metal (hydr)oxides and various attempts have been made
The proton affinity of individual surface groups has been calcu- (1)(2)(3)(4) to relate this overall chemical feature to the general lated with a redefined version of the multi site complexation (MUcharacteristics of the metal (hydr)oxide, treating the inter-SIC) model. In the new approach the proton affinity of an oxygen face as chemically homogeneous. The surfaces of metal originates from the undersaturation of the oxygen valence. The (hydr)oxides are, however, generally heterogeneous; i.e., factors valence and coordination number, which are the basis of several types of metal (hydr)oxides groups are present, and Pauling's definition of bond valence, in combination with the numthe PZC is only the result of a combined action of several ber of coordinating (Me and H) ions, are dominant in determining types of surface groups. It implies that in general from the the proton affinity. The neutralization of an oxygen by Me ion(s) PZC value no direct information can be found about the is calculated on the basis of the actual bond valence, which accounts for structural details, resulting from an asymmetrical distri-proton affinity of individual types of surface groups. Also, bution of charge in the coordination environment. An important the charging curve as a whole will not give this important role in the new version of the MUSIC model is given to the H information a priori, in particular because it is very strongly bonds. The model shows that the proton affinity is determined not influenced by the electrostatic field of the double layer, only by the number of donating H bonds but also by the number masking the individual contributions. With the development of accepting H bonds. The proton affinity of surface groups and of the multi site complexation model (MUSIC) a first atof solution complexes can be understood in one theoretical frametempt has been made to estimate the proton affinity for indiwork, on the basis of a different number of donating and accepting vidual types of surface groups (5), resulting in a predicted H bonds. The MUSIC model predicts the variation in proton affin-PZC value once the chemical composition of the interface ity constants for surface groups in particular those with the same is known. In the MUSIC model the proton affinity of the number of coordinating Me ions but with a different structural individual groups was calculated on the basis of the Pauling position. The model is able to predict on the basis of the proton affinity of the individual groups the correct PZC of Me hydroxides, bond valence (6) in which the charge of the central ion is oxohydroxides, and oxides, and explains previous exceptions. The equally distributed over its surrounding ligands. The MUSIC model can also be applied in general to other minerals. แญง 1996 model predicts that one of the main differences in intrinsic Academic Press, Inc. proton affinity of surface groups is due to the number of Key Words: proton affinity; surface groups; surface charge; PZC; metal ions coordinating to the surface oxygens. Another im-MUSIC model; bond valence; hydrogen bond; oxide; gibbsite; goeportant prediction of the MUSIC model is that the difference thite; rutile; anatase; silica; quartz.
in affinity between the first and second proton that binds to the same type of surface oxygen is very large, about 14 log K units. The MUSIC model was calibrated using oxo and a-FeOOH (goethite). Early electron microscopy work on