Electrostatic and electrokinetic potentials in two polymer aqueous phase systems

Abstract

Two polymer aqueous phase systems have proven useful as partition media for biological cells, the partition coefficients obtained being dependent on cell surface properties. One such property appears to be the surface charge density of cells which prompted this detailed examination of electrostatic potential distributions in dextran/poly(ethylene glycol) two phase systems. The partition coefficients of potassium sulfate and potassium chloride in the phase systems were measured and correlated with the Donnan potential difference between the phases, measured with salt bridges to reversible electrodes. Thermodynamic predictions of the relationship between the two measured quantities were confirmed. The preference of sulfate for the dextran-rich bottom phase, which linearly increases with increasing poly(ethylene glycol) (PEG) concentration, is due to the greater exclusion of sulfate by PEG than by dextran, as indicated by equilibrium dialysis of the polymer against different salt concentrations. Electrokinetic studies of droplets of one phase suspended in the other phase revealed relatively large electrophoretic droplet mobilities which increased linearly with the drop diameter and supralinearly with the sulfate concentration. The sign of the mobility depended on the phase the droplet originated from, but the implied surface charge sign was opposite to that anticipated from the difference in potential between the bulk phases. Assuming the mobility and zeta potential are of the same sign, the simplest potential profile which is consistent with these observations is one in which a dipole potential is present at the phase boundary oriented in such a way as to locally reverse the potential gradient. Levine's partial theory for the electrophoresis of systems of this type was tested and found to be consistent with our results. The correlations obtained suggest that the magnitude of the dipole potential is related to the anion partition coefficient in these systems. © 1984 Academic Press, Inc.