Aggregation kinetics and fractal structure of gamma-alumina assemblages

Suspensions of a variety of different aluminum oxides have previously been shown to require very high concentrations of chloride and nitrate anions (> 0.5 M) to induce rapid aggregation. This high stability has been accredited to the presence of surface forces considered to be due to the formation of highly charged Al-13 Polymeric species at slightly acidic pH's and aluminu m oxyhydroxide gel formation under alkaline conditions. The effect of this stability on the structure of the resulting aggregates is investigated here using well-established static light-scattering techniques. Power law behav ior of scattered light intensity as a function of scattering wave vector is observed in all cases and is suggestive of fractal structure. The fractal dimensions obtained fall within the expected range of 1.8 to 2.3 observed f or colloidal aggregates but do not appear to follow the typical observation s for colloids destabilized by indifferent electrolytes where lower fractal dimensions are associated with rapid (diffusion-limited) aggregation and h igher fractal dimensions wit slower (reaction-limited) aggregation. Indeed, relatively constant fractal dimensions (2.10 to 2.25) are observed over th e range of salt concentrations at which the slow to rapid aggregation rate transformation occurs with, if anything, a slightly higher fractal dimensio n observed for higher aggregation rates. The presence of specifically bindi ng sulfate anions appears to negate the strong near-distance repulsive forc es leading to rapid aggregation at low (I to 2 mM) sulfate concentrations. Significantly lower fractal dimensions (1.85 to 1.91) are observed for aggr egates formed by destabilization using sulfate ions than obtained when chlo ride or nitrate are used with, again, an apparent slight increase in fracta l dimension upon increasing aggregation rate. (C) 2001 Academic Press.