CONFORMATIONAL RELAXATION OF POLYELECTROLYTES AT A SOLID LIQUID INTERFACE

The adsorption of hydrolyzed Polyacrylamide onto a spherical nonporous adsorbent with (SiO)2AlOH surface sites was investigated as a function of the pH in aqueous media. The ζ potential and net charge of the adsorbent were determined as a function of pH by electrophoresis and Potentiometrie titration. The point of zero charge (pzc) of the amphoteric group (SiO)2AlOH is at 4.6. The isotherms obtained from batch experiments display a nonclassical shape which is related to a slow reorganization of the interface with time. Adsorption was then performed by monitoring precisely the supply of polymer to the interface as a function of time. It is shown that the final amount adsorbed depends on the rate of polymer supply to the interface, a feature which is also explained in terms of segmental “stiffness” at the interface. A systematic study of the kinetics of adsorption at various pH was then carried out. Below the pzc point, the surface has a net positive charge, and owing to long-range attractive electrostatic forces, adsorption proceeds at a fast rate. On the contrary, for pH values far above the pzc, the rate of adsorption is slowed down by the repulsive electrostatic forces. Near the pzc, the electrochemical behavior of the surface is influenced by the field energy of the ionized groups in the polymer diffuse layer. This gives rise to a remarkable cooperative effect on the kinetics, the adsorption amount versus time displaying a typical sigmoidal shape. The cooperative effect is explained in the light of simple electrostatic considerations assuming a slow segmental mobility at the interface. © 1990, American Chemical Society. All rights reserved.