PNNL

Abstract

The applicability of separating charges of oxyanions across inner- and outer-Helmholtz planes according to the Charge Distribution model was tested by investigating the theoretical charge distribution in a range of metal-sulphate complexes using the methods of Atoms In Molecules and of the Electron Localisation Function. Density Functional Theory gas-phase geometry optimisation calculations revealed that unbound oxygens of the sulphate molecules contracted to relatively constant S-O bond lengths of 1.432 ± 0.019 (3 ?) Å irrespective of the bond strength with the metal ions. The populations of the valence basins of the unbound oxygens also remained relatively constant, showing that even the strongest complexation induces very little charge distribution across the sulphate molecule. Maps of the Laplacian of the electron density and of the Electron Localisation Function revealed that although charge is relatively localised at oxygen centers there is not necessarily a clear charge separation between the inner- and outer-Helmholtz planes. The Proximity and Smit models are presented as alternative surface complexation schemes to provide a molecularly and electronically consistent depiction of the mineral/solution interface. These models are also presented in their capability in accounting for results from large-scale molecular models. It should nonetheless be emphasized that the Charge Distribution model remains a valuable approach and should have the best applicability at low surface loadings and with molecules with sizes similar to those of the compact layer.