The Yen−Mullins model, also known as the modified Yen model, specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents and consists of the following: The most probable asphaltene molecular weight is ∼750 g/mol, with the island molecular architecture dominant. At sufficient concentration, asphaltene molecules form nanoaggregates with an aggregation number less than 10. At higher concentrations, nanoaggregates form clusters again with small aggregation numbers. The Yen−Mullins model is consistent with numerous molecular and colloidal studies employing a broad array of methodologies. Moreover, the Yen−Mullins model provides a foundation for the development of the first asphaltene equation of state for predicting asphaltene gradients in oil reservoirs, the Flory−Huggins− Zuo equation of state (FHZ EoS). In turn, the FHZ EoS has proven applicability in oil reservoirs containing condensates, black oils, and heavy oils. While the development of the Yen−Mullins model was founded on a very large number of studies, it nevertheless remains essential to validate consistency of this model with important new data streams in asphaltene science. In this paper, we review recent advances in asphaltene science that address all critical aspects of the Yen−Mullins model, especially molecular architecture and characteristics of asphaltene nanoaggregates and clusters. Important new studies are shown to be consistent with the Yen−Mullins model. Wide ranging studies with direct interrogation of the Yen−Mullins model include detailed molecular decomposition analyses, optical measurements coupled with molecular orbital calculations, nuclear magnetic resonance (NMR) spectroscopy, centrifugation, direct-current (DC) conductivity, interfacial studies, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS), as well as oilfield studies. In all cases, the Yen−Mullins model is proven to be at least consistent if not valid. In addition, several studies previously viewed as potentially inconsistent with the Yen−Mullins model are now largely resolved. Moreover, oilfield studies using the Yen−Mullins model in the FHZ EoS are greatly improving the understanding of many reservoir concerns, such as reservoir connectivity, heavy oil gradients, tar mat formation, and disequilibrium. The simple yet powerful advances codified in the Yen−Mullins model especially with the FHZ EoS provide a framework for future studies in asphaltene science, petroleum science, and reservoir studies.

Theoretical calculations are presented for the effect on the HOMO−LUMO gap due to the successive addition of aromatic rings and their different distributions, isomers, for polycyclic aromatic hydrocarbons (PAHs). The study is based on ZINDO/S calculations. PAHs with 1−14 fused aromatic rings (FAR) are considered. The results of these calculations are addressed to a currently existing controversy regarding the number of FAR in asphaltene structures. Asphaltenes are considered as polycyclic aromatic compounds similar to PAHs but containing heteroatoms and alkyl side chains. The theoretical results are compared with fluorescence emission (FE) experimental data. It is found that the asphaltene experimental FE range does not necessarily correspond to different chromophores with different number of FAR but may be different isomers with the same number of FAR. Also, the effect of the presence of alkyl chains and heteroatoms in the asphaltene structures on the HOMO−LUMO gap is almost negligible. We conclude that the FAR region in asphaltenes has PAH chromophores with 5−10 fused rings. The 100% compactness (circular) PAH structures, beyond 10 fused rings, and the 0% compactness (linear or zigzag) PAH structures are not possible for asphaltenes. Relationships between the HOMO−LUMO gap and structural parameters for PAH chromophores in asphaltenes were found. The effect of the number of FAR and Clar sextets, the compactness, and longest dimension on the HOMO−LUMO gap of PAHs is evaluated.

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The number and geometry of the rings in polycyclic aromatic hydrocarbons (PAHs) in petroleum asphaltene has remained unresolved for many years. Many sophisticated imaging and spectroscopic methods have been utilized to narrow the list of candidate structures for asphaltene PAHs. Here, we exploit a canonical property of petroleum asphaltenes, their color, along with their fluorescence emission properties. These universal spectral properties are analyzed through the lens of molecular orbital (MO) calculations, thereby providing quantitative bounds on asphaltene PAH systems. Energetic considerations mandate that these fused aromatic ring systems are predominantly aromatic sextet carbon (within the Clar representation) but not entirely sextet carbon. Matching the ubiquitous asphaltene spectral data with MO calculations shows that asphaltene ring systems predominantly consist of 4-10 rings. PAHs with 6-8 rings are most predominant in petroleum asphaltenes. Not surprisingly, polydispersity is implied in this analysis. These results are very consistent with all direct imaging of asphaltene PAHs. When these results are coupled with known molecular weights of asphaltenes, we find that asphaltene molecules possess primarily one fused ring system per molecule. This finding is independently obtained when combining MO results with spectral dispersion measured for asphaltene diffusion constants. The monomeric molecular structure of asphaltenes is supported, and the archipelago model is refuted.

Previous molecular orbital calculations have shown that polycyclic aromatic hydrocarbons (PAHs) with 4-10 fused rings account for the center electronic band positions for bulk asphaltenes. Here, this work is extended to cover low-energy electronic transitions of crude oils and asphaltenes. The primary determinants in optical absorption band location are shown to be the size and geometry of PAHs. Large PAHs are shown to exhibit optical properties exhibited by asphaltenes and crude oils. Furthermore, these results are consistent with the observed Urbach spectral profiles. The rapidly declining electronic absorption at wavelengths exceeding 600 nm is shown to be consistent with the presence of a few large ring systems. Measurements of concentration and temperature independence of crude oil and asphaltene optical spectra imply that potential contributions to their coloration from charge-transfer and potential free radicals are not significant. Nevertheless, the very small electronic absorbances for very low energy are found to fall outside of the absorption profile of large hydrocarbon PAHs. Future work is indicated to account for addressing these very low absorbances for asphaltenes.