Molecular Controls on Kaolinite Surface Charge

pH-dependent, multisite, surface charge on kaolinite can be explained by proton donor-acceptor reactions occurring simultaneously on Si and Al sites exposed on basal planes and edges. Si site Bronsted acidity at the kaolinite-solution interface differs minimally from that of pure SiO2, whereas Al site acidity increases appreciably over that of pure Al2O3. Increasing temperature decreases the pK values of Al and Si proton-exchange sites. Calculated site densities indicate either an elevated participation of edges or substantial contribution from basal planes in the development of surface charge. Independent evidence from scanning force microscopy points to a higher percentage of edge surface area due to thicker particles and basal surface steps than previously assumed. Thus, no basal plane participation is required to explain the site densities determined from proton adsorption isotherms. Molecular modeling of the proton-relaxed kaolinite structure has been used to establish the elevated acidity of edge Al sites and to independently confirm the crystal-chemical controls on surface reactivity.