Separation of Bulk, Surface, and Topological Contributions to the Conductivity of Suspensions of Porous Particles
Résumé
Electromigration of ions through porous silica particles dispersed in an electrolyte is studied by conductivity measurements. By determining the suspension conductivity at infinite dilution of particles where the Maxwell equation is applicable, the conductivity of the particles is determined. At high ionic strength, this allows calculation of the tortuosity of the particles. The tortuosity is then used to extract the pore conductivity from the particle conductivity under low ionic strength conditions where the surface conductivity is not negligible. Evolution of pore conductivity, which appears to be related to pore size, is not monotonous when ionic strength increases, showing first a decrease at very low ionic strengths, i.e., in conditions of double layers overlap in the pores, followed by an increase to trend toward the bulk conductivity at high ionic strength. This unexpected behavior can be explained by the fact that the initial surface conductivity in pores is mainly due to the protons, provided by spontaneous dissociation of surface silanol sites in water, which are subsequently exchanged by sodium.