Studying interactions between undecane and graphite surfaces by chemical force microscopy and molecular dynamics simulations

Collectors are often used to increase the hydrophobicity of valuable minerals during flotation. Hence, it is necessary to know the forces between them, especially the hydrophobic force, which closely associated with hydrophobic minerals floating, such as graphite, coal, and molybdenite. In the present study, graphite sheets were used as the hydrophobic mineral, and the force characteristics and interfacial adsorption structures of undecane (a model collector) on graphite surface with different hydrophobicities were investigated by chemical force microscopy and molecular dynamics simulations. Undecane experiences repulsive interactions as it approaches hydrophilic graphite; however, an obvious jump-in phenomenon driven by hydrophobic force was observed for hydrophobic graphite, which triggers their adhesion. Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO fitting reveal that the hydrophobic force decays at 1.35 nm in a single-exponential manner. The adhesion force during retraction increases with increasing surface hydrophobicity. The hydrophilic surface adsorbs a large amount of water to form a dense and ordered hydration film that interferes with the adsorption of undecane, while a water-depletion layer exists on the hydrophobic surface with closely adsorbed undecane molecules. This study improves our understanding of the action mechanism of flotation collectors for hydrophobic minerals.