Multi-method determination of the fractal dimension of hematite aggregates

The utilisation of the fractal concept has greatly improved our ability to describe the structure of colloidal aggregates and our understanding of their formation mechanisms. We have determined the fractal dimension of KCl-induced hematite aggregates using three different approaches: (i) dynamic scaling, (ii) TEM image analysis and (iii) static light scattering (SLS). In the regime of diffusion limited aggregation ([KCl]>100 mM), analysis of TEM images of 70 aggregates, whose longest dimension varies more than 40 fold, yields the lower limit of the fractal dimension as 1.68 +/- 0.04. Direct scaling of the mean hydrodynamic size of the aggregates measured by photon correlation spectroscopy gives the upper limit of the fractal dimension as 1.87 +/- 0.07. The value obtained by static light scattering as 1.83 +/- 0.07. In the reaction limited regime, SLS measurements yield a fractal dimension of 2.2 +/- 0.1. These results suggest the aggregates grow mainly through collision with other aggregates, rather than through the addition of primary particles.