  Liquid water is not only of obvious importance but also extremely intriguing,
displaying many anomalies that still challenge our understanding of such an a
priori simple system. The same is true when looking at nanoconfined water: The
liquid between constituents in a cell is confined to such dimensions, and there
is already evidence that such water can behave very differently from its bulk
counterpart. A striking finding has been reported from computer simulations for
two-dimensionally confined water: The liquid displays continuous or
discontinuous melting depending on its density. In order to understand this
behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined
water by means of molecular dynamics simulations. At high density we observe
the continuous melting to be related to the phase change of the oxygens only,
with the hydrogens remaining liquid-like throughout. Moreover, we find an
intermediate hexatic phase for the oxygens between the liquid and a triangular
solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory
for two-dimensional melting. The liquid itself tends to maintain the local
structure of the triangular ice, with its two layers being strongly correlated,
yet with very slow exchange of matter. The decoupling in the behavior of the
oxygens and hydrogens gives rise to a regime in which the complexity of water
seems to disappear, resulting in what resembles a simple monoatomic liquid.
This intrinsic tendency of our simulated water may be useful for understanding
novel behaviors in other confined and interfacial water systems.
