# Microsecond-Scale Molecular Dynamics Simulation of Phase Transition of a Bilayer Ice: Kinetic Constraints in Confined Water

## TL;DR

This paper uses simulations to study how water turns into different types of ice when confined between two surfaces, revealing new insights into the slow and pressure-dependent freezing process.

## Contribution

The discovery of a new bilayer penta-hexa ice phase and its unique kinetic behavior in confined water.

## Key findings

- A new bilayer penta-hexa ice (BL-PHI) phase was identified, consisting of interlocked pentagonal and hexagonal rings.
- BL-PHI forms slowly at intermediate to high lateral pressures (400 to 900 MPa) and has a much higher diffusion activation energy than other bilayer ice phases.
- The transition temperatures of all three bilayer ice phases are pressure-dependent.

## Abstract

In this study, we
investigate the phase behavior of water confined
between two parallel smooth walls by using both classical molecular
dynamics (MD) simulations and machine-learned potential (MLP) MD simulations.
Particular attention is focused toward the water-to-ice phase transition
below the freezing point. Three distinct two-dimensional (2D) bilayer
(BL) crystalline ice phases are observed, namely, bilayer hexagonal
ice (BL-ice I), bilayer very high-density ice (BL-VHDI), and a newly
found bilayer penta-hexa ice (BL-PHI). The latter consists of interlocked
pentagonal and hexagonal rings. The transition from liquid to BL-PHI
is weakly first-order, and typically, the BL-PHI emerges at intermediate
to high lateral pressures (400 to 900 MPa) after microsecond-scale
simulations, highlighting its relatively slow formation process. Compared
to BL-ice I and BL-VHDI, BL-PHI exhibits much higher diffusion activation
energy and hence a much slower freezing rate. Additionally, the transition
temperatures of all three bilayer ices are pressure-dependent. These
findings provide new insights into the complex behavior of nanoconfined
water.