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.