Existence of Multiple Phases of Water at Nanotube Interfaces

Water, because of its anomalous properties, can exhibit complex behavior under strong confinement. At room temperature and pressure, water is assumed to exist in a single phase as a liquid under confinement (e.g., in a carbon nanotube). In this study, using extensive molecular dynamics simulations, we show the existence of multiple phases of water when water meets a nanotube surface under atmospheric conditions (T = 300 K, P = 1 atm). Vapor, high-density ice, and liquid water phases coexist in the region within ∼1 nm from the surface. Structure factor, entropy, pressure, viscosity, and rotational diffusion of water layers near the surface reveal substantial phase anomalies induced by confinement. We show the presence of a new high-density solid-state ice layer (χ = 3.9 g/cm³) with rhombic structure coexisting adjacent to vapor and liquid water. The existence of multiple phases of water near an interface can explain, for example, the slip phenomena, self-filling behavior of a carbon nanotube, and fast transport of water.