Electrostatic and electrokinetic effects on hindered convection in pores

The sieving of macromolecules in ultrafiltration is affected by solute and pore charge, as well as size. A large, relatively rigid molecule such as a globular protein may be viewed as a particle in an electrolyte solution. Charge may influence both its equilibrium partition coefficient and its lag coefficient (G), which is the ratio of particle to fluid velocity. Partitioning had been examined previously for spheres in cylindrical pores by using continuum double layer theory to evaluate the electrostatic potential energy (E). The present objective was to estimate G for particles and pores of like charge. Particle or fluid motion tends to distort the diffuse double layers, an effect termed "relaxation," which increases the drag on the particle. The streaming potential that arises from flow through a charged pore under open-circuit conditions also increases the drag on a confined, stationary particle. These electrokinetic effects were quantified using finite element solutions of the equations of motion, Poisson's equation, and conservation equations for small ions in the electrolyte. It was found that charge effects generally reduce G, with relaxation tending to be the more important contributor. Thus, a freely suspended, charged particle will move through a pore more slowly than an uncharged one of the same size. However, the effects of E on sieving outweigh those of the electrokinetic decrease in G. That is, charge influences sieving mainly by altering the partition coefficient.