Electrostatic and electrokinetic effects on hindered diffusion in pores

When a macromolecule or colloidal particle is in a pore of comparable size, there will be an increase in its drag coefficient due to hydrodynamic interactions with the pore wall, which reduces the particle diffusivity. For charged particles, there can be additional drag due to the distortion of the electrical double layer caused by particle motion. This electrokinetic retardation was evaluated for a sphere in a cylindrical tube, the surfaces being of like charge and constant charge density. This was done using finite element solutions of the equations of motion, Poissonʹs equation, and conservation equations for small ions in the electrolyte. A perturbation scheme was employed involving both the particle Peclet number and the particle or pore surface charge density. The results indicate that electrokinetic retardation might noticeably decrease the intrapore diffusivity of a charged macromolecule, relative to that for an uncharged molecule of the same size. The increase in drag (or reduction in mobility) due to electrical charge was found to be much greater for confined particles than for ones in bulk solution, especially for large Debye lengths. However, the electrostatic decrease in the equilibrium partition coefficient was shown to be a more important determinant of the overall diffusional permeability. In other words, the effects of charge on the partition coefficient of a macromolecule outweighed their effects on the intrapore diffusivity.