MULTIMETHOD DETERMINATION OF THE FRACTAL DIMENSION OF HEMATITE AGGREGATES

The utilisation of the fractal concept has greatly improved our abilit y to describe the structure of colloidal aggregates and our understand ing of their formation mechanisms. We have determined the fractal dime nsion of KCl-induced hematite aggregates using three different approac hes: (i) dynamic scaling, (ii) TEM image analysis and (iii) static lig ht scattering (SLS). In the regime of diffusion limited aggregation ([ KCl]>100 mM), analysis of TEM images of 70 aggregates, whose longest d imension varies more than 40 fold, yields the lower limit of the fract al dimension as 1.68 +/- 0.04. Direct scaling of the mean hydrodynamic size of the aggregates measured by photon correlation spectroscopy gi ves the upper limit of the fractal dimension as 1.87 +/- 0.07. The val ue obtained by static light scattering as 1.83 +/- 0.07. In the reacti on limited regime, SLS measurements yield a fractal dimension of 2.2 /- 0.1. These results suggest the aggregates grow mainly through colli sion with other aggregates, rather than through the addition of primar y particles.