Adsorption of weak polyelectrolytes on amphoteric oxide surfaces

The adsorption of a weakly cationic polyelectrolyte (poly(dimethylaminoethyl methacrylate), (AMA) and a weakly anionic polyelectrolyte (carboxymethylcellulose, CMC), were studied as a function of pH. As substrates we used the (amphoteric) metal oxides TiO2 and Fe2O3. Adsorption of CMC on to colloidal particles was determined from the solution depletion; adsorption of AMA was followed by a reflectometric method. In addition, layer thicknesses were measured. These were determined from the increase in hydrodynamic radius of diffusing colloidal particles. For proper data interpretation, the effects of free polymer on diffusion were also carefully analysed, and it was found that the friction on small particles is intermediate between that exerted by the pure solvent, and that expected on the basis of solution viscosity. The adsorbed amounts showed in both cases a downward trend with increasing polymer charge density, in agreement with an ion-exchange mechanism of adsorption that is expected if electrostatics dominate. AMA displays a maximum adsorbed amount at the point where the polymer charge density becomes so low that electrostatics as the driving force for adsorption vanishes. For CMC there could also be a maximum (at low pH) but there are, as yet, no data at sufficiently low pH. The adsorbed amounts of CMC were insensitive to the molecular weight. Layer thicknesses of adsorbed CMC, however, increased markedly with molecular weight. This may indicate partially irreversible adsorption of CMC.