When water freezes into ice, it usually follows a heterogeneous nucleation process. The most common form of ice under ambient pressure is a hexagonal diamond lattice, but a metastable diamond cubic lattice also exists. In bulk systems, the nucleation medium influences both the crystallization and the morphology of the ice crystals.

Tianshu Li and colleagues, George Washington University, Washington, D.C., USA, have used molecular dynamics (MD) simulations to investigate if water restricted to form 2D ice crystals within a hydrophobic slit nanopore shows different freezing behaviors to a bulk water system. The researchers simulated the slit by placing graphene sheets back to back.

The team found that the freezing behavior in two dimensions is different from that of bulk water systems. Trilayered ice was found to crystallize into a well-ordered, hexagonal structure. 2D ice does not show any metastable cubic morphologies, because the cubic stacking sequence has a lower thermodynamic stability than a hexagonal sequence at the interface.

• Anomalous Stability of Two-Dimensional Ice Confined in Hydrophobic Nanopores,
  Boxiao Cao, Enshi Xu, Tianshu Li,
  ACS Nano 2019.
  https://doi.org/10.1021/acsnano.9b01014