Molecular dynamics study on the effect of surface hydroxyl groups on three-phase wettability in oil-water-graphite systems

In this paper, a hydroxylated graphite surface is generated as a hydrophilic oleophobic material for the application of oil-water separation, and the effects of hydroxyl density on the three-phase wettability are studied in oil-water-graphite systems. We analyze the adsorption of water molecules on the hydroxylated surfaces and obtain the relationship between water-oil-solid interfacial properties and the hydroxyl density, which results from the synthetic effects of the orientation of molecules and hydrogen bonds. With the increase of hydroxyl density, the water-solid contact angle first decreases rapidly, and then remains constant. The density of the hydrogen bond formed between hydroxyls and water molecules in the adsorption layer can explain the regularity of the three-phase wettability. The orientation of the water molecules in the adsorption layer shows insignificant variation, owing to the hydrogen bond network formed between the water molecules; thus, little change is observed in the hydrogen bond density in the adsorption layer.