Water Structure and Hydrogen Bonding at Goethite/Water Interfaces: Implications for Proton Affinities

Molecular dynamics simulations of four crystallographic terminations of goethite (α-FeOOH) in contact with liquid water (300 K, 1 bar) were performed to resolve interfacial water structures in the vicinity of surface (hydr)­oxo groups. Interfacial water molecules adopted highly surface-specific configurations on (010), (100), (110), and (021) planes of goethite. Water molecules generally had weaker hydrogen bond numbers and strengths, as well as smaller self-diffusion coefficients, than their bulk liquid counterparts. Relaxed surface Fe–O and H–acceptor distances and populations were used to estimate proton affinity constants of singly-, doubly-, and triply-coordinated (hydr)­oxo groups using the multisite complexation model. These calculations confirmed that singly coordinated groups are mainly responsible for charge uptake under normal environmental conditions. However, revised proton affinity constants showed that protonation of doubly-coordinated hydroxo groups and one type of triply-coordinated oxo group may be favored in the presence of strongly binding negatively charged ligands. These calculations should facilitate elucidation of surface complexation mechanisms on this environmentally important material.