Direct Measurement of Colloidal Forces between Mica and Silica in Aqueous Electrolyte

A refined methodology which allows attachment of submicrometer colloidal particles to atomic force microscopy cantilevers is described. The interactions between particles of this size and both mica and silica surfaces in aqueous electrolyte at pH 5.8 have been studied. The monotonic repulsive force observed was in all cases well fitted by assuming that the force was due to overlap of electrical double layers. The potential of a cleaved mica surface was found to be significantly less than that of the silica surface. However, the mica potential was found to increase, if the mica surface was immersed immediately (within seconds) in electrolyte after cleavage or after plasma cleaning. This behavior is explained by the finite aqueous solubility of adventitious carbon that spontaneously deposits on mica after cleavage in air. The surface potential of the silica spheres was obtained by fitting symmetric interaction data. The trend in zeta potential of the silica spheres with pH, determined by particle electrophoresis, was similar to that obtained from analysis of force-separation data.