Scientists solved a long-standing mystery by showing that pressure, not tiny spaces alone, determines how water behaves at the nanoscale.
Water is one of the most thoroughly studied substances on Earth, yet scientists have spent decades debating a surprisingly simple question: What happens to water’s chemistry when it is squeezed into spaces only a few molecules wide? These tiny environments exist inside nanoscale pores, membranes, and even biological channels, but researchers have long disagreed over whether confinement alone makes water more or less chemically reactive.
A new study has now provided an answer, revealing that the explanation is far more nuanced than previously believed.
Why Water Splitting Matters
One of water’s most important chemical properties is its ability to split into two charged particles: H3O+ (the hydronium ion) and OH– (the hydroxide ion). This process determines pH, which measures how acidic or alkaline (basic) a solution is. It also plays a central role in acid-base chemistry, influencing everything from the enzymes that power living cells to the electrochemical reactions inside batteries.
The goal of the new research was to determine whether confining water inside nanometer-scale spaces changes how readily this splitting occurs.
Pressure, Not Confinement, Explains the Mystery
In a study published in Science Advances, researchers from the University of Cambridge, Harvard University, CalTech, and the Max Planck Institute for Polymer Research found that the apparent chemical reactivity of nanoconfined water depends strongly on factors such as density, pore size, wall flexibility, and surface chemistry.
However, they discovered that confinement itself is not the true cause.
“When we compared systems under equivalent thermodynamic conditions – specifically at the same chemical potential (the quantity that determines whether a reaction proceeds), the effect of confinement largely disappeared. In other words, the confinement alone does not intrinsically change water’s reactivity. This explains why experiments over the past decade have produced contradictory results,” said Xavier R. Advincula, the study’s lead author.
“The contradictions in the literature were largely because scientists were comparing systems at different effective pressures or densities without realizing it.”
Machine Learning Reveals Water’s Hidden Behavior
To investigate the problem, the team relied on machine learning simulations capable of achieving quantum mechanical accuracy while exploring far more conditions than traditional computational methods can handle.
The researchers examined water trapped between atomically thin sheets of graphene and hexagonal boron nitride (hBN). Although these two materials have nearly identical structures, their surface chemistry differs significantly, making them ideal for comparison.
The simulations also revealed something unexpected. Water confined between these atomically thin layers experiences enormous internal pressures reaching several gigapascals, comparable to pressures deep inside Earth, even though no external force is applied.
Instead, the pressure develops naturally because of van der Waals attraction between the surrounding layers. While this attraction is extremely weak between individual atoms, it becomes remarkably strong across the large surface areas of two-dimensional materials, pulling the sheets together and compressing the trapped water.
Pressure Drives Water Reactivity
These extreme pressures greatly increased the amount of water splitting observed in the simulations. But when the researchers compared the results with ordinary bulk water exposed to the same pressures, they found the behavior was essentially identical.
This showed that the increased reactivity was driven primarily by pressure rather than by confinement itself.
“What surprised us most was how much of the apparent confinement effect could be explained by thermodynamics. Once pressure and chemical potential are properly accounted for, a great deal of the complexity simply falls into place,” said Prof Angelos Michaelides, of the Yusuf Hamied Department of Chemistry at the University of Cambridge.
The Confining Material Still Matters
Although simply trapping water in a tiny space does not inherently change its chemistry, the material surrounding the water can.
The researchers found that in nanometer-scale droplets enclosed by hBN, hydroxide ions (OH–) formed at the droplet edges chemically bond with the surrounding material. This stabilizes the ions and reduces the energy needed for water to split, increasing the amount of dissociation.
Graphene behaved very differently. Because its surface is chemically inert, it does not participate in the reaction, and the same enhancement was not observed.
These findings show that the material surrounding confined water can actively influence its chemical behavior.
“This research provides a new framework for understanding water chemistry at the nanoscale and helps reconcile a decade of apparently conflicting studies,” said Dr. Christoph Schran, of the Theory of Condensed Matter Group at the Cavendish Laboratory.
“More importantly, the work offers a practical design principle for engineering nanoscale chemical environments. Rather than focusing solely on the size of pores or channels, we can tailor water reactivity by choosing a confining material whose surfaces interact with the products of water dissociation and by controlling the pressures generated within confined spaces.”
Potential Applications in Energy Technology
The findings could have important implications for technologies that depend on confined water, including hydrogen fuel cells, batteries, ion-selective membranes, and catalytic systems.
The researchers now plan to study more realistic nanoscale environments, including materials that contain defects and edges, which are common in practical devices. They also hope to compare their predictions with experimental observations using advanced spectroscopic and nanofluidic techniques.
At the same time, the team is screening large families of two-dimensional materials and different surface chemistries in search of materials that can either enhance or suppress water reactivity for specific technological applications.
Reference: “How reactive is water at the nanoscale and how to control it?” by Xavier R. Advincula, Yair Litman, Kara D. Fong, William C. Witt, Christoph Schran and Angelos Michaelides, 24 June 2026, Science Advances.
DOI: 10.1126/sciadv.aeb5772
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