Iron oxides are among the most abundant and reactive minerals in the environment. They develop a surface charge in contact with an aqueous solution, primarily due to protonation of surface oxygen atoms.<br/>On the molecular scale, protons can readily redistribute between surface oxygen atoms in response to a change in conditions (e.g., ion sorption or surface reaction). However, this dynamic aspect of the iron oxide/electrolyte interface is rarely addressed in molecular simulations. Here, we present replica-exchange constant-pH molecular dynamics simulations of archetypal iron oxides (goethite, hematite) in contact with a simple electrolyte solution (e.g., Fig. 1). The constant-pH molecular dynamics approach is a rare event sampling method that allows us to efficiently explore simultaneously protonation space via Monte Carlo sampling and configuration space via classical molecular dynamics.<br/>Our simulations provide a molecular level understanding of how proton dynamics affect electrical double layer properties, and how surface protons can electrostatically couple distant surface sites and their reactivity. We found that Fe(II) sorption and oxidation at one surface site induce an interfacial electrostatic imbalance that is quickly propagated via surface protons to another surface site resulting in its reductive dissolution. This long-range coupled reactivity is essential in predicting the fate of and links between geochemically important reactions that are catalyzed by hydrated iron oxide minerals.

Surface charge dynamics at the iron oxides/electrolyte interface

253rd National Meeting (2017)

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