Imidazole-based Ionic Liquids As Rheological Modifiers of Heavy Crude Oil: an Experimental and Theoretical Study

Two series of imidazolium-based ionic liquids (ILs) were tested as rheological modifiers of heavy crude oils. ILs with the 1-butyl-3-alkylimidazolium cation optimize the effect on viscosity at low concentrations, whereas those with the 1-benzyl-3-alkylimidazolium cation are similarly effective only at higher concentrations, which is due to a stronger IL–IL interaction with respect to the IL–asphaltene interaction. The most effective ILs were [C6bim][PF6], [phetbim][PF6], and [C9bim][NTf2], with viscosity falling in the order of 44.0%, 38.2%, and 32.9%, respectively. An analysis of structural features suggests that the type of anion and the strength of the ion pair interaction play an important role in the capacity of the salts to interact with the crude oil components. In addition, computational methodologies, such as atoms in molecules and non-covalent interactions, were used to analyze the main intermolecular interactions between the most effective [C6bim][PF6] and two different substructures of an asphaltene model. According to these results, hydrogen bonding (F⋯H–C, F⋯H–N, and O⋯H–C), C–H–π, and van der Waals interactions contribute to the breaking up of the π–π stacking interactions between the complexes resulting from the self-association of the aromatic compounds; as a consequence, the crude oil viscosity decreases.