Nanoconfined Water Shows Interfacial Behaviour When Slit Width Accommodates Three or More Layers, Study Finds

Water’s behaviour dramatically changes when it encounters surfaces or becomes squeezed into extremely tight spaces, yet the point at which these two effects diverge has remained a long-standing question. Xavier R. Advincula, Christoph Schran, and Angelos Michaelides, all from the University of Cambridge, now demonstrate a clear distinction between water at surfaces and genuinely confined water. Their research reveals that water maintains characteristics of an open surface until the space between confining walls becomes incredibly narrow, specifically when fewer than three water layers can fit. Below this threshold, the water undergoes a significant structural change, exhibiting enhanced ordering and a fundamentally altered hydrogen-bond network, which has important implications for understanding and controlling water’s behaviour in nanoscale materials and environments.

Specifically, they found that when fewer than three water layers can fit between surfaces, the water undergoes a significant structural change, exhibiting enhanced ordering and a fundamentally altered hydrogen-bond network. This discovery has important implications for understanding and controlling water’s behaviour in nanoscale materials and environments.

Machine Learning Simulates Confined Water Structure

Scientists have pioneered a methodology employing machine-learning molecular dynamics to investigate water confined between graphene sheets, probing slit widths ranging from open interfaces to angstrom-scale confinement. This innovative approach harnessed an advanced machine-learning potential, developed in-house, to simulate water behaviour across these varying confinements. This allowed access to system sizes and trajectory lengths unattainable with conventional methods while maintaining first-principles accuracy, enabling a consistent, molecular-level comparison of density layering, hydrogen bonding, and orientational ordering, key descriptors of interfacial water structure, under different confinement conditions. The team constructed five graphene slit-pore systems, spanning pore widths from approximately 7 to 30 Ångströms, and directly compared these with analogous graphene, water, vacuum systems to isolate the effects of confinement. This work extended previous research by extrapolating density profiles to achieve a bulk-like density at the slit’s center, and the resulting machine-learning potential accurately simulates water behaviour at the atomic scale, capturing subtle structural changes induced by confinement.

Water Structure Transitions in Nanoscale Graphene Slits

This research delivers a detailed molecular-level understanding of how water behaves in extremely confined spaces, distinguishing between water at a simple interface and water truly confined within nanoscale channels. Scientists achieved a breakthrough in characterizing water’s structure within graphene slits, ranging from 6. 41 Ångströms in width, using a combination of first-principles calculations and machine learning potentials. They meticulously examined density layering, hydrogen bonding, and orientational ordering to pinpoint the transition between interfacial and confined behaviour. The experiments revealed a sharp structural transition occurring when the slit width allows for three or more water layers to fit between the graphene sheets. Below this threshold, the water undergoes significant reorganisation, exhibiting enhanced ordering and a restructured hydrogen-bond network. This work establishes a molecular-scale framework for interpreting experimental observations of water in nanoscale systems, providing crucial insights for technologies reliant on water at interfaces and under nanoconfinement.

Water Restructuring Under Extreme Confinement

This research establishes a clear molecular-level understanding of how water behaves at surfaces and under extreme confinement, identifying a sharp structural transition dependent on the space available between confining walls. The team demonstrates that when three or more water layers fit within a narrow space, the water structure closely resembles that of an open surface, maintaining typical density layering, hydrogen bonding, and orientational ordering. However, below this threshold, at the angstrom scale, the water undergoes significant reorganization, exhibiting enhanced ordering and a restructured hydrogen-bond network. These findings are significant because they delineate the boundary between interfacial behaviour and genuine nanoconfinement, providing crucial guidance for predicting and controlling water structure in nanoscale solid-liquid environments. This is particularly relevant for technologies reliant on nanoscale control of water, such as filtration, energy conversion, and electrochemical systems.