Electrostatic and electrokinetic potentials in two polymer aqueous phase systems

Electrophoretic Mobility of a Water-in-Oil Droplet Separately Affected by the Net Charge and Surface Charge Density

Water-in-oil emulsions and droplets exhibit physicochemical properties completely different from those of oil-in-water emulsions and droplets. Thus, directly applying a standard theoretical model to water-in-oil systems cannot describe these anomalous properties. Here, the electrophoretic mobility of a water-in-oil droplet is analytically investigated using Debye-Hückel linearization and neglecting the Marangoni effect. The resulting electrophoretic mobility is shown to be separately dependent on the net charge of the droplet and the surface charge density at the droplet interface. Furthermore, when the net charge is negligible, the electrophoretic mobility is proportional to the surface charge density with a negative coefficient. This indicates that the internal electric double layer inversely contributes to the electrophoresis. This theory is applied to experimental data of water-in-oil emulsions and droplets in the literature, and qualitative and quantitative verification of the theory is discussed.

Protein Transport and Concentration by Electrophoresis in Two-phase Microflows

This work aims at a new class of methods for electric field-mediated separation of biomolecules. In contrast to standard techniques, electrophoretic transport was not performed in a single-phase homogenous fluid, but in a system of stratified layers. The large surface-to-volume ratio characteristic for microfluidic systems allows the interactions of biomolecules with liquid/liquid interfaces to be examined and the suitability of corresponding effects for applications in the field of biomolecular separations and enrichment to be assessed. To perform these types of studies, a micro flow cell was fabricated into which two coflowing immiscible liquid phases can be introduced. Subsequently, the electrophoretic transport of biomolecules driven by an electric field perpendicular to the channel was examined.

To investigate the transport phenomena related to electrophoresis in stratified two-phase systems, aqueous solutions consisting of polyethylene glycol and dextran were prepared, which allowed a stable interface to develop. Transport within one phase and an enrichment of proteins at the phase boundary has been established. In addition, other kinds of fluid combinations such as water and propylene carbonate have been examined, also supporting the enrichment of proteins at the phase boundary.

Donnan potentials in aqueous phase-separated polymer mixtures

A promising approach to texturize water is by the addition of mutually incompatible polymers, leading to phase separation. Here, we demonstrate that the phase stability of aqueous polymer solutions is affected not only by chemical differences between the polymers but also by their electric charge. Direct electrochemical measurements are performed of the electric potential difference between two coexisting phases in aqueous solutions of the charged protein fish gelatin (nongelling) and the uncharged polysaccharide dextran. Charge counteracts demixing because of the entropic cost of confining the counterions to one phase, resulting in a strong shift of the critical point upon an increase of the charge on one of the polymers. Upon phase separation, the charged polymer is spatially confined, and due to the Donnan effect, an interfacial electric potential is developed. A direct proportionality is found between this Donnan potential and the difference in gelatin concentration in the two phases, for which we propose a theoretical explanation. The electrostatics may provide a new handle in the development of stable water-in-water emulsions.

Concentration and size separation of DNA samples at liquid-liquid interfaces

This report introduces a new analytical concept utilizing the mass transfer resistance of a liquid-liquid interface to concentrate and separate DNA samples. DNA molecules can be electrophoretically accumulated at a liquid-liquid interface of an aqueous two-phase system (ATPS) of poly(ethylene glycol) (PEG) and dextran, two polymers that form two immiscible phases in aqueous electrolyte solutions. The detachment of DNA from the interface into the other phase can be triggered by increasing the applied electric field. We experimentally study the size dependence of the detachment process for a broad spectrum of DNA fragments. In a regime where the coiling of the chains does not play a significant role, the process shows a linear dependence on the diffusion coefficient, with shorter DNA chains detaching at lower electric field strengths than larger ones. The concept may enable novel separation protocols for preparative and analytical purposes.

Electrophoretic transport of biomolecules across liquid-liquid interfaces

The mass transfer resistance of a liquid-liquid interface in an aqueous two-phase system composed of poly(ethylene glycol) and dextran is investigated. Different types of proteins and DNA stained with fluorescent dyes serve as probes to study the transport processes close to the interface. A microfluidic device is employed to enable the electrophoretic transport of biomolecules from one phase to another. The results obtained for proteins can be explained solely via the different electrophoretic mobilities and different affinities of the molecules to the two phases, without any indications of a significant mass transfer resistance of the liquid-liquid interface. By contrast, DNA molecules adsorb to the interface and only desorb under an increased electric field strength. The desorption process carries the signature of a thermally activated escape from a metastable state, as reflected in the exponential decay of the fluorescence intensity at the interface as a function of time.

Protein diffusion across the interface in aqueous two-phase systems

We present a detailed study of the diffusive transport of proteins across a fluid phase boundary within aqueous two-phase systems. The aim of the work is to investigate whether local effects at the phase boundary cause a retardation of the diffusive transport between the phases. Possible modifications of interfacial mass transfer could be due to protein adsorption at the phase boundary or local electric fields from electric double layers. Experiments with a microfluidic system have been performed in which protein diffusion (bovine serum albumin and ovalbumin) within a bilaminated configuration of two phases containing polyethylene glycol and dextran is analyzed. A one-dimensional model incorporating phase-specific diffusion constants and the difference in chemical potential between the phases has been formulated. A comparison of experimental and simulation data shows a good overall agreement and suggests that a potential local influence of the phase boundary on protein transport is insignificant for the systems under investigation.

Electrophoretic partitioning of proteins in two-phase microflows

This work reports on protein transport phenomena discovered in partitioning experiments with a novel setup for continuous-flow two-phase electrophoresis consisting of a microchannel in which a phase boundary is formed in flow direction. Proteins can be partitioned exploiting their affinity to different aqueous phases in two-phase systems. This separation process may be enhanced or extended by applying an electric field perpendicular to the phase boundary. In this context, microsystems offer new possibilities, as interfacial forces usually dominate over volume forces, thus allowing a superior control of the formation and arrangement of liquid/liquid phase boundaries. The two immiscible phases which are injected separately into the microchannel are taken from a polyethylene glycol (PEG)-dextran system. The side walls of the channel are partially made of gel material which serves as an ion conductor and decouples the channel from the electrodes, thus preventing bubble generation inside the separation channel. The experiments show that the electrophoretic transport of proteins between the laminated liquid phases is characterized by a strong asymmetry. When bovine serum albumin (BSA) is introduced into the PEG-rich phase, it can easily be transferred into the dextran-rich phase via an applied electric field of low strength or just by diffusion. In the reverse case, up to a certain field strength the transfer to the opposing phase is strongly inhibited. Only if the field strength is further increased will the BSA molecules leave the dextran-rich phase almost completely.

Aqueous two-phase systems for biomolecule separation

Over the past thirty years, aqueous polymer two-phase technology has evolved, both experimentally and theoretically, into a separation science with many useful applications in biomolecule purification and bioconversion. This paper summarizes the developments in the applications of aqueous two-phase systems to biotechnology. The main topics to be considered are the phase diagram and its characteristics, fundamentals of biomolecule partition, large-scale and multi-stage aqueous two-phase biomolecule purification, and extractive bioconversions. The first topic involves a discussion of the thermodynamics of aqueous polymer two-phase formation and how it is influenced by such factors as polymer molecular weight and concentration, temperature, and salt type and concentration. Next, the theoretical and experimental aspects of biomolecule partition in aqueous two-phase systems will be discussed in light of the factors which influence biomolecule partition: polymer concentration and molecular weight; temperature; salt type and concentration; the addition of charged, hydrophobic and affinity derivatives. Having reviewed the fundamentals of phase diagram formation and biomolecule partition, the next two topics are applications of aqueous two-phase technology. The first set of applications involve the large-scale extraction of proteins using one to three equilibrium stages and multi-stage purifications using countercurrent distribution, liquid-liquid partition chromatography and continuous countercurrent chromatography. The second application, and very promising area for future aqueous two-phase technology, is the extractive bioconversion which permits the simultaneous production and purification of a biomolecule.

Electrokinetic demixing of aqueous two-phase polymer/salt systems

Electrokinetic demixing of aqueous two-phase polymer/salt systems is demonstrated, resulting in significant enhancement in demixing rates by about 1-4-fold. The effect of field polarity, field strength, volume ratio, and phase composition on phase demixing has been studied. Further the influence of these parameters on phase demixing could be explained based on the hydrodynamic flow-electroosmotic flow (HEF) model.

Multistage magnetic and electrophoretic extraction of cells, particles and macromolecules

Improved techniques for separating cells, particles, and macromolecules (proteins) are increasingly important to biotechnology because separation is frequently the limiting factor for many biological processes. Manufacturers of new enzymes and pharmaceutical products require improved methods for recovering intact cells and intracellular products. Similarly isolation, purification, and concentration of many biomolecules produced in fermentation processes is extremely important. Often such downstream processing contributes a large portion of the product cost. In conventional methods like centrifugation and even modern methods like chromatography, scale-up problems are enormous, making them uneconomical and prohibitively expensive unless the product is of very high value. Therefore there has been a need for efficient and economical alternative approaches to bioseparation processes to eliminate, reduce, or facilitate solids handling. Magnetic and electric field assisted separations may hold considerable potential for providing a future major improvement in bioseparation technology. In the present review the merits and demerits of the existing methods are discussed. We present mainly our own research on the development of unified multistage extraction processes that are versatile enough to handle cells and particles as well as macromolecules as described below. We describe multistage methods, namely ADSEP (Advanced Separator), MAGSEP (Magnetic Separator), and ELECSEP (Electrophoretic Separator), for quantitatively separating cells, particles, and solutes by using magnetically and electrophoretically assisted extraction processes. To the best of our knowledge, multistage magnetic and electrophoretic separations have not been reported in the earlier literature. The theoretical underpinnings of these separations are crucial to their success and to the identification of their advantages over other separation processes in particular applications. Hence mathematical modeling is stressed here, presenting our own models while also reviewing models reported in the literature. We also present suggestions for future work while analyzing the scale-up and economic aspects of these extraction processes. Commercial uses of the magnetic and electrophoretic processes, having both ground- and space-based research elements, also are presented in this review.

Viscosity prediction of polyethylene glycol-dextran-water solutions used in aqueous two-phase systems

The dynamic viscosities of aqueous polyethylene glycol and dextran, and poly(ethylene glycol)-dextran-water solutions were measured at temperatures of 30, 50 and 70 degrees C. The poly(ethylene glycol) having a relative molecular mass of 8000, and dextran samples with relative molecular masses of 37 500, 494 000, and 2 000 000 were used. A one-parameter Grunberg-like equation proposed earlier by us was used for estimating the values of viscosity of poly(ethylene glycol)-dextran-water solutions. The disposable parameter a for this temperature range was calculated as 1.81 for PEG 8000-dextran 37 500-water solutions, as 2.36 for PEG 8000-dextran 494 000-water solutions, and as 2.57 for PEG 8000-dextran 2 000 000-water solutions. It was observed that the relative errors get larger as the relative molecular mass of the dextran increases but vary between 0.00 and 9.37 in absolute value. In view of the results obtained here and before, we may claim that the proposed model works constantly well at different temperatures giving comparable values for the disposable parameter a.

Bovine serum albumin partitioning in an aqueous two-phase system: effect of pH and sodium chloride concentration

The partitioning of bovine serum albumin (BSA) in a polyethylene glycol 3350 (8% w/w)-dextran 37 500 (6% w/w)-0.05 M phosphate aqueous two-phase was investigated at different pHs, at varying concentrations of sodium chloride at 20 degrees C. The effect of NaCl concentration on the partition coefficient of BSA was studied for the PEG-dx systems with initial pH values of 4.2, 5.0, 7.0, 9.0, and 9.8. The NaCl concentrations in the phase systems with constant pH value were 0.06, 0.1, 0.2, 0.3, and 0.34 M. It was observed that the BSA partition coefficient decreased at concentrations smaller than 0.2 M NaCl and increased at concentrations greater than 0.2 M NaCl for all systems with initial pHs of 4.2, 5.0, 7.0, 9.0, and 9.8. It was also seen that the partition coefficient of BSA decreased as the pH of the aqueous two-phase systems increased at any NaCl salt concentration studied.

Consistent view of electrolytes in aqueous two-phase systems

The effects of electrolytes in aqueous two-phase systems are investigated. It is shown that macroscopic and molecular models give a consistent view of electrolytes at interfaces. The electrostatic potential difference delta psi between coexisting phases is a common property at interfaces even though the phases are strictly electroneutral and delta psi can not be measured. It is shown how delta psi can be quantified under controlled conditions. Additionally, a molecular picture is presented based on computer simulations.

Purification and aqueous two-phase partitioning properties of recombinant Vitreoscilla hemoglobin

Soluble recombinant Vitreoscilla hemoglobin was purified from E. coli lysate by sequential two-phase extraction techniques. Extraction of lysate containing VHb in PEG/dextran gave a 3.6-fold increase in VHb purity in the PEG-rich phase via a size exclusion mechanism. Further extraction of the recovered PEG phase in PEG/sodium sulfate gave an additional 2.0-fold increase in purity in the PEG-rich phase due to an electrostatic mechanism. Final extraction of the PEG phase in PEG/magnesium sulfate gave an additional 1.3-fold increase in VHb purity in the magnesium sulfate-rich phase. The final yield from the extractive purification was 47% with purity of VHb estimated to be greater than 95%. Yields from the sulfate salt extractions are essentially quantitative due to the extreme partitioning behavior of VHb in these systems. VHb partition coefficients as large as 46 in PEG/sodium sulfate and as small as 0.06 in PEG/magnesium sulfate were observed. Similar small partition coefficients were obtained with PEG/manganese sulfate extractions. This dramatic effect of divalent cation content on the partition coefficient of VHb in PEG/sulfate salt systems was investigated by pH and magnesium ion titration experiments. Results show the effect to be largest and nearly constant for pH values greater than 6.0 and diminished at lower pH values. A model based on magnesium ion binding to negatively charged amino acids is shown to correlate with the data well. Based on model formulation and the partitioning behavior of contaminant proteins, the observed effect is expected to be applicable to other proteins.

Electrophoretic transport of solutes in aqueous two-phase systems

Electrophoretic transport of proteins across the interface between the phases of an aqueous polymer two-phase system can be greatly impeded in comparison with transport within the individual phase. This effect can be controlled by modifying the affinity of the protein for a phase by suitable manipulations of such variables as pH. The effect is not caused by differences in the electrophoretic velocity between the two phases, nor by large changes in pH at the interface. An analogy exists between this phenomenon and the related subject of diffusion of electrolytes across the phase interface.

Phase partitioning in space and on earth

In aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system consisting of a PEG-rich phase floating on top of a dextran-rich phase. Biological particles and macromolecules tend to partition differentially between the phases and the liquid-liquid phase interface in these systems. Bioparticle partitioning has been shown to be related to physiologically important surface properties such as membrane charge or lipid composition. Affinity partitioning into the PEG-rich phase can be accomplished by coupling PEG to a ligand having affinity for specific cells or macromolecules. Subpopulations can be identified or separated using multi-step countercurrent distribution (CCD). Incomplete understanding of the influence of gravity on the efficiency and quality of the impressive separations achievable by partitioning, and appreciation for the versatility of this efficient technique, have led to its study for low-gravity biomaterials processing. On Earth, two-phase systems rapidly demix because of density differences between the phases. In low-gravity, demixing has been shown to occur primarily by coalescence. Polymer surface coatings, developed to control localization of demixed phases in low-g, have been found to control electroosmosis which adversely affects electrophoretic separation processes on Earth and in space. In addition PEG-derivatized antibodies have been synthesized for use in immunoaffinity cell partitioning.

Heterogeneity in the surface properties of B16 melanoma cells from sublines with differing metastatic potential detected via two-polymer aqueous-phase partition

When mixed in aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system, consisting of a dextran-enriched lower phase and a PEG-enriched upper phase. Two B16 mouse melanoma cell lines, B16-F1 (low lung colonizing capability) and B16-F10 (high lung colonizing capability) were found to partition differentially into the upper phase in a variety of two-phase systems. Upper-phase partition depends primarily on either hydrophilic (i.e., surface charge density) or hydrophobic (i.e., affinity for the hydrocarbon chain of a PEG-fatty acid ester) cell surface properties, depending on the system used. In single-step partition studies, cells of the B16-F10 subline displayed a greater preference than B16-F1 cells for the upper phase in the hydrophilic system and less preference in systems sensitive to hydrophobic properties. Countercurrent distribution (CCD) experiments, performed with [125I]deoxyuridine DNA-labelled cells, were consistent with single-step partition results. These CCD results demonstrated that B16-F10 cells exhibited greater DNA synthesis than B16-F1 cells and that considerable heterogeneity, in both hydrophobic and hydrophilic surface properties, was present in subpopulations of cells of both sublines. The data also showed considerable enrichment of 125I-specific cell activity in certain sections of the distributions, indicating that differences in cellular DNA synthesis are reflected in the surface properties to which partition is sensitive.