Zeta potential of glass bead packs — Streaming potential apparatus bench-marking and search for a reference value

Most zeta potential data for consolidated porous samples are obtained using an in-house-built apparatus for streaming potential measurements, without an established benchmarking procedure. It is demonstrated that a commonly used method of result comparison with available literature is unreliable. Therefore, zeta potential measurements were conducted on porous soda lime glass bead packs using two experimental setups: an in-house system and a commercial electrokinetic analyzer. The experiments evaluated the influence of measurement configuration and bead size on electrokinetic responses in sodium chloride solutions. Results show that nominally identical solid–liquid systems can yield distinct zeta potential values depending on the experimental setup, highlighting the importance of measurement configuration and system size in comparative studies. Benchmarking across bead sizes revealed an increase in zeta potential magnitude with decreasing bead radius, contradicting predictions from the modified Helmholtz–Smoluchowski equation for porous media. Additional comparisons with glass plates of similar composition indicate that smaller beads exhibit apparent zeta potential overestimation due to surface reactivity and pH variation. The findings emphasize the coupled effects of surface reactivity and system configuration, supported by a simplified capillary bundle model that, for the first time, captures the experimentally observed trends.