Electrokinetic effects on the transport of charged analytes in biporous media with discrete ion-permselective regions.

The influence of external electrical fields on local concentration distributions and the mass transport of ionic background (buffer) species, as well as eluting co- and counterionic tracer molecules, was investigated in a fixed bed of native glass beads by confocal laser scanning microscopy and numerical simulations. Due to the negative surface charge of the porous glass beads and significant electrical double layer overlap, the intraparticle mesopore space becomes ion-permselective. This cation selectivity and the externally superimposed electrical fields induce concentration polarization in the bulk electrolyte solution adjacent to the particles. At the anodic hemisphere of a bead, the actual interplay of convection, diffusion, and electromigration leads to the formation of a convective-diffusion boundary layer with reduced ion concentrations relative to the bulk solution. At the opposite, cathodic hemisphere where counterions leave a bead in the direction of the applied field, electrolyte concentrations increase generating an enriched concentration polarization zone. Complementary data from quantitative confocal laser scanning microscopy and numerical simulations provide insight into the spatial variations of chemical and electrical potential gradients in the hierarchically structured material, including molar flux densities of the background ionic species, and reveal the elution dynamics of co- and counterionic analytes. These results demonstrate that concentration polarization in the external fluid domain, as well as the magnitude and sign of electrophoretic with respect to electroosmotic mobility in the ion-permselective domain, are major local contributions to coupled mass and charge transport, reflecting analyte retention, migration, and dispersion on a macroscopic scale.