Correlation and anticorrelation of fractal aggregates. A model for «spinodal-type» aggregation

We present a simple phenomenological model, based on mass conservation arguments, in order to describe aggregation in dense solutions. It has been previously shown that for diffusion-limited processes scattered intensity distributions exhibit a peak at q≠0 which grows in time and moves to smaller and smaller q vectors. According to the model, each aggregate is surrounded by a depletion region whose size depends on the aggregation kinetics. The form factor for such a cluster should satisfy local mass conservation and hence exhibit a pronounced depression at q=0. the initial stages of aggregation, when the clusters are far apart, are accounted on the basis of a form factor alone. The model shows good agreement with the data and also explains the experimental evidence that the peak does not appear in reaction-limited conditions.