Electroosmotic Flow Phenomena in Packed Capillaries:  From the Interstitial Velocities to Intraparticle and Boundary Layer Mass Transfer

Pulsed field gradient nuclear magnetic resonance studies of electrokinetic flow through a 250 μm i.d. cylindrical fused-silica capillary packed with spherical porous particles ( d p = 41 μm) have revealed the following phenomena and parameters: (i) An electrokinetic wall effect exists due to a mismatch of zeta-potentials associated with the capillary inner wall and the particles surface. It results in a transcolumn velocity profile which depends on the column-to-particle diameter ratio and causes additional longitudinal dispersion. (ii) Compared to the pressure-driven flow through the porous medium, the intraparticle mass transfer rate constant is significantly increased under the influence of a potential gradient. This increase also depends on the buffer concentration via electric double layer overlap. (iii) Fluid molecules in the porous particles remain diffusion-limited in the presence of a pressure gradient. By contrast, intraparticle Peclet numbers above unity have been measured for electroosmotic flow and were found to increase with the applied potential difference. (iv) Interparticle resistance to mass transfer appears to vanish on the pore scale when electric double layers are small compared to the relevant pore dimension.