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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials
Coarse-Grained Molecular Simulation and Nonlinear Manifold Learning of Archipelago Asphaltene Aggregation and Folding Jiang Wang, Mohit Gayatri, and Andrew L. Ferguson J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01634 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018
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The Journal of Physical Chemistry
Coarse-Grained Molecular Simulation and Nonlinear Manifold Learning of Archipelago Asphaltene Aggregation and Folding Jiang Wang,† Mohit Gayatri,‡ and Andrew L. Ferguson∗,¶,†,‡ †Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL 61801, USA ‡Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, USA ¶Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 West Green Street, Urbana, IL 61801, USA E-mail:
[email protected] Phone: (217) 300-2354. Fax: (217) 333-2736
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Abstract Asphaltenes constitute the heaviest aromatic component of crude oil. The myriad of asphaltene molecules fall largely into two conceptual classes: continental – possessing a single polyaromatic core – and archipelago – possessing multiple polyaromatic cores linked by alkyl chains. In this work, we study of the influence of molecular architecture upon aggregation behavior and molecular folding of prototypical archipelago asphaltenes using coarse-grained molecular dynamics simulation and nonlinear manifold learning. The mechanistic details of aggregation depend sensitively on the molecular structure. Molecules possessing three polyaromatic cores show a higher aggregation propensity than those with two, and linear archipelago architectures more readily form a fractal network than ring topologies although the resulting aggregates are more susceptible to disruption by chemical dispersants. The Yen-Mullins hierarchy of selfassembled aggregates is attenuated at high asphaltene mass fractions due to dominance of promiscuous parallel stacking interactions within a percolating network rather than the formation of rod-like nanoaggregates and nanoaggregate clusters. The resulting spanning porous network possesses a fractal dimension of 1.0 at short length scales and 2.0 at long length scales regardless of archipelago architecture. The incompatibility of the observed assembly behavior with the Yen-Mullins hierarchy lends support that high molecular weight archipelago architectures do not occur at significant levels in natural crude oils. Low-dimensional free energy surfaces discovered by nonlinear dimensionality reduction reveal a rich diversity of metastable configurations and folding behavior reminiscent of protein folding, and inform how intramolecular structure can be modulated by controlling asphaltene mass fraction and dispersant concentration.
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The Journal of Physical Chemistry
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Introduction
Asphaltenes are the heaviest aromatic molecules in crude oil containing fused aromatic rings, aliphatic side chains, and, typically, a small number of heteroatoms such as sulfur, nitrogen, oxygen, vanadium, and nickel. 1–6 Defined as a solubility class soluble in toluene and insoluble in n-heptane, 2–7 asphaltenes comprise a myriad of molecular architectures, but typically possess molecular weights in the range 300 g/mol to 1400 g/mol 2,4–13 and structures falling into one of two conceptual classes: continental (or island) – possessing a single polyaromatic core – and archipelago – possessing multiple polyaromatic cores linked by alkyl chains. 11,12,14–16 Asphaltene aggregation is driven by favorable π-π and π-σ interactions between delocalized π electrons within the face and σ electrons at the periphery of the polyaromatic cores. 2,4,10,17 High concentrations and destabilizing conditions can induce large-scale aggregation and precipitation that can foul production equipment and clog well bores and rock formations, and which incur the petroleum industry billions of dollars annually in production inefficiencies. 2,4,5,18,19,19–24 The Yen-Mullins (or modified Yen model) model 9,10 is a hierarchical model of asphaltene aggregation with broad experimental 2,9,10,25–32 and computational 5,7,8,11,23,33–52 support. Under the Yen-Mullins hierarchy, 9,10 asphaltenes are dispersed in crude oil as monomers and small oligomers at low concentration (
