Understanding the intrinsic water wettability of graphite

We report the effect of airborne hydrocarbon contamination on the water wettability of graphite. Graphite is traditionally known to be hydrophobic with water contact angle (WCA) within the 75-95° range. We found that the WCA of highly ordered pyrolytic graphite (HOPG) was 64.4 ± 2.9° when measured within 10 s after exfoliation in air and increased to ca. 90° after exposure to the ambient air. Ellipsometry measurement showed growth of an adsorptive layer on exfoliated HOPG and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) data indicated that the layer is airborne hydrocarbon. Theoretical calculation confirms that adsorption of only a monolayer amount of hydrocarbon is enough to reproduce the hydrophobic behavior previously observed on HOPG. These results indicated that graphite is intrinsically more hydrophilic than previously believed and that surface adsorbed airborne hydrocarbon is the source of hydrophobicity.