Molecular Dynamics Simulation of Monolayer Confined Ice-Water Phase Equilibrium

Confined water became a recent hot topic in water science due to its extremely abundant structural phase behavior. However, there exist few studies focused on the coexistence of two or more confuted water phases and their related properties. We present a methodology for studying the coexistences of two confmed phases of water, based on a series equilibrium molecular-dynamics (MD) simulations using isobaric-isoenthalpic ensembles to iteratively predict the melting temperatures of the low dimensional confined crystal phase of water. The methodology is applied to the coexistence of the monolayer ice and water (described with a simple water model, i.e. SPC/E model) confined in the 0.65 nm size pore, yielding a direct determination of the melting point and extensive atomic-scale characterization for the mono-molecular layer containing the confined ice-water coexistence line. A finite value of lateral pressure (5000 bar) is adopted in the simulation, to mimic the high-pressure environment of the water molecules confmed in the bi-graphene pocket in a recent experiment by Algara-Siller et al. [Nature, 519, 443 (2015)]. The rough structural type and the capillary fluctuation of the line, the microscopic mechanism of the solid-liquid structural transition along the line, as well as the transport of the point defect in the solid side of the coexistence line are identified directly from the MD trajectories. Various profiles of different thermodynamic properties across the coexistence line illustrate the unique features for the in-plane coexistence of the monolayer confuted ice-water system, e.g., the unexpected large width of the crystal-melt transition region, and the compression state along the solid-liquid phase coexistence line. The methodology presented in the current study can be easily applied to the coexistence of multilayer confmed ice and water phases, as well as the many other types of water models beyond the SPC/E used in current work. The achievement of the low dimensional confined ice-water phase coexistence could potentially facilitate the fundamental advancements in thermodynamics and kinetic theories of the low dimensional water science.