Intrinsic Proton Affinity of Reactive Surface Groups of Metal (Hydr)oxides: The Bond Valence Principle

The proton affinity of individual surface groups has been calculated with a redefined version of the multi site complexation (MUSIC) model. In the new approach the proton affinity of an oxygen originates from the undersaturation of the oxygen valence. The factors valence and coordination number, which are the basis of Pauling's definition of bond valence, in combination with the number of coordinating (Me and H) ions, are dominant in determining the proton affinity. The neutralization of an oxygen by Me ion(s) is calculated on the basis of the actual bond valence, which accounts for structural details, resulting from an asymmetrical distribution of charge in the coordination environment. An important role in the new version of the MUSIC model is given to the H bonds. The model shows that the proton affinity is determined not only by the number of donating H bonds but also by the number of accepting H bonds. The proton affinity of surface groups and of solution complexes can be understood in one theoretical framework, on the basis of a different number of donating and accepting H bonds. The MUSIC model predicts the variation in proton affinity constants for surface groups in particular those with the same number of coordinating Me ions but with a different structural position. The model is able to predict on the basis of the proton affinity of the individual groups the correct PZC of Me hydroxides, oxohydroxides, and oxides, and explains previous exceptions. The model can also be applied in general to other minerals.