Mechanisms of Asphaltene Aggregation in Toluene and Heptane Mixtures

The nature of aggregation kinetics of colloidal asphaltene particles in mixture of toluene and heptane was investigated by utilizing a polarizing microscope with appropriate magnification. The concentration of asphaltene in toluene-heptane mixture varied from 1 g/l to 8 g/l. There are two main mechanisms associated with the aggregation process. These mechanisms are diffusion limited aggregation (DLA) and reaction limited aggregation (RLA). Each mechanism has its own characteristics and acts on limited values of asphaltene concentration. At the asphaltene concentrations below the critical micelle concentration (CMC), the DLA mechanism is dominant, while at concentrations above the CMC, at the initial stage of aggregation process, the RLA mechanism is observed and then the mechanism tends toward DLA (crossover behavior). It should be noted that the CMC for asphaltene in the solution is around 3 g/l.