Kinetics of hematite aggregation by polyacrylic acid: Effect of polymer molecular weights

We studied the coagulation kinetics of hematite particles induced by fairly monodisperse polyacrylic acid (PAA) of two different molecular weights (M-w = 1.36 x 10(6), M-w/M-n = 1.53 and M-w = 3.69 x 10(4), M-w/M-n = 1.60) by using photon correlation spectroscopy (PCS) to follow the time evolution of aggregate size. A maximum rate was found at certain polymer/hematite ratios. The maximum rates for the two polymers were exactly the same, which were in turn identical to that of diffusion limited aggregation (DLA) induced by simple electrolytes. In the DLA regime, scaling of the aggregate size with time yielded the same fractal dimension of aggregates for both polymers and simple salts. Moreover, the breadth of the flocculation zone was independent of the molecular weights. However, the optimal dosage, in terms of the total polymer concentration, was displaced slightly to a lower value for the larger polymer. A corresponding displacement was observed in the position of the zero electrophoretic mobility. Thus, the shift in the optimal dosage was most likely due to the higher adsorption affinity of the longer polymer chains. The results strongly demonstrated that polymer bridging was not important as compared with the charge neutralization mechanism. In addition, the observation that the maximal rates were identical in the presence of polymers and simple salts suggested that the macromolecules did not form distinguished patches as described in the ''electrostatic patch'' model. Because of their relatively small charge density, the macromolecules overlapped on the hematite surface, resulting in a quasiuniform charge neutralization. (C) 1997 Elsevier Science B.V.