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

Understanding the surface interactions between elemental selenium (Se-0) and solid substrates is of both fundamental and practical importance in biological systems, water treatment processes, and microelectro mechanical/nanoelectromechanical systems where Se is present. In this work, for the first time, surface interactions between a Se-0 sphere and substrates of varying hydrophobicity, including silica, octadecyltrichlorosilane (OTS) modified silica, and electrodeposited Se-0 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 Se 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 Se-0 sphere and hydrophobic surfaces (i.e., OTS modified silica, and electrode posited Se film), with Se-0-OTS showing relatively larger hydrophobic decay length (D-0 similar to 1.3 nm) than Se-0-Se-0 (D-0 similar to 1 nm). In 0.5 M NaCl, the electrical double layer interaction was significantly compressed. The change of zeta potentials of Se-0, 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 Se-0 and surfaces of varying hydrophobicity in aqueous solutions. (C) 2016 Elsevier B.V. All rights reserved.