Forces between silica surfaces in aqueous solutions of a weak polyelectrolyte

The forces between negatively charged silica surfaces in the presence of a weak polyelectrolyte, poly (2-vinylpyridine), were measured as a function of polymer concentration, salt concentration, solution pH, and surface collision rates. The solubility of the polymer is highly dependent on the solution pH; that is, when the molar concentration of solution protons is equivalent to the molar concentration of pyridine groups the polymer is > 70% protonated and is highly soluble. As the pH increases, the degree of protonation decreases and the polymer becomes insoluble and precipitates from solution. At low polymer concentrations, low salt concentration, and a low pH, the polymer adsorbs strongly with an essentially flat conformation. The forces during compression are well described by DLVO (Derjaguin-Landau-Verwey-Overbeek) theory with no steric forces apparent. During decompression, the adhesive forces are much greater than those between the bare silica surfaces, indicating a strong bridging between the surfaces after contact and a sub-monolayer coverage. At higher polymer concentrations and/or salt levels, a steric interaction is seen during the compression runs and a significant decrease in the adhesion is observed. Both of these results imply a more expanded conformation of the polymer at the surface and a higher surface coverage. Increased collision rates between the surfaces give rise to an increase in the magnitude of the observed steric forces. Such an increase is attributed to an increased apparent stiffness of the chains as the compression rate increases. Measurements in a poor solvent resulted in the appearance of shallow long-range intersegmental attractive force.