How water molecules sit and behave at the solid–liquid interface

Characterizing interfacial solution structure with molecular resolution

Elias Nakouzi

Figure (a) the boehmite crystal structure and (b) 3D FMM capability. (c) 2D and (d) 1D profiles from force gradient maps at the boehmite-water interface. Colored markers in (d) denote force gradient minima that correspond to areas of high-water density above four distinct boehmite(010) crystallographic sites. The first two sub-layers show good agreement with molecular dynamics simulations of the boehmite-water system, but interpreting the additional features required (e) a full-scale simulation of the probe tip approaching the boehmite surface.

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.