Adhesion of iron oxide to silica studied by atomic force microscopy

The interaction of an iron oxide sphere with a silica flat immersed in a dilute electrolyte solution has been studied with an atomic force microscope using the colloid probe technique. The adhesion energy obtained from measurement of the pull-off force, as a function of ionic strength and pH, has been a particular focus. The pH range over which adhesion occurred correlated strongly with electrokinetic data, suggesting that an electrostatic mechanism was dominant. However, the magnitude of the adhesion was significantly less than that anticipated on the basis of the attractive van der Waals and electrostatic forces and the adhesion decreased with increasing ionic strength. These results may be qualitatively explained in terms of the roughness of component surfaces, which increases the effective separation at asperity contact. In addition the pull-off force was not found to be single-valued but dependent on the loading force prior to separation of the surfaces. The presence of a hydration or gel layer, giving rise to a strong short-range repulsion in the neighborhood of the contact points is implicated. These observations highlight deficiencies in currently accepted adhesion models for the description of nonideal, particularly rough,surfaces. (C) 1996 Academic Press, Inc.