Adsorption of ampholytic diblock copolymers from dilute aqueous solution at the solid/liquid interface

The adsorption of the ampholytic diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) (PMAA-b-PDMAEMA) and the corresponding homopolyelectrolytes of the two blocks, PMAA and PDMAEMA, was investigated from dilute aqueous solution on silicon substrates. The adsorbed amount of polymer as a function of pH, polyampholyte concentration, and salt concentration in solution has been determined by ellipsometry at room temperature. As a function of pH the adsorbed amount reaches its maximum at the isoelectric point of the polyampholyte. Some adsorption takes place even in pH ranges where the surface charge has the same sign as the net charge of the polyampholyte. By variation of the polyampholyte concentration, typical adsorption isotherms were determined at several pH values. With increasing salt concentration the adsorbed amount increases, and above a critical concentration it diverges to very large values. The measured dependencies can be explained by the adsorption of one or the other of the two blocks depending on acidity and ionic strength and are in good agreement with theoretical predictions. Adsorption kinetics has also been studied in detail, and the diffusion coefficient of the polyampholyte toward the surface has been obtained in the early state of adsorption as a function of pH, polyampholyte concentration, and salt concentration.