Interactions between elemental selenium and hydrophilic/hydrophobic surfaces: Direct force measurements using AFM

• Surface forces between Se0 and substrates were directly measured for the first time.
• DLVO forces dominate the interactions between Se0 and hydrophilic substrates.
• Increasing salinity and decreasing pH would reduce electrical double layer forces.
• Hydrophobic attraction exists between Se0 and hydrophobic surfaces.

Understanding the surface interactions between elemental selenium (Se0) and solid substrates is of both fundamental and practical importance in biological systems, water treatment processes, and microelectromechanical/nanoelectromechanical systems where Se0 is present. In this work, for the first time, surface interactions between a Se0 sphere and substrates of varying hydrophobicity, including silica, octadecyltrichlorosilane (OTS) modified silica, and electrodeposited Se0 film, were directly measured in aqueous solutions at different pH and salinity conditions using an atomic force microscope (AFM). In 1 mM NaCl, electrical double layer forces and van der Waals forces dominated the interactions between Se0 sphere and hydrophilic silica, and the force-distance profiles could be well fitted by classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Additional hydrophobic attraction was found to play a role in the interactions between Se0 sphere and hydrophobic surfaces (i.e., OTS modified silica, and electrodeposited Se0 film), with Se0-OTS showing relatively larger hydrophobic decay length (D0 ∼ 1.3 nm) than Se0-Se0 (D0 ∼ 1 nm). In 0.5 M NaCl, the electrical double layer interaction was significantly compressed. The change of zeta potentials of Se0, OTS and silica surfaces with pH showed good agreement and same trend as that of the fitted surface potentials based on the force measurements. This work provides useful information regarding the interaction mechanism of Se0 and surfaces of varying hydrophobicity in aqueous solutions.

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