Mineral dissolution is fundamental, yet because of its complexity at the atomic scale remains challenging to quantitatively predict. In contrast to the straightforward measurement of ion release rates, there still are no probes capable of resolving elementary reactions governing ion release rates at interfaces. Molecular simulations are essential and now approaching the sophistication necessary to encompass key aspects of dissolution including ion solvation/desolvation and hydrolysis, bond breaking and reformation, ion diffusion, and detachment at an explicitly described solid/solution interface. To treat these collectively on an equal footing remains accessible primarily just to classical force field molecular dynamics (MD) methods. Even so, most simulations to date have been limited to relatively simple ionic materials, even fewer have been attempted specifically for covalently bonded metal oxides.<br/>Here we report a combined atomic force microscopy (AFM) and MD-based metadynamics simulation study of gibbsite dissolution at high pH. The goal was to achieve registry between microscopic observations of dissolution features on basal (001) surfaces and MD-based predictions of the free-energy landscape for aluminum ion detachment. In situ AFM on quasi-hexagonal gibbsite microplates in 0.2 M NaOH shows the retreat of pre-existing monomolecular step edges, as well as (110) and (100) edge facets. Pit nucleation is observed more frequently at higher NaOH concentrations, consistent with reaction initiation further from equilibrium. Simulations show that the underlying aluminum ion release process is kink formation and kink propagation along step edges. Two individual reactions were found to be rate-limiting for kink formation, the displacement of Al from a step site to a ledge adatom site, and then its detachment from ledge/terrace sites into solution. As a result, a pool of mobile and labile adsorbed Al exists before its release into solution. Because of the gibbsite structure, kink site propagation can occur in multiple directions, consistent with AFM image data revealing no preferred step edge retreat direction and a uniform overall loss of platelet margins. The synergy between microscopy and simulations is a good example of the current state of the art for developing mechanistic models of metal oxide dissolution that ultimately could lead to predictive power.

Linking microscopy and metadynamics simulations to understand gibbsite dissolution at high pH

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.