Hydration behavior under confinement by nanoscale surfaces with patterned hydrophobicity and hydrophilicity

We perform molecular dynamics simulations of water confined between nanoscale surfaces (≈3.2 × 3.2 nm²) with various patterns of hydrophobicity and hydrophilicity at T = 300 K, -0.05 GPa ≤ P ≤ 0.2 GPa, and plate separations 0.5 nm ≤ d ≤ 1.6 nm. We find that the water surface density in the first hydration layer is considerably higher at a hydrophobic patch surrounded by hydrophilic borders than it is at a purely hydrophobic surface with the same area, highlighting the importance of heterogeneity on hydrophobicity at nanoscopic length scales. Increasing the pressure causes a progressive blurring of the difference between interfacial water densities manifest at hydrophilic and hydrophobic surfaces, with only minor differences remaining at 0.2 GPa. At P = -0.05 GPa and d = 0.6 nm, a single layer of hydrophilic sites along the border of the hydrophobic nanoscale plates is sufficient to prevent bulk cavitation, in contrast to the behavior observed in the absence of the hydrophilic sites. At small separation between the nanoscale surfaces (d ≤ 0.7 nm), a single hydrophilic site at the center of the hydrophobic plates prevents complete drying of the confined space, with water molecules remaining next to the hydrophilic site for at least 1 ns.