CATIONIC SURFACTANT INDUCED FRACTAL SILICA AGGREGATES - A LIGHT-SCATTERING STUDY

Fractal silica aggregates are formed when a cationic surfactant, tetradecylpyridinium bromide (TPB), is used a coagulant. To maintain a constant initial surface charge density of silica colloid, a buffer medium of pH = 9.2 was used throughout. Static and dynamic light-scattering measurements were made to study the aggregates' fractal structure in relation to the aggregation kinetics over a broad range of TPB concentrations. Depending on the TPB amount added, three regimes of aggregation-fast, intermediate, and slow aggregation-have been observed for both negatively charged and positively charged silica clusters. In particular, the restructuring of silica fractals has been found to occur rapidly in the limiting regime of fast aggregation as well as in the intermediate aggregation regime, all resulting in a df value of ∼ 2.10. The time variation of the scattered intensity vs scattering vector curves for both quasi-fast and slow regimes can be interpreted in terms of the polydispersity of the cluster distribution and the structure factor of the crossover region. A comparison between the aggregation behaviors of two different colloids by different coagulants, polystyrene latex/NaCl and silica colloid/TPB, has been made with respect to the fractal structure and the growth kinetics. © 1991.