| The adsorbed amounts and interfacial conformations of weak cationic polyelectrolytes (2.5 k and 31 k of poly (dimethylaminoethyl methacrylate), DMAEMA) on colloidal silica were examined in the context of the influence of chain conformation, and the roles of ionization and counterions. The mobility of the polymer in solution is altered at the Bjerrum length, indicating counterion condensation and a conformation change. Variations in pH altered the backbone and surface charge densities, changing the density of contact points for adsorption. Variations in ionic strength screened repulsions among adsorbed oligomers, an effect most apparent at low ionic strengths. NMR solvent relaxation data provided substantial evidence for differences in the anchoring of DMAEMA oligomers and adsorbed trains of a nonionic homopolymer such as polyethylene oxide. DMAEMA adsorbs at a small number of discrete points on side chains while for polyethylene oxide, every monomer on the main backbone can potentially adhere to the surface, giving a less mobile interface from the perspective of the solvent. The other important feature of this model system was that the polycation charge density was greater than that of the substrate. We demonstrate, quantitatively, that the charge overcompensation observed on the isotherm plateau can be directly attributed to the denser positive charge on the adsorbing polycation, and that the ultimate coverage obtained corresponds to the adsorption of one oligomer onto each underlying negative silica charge, when the silica charge is most sparse, at pH 6. This limiting behavior breaks down at higher pHs where the greater silica charge density accommodates single chains adsorbing onto multiple negative sites. Ultimately at the highest pH's, a regime is approached where coil size, not charge distribution, limits the ultimate coverage. Surface charge enhancement is not a cause for charge overcompensation, but a consequence. High affinity adsorption isotherms at dilute concentration were found for high molecular weight DMAEMA. As pH increases, the bound mass decreases more rapidly than the adsorbed amount, suggesting that the DMAEMA layers may contain loops and tails of higher mobility, before adsorption disappears altogether. |
