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, an d surface collision rates. The solubility of the polymer is highly dep endent on the solution pH; that is, when the molar concentration of so lution protons is equivalent to the molar concentration of pyridine gr oups 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 concentratio ns, 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 w ith no steric forces apparent. During decompression, the adhesive forc es are much greater than those between the bare silica surfaces, indic ating a strong bridging between the surfaces after contact and a sub-m onolayer coverage. At higher polymer concentrations and/or salt levels , a steric interaction is seen during the compression runs and a signi ficant decrease in the adhesion is observed. Both of these results imp ly a more expanded conformation of the polymer at the surface and a hi gher surface coverage. Increased collision rates between the surfaces give rise to an increase in the magnitude of the observed steric force s. Such an increase is attributed to an increased apparent stiffness o f the chains as the compression rate increases. Measurements in a poor solvent resulted in the appearance of shallow long-range intersegment al attractive force.