Effect of different crystal faces on experimental interaction force and aggregation of hematite

Charging is a basic property of the solid/solution interface of minerals. The charging at different crystal faces depends on the surface chemical composition, that is, the type and number of proton-reactive surface groups. Atomic force microscopy has provided direct information on the pH-dependent charging properties of individual crystal faces (Eggleston and Jordan, Geochim. Cosmochim. Acta, 1998) of hematite (001 face) and quartz (101 face). This promising scanning force technique may enable the testing of structure-reactivity relations as applied in the MUlti Site Complexation model (MUSIC). The published pH-dependent variation of the interaction forces has been evaluated. The large experimental difference in the pH dependence of the interaction force is in accord with the expected proton affinity behavior of both crystal faces, as predicted with the MUSIC model. The reactivity of the 001 face of hematite is quite exceptional, because the surface groups only charge at extremely high or low pH values in contrast to the generally observed (overall) charging behavior of hematite. The difference in reactivity of the crystal faces of hematite has implication for the rate of aggregation. The presence of the 001 face leads to a less pH-dependent rate of aggregation below the point of zero charge, in accord with experimental observations (Schudel et al., J. Colloid Interface Sci., 1997). Model analysis further shows that the interaction forces acting on the 001 faces of hematite are sensitive to the presence of surface defects, in contrast to the forces acting on the 101 faces of quartz.