The effect of the discrete nature of ions on the excess adsorption of Na+ and Cl− at the silica–aqueous interface

Wet pastes of silica, prepared by centrifugation of dispersions (with a salt concentration of 100 mM NaCl) that subsequently were decanted the supernatant, were analysed using X-ray photoelectron spectroscopy. The uptake of Na and Cl ions by the wet pastes were quantified and compared with predictions based on the Gouy–Chapman (GC) concept, the triple layer model (TLM), and a modified TLM, which takes into account the discrete ion effect. The most striking feature of the results is that the Cl uptake increases by a factor of ∼2 with increasing surface counter charge, which has been explained on the basis of the discrete nature of ions.