First-Principles Molecular Dynamics Simulations of the Spontaneous Freezing Transition of 2D Water in a Nanoslit

As with bulk ices, two-dimensional (2D) ices exhibit diverse crystalline structures, and the majority of these 2D structures have been predicted based on classical molecular dynamics (MD) simulations. Here, the spontaneous freezing transition of 2D liquid water within hydrophobic nanoslits is demonstrated for the first time using first-principles MD simulations. Various 2D ices are observed under different lateral pressure and temperature conditions. Notably, the liquid water confined to a 6.0 A-wide nanoslit can spontaneously freeze into a monolayer ice consisting of an array of zigzag water chains at 2.5 GPa and 250 K. Moreover, within an 8.0 A-wide nanoslit and at 4.0 GPa and 300 K, a previously unreported bilayer ice forms spontaneously that has a structure resembling that of the double surface layers of bulk ice-VII. Both 2D crystalline ices do not obey the ice rule, suggesting first-principles simulation can access a certain phase space that is not easily approached using classical simulations.

Automated summary

First-principles MD simulations demonstrate the spontaneous freezing transition of 2D liquid water within hydrophobic nanoslits, leading to the observation of various 2D ices under different pressure and temperature conditions. The study reveals the formation of a monolayer ice with zigzag water chains and a bilayer ice resembling double surface layers of bulk ice-VII in different width nanoslits at specific pressure and temperature conditions. These 2D crystalline ices do not obey the ice rule, highlighting the unique phase space accessible through first-principles simulations.