Coefficients aio and bio are constants, the values of which depend upon the particular compositions of the two-phase systems under comparison.     

Experimental and thermodynamic modelling of systems containing water and ethylene glycol: Application to flow assurance and gas processing

Accurate knowledge of hydrate phase equilibrium in the presence of inhibitors is crucial to avoid gas hydrate formation problems and to design/optimize production, transportation and processing facilities. In this communication, we report new experimental dissociation data for various systems consisting of methane/water/ethylene glycol and natural gas/water/ethylene glycol. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the hydrate dissociation conditions of methane and natural gases in the presence of distilled water or ethylene glycol aqueous solutions. Predictions of the developed model are validated against independent experimental data and the data generated in this work. A good agreement between predictions and experimental data is observed, supporting the reliability of the developed model.     

Measurement and correlation of partition coefficients of hydrolytic enzymes for dextran+poly (ethylene glycol) + water aqueous two-phase systems at 20°C

The partition coefficients of hydrolytic enzymes, α-amylase, β-amylase and glucoamylase for the dextran (DEX)+poly(ethylene glycol) (PEG)+water aqueous two-phase systems with various polymer molecular weights were measured at 20°C. The partition coefficients obtained were correlated by using a mod fied Flory-Huggins equation which was empirically proposed for aqueous systems. The interaction parameters required were determined by fitting the model to the experimental data. The partition coefficients could be correlated with good agreement.     

Calculation of the Density and Activity of Water in ATPS Systems for Separation of Biomolecules

In this study, the perturbed hard sphere chain equation of state is utilized to calculate the activity of water in binary and ternary solutions of polyethylene glycol (PEG), salt and water. The liquid density of the binary and ternary solutions is also predicted. To estimate the water activity in PEG–water binary systems, a linear correlation is obtained for the binary interaction parameter between water and PEG. Then, using this correlation and without introducing any additional binary parameters, the water activities are predicted in ternary solutions of water, salt and PEG with different molecular weights (MW). Our results show that the mean absolute average relative deviation (AARD %) of water activity for binary PEG–water solutions in 298 K is 0.73 %. In addition, the water activity in ternary solutions of water and two PEGs with different MW is predicted within 0.31 % AARD %. Furthermore, the AARD % for prediction of water activities in binary PEG–water solutions over the temperature range 308–338 K is 0.41 %. Also, the water activities of aqueous two-phase systems are predicted with AARD % = 0.64 %. In this regard, no adjustable parameters were correlated between salt and PEG. Finally, liquid densities were predicted in binary solutions of water–PEG and ternary solutions of water–PEG–salt.     

Thermodynamic representation of phase equilibrium behavior of aqueous solutions of amino acids by the modified Wilson model

The polymer–electrolyte Wilson model [R. Sadeghi, J. Chem. Thermodyn. 37 (2005) 323–329] which has a molecular thermodynamic framework has been extended to model the vapor–liquid and liquid–solid equilibrium behavior of amino acids and small peptides in aqueous solutions as functions of temperature, ionic strength and amino acid compositions. The utility of the model is demonstrated with a successful representation of the activity coefficients and the solubility of several amino acids in different aqueous solutions and the results are compared with those obtained from the NRTL model.     

PVTx measurements for N2O + CH3F, N2O + CH2F2, and N2O + CHF3 binary systems

Isochoric PVTx measurements have been performed for the binary system of nitrous oxide + CH₃F (R41), +CH₂F₂ (R32), and +CHF₃ (R23) using a new experimental set-up. The experiments covered both the two-phase region and the superheated vapor region and were performed within the temperature range 214–358 K and within a pressure range from 270 to 5600 kPa. Data have been collected for not less than four compositions for each system. The vapor–liquid equilibrium data were derived correlating the experimental data by means of the Carnahan–Starling–De Santis equation of state. The studied systems show a positive deviation from the Raoult's law. The results obtained were compared with the Burnett PVTx data. The two methods showed a mutual consistency within an acceptable margin of error. No other experimental PVTx data were found in the literature for these binary systems.     

Enantioselective extraction of mandelic acid enantiomers by aqueous two-phase systems of polyethylene glycol and ammonium sulfate containing ��-cyclodextrin as chiral selector

A new chiral separation technology, aqueous two-phase extraction, was proposed for the separation of racemic mandelic acid. The distribution behavior of mandelic acid enantiomers was investigated in aqueous two-phase systems composed of polyethylene glycol and ammonium sulfate containing ??-cyclodextrin as chiral selector. The influences of the pH, the mass fraction of polyethylene glycol and ammonium sulfate, the polymerization degree of polyethylene glycol, the initial concentration of ??-cyclodextrin, mandelic acid enantiomers and extractive temperature on the distribution behavior were studied respectively. The results show that ??-cyclodextrin is inclined to recognize L-enantiomer; under the optimized conditions, the separation factor reaches 2.46 and the enantiomeric excess is 42.13% in the top phase and 40.43% in the bottom phase, respectively. Aqueous two phase chiral-extraction with strong chiral separation ability, plays great role in preparative separation of racemic compounds and is important for the development of aqueous two phase extraction technique.     

Purification of papain by metal affinity partitioning in aqueous two‐phase polyethylene glycol/sodium sulfate systems

A simple and inexpensive aqueous two‐phase affinity partitioning system using metal ligands was introduced to improve the selectivity of commercial papain extraction. Polyethylene glycol 4000 was first activated using epichlorohydrin, then it was covalently linked to iminodiacetic acid. Finally, the specific metal ligand Cu²⁺ was attached to the polyethylene glycol‐iminodiacetic acid. The chelated Cu²⁺ content was measured by atomic absorption spectrometry as 0.88 mol/mol (polyethylene glycol). The effects on the purification at different conditions, including polyethylene glycol molecular weight (2000, 4000, and 6000), concentration of phase–forming components (polyethylene glycol 12–20% w/w and sodium sulfate 12–20%, w/w), metal ligand type, and concentration, system pH and the commercial papain loading on papain extraction, were systematically studied. Under optimum conditions of the system, i.e. 18% w/w sodium sulfate, 18% w/w polyethylene glycol 4000, 1% w/w polyethylene glycol‐iminodiacetic acid‐Cu²⁺ and pH 7, a maximum yield of 90.3% and a degree of purification of 3.6‐fold were obtained. Compared to aqueous two phase extraction without ligands, affinity partitioning was found to be an effective technique for the purification of commercial papain with higher extraction efficiency and degree of purification.     

Carbon dioxide-water phase equilibria results from the Wong-Sandler combining rules

A model based upon the Peng-Robinson equation of state with the Wong-Sandler mixture combining rule (W-S MCR) can correlate phase equilibria in CO₂ + H₂O. The W-S MCR requires two energy parameters for liquid behavior and one interaction parameter for gas behavior, k_ij. In this paper, we present expressions for the energy parameters which cover a wide temperature range, and we use a new procedure to obtain k_ij by relating it to experimental cross second virial coefficients, B_ij. The three-phase pressures calculated for this system using our proposed model agree with the experimental data within a fraction of 1 bar. The correlated phase behavior of CO₂ + H₂O appears to be accurate over the ranges 1 – 1000 bar and 298.15–623.15 K. The proposed model also confirms the advantage of using the W-S MCR for phase equilibrium calculations.     

A new extraction method for the preparation of zinc oxide nanoparticles in aqueous two-phase systems

An extraction method was suggested for the preparation of differently shaped nano- and micrometer- sized zinc oxide particles at the interface boundary of aqueous two-phase systems based on polyethylene glycol (polyethylene oxide) and sodium sulfate using NaOH and aqueous ammonia as precipitating agents.     

Synthesis of amphiphilic poly(ether-amide) dendrimer endcapped with poly(ethylene glycol) grafts and its solubilization to salicylic acid

An amphiphilic dendrimer (DPEA-PEG) grafting polyethylene glycol at the terminals was prepared by endcapping of dendritic poly(ether-amide) (DPEA) with isocyanate terminated linear polyethylene glycol (PEG-NCO). The molecular structure was verified by gel permeation chromatography (GPC), ¹H NMR and FT-IR. The micelle characteristic of DPEA-PEG in water was investigated. The critical micelle concentration (CMC) was determined by a fluorescence technique to be 55.5 mg/L. The hydrodynamic radius of micelles was measured by dynamic light scattering (DLS) to be 76.2 nm. The UV–vis spectrum showed that the solubility of salicylic acid increased from 1.91 to 2.78 mg/L when the concentration of DPEA-PEG attained 5 mg/mL in an aqueous solution.     

Controlled growth of calcium oxalate crystal in bicontinuous microemulsions containing amino acids

The growth of calcium oxalate (CaC₂O₄) crystal in water channels of three kinds of bicontinuous microemulsions, consisted of P-octyl polyethylene glycol phenylether (OP)/n-amyl alcohol/cyclohexane/water and above microemulsions containing tryptophan (Trp) or aspartic acid (Asp) has been studied. The products were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The results indicated that both surfactant and amino acids all could prompt the growth of calcium oxalate monohydrate (COM) crystal, but the crystal morphology varied with the different microemulsions, pH values of the aqueous solution in channels and concentrations of the reactants. Various crystal morphologies such as butterfly-like, hollow and spiny spherical could be observed easily. A model of molecular identification – organized assembly – pervasion-combination balance was proposed to explain the formation mechanism of CaC₂O₄ crystals in the microemulsions containing Asp.     

Extraction of humic acids in two-phase aqueous polymeric systems

Extraction of humic acids isolated from various soils was studied in polyethylene glycol(NH₄)₂SO₄, polyethylene glycol-dextran, and polyethylene glycol-dextran sodium salt two-phase aqueous polymeric systems. It was shown that, in all systems, humic acids are extracted into the polyethylene glycolrich phase. It was studied how the composition of the extraction system, molecular weight of the polymer, and the sample nature affect the distribution ratios of humic acids.     

Measurement and correlation of the solubility of MnSO4·H2O in ethanol + water + MgSO4·7H2O solutions

Data on the solubility of manganese sulphate monohydrate in water, and in aqueous alcohols is essential for salting-out crystallisation studies. The solubilities of the quaternary system MnSO₄·H₂O + MgSO₄·7H₂O + H₂O + EtOH were determined in the temperature range 293.2–323.2 K over the ethanol mole fraction range of 0.00–0.12. The solubility data were used for modelling with the modified extended electrolyte non-random two-liquid (NRTL) equation. The present extension uses ion-specific parameters instead of the electrolyte-specific NRTL binary interaction parameters. This approach has feasibility for many electrolytes and mixed aqueous solution systems in principle. The model was found to correlate the solubility data satisfactorily.     

Electroextraction

Since both phases of aqueous two-phase systems are electrically conductive, application of electric fields in these systems gives rise to electrokinetic mass transfer of charged species. We have shown that an aqueous two-phase system can be used as a medium for electrophoretic separation with the liquid-liquid interface providing stability against convection and facilitating product recovery. Proteins have been directed into either the top or bottom phases of polyethylene glycol/dextran aqueous two-phase systems using 20–50 V/cm electric fields perpendicular to the phase interfaces. Binary protein mixtures have been separated in both batch and continuous modes by operating between isoelectric points and directing oppositely charged proteins into separate phases. Preliminary results on focusingE. coli cells into one phase while directing a proteinaceous product into the other phase indicate that two-phase electrophoresis may prove useful for commercial-scale recovery of proteins from fermentation broth. Recent studies in our laboratory indicate that the electrostatic potential profile near the interface of conducting liquid-liquid systems can influence electrophoretic migration of charged species across the interface. We have observed that phase systems with high Donnan potentials hinder electrophoretic mass transfer across liquid-liquid interfaces, whereas phase systems with low Donnan potentials do not.