ON THE NATURE OF THE ENERGETIC SURFACE HETEROGENEITY IN ION ADSORPTION AT A WATER OXIDE INTERFACE - THE BEHAVIOR OF POTENTIOMETRIC, ELECTROKINETIC, AND RADIOMETRIC DATA

The fact that the surfaces of the actual solids are energetically heterogeneous has long been realized by scientists investigating the adsorption of gases and nonelectrolyte mixtures onto oxides. However, that important factor was almost totally ignored in the studies of ion adsorption at the water/oxide interface. The authors show the reason for that. These are special features of the most frequently investigated experimental data-the titration curves. Because of some compensation effects, these composite adsorption isotherms are almost insensitive to surface energetic heterogeneity. It is also shown that electrokinetic data and radiometrically measured individual adsorption isotherms of ions are very sensitive, and make even discrimination among various heterogeneity models possible. The most realistic model seems to be that assuming small correlations between the adsorption energies of different ions, adsorbed on various surface sites distributed on an oxide surface at random. That heterogeneity model is able to explain the differences between the experimentally estimated point of zero charge and isoelectric point values.