Two-Dimensional Water in Graphene Nanocapillaries Simulated with Different Force Fields: Rhombic Versus Square Structures, Proton Ordering, and Phase Transitions

TL;DR

This study uses molecular dynamics simulations with various force fields to explore the structures, proton ordering, and phase transitions of monolayer ice confined between graphene sheets, revealing diverse arrangements and transition behaviors.

Contribution

It systematically compares different water force fields in simulating quasi-2D ice, uncovering structural diversity and phase transition mechanisms not fully explored before.

Findings

Identified nearly square and rhombic hydrogen bond patterns.

Observed second-order phase transitions between different ice structures.

Detected melting-like transitions from ice to liquid water at high temperatures.

Abstract

Hydrogen bond patterns, proton ordering, and phase transitions of monolayer ice in two-dimensional hydrophobic confinement are fundamentally different from those found for bulk ice. To investigate the behavior of quasi-2D ice, we perform molecular dynamics simulations of water confined between fixed graphene plates at a distance of 0.65 nm. While experimental results are still limited and theoretical investigations are often based on a single force field model, this work presents a systematic study using different water force fields, i.e. SPCE, TIP3P, TIP4P, TIP4P/ICE, TIP5P. The water occupancy of the graphene capillary at a pressure of 1000 MPa is determined to be between 13.5 and 13.9 water molecules per square nanometer, depending on the choice of the water force field. To ensure complete sampling of the configurational space and to overcome barriers separating metastable structures, these simulations are based on the replica exchange molecular dynamics technique. We report different tetragonal hydrogen bond patterns which are classified as nearly square or as rhombic. While many of these arrangements are flat, in some cases puckered arrangements are found, too. Also the proton ordering of the quasi-2D water structures is considered, allowing to identify them as ferroelectric, ferrielectric or antiferroelectric. For temperatures between 200 K and 400 K we find several second-order phase transitions from one ice structure to another, changing in many cases both the arrangements of the oxygen atoms and the proton ordering. For temperatures between 400 K and 600 K there are melting-like transitions from a monolayer of ice to a monolayer of liquid water. These first-order phase transitions have a latent heat between 3.4 and 4.0 kJ/mol.