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 m icroscope using the colloid probe technique. The adhesion energy obtai ned from measurement of the pull-off force, as a function of ionic str ength and pH, has been a particular focus. The pH range over which adh esion occurred correlated strongly with electrokinetic data, suggestin g that an electrostatic mechanism was dominant. However, the magnitude of the adhesion was significantly less than that anticipated on the b asis of the attractive van der Waals and electrostatic forces and the adhesion decreased with increasing ionic strength. These results may b e qualitatively explained in terms of the roughness of component surfa ces, which increases the effective separation at asperity contact. In addition the pull-off force was not found to be single-valued but depe ndent on the loading force prior to separation of the surfaces. The pr esence of a hydration or gel layer, giving rise to a strong short-rang e repulsion in the neighborhood of the contact points is implicated. T hese observations highlight deficiencies in currently accepted adhesio n models for the description of nonideal, particularly rough,surfaces. (C) 1996 Academic Press, Inc.