Salt-induced fast aggregation of polystyrene latex

We study fast, salt-induced aggregation of polystyrene spheres by means of low-angle static light scattering covering two decades of scattered wave vector q. The measurements are taken at a fixed salt concentration, varying the monomer concentration from c0=1×109 to 5×1010 cm-3. During each run, the fractal dimension df of the clusters and the time evolution of the average radius RG and average mass Mw are determined. At larger concentrations we consistently find df=1.61-0.02, but, as the concentration is decreased, df grows to 1.83-0.02. The value of df is determined both from the asymptotic behavior of the scattered intensity I(q) and from the relation between the average mass and radius, I(0)RGfd. Good agreement is found between the two sets of data. The earlier portions of the curves for RG as a function of time t are compatible with a power-law growth and collapse onto a master curve when the reduced time T=c0t is used. Noticeable deviations from power-law growth are, however, observed at later times. Finally, we show that for the lowest concentrations the reactions stop when the clusters attain a maximum diameter. We present arguments showing that reaction termination is due to sedimentation, the time required to diffuse across intercluster distance becoming longer than the settling time through the sample due to sedimentation.