Flocculation of hematite with polyacrylic acid: Fractal structures in the reaction- and diffusion-limited aggregation regimes

The structure of hematite aggregates in the presence of fairly monodisperse polyacrylic acid (PAA) with two different molecular weights (M-w = 1.36 x 10(6), M-w/M-n = 1.53; M-w= 3.69 x 10(4), M-w/M-n = 1.60) was studied using static light scattering (SLS). The fractal dimensions were calculated from the scattering exponents, after taking into account the finite size of agg regates, using exponential and Gaussian cutoff functions. Three flocculatio n regimes, namely, pre-DLA, DLA (diffusion-limited aggregation), and post-D LA, were defined based on the polymer concentration. In the DLA. regime, fr actal dimension values, D-f = 1.84 +/- 0.02 and 1.73 +/- 0.02, were obtaine d using exponential and Gaussian cutoff functions, respectively. A fractal dimension of approximately 2.0 was found, as expected, in the pre-DLA regim e (at PAA concentrations lower than the optimal dosage for a DLA regime) wh ere the flocculation rate was reaction limited. In contrast, in the post-DL A regime, the flocculation was slow but the structure of aggregates was as tenuous as in the DLA regime with a fractal dimension D-f approximate to 1. 8. Moreover, for all three regimes, the D-f values were independent of the molecular weights of PAA. The lower fractal dimension in post-DLA was proba bly due to the increased concentration of polymer chains between adjacent p articles in aggregates. The steric hindrance favored tip-to-tip aggregation , leading to a more tenuous structure. (C) 1998 Academic Press.