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

Abstract

We perform systematic molecular dynamics simulations of water confined between two nanoscale plates at $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We investigate the effect of pressure $(\ensuremath{-}0.15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}\ensuremath{\leqslant}P\ensuremath{\leqslant}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ and plate separation $(0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ensuremath{\leqslant}d\ensuremath{\leqslant}1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ 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., $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $P\ensuremath{\approx}\ensuremath{-}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $P\ensuremath{\approx}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$), until, at high $P$, no capillary evaporation occurs. For $d\ensuremath{\approx}0.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $P\ensuremath{\geqslant}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, 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.