Microsecond-Scale Molecular Dynamics Simulation of Phase Transition of a Bilayer Ice: Kinetic Constraints in Confined Water.

In this study, we investigate the phase behavior of water confined between two parallel smooth walls by using both classical molecular dynamics (MD) simulations and machine-learned potential (MLP) MD simulations. Particular attention is focused toward the water-to-ice phase transition below the freezing point. Three distinct two-dimensional (2D) bilayer (BL) crystalline ice phases are observed, namely, bilayer hexagonal ice (BL-ice I), bilayer very high-density ice (BL-VHDI), and a newly found bilayer penta-hexa ice (BL-PHI). The latter consists of interlocked pentagonal and hexagonal rings. The transition from liquid to BL-PHI is weakly first-order, and typically, the BL-PHI emerges at intermediate to high lateral pressures (400 to 900 MPa) after microsecond-scale simulations, highlighting its relatively slow formation process. Compared to BL-ice I and BL-VHDI, BL-PHI exhibits much higher diffusion activation energy and hence a much slower freezing rate. Additionally, the transition temperatures of all three bilayer ices are pressure-dependent. These findings provide new insights into the complex behavior of nanoconfined water.