Electrokinetic Demixing of Aqueous Two-Phase Systems. 3. Drop Electrophoretic Mobilities and Demixing Rates

Scale-up of aqueous two-phase extraction, which is useful in the isolation and
purification of certain bioproducts, is limited by the slow demixing rates of the two
aqueous phases. Electrokinetic demixing has been shown to increase by more than
5-fold the demixing rates of systems up to 100 mL in volume in a manner that depends
on field strength, field polarity, pH, and phase composition. The present study is an
attempt to relate demixing rates to droplet electrokinetic mobilities which were
measured microscopically and inferred from demixing data. A clear dependence of
demixing rate was observed on drop electrophoretic mobility and pH. The electrophoretic
mobility of individual phase droplets suspended in the other phase was
measured for poly(ethylene glycol)/Dextran systems using a microelectrophoresis unit
and compared with mobilities predicted by electrokinetic theory. We confirmed earlier
reports that the droplet electrophoretic mobility increased with increasing drop
diameter and explained this increase on the basis of an internal electroosmotic flow
model. Effective electrophoretic mobilities were estimated from electrokinetic demixing
data in a 100-mL column and compared with predicted as well as experimentally
measured values of electrophoretic mobility. The mobilties increased with increased
phosphate ionization due to change in pH irrespective of the sign (or polarity) of the
applied electric field. The electroosmotic flow model could explain satisfactorily the
following two paradoxes: (1) the direction of migration of drops is the opposite of that
predicted by colloid electrokinetics and (2) the phase demixing rate increased
irrespective of the sign of the applied electric field.