Adsorption and flocculation behavior of cationic polyacrylamide and colloidal silica

The adsorption of cationic polyacrylamide and silica nanoparticle systems onto a model surface has been compared with the adsorption and flocculation of a fiber suspension. An increase in ionic strength affects the polyelectrolyte adsorption in different ways in these two systems. With a silica surface, an increase in the ionic strength leads to a continuous increase in the adsorption. On a cellulose fiber, the adsorption increases at low ionic strength (1-10 mM NaCl) and then decreases at higher ionic strength (10-100 mM NaCl). When polyelectrolyte is added to the fiber suspension, flocculation occurs with an optimum around a polyelectrolyte addition of 0.4 mg g(-1). Above this addition level the flocculation effect is reduced and dispersion occurs when more than 1.2 mg g(-1) polyelectrolyte is added. This behavior suggests a bridging flocculation, the dispersion being explained by electrosteric stabilization of the suspension. Addition of silica nanoparticles to the fiber suspension leads to a greater degree of flocculation than in a single polyelectrolyte system. The adsorption of nanoparticles to a polyelectrolyte-covered silica surface increases both the adsorbed amount and the thickness of the adsorbed film. A more extended silica particle, giving a more extended polyelectrolyte-silica film, also results in a higher degree of fiber flocculation. (C) 2003 Elsevier Science B.V. All rights reserved.