Layering Governs Phase Transitions in Nanoconfined Water.

At molecular-scale confinement, water organizes into discrete layers whose collective rearrangements control its phase behavior. Building on the well-established oscillations of pressure, diffusivity, and other thermodynamic quantities with channel height h, we show that these oscillations originate from two distinct microscopic transition pathways in the hydrogen-bond (HB) network. Using extensive molecular dynamics simulations, we identify a constant-layer phase transition (CLPT), in which freezing develops continuously within a fixed number of layers via the gradual strengthening of intralayer HBs, and a variable-layer phase transition (VLPT), in which layer-splitting instability abruptly collapses the global HB network and melts the ice as the layer number changes. These mechanisms alternate with an increase in confinement height, producing periodic freezing-melting cycles and a ρ-h phase diagram composed of integer-layer ice bands bounded by CLPT and VLPT lines. Motivated by this picture, we develop a minimal Landau free energy model in which the quadratic coefficient oscillates with channel height, encoding geometric commensurability between molecular size and confinement. This oscillatory Landau framework qualitatively reproduces the simulated phase boundaries and demonstrates that layering acts as an active thermodynamic field governing ordering transitions in strongly confined liquids.