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We perform systematic molecular dynamics simulations of water confinedbetween 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 thephase behavior of water when the plates are either hydrophobic or hydrophilic.When water is confined between hydrophobic plates, capillary evaporation occursbetween the plates at low enough P. The threshold value of d at which thistransition 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 nmand P >= 0.1 GPa, the system crystallizes into a bilayer ice.  A P-d phase diagram showing the vapor, liquid, and bilayer ice phases isproposed. When water is confined by hydrophilic (hydroxylated silica) plates,it remains in the liquid phase at all P and d studied. Interestingly, weobserve, for this case, that even at the P at which bulk water cavitates, theconfined water remains in the liquid state. We also study systematically thestate of hydration at different P for both kinds of plates. For the range ofconditions studied here, we find that in the presence of hydrophobic plates theeffect of P is to enhance water structure and to push water molecules towardthe plates. The average orientation of water molecules next to the hydrophobicplates does not change upon pressurization. In contrast, in the presence ofhydrophilic plates, water structure is insensitive to P. Hence, our resultssuggest 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.Comment: accepted for publication in Phys. Rev. E. Figs. 1, 3, 4, and 6 are in  low resolution to avoid large fig. file

Effect of pressure on the phase behavior and structure of water confined between nanoscale hydrophobic and hydrophilic plates