Formation of Noncovalent Complexes between Complex Mixtures of Polycyclic Aromatic Hydrocarbons (Asphaltenes) and Substituted Aromatics Studied by Fluorescence Spectroscopy

Quantification of the formation of molecular complexes between simple organic aromatic molecules is straightforward for pure components of definite composition; however, it becomes challenging when the interacting species are complex, intractable mixtures of thousands of polycyclic aromatic hydrocarbons (PAHs). Using the example of asphaltenes, a complex mixture of oil-derived PAHs, we demonstrate the potential of fluorescence spectroscopy for quantification of molecular processes that are at the core of their aggregation. It is found that small electron-deficient aromatic additives, especially surfactant-like additives with electron-withdrawing functional groups, interact strongly with the PAHs and lead to the formation of molecular complexes. Computational modeling revealed that as we increase the number of PAH molecules in the cluster, cohesive π-πstacking interactions between PAHs dominate in preference to adhesive hydrogen bonding or π-πstacking interactions between the PAHs and additives.