Variation of Critical Crystallization Pressure for the Formation of Square Ice in Graphene Nanocapillaries

TL;DR

This study uses molecular dynamics simulations to explore how the size of graphene sheets influences the critical pressure needed for square ice formation in nanocapillaries, revealing size-dependent thresholds and behaviors.

Contribution

It demonstrates the size dependence of critical crystallization pressure for square ice formation in graphene nanocapillaries, providing insights into nanoscale ice formation mechanisms.

Findings

Critical pressure decreases with increasing graphene size.

Threshold size below which square ice cannot spontaneously form.

Fluctuations in critical pressure diminish as graphene size grows.

Abstract

Two-dimensional square ice in graphene nanocapillaries at room temperature is a fascinating phenomenon and has been confirmed experimentally. Instead of temperature for bulk ice, the high van der Waals pressure becomes an all-important factor to induce the formation of square ice and needs to be studied further. By all-atom molecular dynamics simulations of water confined between two parallel graphene sheets, which are changed in size (the length and the width of the graphene sheets) over a wide range, we find that the critical crystallization pressure for the formation of square ice in nanocapillary strongly depends on the size of the graphene sheet. The critical crystallization pressure slowly decreases as the graphene size increases, converging to approximately macroscopic crystallization pressure. The unfreezable threshold for graphene size is obtained by estimating the actual pressure and it is difficult to form the square ice spontaneously in practice when the graphene sheet is smaller than the threshold. Moreover, the critical crystallization pressure fluctuates when the graphene size is minuscule, and the range of oscillation narrows as the sheet size increases, converging to the macroscopic behavior of a single critical icing pressure for large sheets. The graphene size also affects the stability and crystallization time of square ice.