Adhesive forces between adsorbed anionic polyelectrolyte layers in high ionic strength solutions

Direct interaction forces between a hematite surface and a silica sphere in the presence of high-molecular weight anionic polyelectrolytes, sodium polyacrylate (NaPAA), an acrylamide/acrylate copolymer, an hydroxamate copolymer and starch have been measured using the atomic force microscope (AFM). In aqueous solution at high pH, the interaction between the bare oxide surfaces is well described by DLVO theory and is dominated by a repulsive electrostatic force. Addition of a high-molecular weight polyelectrolyte did not significantly alter the forces on approach. There was no sign of steric repulsion prior to the distance of closest approach. Therefore, it appears that these molecules are adsorbed to the surface in a flat conformation. On separation, the adhesive force between the surfaces was observed to change in magnitude as a function of polyelectrolyte type and concentration. Elastic minima that were found to extend to large surface separations were observed to occur on separation of the surfaces for all polyelectrolytes studied. At high-polyelectrolyte concentration, the interaction on both approach and separation of the surfaces was repulsive. Specific interaction of the functional groups with the metal ion of the oxide surface appears to be responsible for the differences in the adhesive force. Hydroxamate and starch molecules were found to produce the strongest and most extended adhesive forces.