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.

Complementary neutron and X-ray small angle scattering results give prominent information on the asphaltene nanostructure. Precise SANS and SAXS measurements on a large q-scale were performed on the same dilute asphaltene-toluene solution, and absolute intensity scaling was carried out. Direct comparison of neutron and X-ray spectra enables description of a fractal organization made from the aggregation of small entities of 16 kDa, exhibiting an internal fine structure. Neutron contrast variation experiments enhance the description of this nanoaggregate in terms of core-shell disk organization, giving insight into core and shell dimensions and chemical compositions. The nanoaggregates are best described by a disk of total radius 32 Å with 30% polydispersity and a height of 6.7 Å. Composition and density calculations show that the core is a dense and aromatic structure, contrary to the shell, which is highly aliphatic. These results show a good agreement with the general view of the Yen model (Yen, T. F.; et al. Anal. Chem.1961, 33, 1587-1594) and as for the modified Yen model (Mullins, O. C. Energy Fuels2010, 24, 2179-2207), provide characteristic dimensions of the asphaltene nanoaggregate in good solvent.

Ultracentrifugation has been used to produce asphaltene fractions of reduced polydispersity. The structure of these asphaltene fraction solutions has been investigated using viscosity and X-ray scattering (SAXS) measurements as a function of concentration. The relative viscosities of the solutions were found to be fraction-dependent: intrinsic viscosities, radii of gyration, and second viriel coefficients followed a power law with molar mass Mw. A flat disc model succeeded in describing scattering data but failed to take viscosity data into account. By contrast, a fractal model has been found to be consistent with dependence of all measured parameters. Asphaltene-in-toluene solutions were found to form nanometric mass fractal aggregates of fractal dimension 2.1, which in consequence trapped solvent. When, instead of concentration, effective volume fractions are used, the relative viscosities of fractions merge on a master curve which can be fitted by a hard sphere model. In addition, the reduced osmotic moduli deduced from scattering measurements of the different solutions, when expressed as a function of a concentration adimensional parameter, merge again on a master curve which is in accordance with the hard sphere behavior. The viscosities of solutions can be fully predicted from structure considerations if the ratio of hydrodynamic to gyration radius is taken as 0.6. This ratio is found consistent with the fractal description of the aggregates.

A series of viscosimetric and small-angle neutron scattering experiments on asphaltenes diluted in mixed toluene/heptane solvents has been conducted, with the purpose of characterizing the size, molecular weight, and internal structure of asphaltene aggregates as a function of solvent conditions. With increasing flocculant (i.e., heptane) content in the solvent, the intrinsic viscosities of asphaltene aggregates first decreased, went through a minimum for heptane fractions ≈ 10−20%, and then increased at the approach of flocculation. These variations paralleled those of the volume of aggregate occupied per unit mass of asphaltene, a behavior reminiscent of the Flory−Fox relationship for polymers in a solvent. This volume, proportional to the cubed radius of gyration of the aggregates divided by their molecular weight, was determined from the neutron scattering data. For increasing heptane fractions in the solvent, the molecular weight of the aggregates increased with their radius of gyration according to a power law, the exponent being in the range of 2. This exponent also characterized the self-similar internal structure of the asphaltene aggregates. With due care to the possible systematic effects of the strong polydispersity of these aggregates, these results are discussed in light of recent models of colloidal aggregation.