Dynamic behaviors of interfacial water on the self-assembly monolayer (SAM) heterogeneous surface.

Dynamic behaviors of water molecules near the surface with mixed hydrophobic and hydrophilic areas are studied by molecular dynamics simulation. More specifically, the diffusion coefficient and hydrogen bond lifetime of interfacial water on the self-assembly monolayer composed of hydrophobic and hydrophilic groups and their dependence on the mixing ratio are studied. The diffusion dramatically slows down, and the hydrogen bond lifetime considerably increases when a few hydrophilic groups are added to the hydrophobic surface. When the percentage of hydrophilic groups increases to 25%, the behavior of interfacial water is similar to the case of the pure hydrophilic surface. The sensitivity to the hydrophilic group can be attributed to the fact that the grafted hydrophilic groups can not only retard the directly bound water molecules but also affect indirectly bound water by stabilizing hydrogen bonds among interfacial water molecules.