Interfacial charging phenomena of aluminum (hydr)oxides

The interfacial charging of AI(OH)(3) (gibbsite and bayerite) and Al2O3 has been studied. For Al(OH)3 it can be shown that the very strong variation i n charging behavior for different preparations is related to the relative p resence of differently reacting crystal planes. The edge faces of the hexag onal gibbsite crystals are proton reactive over the whole pH range, in cont rast to the 001 plane, which is mainly uncharged below pH = 10. On this 001 face only doubly coordinated surface groups are found, in contrast to the edges which also have singly coordinated surface groups. The results are fu lly in agreement with the predictions of the Multi site complexation (MUSIC ) model. The proton adsorption, electrolyte ion adsorption, and shift of th e IEP of gibbsite and aluminum oxide have been modeled simultaneously. For gibbsite, the ion pair formation of Na is larger than that of Cl, as is evi denced by modeling the experimentally observed upward shift of the IEP and charge reversal at high electrolyte concentrations. All these experimental results can be satisfactorily modeled with the MUSIC model, including the e xperimental surface potential of aluminum oxide (ISFET).