  We perform systematic molecular dynamics simulations of water confined
between two nanoscale plates at T=300 K. We investigate the effect of pressure
(-0.15 GPa <= P <= 0.2 GPa) and plate separation (0.4 nm <= d <= 1.6 nm) on the
phase behavior of water when the plates are either hydrophobic or hydrophilic.
When water is confined between hydrophobic plates, capillary evaporation occurs
between the plates at low enough P. The threshold value of d at which this
transition occurs decreases with P (e.g., 1.6 nm at P ~ -0.05 GPa; 0.5 nm at P
\~ 0.1 GPa), until, at high P, no capillary evaporation occurs. For d ~ 0.6 nm
and P >= 0.1 GPa, the system crystallizes into a bilayer ice.
  A P-d phase diagram showing the vapor, liquid, and bilayer ice phases is
proposed. When water is confined by hydrophilic (hydroxylated silica) plates,
it remains in the liquid phase at all P and d studied. Interestingly, we
observe, for this case, that even at the P at which bulk water cavitates, the
confined water remains in the liquid state. We also study systematically the
state of hydration at different P for both kinds of plates. For the range of
conditions studied here, we find that in the presence of hydrophobic plates the
effect of P is to enhance water structure and to push water molecules toward
the plates. The average orientation of water molecules next to the hydrophobic
plates does not change upon pressurization. In contrast, in the presence of
hydrophilic plates, water structure is insensitive to P. Hence, our results
suggest that upon pressurization, hydrophobic plates behave as `soft' surfaces
(in the sense of accommodating pressure-dependent changes in water structure)
while hydrophilic walls behave as `hard' surfaces.

Chemical Physics;Soft Condensed Matter;Computational Physics