Water has been studied more than almost any other substance, yet scientists have long debated a surprisingly simple question: What happens to its chemistry when it is squeezed into spaces only a few molecules wide?
Those tiny spaces exist throughout nature and technology, including nanoscale pores, membranes, and biological channels. A new study has now found that the answer is more nuanced than researchers once believed, helping resolve years of conflicting results.
Why Water Splitting Matters
One of water’s defining 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, and plays a central role in acid-base chemistry. It influences everything from the enzymes that keep your cells functioning to the reactions that occur inside batteries.
Scientists wanted to determine whether confining water to spaces just billionths of a meter across changes how readily this splitting occurs.
Their findings, published in Science Advances, suggest that the apparent chemical reactivity of nanoconfined water depends strongly on factors such as density, pore size, wall flexibility, and surface chemistry.
“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 the Missing Piece
To explore the problem, the researchers relied on machine learning simulations that reproduce quantum mechanical accuracy while allowing them to study a much broader range of conditions than traditional computational methods.
The team examined water trapped between sheets of graphene and hexagonal boron nitride (hBN). Although both materials are only one atom thick and share a similar structure, their surface chemistry is very different.
The simulations also revealed that water droplets confined between these materials experience extremely high internal pressures. Water trapped between graphene or hBN sheets can reach pressures of several gigapascals, similar to those found deep inside Earth, even though no external force is applied.
Instead, the pressure develops naturally because of van der Waals attraction between the atomically thin layers. While the force between individual atoms is weak, it becomes remarkably strong across the large surface area of two dimensional materials, pulling the sheets together and compressing the water trapped between them.
Pressure, Not Confinement, Drives Water Reactivity
The researchers found that these intense pressures greatly increase the splitting of water molecules.
However, when they compared confined water with ordinary bulk water exposed to the same pressure, both behaved in essentially the same way. This showed that the increased reactivity comes primarily from pressure itself rather than confinement alone.
“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.
Surface Chemistry Still Plays an Important Role
Although simply squeezing water into tiny spaces does not inherently make it more reactive, the surrounding material can still influence its chemistry.
In water droplets confined by hBN, hydroxide ions (OH– ) that formed around the edges bonded chemically with the surrounding material. This stabilized the ions, lowered the energy required for water to split, and increased the amount of dissociation.
The same effect was not observed with graphene because its chemically inert surface does not participate in the reaction.
The results show that the material surrounding confined water can actively shape 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.
Next, the researchers plan to study more realistic environments that include defects and edges commonly found in practical materials. They also hope to compare their predictions with laboratory measurements using advanced spectroscopic and nanofluidic techniques.
At the same time, the team is screening large families of two dimensional materials and surface chemistries to identify combinations that can either enhance or suppress water reactivity for specific technological applications.