Deciphering the water structure and dynamics on mineral surfaces is critical in understanding the underlying chemical reactions responsible for ion exchange and adsorption mechanisms, and mineral dissolution and growth rates. In this study, quasielastic neutron scattering (QENS) is utilized to measure the diffusion dynamics of surface-bound water in nanopowder calcite that possess predominantly the (104) crystal face. However, due to the mineral-water interfacial complexity, numerous types of water diffusional motions associated to various surface sites are presented in the measured QENS spectrum, which makes it impossible to infer the precise water motions without a molecular modeling. Classical molecular dynamics (CMD) is well-suited for both time- and length-scales to overcome the challenge of inferring specific rate constants from a QENS spectra and reveal the individual water exchange dynamics on the surface of a calcite nanoparticle. Since the CMD simulations are strongly dependent on the implemented forcefield, a combination of calcite – water forcefields are utilized to get the best estimate of water mobility and is benchmarked against the QENS data to validate the quality of the forcefields. The computed/fitted QENS signals is deconvoluted to contain three types of water diffusional components, which include the monodentate water binding to surface calcium and carbonate sites, and lateral water exchanges between sites. Regardless of water types, the mobility of all water molecules is slowing down as the temperature cools to 240 K. In addition, exploration of water diffusivities and site-specific water orientations suggest that certain bonding orientations are preferred. The improved understanding resulting from this study will enhance our predictive capability for water exchange rates on calcite surfaces, which is fundamental for understanding the rate-limiting steps during carbonate mineral dissolution and growth.

Structure and dynamics of water on Calcite (104) surface studied by quasielastic neutron scattering and molecular dynamics simulations

ACS Fall 2021

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