top or the bottom phases of PEG/DX system employing 20–50 Vm –1 electric
fields perpendicular to the phase interfaces. They could achieve separation of
binary mixtures in both batch and continuous modes by operating between isoelectric points and directing oppositely charged proteins into opposite phases.
Recently several studies have been reported regarding improved extractive
separation by the application of electric fields to both traditional organic solvent extraction systems and ATPSs [126–130]. Scott and Wham [127] applied an
electric field to create emulsion with high interfacial area and contacting with
the continuous phase and also to induce coalescence in a novel counter current
extractor. Increased mass transfer was achieved due to the altered convection
currents within and around the oscillating aqueous drops dispersed in a continuous, nonconducting, organic phase under the influence of a pulsating
electric field [129]. Electroextraction with an applied field of 250 Vm –1 was
successfully employed to recover citric acid from water using n-butanol
(saturated with water) as solvent [131].
Levine and coworkers [132, 133] have reported that electrophoretic transport
of proteins across the interface of ATPS is greatly impeded in one direction.
They have indicated that the electrophoretic transfer of proteins is readily
achieved if the protein is migrating from its less-preferred phase (which is not
favorable according to its equilibrium partition behavior) to the morepreferred phase. They also observed that the protein does not migrate in the
opposite direction, that is, from the more-preferred phase to the less-preferred
phase under similar conditions. However, Theos and Clark [126] have shown
that the protein can be made to transfer across the aqueous two-phase
boundary in both directions.
Scale-up of ATPE, which is useful in the isolation and purification of bioproducts [134, 135], depends to a large extent upon the rapid demixing of the
phases with the desired product concentrated in one of them. Pairs of phases
involved in ATPE are characterized by high viscosities, low interfacial tensions,
and similar densities [70, 80]. These properties lead to slow demixing rates of
these phases which has been counteracted by centrifugation, column contacting, or electrokinetic demixing – each having its own drawbacks [85].
Electrokinetic demixing has been shown to increase demixing rates of ATPSs
(up to 100 ml) more than fivefold in a manner that depends on field strength,
field polarity, concentration of partitioning anion, and phase composition [91,
92]. Electrokinetic demixing is also potentially useful in situations such as low
gravity where an additional force has to be introduced for the demixing of the
equilibrated phases [136]. Operation of electrokinetic demixing on a commercial scale requires further understanding of the fundamentals involved in
the process, which is the motivation for our recent work where the objective was
to identify the mechanism of enhanced demixing [137].
When two polymers are dissolved in aqueous solution at concentrations that
cause phase separation, certain dissolved ions such as phosphate are unequally
partitioned between the phases [138] leading to an electrical potential across
the interface [139] and an apparent electrokinetic potential at the surface of the
dispersed phase droplets [140, 141]. As a consequence of the latter, droplets of
dispersed phase move in the continuous phase in the presence of an externally
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
175
fields perpendicular to the phase interfaces. They could achieve separation of
binary mixtures in both batch and continuous modes by operating between isoelectric points and directing oppositely charged proteins into opposite phases.
Recently several studies have been reported regarding improved extractive
separation by the application of electric fields to both traditional organic solvent extraction systems and ATPSs [126–130]. Scott and Wham [127] applied an
electric field to create emulsion with high interfacial area and contacting with
the continuous phase and also to induce coalescence in a novel counter current
extractor. Increased mass transfer was achieved due to the altered convection
currents within and around the oscillating aqueous drops dispersed in a continuous, nonconducting, organic phase under the influence of a pulsating
electric field [129]. Electroextraction with an applied field of 250 Vm –1 was
successfully employed to recover citric acid from water using n-butanol
(saturated with water) as solvent [131].
Levine and coworkers [132, 133] have reported that electrophoretic transport
of proteins across the interface of ATPS is greatly impeded in one direction.
They have indicated that the electrophoretic transfer of proteins is readily
achieved if the protein is migrating from its less-preferred phase (which is not
favorable according to its equilibrium partition behavior) to the morepreferred phase. They also observed that the protein does not migrate in the
opposite direction, that is, from the more-preferred phase to the less-preferred
phase under similar conditions. However, Theos and Clark [126] have shown
that the protein can be made to transfer across the aqueous two-phase
boundary in both directions.
Scale-up of ATPE, which is useful in the isolation and purification of bioproducts [134, 135], depends to a large extent upon the rapid demixing of the
phases with the desired product concentrated in one of them. Pairs of phases
involved in ATPE are characterized by high viscosities, low interfacial tensions,
and similar densities [70, 80]. These properties lead to slow demixing rates of
these phases which has been counteracted by centrifugation, column contacting, or electrokinetic demixing – each having its own drawbacks [85].
Electrokinetic demixing has been shown to increase demixing rates of ATPSs
(up to 100 ml) more than fivefold in a manner that depends on field strength,
field polarity, concentration of partitioning anion, and phase composition [91,
92]. Electrokinetic demixing is also potentially useful in situations such as low
gravity where an additional force has to be introduced for the demixing of the
equilibrated phases [136]. Operation of electrokinetic demixing on a commercial scale requires further understanding of the fundamentals involved in
the process, which is the motivation for our recent work where the objective was
to identify the mechanism of enhanced demixing [137].
When two polymers are dissolved in aqueous solution at concentrations that
cause phase separation, certain dissolved ions such as phosphate are unequally
partitioned between the phases [138] leading to an electrical potential across
the interface [139] and an apparent electrokinetic potential at the surface of the
dispersed phase droplets [140, 141]. As a consequence of the latter, droplets of
dispersed phase move in the continuous phase in the presence of an externally
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
175