Universal Behavior of Asphaltene Aggregation in Hydrocarbon Solutions

We have investigated the aggregation kinetics of asphaltenes of different origin and of different vanadium content in toluene+ n-heptane mixtures by dynamic light scattering (photon-correlation spectroscopy) adapted to opaque fluids. Two types of aggregation kinetics have been observed: diffusion-limited aggregation (DLA) and reaction-limited aggregation (RLA) Colloidal properties of asphaltenes have heen found to depend on the content of vanadium. At asphaltene concentrations in toluene below the critical micelle concentration of about 3 g L−1 the aggregation kinetics observed appears to be solely limited by diffusion. Far above the critical micelle concentration, the 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 micelleconcentration, a crossover between the two types of aggregation kinetics occurs.Besides, the type of the aggregation kinetics was also found to depend on the vanadium content in asphaltenes. We propose a simple theoretical expression for the crossover kinetics and a universal scaled description of the observed phenomena.