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.