Molecular modeling of the surface charging of hematite - II. Optimal proton distribution and simulation of surface charge versus pH relationships

A parameterized classical potential model for the interaction of water and hydroxide with iron oxide was used to calculate the optimal proton arrangem ent and proton binding energies on the (012) surface of hematite. Energy mi nimization calculations with the parameterized potential model indicate tha t approximately 75% of adsorbed water molecules are dissociated on this sur face, in agreement with recent TPD and HREELS measurements. Surface protona tion/deprotonation energies were calculated from the predicted optimal arra ngement of protons on the neutral (012) surface. A supercell geometry with translational symmetry in two dimensions and finite in the third dimension (2-D PBC) was assumed. The calculated surface protonation energies were the n used to model the experimentally observed surface-charging curve of hemat ite in aqueous solution. Excellent agreement was found between the calculat ed and measured surface charge for ionic strengths ranging from 0.001 to 0. 1 M. Our calculations favor the value of 8.5 for the pH of zero charge of h ematite over the more recent result of 6.7. (C) 1999 Elsevier Science B.V. All rights reserved.