been made to predict proton affinity constants of individual An improved method has been developed for the calculation of surface groups for metal (hydr)oxides. Hiemstra and van proton affinities of individual surface groups of metal (hydr)ox-Riemsdijk were the first to predict the proton affinities for ides. The method is a refinement of the multi site complexation different types of surface groups. In this so-called MUSIC (MUSIC) model. In the MUSIC model, Pauling bond valences are model a relation is found between the proton affinity of a used to predict the individual proton affinities of surface groups of surface group and the summation of all bond valences of metal (hydr)oxides. One of the major changes in the refined MUthe bonds with underlying metal ions (6, 7).
SIC model is that the Pauling bond valences are redefined with a
In the model it is assumed that the charge of a central ion method in which the bond valence is related to the bond length.
is equally distributed over its ligands which implies the use In this paper, the refined MUSIC model is applied to three different iron (hydr)oxides (goethite, lepidocrocite, and hematite). The of Pauling bond valences (8). This prerequisite, however, resulting proton affinities can be combined with the crystal strucis only met for a few metal (hydr)oxides. In this study the ture and morphology in order to describe the experimental chargactual bond valences will be used, which can be calculated ing curves. The charging curves could be described very well for from a relationship between the bond length and the bond minerals with a well-known (goethite) or reasonably well-known valence of a particular bond (9, 10). The new method will (lepidocrocite) crystal morphology. For crystals of which the morbe applied to three different iron (hydr)oxides, namely, goephology is less well known (hematite), the refined MUSIC model thite, lepidocrocite, and hematite. For the prediction of the can be a powerful support for a suggested morphology. แญง 1998 individual proton affinity constants, the exact metal-oxygen Academic Press distances in the iron(hydr)oxide crystals are obtained from