KINETICS OF HEMATITE AGGREGATION BY POLYACRYLIC-ACID - EFFECT OF POLYMER MOLECULAR-WEIGHTS

We studied the coagulation kinetics of hematite particles induced by f airly monodisperse polyacrylic acid (PAA) of two different molecular w eights (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 cert ain polymer/hematite ratios. The maximum rates for the two polymers we re 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 fract al dimension of aggregates for both polymers and simple salts. Moreove r, the breadth of the flocculation zone was independent of the molecul ar 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 dos age was most likely due to the higher adsorption affinity of the longe r polymer chains. The results strongly demonstrated that polymer bridg ing was not important as compared with the charge neutralization mecha nism. In addition, the observation that the maximal rates were identic al in the presence of polymers and simple salts suggested that the mac romolecules did not form distinguished patches as described in the ''e lectrostatic patch'' model. Because of their relatively small charge d ensity, the macromolecules overlapped on the hematite surface, resulti ng in a quasiuniform charge neutralization. (C) 1997 Elsevier Science B.V.