Discreteness of charge and solvation effects in cation adsorption at the oxide/water interface

Streaming potential data for the vitreous silica/aqueous electrolyte interface have been obtained to yield the ζ potential variation with pH as a function of the concentration of KNO3, Ba(NO3)2 and La(NO3)3 solutions. The results have been interpreted in terms of the Gouy-Chapman-Stern theory, modified to include discreteness of charge effects, to account for the observed maxima in |ζ| with pH for adsorbed Ba2+ and La3+. For satisfactory agreement between theory and experiment it was necessary to add a term to account for the change in the free energy of solvation as the multicharged cations adsorb at the silica/water interface.