Relationship between interaction forces and the structural compactness of depletion flocculated colloids

The addition of non-adsorbing polymers to a colloidal dispersion can induce particle aggregation through depletion flocculation. Small-angle static light scattering has been used to show that the structure of aggregates formed upon the addition of non-adsorbing poly(acrylic acid) to a dilute aqueous dispersion of colloidal polystyrene latex particles exhibited power-law scattering behaviour. The observed floc mass fractal dimension, dF, was found to be dependent upon the polyacid concentration. Increasing the polyacid concentration resulted in lower fractal dimensions, with dF levelling off at high polymer concentrations. This structural behaviour was attributed to the deepening in the secondary potential energy well and hence higher particle sticking efficiencies upon collisions when the polymer concentration was raised. This is directly supported by atomic force microscopy data, which showed that the interaction force curves between charged surfaces for solutions of the non-adsorbing poly(acrylic acid) exhibited attractive secondary minima, the depths of which increased with increasing polymer concentration, saturating at high polyacid concentrations. In contrast to the much lower values obtained via salt-induced aggregation (dF=1.78 to 2.20), significantly higher mass fractal dimensions and hence greater structural compactness were observed for these depletion-flocculated flocs.