When Is Nanoconfined Water Different From Interfacial Water?

Water behaves very differently at surfaces and under extreme confinement, but the boundary between these two regimes has remained unclear. Despite evidence that interfacial effects persist under sub-nanometre confinement, the molecular-scale behaviour and its evolution with slit width remain unclear. Here, we use machine-learning molecular dynamics with first-principles accuracy to probe water at graphene surfaces across slit widths ranging from the open-interface limit to angstrom-scale confinement. We find that water undergoes a sharp structural transition: when three or more water layers fit between the walls, the structure of the graphene-water interface is effectively indistinguishable from that in an open system, with density layering, hydrogen bonding, and orientational ordering retaining interfacial character. Below this threshold, however, angstrom-scale confinement strongly reorganises the liquid, producing enhanced ordering, a restructured hydrogen-bond network, and modified orientational motifs. These results establish a molecular-level picture that clearly separates interfacial behaviour from genuine nanoconfinement and provide guidance for predicting and controlling the structure of water in nanoscale solid-liquid environments.

💡 Research Summary

This paper presents a clear molecular-level distinction between “interfacial water” and “genuinely nanoconfined water” by systematically investigating the structural evolution of water confined within graphene slit pores of varying widths, from angstrom-scale confinement to the open interface limit.

The authors employ machine-learning molecular dynamics (ML-MD) using a MACE potential trained on revPBE-D3(0) density functional theory data. This approach provides first-principles accuracy while enabling the simulation of large systems and long timescales necessary for robust statistical analysis. The study compares two sets of systems: 1) Graphene-water-graphene slit pores of five widths (XS: 6.91 Å, S: 9.41 Å, M: 12.20 Å, XL: 19.41 Å, XXL: 29.41 Å), and 2) Corresponding open graphene-water-vacuum interfacial systems, created by removing one confining graphene sheet.

The central finding is a sharp structural transition governed by the number of water layers that can fit between the graphene walls. In the “weak confinement” regime (pore widths accommodating three or more water layers, approximately ≥12 Å), the water structure adjacent to each graphene surface is effectively indistinguishable from that at an open graphene-water interface. Key descriptors—including the position and intensity of the first density peak, the average number and distribution of hydrogen bonds (H-bonds) per interfacial molecule, and the orientational ordering of O-H bonds relative to the surface—all converge to their interfacial limits. This indicates that the influence of the opposing confining wall becomes negligible, and the system behaves like two independent, overlapping interfaces.

In contrast, in the “strong confinement” regime (pores narrower than ~10 Å, accommodating two or fewer layers), water undergoes a fundamental reorganization. The density layering becomes more pronounced and compressed. More significantly, the hydrogen-bond network is restructured; for example, in the S pore, the average H-bonds per interfacial molecule increases, and the population of fully coordinated DDAA motifs rises substantially, indicating a shift towards a more ordered, crystal-like network. The orientational distributions also deviate markedly from the interfacial pattern. In the extreme XS pore, removing one wall leads to the formation of discrete water clusters rather than a continuous film, highlighting how strong confinement can induce qualitatively different states.

The study concludes that the boundary between interfacial and nanoconfined behavior lies at the point where roughly three molecular layers of water can exist between the walls. Below this threshold, angstrom-scale confinement dominates and creates a distinct structural regime characterized by enhanced ordering and a restructured H-bond network. These results provide a concrete molecular-scale framework for interpreting experiments and designing technologies that rely on water in nanoscale environments, such as nanofluidic devices, filtration membranes, and electrochemical systems.