Theoretical-experimental evaluation of rheological behavior of asphaltene solutions in toluene and p-xylene: Effect of the additional methyl group

This study is focused on understanding the effect of the additional methyl group (-CH3) of p-xylene, in comparison with toluene, on the rheological behavior of model solutions of asphaltenes. For this, three concentrations of asphaltenes in both solvents were evaluated experimentally and theoretically. Experimental results show an increase of the viscosity as the asphaltene concentration increase, being up to 739% higher in p-xylene than in toluene. Molecular dynamics (MD) simulations were conducted to understand the difference in the rheological behavior of asphaltene model solutions. The viscosity, volumetric fraction of aggregates, coordination number (CN), as well as the asphaltene-asphaltene and asphaltene-solvent interaction energies were calculated. Experimental viscosities were accurately reproduced by MD simulations, with average deviation lower than 7.4% and 10.9% for toluene and p-xylene solutions, respectively. The volumetric fraction exhibits a similar trend regarding the increase in viscosity, whereby the results obtained are in agreement with the Einstein theory modified for concentrated suspensions, since an increase in the volumetric fraction promotes viscosity increases. On the other hand, from the theoretical evaluation, it was found out that the additional methyl group in the p-xylene structure regarding toluene, promotes significant differences between the interaction energies of asphaltene aggregates. This is a consequence of an increasing of the aggregates size, which contributes to the increase of volumetric fraction and therefore the viscosity. Finally, this study highlights the importance of the asphaltene-solvent relationship, from an atomistic point of view, since small changes in aggregates volume promotes significantly changes in viscosity. (C) 2020 Elsevier B.V. All rights reserved.