Influence of Ionic Liquids on the Aggregation and Preaggregation Phenomena of Asphaltenes in Model Solvent Mixtures by Molecular Dynamics Simulations and Quantum Mechanical Calculations: The Effect of Cation's Shape and Size

The merits of ionic liquids (ILs) as additives to influence the solid-liquid behavior of asphaltenes in crude oil models have been well-established in specialized literature. The direct interaction between ionic liquids and asphaltenes is recognized as a key factor for the role played by these additives in the asphaltene aggregation process, which depends on the structural details of the ionic liquids used (and also on the model considered for the asphaltenes). In a previous paper, we presented a systematic molecular dynamics study of the effect of 1-alkyl-3-methyl imidazolium-based ionic liquids on the asphaltene preaggregation phenomenon, focusing on the effect of the alkyl chain length of the cation and on the size of the anion. In this work, this study is extended to evaluate the effect of the shape and size of the cation head. The preaggregation phenomena in toluene/n-heptane mixtures and the asphaltene/IL interactions are studied here by molecular dynamics simulations and density functional theory (DFT) calculations, using N-alkylpyridinium, N-alkyl isoquinolinium, and 1-alkyl-3-methyl benzimidazolium chloride ILs, with alkyl chains containing between 4 and 10 carbon atoms as additives. These additives show a general dispersing effect in n-heptane-rich systems, with the N-alkyl isoquinolinium-based ILs with longer alkyl chains (especially C-8 but also C-10) being the most active ones. The asphaltene/IL interaction is found to be enhanced by cations with two condensed aromatic rings instead of one, which seems to highlight the importance of the pi-pi stacking contact. However, the most interactive family of ILs (1-alkyl-3-methyl benzimidazolium chloride) is also the one that induces asphaltene aggregation, especially for shorter alkyl side chains (with 33% larger aggregates than the systems without additive for mixtures with equal proportions of the two solvents), probably representing a binder effect for asphaltenes. Although by molecular simulation, the N-alkyl isoquinolium-based ILs showed the clearest dispersing effect toward asphaltenes and the most intense interactions with them, they also presented the lowest interaction energies of the asphaltene-IL dimers obtained by DFT, and this was found to be the consequence of a specific cation-anion interaction only for this family of ILs, which is close to a covalent bond. This work is a valuable addition to the understanding of the molecular interactions that govern asphaltene aggregation in the presence of additives, and contributes to the selection and design of better additives to prevent or induce asphaltene aggregation in crude oil.