Asymmetric hydrophilic/hydrophobic nanoconfinement directs novel two-dimensional ice structures and phase transitions

Understanding the phase and dynamic behaviors of nanoconfined water is of critical importance for both fundamental scientific research and technological applications. Although numerous studies have investigated nanoconfined water systems, most have exclusively focused on symmetric hydrophobic confinement. In contrast, the phase behavior of water under asymmetric hydrophobic/hydrophilic confinement remains poorly understood. Here, we systematically studied the compression properties, phase diagram, and freezing/melting transitions of two-dimensional (2D) water/ice (monolayer to trilayer) confined between mica and graphene nanoslits. We establish a compression phase diagram in the plane of nanocapillary width and pressure, revealing that symmetry breaking of water-surface interactions induces unique 2D ice structures. In particular, we report four previously unidentified 2D ice phases: monolayer triangular ice (ML-TI), bilayer AA- and AB-stacked triangular/hexagonal mixed ice (BL-AB-THMI and BL-AA-THMI), and trilayer triangular/hexagonal mixed ice (TL-THMI). These structures emerge from the synergistic interplay between the templating effect of the mica surface and confinement effects. Our study fills a fundamental gap in the physics of asymmetric nanoconfinement, provides new mechanistic insights into structural transitions, and offers guidance for nanotechnology applications.