Depletion flocculation induced by synergistic effects of nanoparticles and polymers

The depletion flocculation of stable suspensions of charged microparticles (1.2 μm diameter polystyrene) by mixtures of silica nanoparticles (7 nm) and poly(acrylic) acid (PAA, Mn = 24 500) was studied. The experiments revealed a very clear critical flocculation concentration of PAA that was lowered by the addition of the silica nanoparticles. Equilibrium pair potentials between the particles, calculated from force profiles measured between a silica particle and plate using colloid probe atomic force microscopy, showed clear a synergistic effect in that the magnitude of the depletion and structural forces produced in the mixed PAA/nanoparticle system was significantly greater than the sum of the forces in systems containing only PAA or only nanoparticles. This effect arises from adsorption of the PAA onto the nanoparticles, creating much larger charged complexes. Comparing the calculated pair potentials to the observed flocculation results revealed that flocculation occurred once the magnitude of the secondary energy well formed by the attractive depletion force exceeded 5 kT.