When Is Nanoconfined Water Different From Interfacial Water?

TL;DR

This study uses machine-learning molecular dynamics to identify a sharp transition in water behavior at graphene surfaces, distinguishing between interfacial effects and true nanoconfinement at the molecular level.

Contribution

It provides the first detailed molecular-level characterization of water behavior across a range of slit widths, clarifying the boundary between interfacial and nanoconfined water.

Findings

Water retains interfacial properties with three or more layers.

Below this threshold, water shows enhanced ordering and restructured hydrogen bonds.

The results distinguish interfacial effects from nanoconfinement at the molecular scale.

Abstract

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.