CONVECTIVE DIFFUSION IN POROUS MEMBRANES WITH ADSORBED CHARGES

We have studied experimentally the effects that the charge adsorbed on the membrane pore walls exerts on the convective diffusion through a porous membrane. The plot of the logarithm of the bulk concentration ratio vs convective flow through the membrane is usually employed to determine the membrane parameter A/L (effective membrane area/membrane thickness). It is shown that the deviations from the linear behavior of this plot give us information on the adsorbed charge concentration. A simple model based on the Nernst-Planck equation for the ion fluxes and the Navier-Stokes equation for the pore fluid velocity has proved to be useful for the understanding of the experimental trends observed over a range of membrane pore radius and electrolyte concentration. Our study can be seen as an independent confirmation of a rather general result: ion adsorption to the pore walls of inert, porous membranes can significantly affect the transport phenomena, especially in the limit of low electrolyte concentration.