Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach: I. Model description and evaluation of intrinsic reaction constants

Abstract

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 OH 2 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-p K 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-p K model and the recently presented 1-p K 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.