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

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.