Asphaltenes, subfractions A1 and A2 aggregation and adsorption onto RH-SiO2 nanoparticles: Solvent effect on the aggregate size

Abstract

The aggregation and adsorption of asphaltenes in dilute solutions has been extensively studied and is now well established. At low concentrations, asphaltenes are mainly found in the form of aggregates of approximately 3 nm in size. One strategy to simplify the study of these systems has been to fractionate the asphaltenes into subfractions called A1 and A2. The nature of these subfraction aggregates, their behavior with the concentration and their interaction with other aggregates and surfaces remain under debate. This work presents experimental results on the aggregation and adsorption of asphaltenes and their subfractions A1 and A2 onto SiO2 nanoparticles from rice husks (RH-SiO2), and the evidence allows us to infer that the aggregates are in equilibrium in solution over a wide range of concentrations. The size distribution for each subfraction does not change with dilution but is sensitive to solvent media. Subfraction A1 has a very similar behavior to asphaltenes, while subfraction A2 tends to form larger aggregates. Larger aggregates more likely adsorb on surfaces than small aggregates. According to the results, the interaction of the aggregates in different solvents can be studied as a phase change phenomenon, where the aggregate can be considered a partially immiscible phase in a solvent. The theoretical principles are described, and a hypothesis is elaborated in these terms.