ASSESSING THE INFLUENCE OF SPECTATOR IONS ON THE KINETICS AND MECHANISMS OF LIGAND EXCHANGE AT THE CALCITE (104)-WATER INTERFACE

In situ imaging techniques have been instrumental in shaping understanding of interfacial processes, and continue to yield new insights into the complexities of mineral dissolution and growth. However, as these methods provide only indirect information about the chemical processes underlying surface reactivity, little is known about the molecular scale mechanisms of ion attachment and detachment, even for mineral surfaces that have been thoroughly investigated. Recognizing such limitations, experimental investigations of interfacial processes now frequently include insights derived from molecular simulations. However, our capacity to simulate rare events in such complex systems is limited by our ability to accurately characterize multi-dimensional energy landscapes, and to identify important stable and metastable states and the transition pathways between them. Current approaches generally require either some degree of a priori knowledge of the energy landscape (i.e. important collective variables), or possession of an initial reactive pathway. These requirements place significant restrictions on the ability to probe complex dynamic processes with atomistic simulations. Our group is developing a path sampling method, based on the Forward Flux Sampling (FFS) procedure, that offers several advantages over existing approaches. We have recently demonstrated the validity of this approach across six orders of magnitude by computing accurate water exchange rates on on Li⁺, Ca²⁺ and Mg²⁺. In this study, we investigate the kinetics of ligand exchange on calcium ions at the calcite (104)-water interface. FFS simulations are performed to characterize the rates and mechanisms of water exchange and ion detachment, and the influence of NaCl and KCl solutions on these processes.