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O: Oberflächenphysik

O 15: Postersitzung (Adsorption an Oberfl
ächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Rastersondentechniken, Zeitaufgelöste Spektroskopie, Methoden)

O 15.7: Poster

Freitag, 4. März 2005, 17:00–20:00, Poster TU D

Study of the effect of electrolyte ions on the forces between hydrophobic polymer surfaces in aqueous solutions — •Astrid Drechsler and Karina Grundke — Leibniz-Instititut für Polymerforschung e.V., Hohe Straße 6, 01069 Dresden

Interfacial phenomena are strongly influenced by adsorption processes. In aqueous environments, even simple electrolyte ions modify the interactions of polymer surfaces. In the present study, the forces between a flat polystyrene surface and a colloidal polystyrene sphere have been measured directly in water and solutions of KCl and KOH using a scanning force microscope. At distances > 5...50 nm, all force-distance curves show an electrostatic repulsion caused by an electrical double-layer of electrolyte ions adsorbed to the surface. The surface potential of the polystyrene surface calculated from the force-distance curves coincides with its zeta potential. The decay lenght of the electrostatic repulsion diminishes with increasing electrolyte concentration due to the screening of the surface potential by the mobile ion layer. At distances < 5...20 nm, in most of the solutions an attractive force of variable range and strength dominates, the so-called ’hydrophobic force’. We attribute it to the existence of nanoscopic air bubbles on the hydrophobic polymer surfaces. It is not affected by the KCl concentration. In KOH concentrations <= 10-4 M, this attraction disappears. Instead, a "soft" steric repulsion occurs. The adhesion force decreases steadily with increasing KOH concentration. It is not clear if these phenomena are caused by air nanobubbles or if the nanobubbles are replaced by a strongly adsorbed layer of OH– and counterions.