Although graphitic materials were thought to be hydrophobic, recent experimental results based on contact angle measurements show that the hydrophobicity of graphitic surfaces stems from airborne contamination of hydrocarbons. This leads us to question whether a pristine graphitic surface is indeed hydrophobic. To investigate the water wettability of graphitic surfaces, we use molecular dynamics simulations of water molecules on the surface of a single graphene layer at room temperature. The results indicate that a water droplet spreads over the entire surface and that a double-layer structure of water molecules forms on the surface, which means that wetting of graphitic surfaces is possible, but only by two layers of water molecules. No further water layers can cohere to the double-layer structure, but the formation of three-dimensional clusters of liquid water is confirmed. The surface of the double-layer structure acts as a hydrophobic surface. Such peculiar behavior of water molecules can be reasonably explained by the formation of hydrogen bonds: The hydrogen bonds of the interfacialwater molecules form between the first two layers and also withineach layer. This hydrogen-bond network is confined within the doublelayer, which means that no “dangling hydrogen bonds”appear on the surface of the double-layer structure. This formationof hydrogen bonds stabilizes the double-layer structure and makesits surface hydrophobic. Thus, the numerical simulations indicatethat a graphene surface is perfectly wettable on the atomic scaleand becomes hydrophobic once it is covered by this double layer ofwater molecules.
展开▼
