Validation of Heteroatoms in Asphaltene Molecular Structures Obtained by Atomic Force Microscopy Using Electronic Transitions Calculations Combined with the Y-Rule

Rendering accurate structural characterization of asphaltenes enhances our knowledge of the chemical processes that occur in oil reservoirs. Furthermore, the establishment of structure–reactivity and structure–function correlations is made possible by accurate structural characterization of asphaltenes. In addition, the accurate structural characterization of asphaltenes permits one to carry out molecular simulations to understand the effect of asphaltenes in the formation/breakage of emulsions and in the change of wettability; both processes are related to interfacial phenomena. Furthermore, the accurate structural characterization of asphaltenes allows reservoir evaluation using asphaltene thermodynamics. In 2015 and 2017, IBM–Zurich and collaborators obtained atomic force microscopy (AFM) images of coal-derived and oil asphaltenes from different sources. For the case of the coal-derived asphaltenes, the AFM images can straightforwardly be translated into chemical structures. However, this task has proven to be difficult in the case of the AFM images obtained for oil asphaltenes. In the translation of the images to chemical structures, there are sections in the polycyclic aromatic hydrocarbon (PAH) region, marked with an “X”, which denotes unknown heteroatoms. This is due to the lack of ideal imaging conditions, such as a combination of tip, substrate, and mobility, in particular for N- and S-containing molecules, which are frequently encountered in asphaltenes. Therefore, the identification of heteroatoms in the AFM remains difficult. The IBM–Zurich asphaltene structures are now being used by researchers around the world for the calculation of chemical properties of asphaltenes using computational chemistry and molecular simulations, but the nature of the X atom has not been revealed in a systematic study. Hence, in the present contribution, the elucidation of the X atoms and validation of the AFM chemical structures for oil asphaltenes is carried out by calculating, at the ZINDO/S level, the frontier molecular orbitals energy gap for all the structures with different nitrogen and sulfur species and all their probable X combinations. The obtained frontier orbitals energy gap is compared with the well-known experimental fluorescence emission data of asphaltenes to conclude the most likely nature of X. We hope this work will assist research groups by providing validated AFM-asphaltene structures with no unknown heteroatoms for molecular simulation of properties or molecular dynamics, either all-atoms or coarse grain, or for coarse grain dissipative particle dynamics simulations. In addition, all the structures were analyzed by the most predominant number of fused aromatic rings (nFAR), π-electronic distribution in resonant sextets (NR) obtained with the Y-rule, percentage of aromatic sextet (%NR), and Y-rule mapping (%NR/nFAR). It is found that, in general, X = S increases the stability of the asphaltene structure, compared with X = NH or N.