Critical behavior of a water monolayer under hydrophobic confinement

TL;DR

This study uses simulations to show that water monolayers under hydrophobic confinement exhibit a 2D liquid-liquid critical point, with a unique 2D-3D crossover influenced by water's hydrogen bonding network.

Contribution

It reveals a novel 2D-3D crossover in water's critical behavior under confinement, differing from simple liquids, due to water's hydrogen bond cooperativity.

Findings

Water monolayer undergoes a liquid-liquid phase transition ending in a 2D Ising universality class.

A 2D-3D crossover occurs at L/h~50, much higher than simple liquids.

The hydrogen bond network's cooperativity explains the unique crossover behavior.

Abstract

The properties of water can have a strong dependence on the confinement. Here, we consider a water monolayer nanoconfined between hydrophobic parallel walls under conditions that prevent its crystallization. We investigate, by simulations of a many-body coarse-grained water model, how the properties of the liquid are affected by the confinement. We show, by studying the response functions and the correlation length and by performing finite-size scaling of the appropriate order parameter, that at low temperature the monolayer undergoes a liquid-liquid phase transition ending in a critical point in the universality class of the two-dimensional (2D) Ising model. Surprisingly, by reducing the linear size L of the walls, keeping the walls separation h constant, we find a 2D-3D crossover for the universality class of the liquid-liquid critical point for L/h~50, i.e. for a monolayer thickness that is small compared to its extension. This result is drastically different from what is reported for simple liquids, where the crossover occurs for L/h ~ 5, and is consistent with experimental results and atomistic simulations. We shed light on these findings showing that they are a consequence of the strong cooperativity and the low coordination number of the hydrogen bond network that characterizes water.