Zero-point-of-charge prediction from crystal chemistry and solvation theory

Abstract

The pristine point of zero charge (pHPPZC) is a property of each solid in water that is widely used in the interpretation of adsorption processes and dissolution rates. Considerable effort has gone into attempting to calculate the pHPPZC values of oxides and silicates based on crystal chemistry and electrostatic models of the interaction between protons and OH surface groups. However, substantial discrepancies remain between calculated and measured values of the pHPPZC even for simple oxides such as quartz. This implies that the widespread use of real and fictive components (e.g., SiIVO2 and Al2IVO3) to interpret the surface characteristics of silicates is unwarranted. In the present paper, I show that by adding electrostatic solvation theory to crystal chemical and electrostatic models, the differences between pHPPZC values of crystalline solids can be accurately quantified. The new model sums free energy contributions associated with proton solvation, electrostatic repulsion of protons by cations underlying the surface, and electrostatic attraction of protons by oxygen anions near the surface. It results in a theoretical dependence of the pHPPZC on the dielectric constant of the kth solid (ϵk) and the ratio of the Pauling electrostatic bond strength to the cation-hydroxyl bond length (srM-OH), according to the equation pHPPZC = − 0.5(ΔΩr2.303RT)(1ϵk) − B(srM-OH) + log KH+″, in which ΔΩr, B, and KH+″ are constants. This relationship describes the pHPPZC values of oxides and silicates to better than ±0.5 and enables prediction of surface protonation reactions from the properties of the underlying crystal structure alone. These results suggest that the bonding of protons at the crystal-water interface is more analogous to the bonding in the bulk crystal structure than to the bonding in analogous aqueous complexes emphasized in other studies.