Reversible aggregation of particles with short oligomeric sidechains at the surface studied with Langevin dynamics

We investigate nanoscopic aggregation of asphaltene nanoaggregates by means of Brownian dynamics simulations. Nanoaggregates that represent clusters of 6-10 asphaltene molecules are modelled as spherical particles with an attractive core consisting of polyaromatic hydrocarbons rings and short aliphatic tails that protrude from the surface, as in a spherical brush. As two such nanoaggregates approach each other, the steric repulsion that arises from interactions between the tails, is modelled using Alexander-de Gennes theory and Derjaguin approximation, while the attraction upon contact between polyaromatic cores, caused by van der Waals and other interactions is modelled as a Morse potential. The aggregation occurs when the steric barrier is overcome and the cores come into contact. Our simulations show that, with shallow interaction potentials, that is, with weak attractions and low barriers, the nanoaggregates form small agglomerates with ramified structure that are in metastable equilibrium with uniform solutions. These agglomerates can be described with a fractal dimensions below 1.5, which may represent soluble asphaltenes. The increase in attraction strength promotes aggregation and destabilizes the systems as in the case of dilution with a flocculant, for which several aggregation regimes are observed. Increasing the well-depth and/or barrier for nanoaggregate interaction leads to classical diffusion limited and reaction limited regimes. We find that with high barrier the clusters formed are less close-packed, and may represent the weak gel state typical for bitumen gel.