PNNL

The Science                                 

The solution structure at mineral interfaces is key to understanding interfacial phenomena yet remains poorly understood. In this study, researchers used three-dimensional fast force mapping (3D FFM) to visualize the hydration structure at the boehmite (AlOOH)-water interface. They then benchmarked these results against molecular dynamics (MD) simulations. The scientists discovered that water molecules within one nanometer of the boehmite-solution interface exhibit a periodic, layered pattern. This pattern is templated by the underlying crystal lattice of the mineral. Their results reveal four laterally structured water layers with the highest water densities adjacent to the boehmite hydroxyl groups. These findings uncover a complex relationship between site-specific chemistry, water density, and long-range particle interactions.

The Impact

At the solid-liquid interface, solvent molecules and ions arrange into organized patterns. Their arrangement influences a range of processes at this interface, including catalysis, ion adsorption, and electron transfer. Despite its importance, little is known about the solution structure at this interface, especially at the molecular scale. 3D FFM is an emerging technique for investigating interfacial mineral-solution structures with subnanometer resolution. This research resolves the interfacial solution structure at the boehmite surface and provides important context for understanding and interpreting 3D FFM data for resolving such structures. Interfacial structure influences boehmite dissolution, which is important for reducing the volume of high-level radioactive waste sludge in underground storage tanks at the Hanford Nuclear Reservation. Additionally, boehmite has potential applications in energy storage, optoelectronics, and catalysis, thus resolving its interfacial structure helps turn this potential into reality.   

Summary

Researchers used 3D FFM to map the distribution of water at the boehmite-solution interface. They then employed MD simulations to investigate the effects of probe size, chemistry, and interactions on the measurements to interpret the experimental data. The results reveal that the first water layer adsorbs at the sites adjacent to the boehmite hydroxyl groups. This is followed by a periodic, layered pattern of water density distribution that extends approximately one nanometer. Fourier transform analyses of the data show that the interfacial solution structure exhibits boehmite crystallographic symmetry and is, thus, templated by the underlying boehmite lattice. The first two sub-layers visualized by 3D FFM show good agreement with MD simulations. However, interpreting additional features requires a full-scale simulation of the probe approaching the boehmite surface. This study improves on existing models for 3D FFM data interpretation, highlights the limitations of this technique, and presents a roadmap for visualizing interfacial regions with lattice resolution.

Contact

James J. De Yoreo
Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory  (PNNL)
james.deyoreo@pnnl.gov

Funding

This study was supported by:

• IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences (BES) program
• Laboratory Directed Research and Development Program at PNNL
• The Linus Pauling Distinguished Postdoctoral Fellowship program
• BES Materials Sciences and Engineering Division, Synthesis and Processing Sciences program
• BES Chemical Sciences, Geosciences, and Biosciences Division, Condensed Phase and Interfacial Molecular Science program

A portion of this work was performed using PNNL Research Computing and the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy, Office of Science, Biological and Environmental Research program and located at PNNL.