Effect of temperature on the structure and phase behavior of water confined by hydrophobic, hydrophilic, and heterogeneous surfaces.

We perform molecular dynamics simulations of water confined between atomically detailed hydrophobic, hydrophilic, and heterogeneous (patchy) nanoscale plates. We study the effects of temperature 220 <or= T <or= 300 K on confined water's behavior at various pressures -0.2 <or= P <or= 0.2 GPa and plate separations 0.5 <or= d <or= 1.6 nm. Combining this with our earlier results on the same system [Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2006, 73, 041604; Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. J. Phys. Chem. C, 2007, 11, 1323], where pressure was varied at constant temperature, allows us to compare water's behavior in nanoscale confinement, upon isobaric cooling and isothermal compression, corresponding to paths of interest in protein denaturation. At a fixed temperature, water confined between hydrophobic plates can form vapor, liquid, or crystal (bilayer ice) phases, depending on the values of P and d. The P-d phase diagrams at T = 300 K and T = 220 K show that cooling suppresses the vapor phase and stabilizes the liquid and crystal phases. The critical separation d(c)(P), below which vapor forms, shifts to lower values of d and P upon cooling. The density profiles show that, upon cooling, water approaches the hydrophobic plates. Hence, the effective hydrophobicity of the plate decreases as T decreases, consistent with the suppression of the vapor phase upon cooling. However, both the orientation of water's molecules at the interface and the water contact angle on the hydrophobic surface show practically no temperature dependence. Simulations of water confined by heterogeneous plates decorated with hydrophobic and hydrophilic patches reveal that cooling leads to appreciable blurring of the differences between water densities at hydrophobic and hydrophilic surfaces. This observation, together with remarkable similarities in confined water's response to isobaric cooling and to isothermal compression, suggests that the invasion of hydrophobic cavities by water is an important mechanism underlying both pressure and cold denaturation of proteins.