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 ra tes of the two aqueous phases. Electrokinetic demixing has been shown to in crease by more than 5-fold the demixing rates of systems up to 100 mL in vo lume in a manner that depends on field strength, field polarity, pH, and ph ase composition. The present study is an attempt to relate demixing rates t o droplet electrokinetic mobilities which were measured microscopically and inferred from demixing data. A clear dependence of demixing rate was obser ved on drop electrophoretic mobility and pH. The electrophoretic mobility o f individual phase droplets suspended in the other phase was measured for p oly(ethylene glycol)/Dextran systems using a microelectrophoresis unit and compared with mobilities predicted by electrokinetic theory. We confirmed e arlier reports that the droplet electrophoretic mobility increased with inc reasing drop diameter and explained this increase on the basis of an intern al electroosmotic flow model. Effective electrophoretic mobilities were est imated from electrokinetic demixing data in a 100-mL column and compared wi th predicted as well as experimentally measured values of electrophoretic m obility. The mobilties increased with increased phosphate ionization due to change in pH irrespective of the sign (or polarity) of the applied electri c field. The electroosmotic flow model could explain satisfactorily the fol lowing two paradoxes: (1) the direction of migration of drops is the opposi te of that predicted by colloid electrokinetics and (2) the phase demixing rate increased irrespective of the sign of the applied electric field.