The structure of liquid water at an extended hydrophobic surface

TLDR

Molecular dynamics simulations of liquid water between flat hydrophobic surfaces reveal a surface arrangement that balances hydrogen‑bond maximization and packing density. A detailed analysis shows that the liquid’s structural properties farther from the surface are governed by this surface structure. The surfaces generate density oscillations extending at least 10 Å and orientational preferences extending at least 7 Å, with nearest‑surface water exhibiting dangling hydrogen bonds and the hydration structure of large hydrophobic surfaces differing markedly from that of small ones.

Abstract

Molecular dynamics simulations have been carried out for liquid water between flat hydrophobic surfaces. The surfaces produce density oscillations that extend at least 10 Å into the liquid, and significant molecular orientational preferences that extend at least 7 Å into the liquid. The liquid structure nearest the surface is characterized by ‘‘dangling’’ hydrogen bonds; i.e., a typical water molecule at the surface has one potentially hydrogen-bonding group oriented toward the hydrophobic surface. This surface arrangement represents a balance between the tendencies of the liquid to maximize the number of hydrogen bonds on the one hand, and to maximize the packing density of the molecules on the other. A detailed analysis shows that the structural properties of the liquid farther from the surface can be understood as effects imposed by this surface structure. These results show that the hydration structure of large hydrophobic surfaces can be very different from that of small hydrophobic molecules.