  Water in nanoscale cavities is ubiquitous and of central importance to
everyday phenomena in geology and biology. However, the properties of nanoscale
water can be remarkably different from bulk, as shown e.g., by the anomalously
low dielectric constant of water in nanochannels [1], near frictionless water
flow [2], or the possible existence of a square ice phase [3]. Such properties
suggest that nanoconfined water could be engineered for technological
applications in nanouidics [4], electrolyte materials [5], and water
desalination [6]. Unfortunately, challenges in experimentally characterising
water on the nanoscale and the high cost of first-principles simulations have
prevented the molecular level understanding required to control the behavior of
water. Here we combine a range of computational approaches to enable a
first-principles level investigation of a single layer of water within a
graphene-like channel. We find that monolayer water exhibits surprisingly rich
and diverse phase behavior that is highly sensitive to temperature and the van
der Waals pressure acting within the nanochannel. In addition to multiple
molecular phases with melting temperatures varying non-monotonically by over
400 degrees with pressure, we predict a hexatic phase, which is an intermediate
between a solid and a liquid, and a superionic phase with a high electrical
conductivity exceeding that of battery materials. Notably, this suggests that
nanoconfinement could be a promising route towards superionic behavior at
easily accessible conditions.
