Crossover kinetics of asphaltene aggregation in hydrocarbon solutions

We have investigated the kinetics of asphaltene aggregation in toluene+n-heptane mixtures with a dynamic light-scattering method adapted to opaque fluids. Two types of aggregation kinetics have been observed: diffusion-limited aggregation and reaction-limited aggregation. At asphaltene concentrations in toluene below the critical micelle concentration of about 3 g l−1 the aggregation kinetics appears to be solely limited by diffusion. Far above the critical micelle concentration reaction-limited aggregation takes place at least in the initial stage of particle growth. At a concentration of asphaltenes in toluene slightly above the critical micelle concentration, a crossover between these two types of aggregation kinetics has been observed. We propose a simple theoretical expression for the crossover kinetics and also a universal scaled description of the observed phenomena.