Asphaltene Mesoscale Aggregation Behavior in Organic Solvents-A Brownian Dynamics Study.

Significant advances have been achieved in understanding the main molecular mechanisms leading to asphaltene aggregation. However, the existing computational deficiency of molecular dynamics simulations did not allow full reproduction of the complex aggregation behavior of asphaltene in the past. In this work, we use the Brownian dynamics simulation to investigate asphaltene aggregation behavior on larger length and time scales that have not been previously accessed by molecular simulations. This enabled us to completely render the formation of clusters of asphaltene nanoaggregates and the resulting fractal or network of aggregates during the aggregation process. Asphaltene aggregation is studied at several volume fractions (ϕ = 1-7%) of asphaltene nanoaggregates in two solvents including heptane and heptol (i.e., a mixture of heptane and toluene). Our simulation results support the aggregation hierarchy proposed in the Yen-Mullins model (Mullins, Annu. Rev. Anal. Chem. 2011, 4, 393-418.) by demonstrating that asphaltene nanoaggregates form small clusters with an aggregation number of 7-8 and an average gyration radius of ∼4.0 nm capable of forming either fractal aggregates with a fractal dimension of 1.93-2.04 at low ϕ or percolating networks of aggregates at high ϕ. Percolating structures are observed at ϕ = 7% in both solvents. In heptol, the structures mainly percolate along two directions, whereas in heptane, they can percolate along three directions (i.e., x, y, and z). The self-diffusion coefficient ( D) significantly decreases as ϕ increases. Generally, D is larger in heptol than in heptane, but this difference diminishes as ϕ increases, approaching to almost the same value at ϕ = 7%.