Computational simulations of solvation force of water under different hydrophobic interactions

Surfaces in water would bear hydration repulsion or hydrophobic attraction when separation is small. However, the interaction mechanism of hydrophobic surfaces is still unclear though they are very important when the surfaces are in nanometer separation. With molecular dynamics simulations, the solvation force of water molecules between graphene surfaces of different hydrophobicity is analyzed. Important features of the step-like solvation force oscillatory behavior during the compression within a distance of ~1.5 nm indicate that water is squeezed out layer-by-layer. The hydrophobicity of the graphene surfaces is shown to be an important parameter that influences the solvation force of water molecules. We find that the solvation force decreases when the hydrophobicity of the graphene surfaces increases. Detailed analysis of the water density distributions and the water molecule orientation between graphene surfaces show that changing hydrophobicity would influence the water structure. As the graphene surface becomes more hydrophobic, the water molecules become less ordered and the concentration will also decrease to some extent, which can account for the attenuation of the solvation force.