Surfaces, not confinement, rule until the thinnest limits

Researchers at the Max Planck Institute for Polymer Research have upended assumptions about how water behaves when squeezed into atom-scale spaces. By applying spectroscopic tools together with the machine learning simulation technique to water confined in a space of only a few molecules thick, the team, led by Mischa Bonn, found that water’s structure remains strikingly “normal” until confined to below a nanometer, far thinner than previously believed.

The research, “Interfaces Govern the Structure of Angstrom-Scale Confined Water Solutions,” was published in Nature Communications.

Peering into the structure of a layer of water molecules that is only a few molecules thick is a formidable scientific challenge. The team fabricated a nanoscale capillary device by trapping water between a single layer of graphene and a calcium fluoride (CaF₂) substrate. They then wielded cutting-edge vibrational surface-specific spectroscopy—capable of detecting the microscopic structure of confined water, including the orientation and hydrogen-bonding of water molecules—to “see” the elusive few layers of water.

The researchers observed that even when the water was confined to three molecular layers—a space barely wider than the molecules themselves—the properties of water at the center still mimicked those of ordinary bulk water in contact with two surfaces. Interfacial effects, determined by the graphene sheet and CaF₂ substrate, overwhelmingly dictated the arrangement and behavior of the molecules.

Only when reduced to truly angstrom-scale dimensions, where water is thinner than two layers, did the actual confinement begin to dominate and reorganize the liquid on a structural level. Prediction of machine-learning-based simulation techniques could validate assumptions used in the spectroscopic measurement, nicely reproduce the observation, and confirm the conclusions.

“This research changes our perspective on confined water,” explained first author Yongkang Wang. “Our findings are relevant to most practical scenarios—like water in nanochannels, membranes, or between layered materials—where it is the surfaces that dictate water’s properties, not the spatial confinement itself, except at vanishing (approaching molecular) thicknesses.”

Broad implications for technology, biology, and materials science

These insights carry major implications for a wide range of fields, from nanofluidics and geology to biology and advanced materials. The results clarify that most nanoconfined water on Earth or in technological devices—such as in membranes, nanofluidic circuits, or biological pores—remains governed by interfacial phenomena, even under extreme confinement. Only for water slivers less than a single nanometer thick do the rules truly change.

Pushing the frontiers of water science

“Our results set a new benchmark,” said corresponding author Yuki Nagata. “If you’re working with so-called ‘nanoconfined water,’ it is the surface chemistry—not just geometry—that determines its properties, unless confinement is pushed to the very limit.”

The ability to probe and understand just a few layers of water molecules—the region of greatest scientific and technological mystery—marks a significant advance for the field. This work not only resolves key theoretical debates but also points the way for designing future nanodevices, materials, and perhaps even methods for controlling water’s properties at unprecedented precision.