Continuous melting through a hexatic phase in confined bilayer water

TL;DR

This study uses molecular dynamics simulations to reveal that nanoconfined bilayer water exhibits a continuous melting process involving an intermediate hexatic phase, with decoupled oxygen and hydrogen behaviors, advancing understanding of 2D water phase transitions.

Contribution

It demonstrates the presence of a hexatic phase in confined bilayer water and details the decoupled melting behavior of oxygens and hydrogens, providing new insights into 2D water phase transitions.

Findings

Identification of a hexatic phase in confined bilayer water.

Decoupled melting of oxygens and hydrogens.

Evidence of a continuous melting process at high density.

Abstract

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquid-like throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated, yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.