Force interactions between calcium carbonate and chemically functionalized colloidal probes: The effect of calcium ion and polyelectrolyte addition
An understanding of fundamental interactions of micron-sized pigments in papermaking and paper coating processes is important in controlling the interactions between the pigment and cellulose fibers, flocculants and binder. This research established, for the first time, the effects of calcium ion concentration and anionic polyelectrolyte (sodium salt of polyacrylic acid) on interactions between a freshly-cleaved calcite (CaCO3) surface and a chemically-functionalized colloidal probe by direct force measurements using an atomic force microscope. In the absence of polyelectrolyte, adhesive interactions increased with increasing polarity of the probe. The adhesive interaction between calcite and a carboxylate-terminated probe increased strongly with increasing Ca2+ ion concentration and leveled off at about 0.1 M after most available Ca2+ adsorption sites had been occupied. The extent of steric interactions in the presence of polyacrylate depended on polymer concentration and suggested a flat polymer conformation up to monolayer coverage, and an extended polymer conformation above full coverage. Increasing the Ca2+ ion concentration caused a significant compression of the polyacrylate layer, Adhesion between CaCO3 and carboxylate-terminated probes increased with increasing polyelectrolyte concentration. In addition, for any given polymer concentration, adhesion increased also with increasing Ca2+ ion concentration, suggesting a Ca2+ ion mediated polymer bridging mechanism. For a given ion concentration, adhesion force leveled off or peaked at approximately monolayer surface coverage of polymer. These results will be critically discussed in context of published data on retention of dispersed and undispersed calcium carbonate pigments and in context of published data on the dispersion of calcium carbonate with adsorbed polyacrylates.
