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 li ght scattering has been used to show that the structure of aggregates forme d upon the addition of non-adsorbing poly(acrylic acid) to a dilute aqueous dispersion of colloidal polystyrene latex particles exhibited power-law sc attering behaviour. The observed flee mass fractal dimension, d(F), was fou nd to be dependent upon the polyacid concentration. Increasing the polyacid concentration resulted in lower fractal dimensions, with d(F) levelling of f at high polymer concentrations. This structural behaviour was attributed to the deepening in the secondary potential energy well and hence higher pa rticle sticking efficiencies upon collisions when the polymer concentration was raised. This is directly supported by atomic force microscopy data, wh ich showed that the interaction force curves between charged surfaces for s olutions of the non-adsorbing poly(acrylic acid) exhibited attractive secon dary minima, the depths of which increased with increasing polymer concentr ation, saturating at high polyacid concentrations. In contrast to the much lower values obtained via salt-induced aggregation (d(F) = 1.78 to 2.20), s ignificantly higher mass fractal dimensions and hence greater structural co mpactness were observed for these depletion-flocculated flocs. (C) 2000 Els evier Science B.V. All rights reserved.