Effect of pH and Salt on the Adsorption and Interactions of an Amphoteric Polyelectrolyte

Abstract

Adsorption and surface force measurements were carried out on gelatin (MW ~100 000) adsorbed on mica from aqueous NaCl solutions. Measurements were made over a range of ionic strengths and pH. The results show that gelatin adsorbs via discrete ionic bonds formed between negative surface groups on mica and positive basic groups on gelatin; these exist even when gelatin is net negatively charged (at a pH above the IEP of 5.0). The phenomenon 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. © 1992, American Chemical Society. All rights reserved.