Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach

At the solid/solution interface of (hydr)oxides various types of surface groups exist, each reacting according to its own affinity constant (K) for protons. A model is presented that estimates the value of the log K of various types of surface groups (singly, doubly, and triply metal-coordinated O(H) and OH(H) surface groups) of (hydr)oxides. The intrinsic affinity constants (K) depend on many factors, e.g., the valence of the central cation (Me) of the (hydr)oxides, its electron configuration, and the MeH distance of the reacting surface group. Besides these also the number of surrounding ligands, the number of central cations coordinating with a ligand, and the type of reacting ligand (an oxo or hydroxo species) determine the proton affinity constant. Proton adsorption reactions can in principle be considered as a two-step proton adsorption reaction, forming OH and OH2 species at the surface. Analysis of the calculated affinity constants shows, however, that generally a surface group will react in a limited pH range (for instance pH 3–10) only according to a one-step protonation reaction (1-pK model). A general MUltiSIte Complexation model (MUSIC) is presented, which is based on crystallographic considerations. The new site binding model (MUSIC) can unify the classical 2-pK model and the recently presented 1-pK model, both being special cases of the model described here. The surface charge of a surface with more than one type of surface group can be described with one proton adsorption reaction and one discrete K for each type of surface group.