EFFECT OF PH AND SALT ON THE ADSORPTION AND INTERACTIONS OF AN AMPHOTERIC POLYELECTROLYTE

Adsorption and surface force measurements were carried out on gelatin (MW approximately 100 000) adsorbed on mica from aqueous NaCl solutions. Measurement, were made over a range of ionic strenghts and PH. The results show that gelatin adsorbs via discrete ionic bonds formed between negative surface on mica and positive basic groups on gelatin; these exist even hen gelatin is net negatively pH above the IEP of 5.0). The phenemenon of strong adsorption even when both the polymer and surface are net negatively charged does not appear to occur with macromolecules containing only acidic groups. The forces were generally repulsive: at separations beyond layer overlap the repulsion is dominated by an electrostatic "double-layer" force, while closer in it is dominated by a "brush-like" steric interaction. We discuss the effects of pH and salt on the configurations, the interaction forces, and the dynamics of the adsorbed coils.