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 corr esponding homopolyelectrolytes of the two blocks, PMAA and PDMAEMA, was inv estigated from dilute aqueous solution on silicon substrates. The adsorbed amount of polymer as a function of pH, polyampholyte concentration, and sal t concentration in solution has been determined by ellipsometry at room tem perature. As a function of pH the adsorbed amount reaches its maximum at th e 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 o f the polyampholyte. By variation of the polyampholyte concentration, typic al adsorption isotherms were determined at several pH values. With increasi ng salt concentration the adsorbed amount increases, and above a critical c oncentration it diverges to very large values. The measured dependencies ca n be explained by the adsorption of one or the other of the two blocks depe nding on acidity and ionic strength and are in good agreement with theoreti cal predictions. Adsorption kinetics has also been studied in detail, and t he diffusion coefficient of the polyampholyte toward the surface has been o btained in the early state of adsorption as a function of pH, polyampholyte concentration, and salt concentration.