System Of Polyethylene Glycol Research Articles

Solubility and Henry's law constant of sulfur dioxide in aqueous polyethylene glycol 300 solution at different temperatures and pressures

In this work, isothermal gas–liquid equilibrium (GLE) data are reported for the system of SO2+N2+polyethylene glycol (PEG) 300+water (H2O) at five different reaction temperatures (from 298.15 to 318.15K) and different pressures (from 110.34 to 142.03kPa) with SO2 partial pressures in the range of 0 to 180Pa. Measurements were carried out by a saturation method using a glass absorption apparatus, which was controlled at constant temperatures by a thermostatic circulation bath with a Beckmann thermometer. The GLE data were obtained with uncertainties within ±0.02K for temperature, ±0.1kPa for total pressures, and ±0.05 for SO2 concentration in the gas phase and ±0.003 for SO2 concentration in the liquid phase. The measurements showed that the solubility of dilute SO2 in the system of PEG (1)+water (2) increased the with increasing PEG concentration in the mass fraction range of w1=(0.50–1.00). The solubility of SO2 in the system of PEG (1)+water (2) presented an extreme minimum at the mass fraction of w1=0.50 of 152.3gm−3 when SO2 in the gas phase was designed at ySO2=5×10−4. The solubility of dilute SO2 in the system of PEG (1)+water (2) increased with decreasing PEG concentration in the mass fraction range of w1=(0.05–0.50). The solubility of SO2 in the system of PEG (1)+water (2) presented an extreme minimum at the mass fraction of w1=0.05 of 270.7gm−3 when SO2 in the gas phase was designated at ySO2=5×10−4. Henry's law constants were fitted with a dynamic system based on the GLE data. Using the solubility data, the partial molar thermodynamic functions of solution, enthalpy and entropy, were calculated. Conclusions drawn from this work may be used to provide important GLE data for the design and operation of the absorption and desorption processes of PEG solutions in flue gas desulfurization.

Effect of Permeation Enhancers on the Release Behavior and Permeation Kinetics of Novel Tramadol Lotions

Purpose: The aim of this research work was to formulate, characterize and evaluate the in vitro permeation behavior of tramadol lotion containing propylene glycol (PG) and polyethylene glycol (PEG) as permeation enhancers.Methods: The permeation experiments were conducted in vitro using full thickness rabbit skin in Franz diffusion cells. The donor compartment was filled with PBS (phosphate buffered saline) at pH 7.4 ± 0.1. The receptor phase was continuously stirred PBS (pH 7.4) at 37 °C ± 0.5. The amount of tramadol permeated into the receptor phase was determined spectrophotometrically at 271 nm. Various permeation parameters such as permeation coefficient (Kp), diffusion coefficient (D), flux (J), input rate,and enhancement ratio were obtained using Fick’s diffusion laws.Results: Permeation increased with increase in the concentrations of both enhancers tested. Maximum cumulative amount permeated for control lotion (Lc) was 357 ìg/cm2/min with input rate 0.574 ìg/min and lag time (tlag) of 34.93 min, while for the optimum test lotion (L4, containing 8 % PG/PEG in ratio of 1:1 v/v), it was 926 ìg/cm2/min, 1.482 ìg/min and 58.36 min, respectively. The significantly (p < 0.05) higher permeability shown by the test lotion L4 can be attributed, in part, to the interaction of PG withintercellular lipids leading to the disruption of their organization and increasing their fluidity, and also partly as a result of solubilization of lipid bilayers by PEG.Conclusion: A binary system of PG and PEG in lotion can be successfully utilized for the permeation enhancement of tramadol.Keyword: Tramadol, Transdermal delivery, Permeation, Propylene glycol, Polyethylene glycol, Rabbit skin.

The Polyethyleneglycol Dilemma: Advantage and Disadvantage of PEGylation of Liposomes for Systemic Genes and Nucleic Acids Delivery to Tumors

Gene and nucleic acid therapy is expected to play a major role in the next generation of agents for cancer treatment. We have recently developed a multifunctional envelope-type nano device (MEND) for use as a novel nonviral gene delivery system. The modification of polyethyleneglycol (PEG), i.e., PEGylation, is a useful method for achieving a longer circulation time for the delivery of MEND to a tumor via the enhanced permeability and retention (EPR) effect. However, PEGylation strongly inhibits cellular uptake and endosomal escape, which results in significant loss of activity of the delivery system. For successful nucleic acid delivery for cancer treatment, the crucial problem associated with the use of PEG, i.e., the "PEG dilemma" must be resolved. In this review, we describe the development and applications of MEND and discuss various strategies for overcoming the PEG dilemma based on the manipulation of both pharmacokinetics and intracellular trafficking of cellular uptake and endosomal release. To increase cellular uptake, target ligands including proteins, peptides, antibodies and aptamers that recognize molecules specifically expressed on tumors are first introduced. Second, cleavable PEG systems are described. The cleavage of PEG from carriers was achieved in response to the intracellular environment as well as the tumor microenvironment, which improvs cellular uptake and endosomal escape. Then, endosomal fusogenic peptides are discussed. Finally, pH-sensitive liposomes using pH-sensitive lipids are described.

Novel self-assembling systems based on amphiphilic phosphonium salt and polyethylene glycol. Kinetic arguments for synergetic aggregation behavior

Structural behavior and catalytic activity toward basic hydrolysis of O-alkyl-O-p-nitrophenyl chloromethyl phosphonates (alkyl=ethyl (S1) and hexyl (S2)) of mixed cetyl triphenyl phosphonium bromide (TPPB)–polyethylene glycol (PEG) systems are studied. The interdependence of aggregates and substrates is revealed from symbate changes in their self-diffusion coefficients determined by NMR FT-PGSE method. Much lower zeta-potentials of mixed systems as compared to single TPPB micelles are found, although the counterion binding of aggregates decreases with the addition of PEG. A 1.5–2-fold increase in the reactivity of phosphonates is shown to occur in mixed TPPB–PEG systems versus single surfactant micelles. In addition, a polymer induced shift of the onset of the rate acceleration toward the lower concentrations is observed. These findings provide strong evidences for synergetic solution behavior in the TPPB–PEG systems. The quantitative treatment of kinetic data in terms of pseudophase model sheds light on the factors of catalytic action. In the case of hydrolysis of S1, the more favorable microenvironment is responsible for the higher catalytic effect of mixed systems as compared to single TPPB micelles, while the factor of concentration decreases. The opposite trend is observed for hydrolysis of phosphonate S2, for which an increase in the micellar rate effect with the PEG addition is mainly contributed by the growth of the factor of concentration.

Competitive Adsorption between Polyethylene Glycol and Acid Brilliant Red on Expanded Graphite

Expanded graphite (EG) adsorbent was prepared with graphite as raw materials, potassium permanganate as oxidant, and vitriol as intercalation compound. Polyethylene glycol (PEG) 10000 and Acid brilliant red 3B (ABR) with obvious difference in molecular weights were selected as organic adsorbates, and their adsorption and competitive adsorption thermodynamics and kinetics on EG were investigated. The results are: In both single component system and dual component system, the adsorption and competitive adsorption isotherm of PEG 10000 and ABR on EG are all type I. In dual component system, the existence of another component would decrease the single component adsorbance, which might be caused by the competitive adsorption of these adsorbates for the same adsorbing site on EG. Adsorption and competitive adsorption are spontaneous, and the increase of ionic strength, temperature and expanded volume of the adsorbent would cause the increase of adsorbance. No matter the single component system or the dual component system, the adsorption kinetics models can all be well described with pseudo second-order model. Adsorption rate increases with the increase of temperature. Physical adsorption is the main action between EG with these adsorbates.

Zero order drug delivery from double-layered porous films: release rate profiles from ethyl cellulose, hydroxypropyl cellulose and polyethylene glycol mixtures.

Abstract Laminated double-layered films comprising a drug-containing and drug-free layer were prepared using tripelennamine, barbitone, salicylic acid and caffeine dispersed in hydroxypropylcellulose (HPC) attached to ethyl cellulose (EC) films containing various proportions of polyethylene glycol (PEG) or HPC to enhance permeability. Drug release in vitro followed zero-order kinetics, rate constants being dependent on the thickness of the drug-free membrane, which was rate-controlling. Thickness-corrected zero order constants were independent of drug-loading, which did, however, control the duration of release. Permeability coefficient measurements on the same rate-controlling films used as single barrier membranes enabled the effective drug concentration (Co) at the interface between the laminated membranes to be estimated; Co was independent of drug loading and was of the order expected from the aqueous solubilities of the drugs. Release rates were enhanced by addition of hydrophilic polymer to the rate-controlling membranes, either linearly with fraction of additive for PEG to 0·6 or HPC to 0·4, or logarithmically for HPC from 0·4 to 0·8. Enhancement coefficients, which were different for each system, reflected the different mechanisms of hydrophilic polymer action. PEG was leached out rapidly, pores being formed in the matrix. In contrast, HPC was largely retained, so that the enhancement was less. The logarithmic enhancement stemmed from formation of swollen hydrated channels, which, unlike the low HPC fractions or the PEG systems, allowed entry of buffer ions, so that only in these channel systems were the release rates altered by change of the external pH.

Amine Functionalization of Collagen Matrices with Multifunctional Polyethylene Glycol Systems

A method to functionalize collagen-based biomaterials with free amine groups was established in an attempt to improve their potential for tethering of bioactive molecules. Collagen sponges were incorporated with amine-terminated multifunctional polyethylene glycol (PEG) derivatives after N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide and N-hydroxysuccinimide (EDC/NHS) cross-linking. The extent of the incorporation of different amounts and different numbers of active moieties of amine-terminated PEG systems into the collagen scaffolds was evaluated using ninhydrin assay, Fourier transform infrared spectrophotometry (FTIR), collagenase degradation assay, denaturation temperature measurements, and in vitro cell studies. A 3% 8-arm amine-terminated PEG was found to be the minimum required effective concentration to functionalize EDC/NHS stabilized collagen scaffolds. EDC/NHS stabilized scaffolds treated with 3% 8-arm amine-terminated PEG exhibited significantly improved denaturation temperature and resistance to collagenase degradation over non-cross-linked scaffolds (p < 0.002). Biological evaluation using 3T3 cells demonstrated that the produced scaffolds facilitated maintenance of the cells' morphology, metabolic activity, and ability to proliferate in vitro. Overall, our results indicate that amine-terminated PEG systems can be used as means to enhance the functionality of collagenous structures.

Partition equilibrium of phenol red in aqueous polymer–surfactant system: Determination of critical aggregation concentration

Acid–base equilibrium of phenol red (PR), a sulphonephthalein dye, was studied spectrophotometrically in aqueous media containing water soluble nonionic polymers viz., polyvinyl alcohol (PVA) and polyethylene glycol (PEG) in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A partition equilibrium method was utilized to determine the equilibrium constant of partition of the PR between micellar pseudo phase and aqueous phase and the critical aggregation concentration (CAC) of SDS in buffered aqueous systems containing the neutral polymers. The pH dependent association constants, K ass of PR with SDS–PVA, and SDS–PEG system increase in the order PEG 200 < PEG 400 < PEG 600 < PVA. The value of K ass was found to be greater in Tris buffer system than in phosphate buffer system of same pH. Interaction of the dye with the aqueous SDS–PVA and SDS–PEG systems is found to be endothermic and entropy oriented.

Rheological properties of reversible thermo-setting in situ gelling solutions with the methylcellulose–polyethylene glycol–citric acid ternary system (2): Effects of various water-soluble polymers and salts on the gelling temperature

The influences of various salts and water-soluble polymers on the phase transition temperature of thermo-setting gels prepared by combining methylcellulose (MC)–sodium citrate (SC)–polyethylene glycol (PEG) at appropriate ratios (the MC–SC–PEG system) were investigated. Concerning cations, comparison of the phase transition temperature between SC and tripotassium citrate (PC) showed a rapid increase in the viscosity of SC between 20 °C and 25 °C and an increase in the viscosity of PC between 30 °C and 35 °C. Concerning the valency of anions, comparisons among SC, disodium tartrate dihydrate (ST), disodium maleate hemihydrates (SM), and sodium sulfate (SS) showed a rapid increase in the viscosity of trivalent SC between 20 °C and 25 °C and changes in the viscosity of the three bivalent sodium salts (ST, SM, and SS) at ≥30 °C. Thus the phase transition temperature decreased with an increase in the valency of anions. Subsequently, the influences of various water-soluble polymers on the gelling temperature were compared. Using polyvinylpyrrolidone (PVP) instead of PEG, the gelling temperature decreased with an increase in the PVP concentration even without the addition of SC. Unlike PVP, the addition of xanthan gum as a viscosity-increasing polysaccharide did not reduce the gelling temperature irrespective of its concentration. Temperature-associated changes in viscosity were observed at a fixed SC concentration with changes in the concentration of PVP or PEG. The gel phase transition temperature increased from 46 °C to 50 °C in gels not containing PVP or PEG. The viscosity did not differ between the addition of PVP or PEG at a low concentration and its absence. However, the viscosity clearly changed after the addition of each agent at a high concentration.

Modification of NRTL-NRF model for computation of liquid–liquid equilibria in aqueous two-phase polymer–salt systems

The aqueous two-phase system (ATPS), a polymer and a salt, is widely used as an extraction process for separation of biomolecules in down stream processing. At this work, using local area fraction, the local composition non-random factor (NRF) model of Haghtalab and Vera is extended for aqueous binaries of electrolyte, polymer and ternaries of aqueous polymer–salt solutions. Using area fraction of species, a new function of excess Gibbs energy has been developed for multicomponent aqueous polymer–electrolyte systems. The modified NRTL-NRF model is applied to correlate the phase behavior of aqueous two-phase polymer–salt systems of PEG + Na 2SO 4 + H 2O, PEG + K 2HPO 4 + H 2O and PEG + (NH 4) 2SO4 + H 2O at 25 °C. The effect of pH solution and molecular weight of polyethylene glycol (PEG) is investigated and the tie lines with binodal curves for the fifteen systems are shown. The comparison of the results of the present model with the experiment shows that the modified NRTL-NRF model is more accurate.

Purification of an α-amylase inhibitor in a polyethylene glycol/fructose-1,6-bisphosphate trisodium salt aqueous two-phase system

A novel aqueous two-phase system of polyethylene glycol (PEG)/fructose-1,6-bisphosphate (FBP) trisodium salt was employed to purify an α-amylase inhibitor (α-AI) from wheat flour after failure in use of the conventional aqueous two-phase systems of PEG/(NH 4) 2SO 4 and PEG/dextran (DEX). Phase compositions including PEG molecular weight and concentration of PEG, fructose-1,6-bisphosphate trisodium salt concentration, as well as pH of the system affected the partition of α-AI. The inhibitor was obtained with a purification factor of 3.2 and recovery of 79% by using 11.7% (w/w) of PEG2000 and 19% (w/w) of fructose-1,6-bisphosphate trisodium salt at pH 7.0 in the top phase.

Purification of plasmid DNA with aqueous two phase systems of PEG 600 and sodium citrate/ammonium sulfate

Polymer/salt aqueous two phase systems (ATPS) based on polyethylene glycol (PEG) 600, sodium citrate and ammonium sulfate were used to partially purify plasmid DNA (pDNA) from Escherichia coli alkaline lysates. The effect of pH and lysate load on the binodal curve was analyzed and tie-lines were determined in order to establish the optimal conditions for ATPS formation. A series of extraction experiments were performed at pH 6.9 using a 20% (w/w) lysate load and systems with 16.5% (w/w) salt, 19.0% (w/w) PEG 600, and a tie-line length of 37.1% (w/w). Under these conditions, plasmid DNA was recovered in the salt-rich bottom phase. However, whereas with sodium citrate-based systems recovery yields closer to 100% were obtained, the use of ammonium sulfate enabled higher purification although with lower recoveries. Thus, the performance of ATPS prepared by combining sodium citrate and ammonium sulfate was also evaluated. A mixture of 25% (w/w) ammonium sulfate and 75% (w/w) sodium citrate offered a good compromise between plasmid recovery (91.1%) and purity (17.2%). Multi-step extraction procedures were evaluated in order to improve the process performance. Although the majority of the impurities were removed in the first step, incremental increases in the purity were obtained with the inclusion of extra steps. The top to bottom phase volume ratio was increased in order to increase plasmid concentration in the bottom phase. Although this was achieved using a phase ratio of 4, it was not possible to concentrate plasmid relatively to the starting lysate. Overall, the results show that ammonium sulfate, a salt which has a high environmental impact, can be partially replaced in ATPS by sodium citrate, without significant decrease in the performance of plasmid purification.

Optimization of methods for using polyethylene glycol as a non-permeating osmoticum for the induction of microspore embryogenesis in the Brassicaceae

The objective of this work was to enhance the quality and quantity of microspore-derived embryos of cruciferous species by using polyethylene glycol (PEG) to replace sucrose in the culture medium. The main advantage in using PEG is that it produces embryos that are morphologically more similar to zygotic embryos and have enhanced germination capabilities. When microspores were cultured in full strength NLN medium supplemented with 25% (w/v) PEG, the addition of 3 ml of full strength NLN with 13% (w/v) sucrose at 14 d was beneficial for embryo quality and quantity. Experiments showed that this PEG system could be used for a number of Brassica napus cultivars, as well as a number of other cruciferous species. PEG enhanced microspore embryogenesis in B. nigra, Crambe abyssinica, and Raphanus oleifera. Microspore-derived embryos were obtained from all cruciferous species evaluated (B. alboglabra, B. carinata, B. juncea, B. rapa, B. nigra, R. oleifera, Crambe abyssinica, Sinapis alba) using either sucrose or PEG as the osmoticum. Microspore embryogenesis was induced in B. napus in PEG-based cultures without a 32°C heat shock (i.e., 4, 15, 18, and 24°C). These temperature conditions were non-inductive when sucrose was used as the osmoticum. Spontaneous chromosome doubling occurred in 64–92% of the regenerated plants when PEG was used in the NLN culture medium, whereas in culture medium containing sucrose, the spontaneous doubling rate was 2–18%.

Improvement of dissolution rate of arbidol hydrochloride from solid dispersion prepared with PEG system by fusion method

Solid dispersions (SDs) of arbidol hydrochloride (AH) with polyethylene glycol (PEG) of different grades as dispersion carriers were prepared by applying the fusion method. Powder X-ray diffraction, differential scanning calorimetry (DSC), and Fourier transformation-IR spectroscopy (FT-IR) were utilized to evaluate the physicochemical properties of the SDs. The dissolution property of AH in the SDs was markedly enhanced in pH 1.2 buffer solution. After 3 min of dissolution, the percent concentrations of AH in the SDs with PEG 8000 and PEG 4000 were 59.03 and 53.56%, corresponding to a 15.44- and 14.01-fold increase as compared to pure AH. The water vapor adsorption test showed that the amount adsorbed on the surface of SDs had a marked increase as compared to the corresponding physical mixtures (PMs) and pure AH. Therefore, we propose that the fusion method, using PEG systems as the carriers, can be utilized to prepare SDs of AH, which will remarkably increase the wettability, solubility, and dissolution rate of AH.

The Role of SPS, MPSA, and Chloride in Additive Systems for Copper Electrodeposition

This paper documents an experimental study to compare the behavior of bis-(3-sodiumsulfopropyl) disulfide (SPS)–Cl–polyethylene glycol (PEG) and 3-mercaptopropanesulfonic acid sodium salt (MPSA)-Cl-PEG additive systems during copper electrodeposition. These systems are analogous to those used industrially to plate copper lines onto integrated circuits. Galvanostatic experiments show that in the absence of chloride ion, either SPS or MPSA added to the plating solution inhibited copper deposition. Upon the addition of , a rapid transition from inhibition to acceleration was observed for both additives, illustrating the critical role played by the chloride ion in these systems. Potentiostatic experiments performed for the Cl–PEG–SPS and Cl–PEG–MPSA additive systems show that the potential dependency of the Cl–PEG–SPS system was much stronger than that of the MPSA–Cl–PEG system. Differences between the SPS and MPSA additive systems suggest that acceleration may occur through an MPSA pathway. Finally, results with chemical compounds similar to MPSA indicate that the thiol group is associated with inhibition and that a synergistic accelerating relationship exists between the sulfonate group and chloride, presumably due to complex formation.

Studies on thermal activities of fabrics treated by polyethylene glycol

In this study, the special adjusting-temperature function of polyethylene glycol (PEG) with low molecular weight was introduced. PEGs and a two-group mixing system of PEGs of different molecular weights were added to fabrics, respectively, and the thermal activities of modified fabrics were studied. In addition, the thermal stability of PEG and fabric at a higher curing temperature was also discussed in detail. The results showed that the thermal properties of PEG decreased after being crosslinked to fabrics and the thermal activity parameters of treated fabrics could be changed and adjusted by selecting an appropriate two-group mixing system. Some thermolysis and thermooxidative degradation of PEG and fabric used in the investigations might take place at higher curing temperatures. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2288–2292, 2003

Thickness of non-ionic polymer layers carried by inorganic adsorbents in the case of the TiO 2—poly ethyleneglycol system

Abstract The variation of the quantity of poly(ethyleneglycol) adsorbed on TiO 2, which follows a logarithmic law and is consistent with the exponential decrease in non-ionic polymer density observed by Hesselink at the solid-liquid interface, results in reduced ζ potential values. This decrease is uniquely due to the shift of the shear plane with a distance x equal to the thickness of the adsorbed polymer layer. A law, allowing the calculation of the thickness x from the adsorbed quantity of polymer Γ and the ζ potential, was suggested. A coefficient Γα relating the adsorption α to x was defined.

Enhancement of Dissolution and Bioavailability of Piroxicam in Solid Dispersion Systems

Solid dispersion systems of water-insoluble piroxicam in polyethylene glycol (PEG) 4000 and in urea were prepared by fusion and solvent methods and were characterized in this study. The in vitro dissolution studies showed that the dispersion systems containing piroxicam and PEG4000 or urea gave faster dissolution than the corresponding simple mixtures. The differential scanning calorimetry (DSC) study indicated that the piroxicam-PEG system prepared by the fusion method is a solid dispersion, while the piroxicam-urea system prepared by the solvent method is likely to be a solid solution of piroxicam in urea. The storage testings showed that all dispersions were stable, except that uptake of water during storage may occur in the PEG system. A single-dose study with rabbits showed that the dispersion systems provided statistically significant to a higher extent and rate of bioavailability than the corresponding physical mixture (p < 0.05).

Species Effects on Wood-Liquefaction in Polyhydric Alcohols

Summary Four softwood and three hardwood species were liquefied using polyethylene glycol (PEG) or glycerol-containing PEG solvent systems as liquefaction reagents under defined reaction conditions: wood / liquefaction reagent / sulfuric acid = 3.00/9.00/0.27 gram, at 150 °C. The percentage residues and hydroxyl numbers were determined as an estimate of polyol value of the solution. The PEG system yielded 10–30% residual materials even under the best reaction conditions since low -OH group content resulted to re-condensation of liquefied wood (LW). The hydroxyl numbers linearly decreased from 210 to 100 mg KOH/g on extended (150 min) reaction time. On the other hand, addition of 10% glycerol to the PEG system resulted in small amount of unliquefied residues (~ 3%) and a stable hydroxyl number, regardless of the wood species tested. In case of 60 min reaction time, the mean residue value was 5.8% and a corresponding mean hydroxyl number of 216.1 mg KOH/g was obtained. This system is deemed suitable for liquefaction of a wide variety of wood species.

Affinity Partitioning of Metal Ions in Aqueous Polyethylene Glycol/Salt Two-Phase Systems with PEG-Modified Chelators

Abstract The systematic partitioning of metal ions in the presence of polyethylene glycol (PEG) chelate derivatives in PEG/salt two-phase systems is described. Results show that the partitioning is enhanced significantly, and this effect is apparently based on the characteristics of binding of the chelate and the nature of the metal ion in addition to the effect of the attached PEG molecule. Previous work with PEG-iminodiacetic acid has been extended with the chelating derivatives PEG-TED [tris(carboxymethyl)ethylene diamine], PEG-TREN-Cm [carboxy-methylated-tris(2-aminoethyl)amine], and PEG-ASP (aspartic acid). Their effectiveness to selectively partition the metal ions is demonstrated and discussed. The partition experiments were performed using two-phase systems of PEG (8000 MW), sodium sulfate, the PEG-chelating derivatives, and different concentrations of copper, nickel, and cobalt as model metal ions. Partition coefficients (ratio of top to bottom metal concentrations) greater than 1 were obtained for the metal ions with all the chelating derivatives tested. Selective enhancement of partitioning was observed in all cases. Based on the experimental results and theoretical principles, the method shows potential as an alternative mode to remove efficiently and selectively metal ions from solutions using aqueous two-phase systems and PEG-chelate derivatives.