Pressure explains much of water’s nanoscale chemistry, study finds

Water squeezed into nanometer-wide spaces does not appear to become chemically unusual because of confinement alone, according to researchers from Cambridge, Harvard, Caltech and the Max Planck Institute for Polymer Research. Their study in Science Advances offers an explanation for why previous experiments on nanoscale water have pointed in different directions.

The work examines a basic question in chemistry: how readily water splits into hydronium ions, H₃O⁺, and hydroxide ions, OH⁻. That reaction sets pH and shapes acid-base chemistry in systems ranging from enzymes to battery electrodes, the University of Cambridge said.

Researchers have long studied what happens when water is held in spaces only a few molecules across, such as tiny pores, membranes and biological channels. According to the Cambridge-led team, earlier disagreements often arose because comparisons were made between systems that were not under the same thermodynamic conditions.

Xavier R. Advincula, the study’s lead author, said the effect of confinement largely vanished when the team compared systems at the same chemical potential, a thermodynamic quantity tied to whether reactions proceed. He said many conflicting reports came from comparing water at different effective pressures or densities without recognizing the difference.

Simulations separate pressure from confinement

The team used machine-learning simulations designed to reach quantum-mechanical accuracy while testing more conditions than conventional calculations can easily cover, according to Cambridge. The researchers modeled water held between sheets of graphene and hexagonal boron nitride, or hBN, two atomically thin materials with similar structures but different chemistry at their surfaces.

The simulations showed that tiny water droplets trapped between those sheets can face pressures of several gigapascals, Cambridge said. Those pressures are comparable to conditions deep inside Earth, even though no outside force is applied.

According to the researchers, the pressure comes from van der Waals attraction between the confining layers. Individually, those attractions are weak, but across the broad area of two-dimensional materials they can pull the sheets together and compress the trapped water.

The study found that these high pressures increased water dissociation. But when the team compared confined droplets with bulk water at matching pressures, the same pattern appeared, indicating that pressure accounted for much of the increased reactivity.

Angelos Michaelides of the University of Cambridge’s Yusuf Hamied Department of Chemistry said the surprising result was how much of the apparent nanoscale effect could be explained by thermodynamics once pressure and chemical potential were included.

Wall chemistry still matters

The researchers also found that the material surrounding the water can change the reaction when it participates chemically. In hBN-confined droplets, hydroxide ions at the droplet edges bonded to the surrounding material, stabilizing the ions and making water splitting more favorable, Cambridge said.

That behavior did not appear with graphene, which the researchers described as chemically inert in this setting. The contrast suggests that nanoscale water chemistry can be controlled by choosing surfaces that interact with the products of water dissociation.

Christoph Schran of the Theory of Condensed Matter Group at Cambridge’s Cavendish Laboratory said the work gives researchers a framework for reconciling a decade of differing studies. He said it also points to a design principle: engineers should consider both the pressures generated in confined spaces and the chemistry of the confining material, rather than pore size alone.

Cambridge said the findings could help technologies that use confined water, including hydrogen fuel cells, batteries, ion-selective membranes and catalysts. The researchers plan to study more realistic environments, including materials with defects and edges, and to compare their predictions with spectroscopic and nanofluidic measurements.

The team is also examining broader sets of two-dimensional materials and surface chemistries to identify candidates that could increase or reduce water reactivity for specific applications, according to Cambridge.

This story draws on original reporting from Phys.org.