How water molecules sit and behave at the solid–liquid interface
The molecules in that glass of water next to you are quite active! They move around, interact, and react with each other, and collectively form the liquid we all know and cherish. However, things get even more complicated when water molecules are within a couple nanometers of a solid surface.
At the solid–liquid interface, those water molecules are not like their bulk counterparts. Instead, they assemble into ordered patterns whose details remain a mystery. These hydration layers are a few water molecules thick and play an important role in a variety of interfacial phenomena, such as chemical reactions at catalytically active sites, charge transfer processes at battery electrodes, and ion adsorption at mineral surfaces. Accordingly, understanding the structure and dynamics of interfacial water is crucial for a multitude of topics, from geochemical cycling and environmental remediation to materials synthesis and chemical production.
One case study of particular interest is the boehmite–water interface. Boehmite is an aluminum-based mineral, AlOOH, (Figure a) that is prevalent in the waste tanks at the Hanford site, a legacy of the Cold War era. Formed as a side-product of plutonium production operations over decades, these nanosized boehmite particles interact and aggregate into complex shapes that influence the rheological properties of the slurries in the nuclear waste in ways that complicate their safe treatment and disposal. While a previous study examined the shape of boehmite aggregates,1 the researchers reached a stumbling block for accurately predicting the boehmite aggregation shapes. Existing models do not account for the ordering of water molecules close to the boehmite interface and their role in when and how boehmite aggregates.
In fact, very few techniques can characterize solid–liquid interfaces with molecular resolution, which is why our team is excited about a recently developed capability, 3D fast force mapping (3D FFM). Based on atomic force microscopy technology, this technique employs a nanosized probe that navigates the interfacial region and records the molecular forces it experiences from the local surroundings with remarkable sensitivity (Figure b). As it penetrates the water layers close to the surface, the probe creates a three-dimensional force map that is intimately related to the local distribution of water molecules. The concept is simple and the promise is great: 3D FFM provides a pathway to visualize how water molecules sit and behave at the interface.
Using 3D FFM, our team investigated the molecular details of the boehmite–aqueous interface (Figure c). We observed a complex pattern of forces from the layered water molecules within one nanometer of the surface. Interestingly, this fluid phase shows the same lattice symmetries as the underlying solid. In other words, the rows of ordered atoms in the boehmite itself determine where the water molecules prefer to sit. Specifically, the first water layer adsorbs at the sites adjacent to the boehmite hydroxyls (Figure d), a chemical group that interacts favorably with water molecules via hydrogen bonding.
These experimental data were coupled to a rigorous suite of molecular dynamics computer simulations (Figure e) that aimed to address the nanosized “elephant in the room”: how do the forces measured by the probe translate to the positions of water molecules? We investigated the effects of the probe size, chemistry, and interactions on the measurements, thus significantly improving current models for 3D FFM data interpretation.
To date, only a handful of research groups have used 3D FFM, but we anticipate that will change soon as the technique becomes more accessible. Further studies will provide a detailed understanding of interfacial hydration layers, improving current models and eventually resolving how boehmite aggregation influences the rheological behavior of tank waste.
1) E. Nakouzi, J. A. Soltis, B. A. Legg, G. K. Schenter, X. Zhang, T. R. Graham, K. M. Rosso, L. M. Anovitz, J. J. De Yoreo, J. Chun. Impact of Solution Chemistry and Particle Anisotropy on the Collective Dynamics of Oriented Aggregation ACS Nano , 12, 10114-10122, 2018 https://doi.org/10.1021/acsnano.8b04909
2) E. Nakouzi, A. G. Stack, S. Kerisit, B. A. Legg, C. J. Mundy, G. K. Schenter, J. Chun, J. J. De Yoreo, Moving beyond the Solvent-Tip Approximation to Determine Site-Specific Variations of Interfacial Water Structure through 3D Force Microscopy, J. Phys. Chem. C, 125, 1282–1291, 2021 https://doi.org/10.1021/acs.jpcc.0c07901
Design of the study, collection, and analysis of three-dimensional fast force mapping (3D FFM) data, MD analysis of interactions between a Leonard-Jones object and boehmite, and integration of all measurements and simulations were supported as part of IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES). Development of the 3D FFM analysis and visualization capability was supported by the Laboratory Directed Research and Development Program (LDRD) at Pacific Northwest National Laboratory (PNNL) through the Linus Pauling Distinguished Postdoctoral Fellowship program to which E.N. is grateful for support. Incorporation of the concepts of nanoscale hydrodynamics was supported by the LDRD Program at PNNL. Development of the 3D FFM measurement capability was carried out at PNNL with support from the BES Division of Materials Science and Engineering, Synthesis and Processing Sciences Program. Development of concepts for and analyses of long-range forces was carried out at PNNL with support from the BES Chemical Sciences, Geosciences, and Biosciences Division (CGSB), Chemical Physics and Interfacial Sciences Program. The design and execution of the MD simulations of the boehmite−water−silica system were carried out at PNNL with support from the BES, CGSB, Geosciences Program. These simulations were performed using PNNL Institutional Computing and the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated for DOE by Battelle Memorial Institute under contract no. DEAC05-76RL0-1830. Nathan Johnson created the graphic in Figure b.