Effect of intraparticle porosity and double layer overlap on electrokinetic mobility in multiparticle systems

We have performed a systematic experimental study of electrokinetic mobility depending on the mobile phase ionic strength (concentration of Tris buffer) using different, well-defined porous and, for reference behavior, nonporous silica-based spherical microparticles. Effects of both an intraparticle electrical double layer overlap and the porosity on electrophoretic mobility in dilute suspensions and electroosmotic mobility in fixed beds of the devised (non)porous particles were investigated. The electrokinetically consistent results demonstrate a substantially different behavior for the porous (i.e., permeable and conducting) particles with respect to nonporous (solid) spheres. It may be related to a porous particle's dipole coefficient via the intraparticle void volume and, strongly depending on mobile phase ionic strength, to the actual magnitude of electroosmotic flow jetted through a particle's interior. In contrast to the normal electrical double layer behavior observed for the solid spheres (continuous decrease of mobility with ionic strength), these competitive contributions give rise to pronounced maxima in the mobility of the porous spheres. Our results are in qualitative agreement with the theoretical analysis of Miller et al. [J. Colloid Interface Sci. 1992,153,237] which, depending on the double layer interaction within a porous aggregate of solid spheres, predicts (significantly) lower or higher mobilities with respect to an impermeable and nonconducting particle.