The Influence of Charge Variation on the Adsorbed Configurationof a Model Cationic Oligomer onto Colloidal Silica

The adsorbed amounts and interfacial conformations of a low-molecular-weight oligomer of a weak cationic polyelectrolyte (15-unit chain of dimethylaminoethyl methacrylate, DMAEMA) on colloidal silica were examined, with the ultimate intent of providing perspective on the adsorption of high-molecular-weight weak polyelectrolytes. At all but the lowest ionic strengths or highest pHs, over the full range of adsorbed oligomer amounts on each isotherm, the interfacial conformation was relatively insensitive to coverage with train fractions from 0.8 to 1. This occurs because short oligomeric chains are not capable of forming long loops or tails found in highermolecular-weight homopolymer layers. Variations in pH altered the backbone and surface charge densities, changing the density of contact points for adsorption. This was apparent in the NMR solvent relaxation behavior, which suggested the tightest binding near pH 6, and a sharp drop in the train fraction at high pH, just as the adsorption began to diminish with reduced backbone protonation. 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 oligomer and adsorbed trains of a nonionic homopolymer such as polyethylene oxide (T. Cosgrove, M. A. Cohen Stuart, and G. P. van der Beek, Langmuir 7, 327 (1991)). DMAEMA adsorbs at a small number of discrete points on side chains, while with polyethylene oxide the literature suggested every monomer on the main backbone can potentially adhere to the surface, giving a less mobile interface from the perspective of the solvent. C © 2001 Academic Press