Effect of colloidal silica and electrolyte on the structure of an adsorbed cationic polyelectrolyte layer

The interactions between an adsorbed high molecular weight cationic polyacrylamide (C-PAM) and anionic components have been investigated by ellipsometry. The anionic components used were non-aggregated and microaggregated anionic colloidal silica particles (ACS) and an anionic polyacrylamide (A-PAM). The C-PAM was adsorbed to a silica surface and the different anionic components were then added to the system. The effect of adding an electrolyte (NaCl) to some of these systems was also investigated. The thickness of the adsorbed layer, the adsorbed amount and some kinetic aspects of the adsorption and desorption processes were studied. A three-fold expansion was observed for the C-PAM layer when non-aggregated and microaggregated ACS was added. When large amounts of NaCl are added there is a great decrease in layer thickness for a system with non-aggregated ACS. A system with microaggregated ACS particles is less affected by an increase in the electrolyte concentration. More ACS is adsorbed to a C-PAM layer in the presence of NaCl than in its absence. When A-PAM is added to the C-PAM layer, the thickness and adsorbed amount is much less influenced than if ACS is added. A small increase in adsorption and in layer thickness is first observed and then a desorption process starts. Information about the conformation of the adsorbed cationic polyacrylamide has also been obtained. The ellipsometry measurements indicate that the structure of the adsorbed layer changes as the adsorption process proceeds. The polymers adsorbed first to the surface give a more dense layer than the polymers adsorbed last. (C) 1999 Elsevier Science B.V. All rights reserved.