Fractal heteroaggregation of oppositely charged colloids

Floc structures resulting from selective heteroaggregation of positively an d negatively charged colloids are investigated as a function of number rati o and shear conditions at pH 6. Negatively charged silica and positively ch arged alumina-coated silica undergo rapid aggregation due to attractive ele ctrostatic interactions. At either extreme in number ratio, growth is termi nated 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 distributio n of positive and negative charges is more uniform. Structure evolution of large heteroaggregates is assessed by static light scattering. Shear strong ly influences the packing geometry and the tendency for the aggregates to u ndergo restructuring. At high shear (N-Re > 2000), heteroaggregates show re latively dense packing and do not restructure. Fractal dimension D-f decrea ses 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 restructu ring occurs. The lowest observed fractal dimension is 1.79. In the absence of applied shear, heteroaggregates with D-f = 1.79 rearrange to more compac t structures with D-f = 1.88. (C) 2000 Academic Press.