Rejection of polyelectrolytes from microporous membranes

Abstract

The rejection coefficient for rigid (bovine serum albumin) and flexible (hydrolyzed polyacrylamide) polyelectrolytes during ultrafiltration was experimentally determined by a transient material balance technique. Parameters such as supporting electrolyte (NaCl) concentration, pore radius, and pore flow velocity were varied to study their effects on the degree of rejection. The membranes were fabricated by the track-etch process and offered the advantage of a well-defined pore structure in which constrictions and tortuosity effects were minimal. At a given pore radius, the NaCl concentration strongly affected the rejection coefficient by altering the long-range electrostatic forces between pore wall and macromolecule for the rigid polyelectrolyte, and by substantially altering the molecular conformation of the flexible polyelectrolyte. In all cases, the rejection coefficient increased as the ionic strength decreased. With the flexible macromolecules, the shear rate in the pore also appeared to be a significant factor. Our most surprising result is that the flexible polyelectrolyte was able to pass through pores of radius less than one-half the hydrodynamic (Einstein) radius of the macromolecule, indicating that flexible macromolecules cannot be modeled as rigid spheres in predicting the rejection coefficient.