Fractal heteroaggregation of oppositely charged colloids

Floc structures resulting from selective heteroaggregation of positively and negatively charged colloids are investigated as a function of number ratio and shear conditions at pH 6. Negatively charged silica and positively charged alumina-coated silica undergo rapid aggregation due to attractive electrostatic interactions. At either extreme in number ratio, growth is terminated at an early stage, presumably because the aggregates acquire the same sign of charge, eliminating the driving force for further aggregation. For intermediate number ratios, extensive growth occurs, since the distribution of positive and negative charges is more uniform. Structure evolution of large heteroaggregates is assessed by static light scattering. Shear strongly influences the packing geometry and the tendency for the aggregates to undergo restructuring. At high shear (N-Re > 2000), heteroaggregates show relatively dense packing and do not restructure. Fractal dimension D-f decreases from 2.64 to 2.26 as the number of positive particles is increased. At low shear (N-Re < 200), packing of the particles is more open and restructuring occurs. The lowest observed fractal dimension is 1.79. In the absence of applied shear, heteroaggregates with D-f = 1.79 rearrange to more compact structures with D-f = 1.88. (C) 2000 Academic Press.