Density Functional Theory Investigation of the Contributions of π–π Stacking and Hydrogen-Bonding Interactions to the Aggregation of Model Asphaltene Compounds

Abstract

We performed density functional theory (DFT) calculations using the WB97Xd functional with a dispersion correction term and the 6-31G(d,p) basis set to study the contributions of π–π stacking and hydrogen-bonding interactions to the aggregation of asphaltene model compounds containing a 2,2′-bipyridine moiety covalently bonded to one (monosubstituted) and two (disubstituted) aromatic hydrocarbon moieties (phenyl, naphthyl, anthracyl, phenanthryl, and pyrenyl) through ethylene tethers. In these compounds, the N atoms of the 2,2′-bipyridine moiety provide lone pairs for hydrogen bonding to water molecules present in solution. The aggregation strength of the homodimers of these model compounds is evaluated in terms of the aggregation energies, enthalpies, and ΔG 298, as well as the π–π interaction distances. Geometry optimization and thermochemistry analysis results show that the homodimers of both mono- and disubstituted compounds are stable and have a negative ΔG 298 of aggregation because of π–π stacking interactions. Two water bridges containing one, two, or three water molecules per bridge span between two monomers and provide additional stabilization of the homodimers because of hydrogen bonding. The stabilization of the monosubstituted homodimers is the largest with two water molecules per bridge, whereas the stabilization of the disubstituted homodimers is the largest with three water molecules per bridge. The calculated 1H nuclear magnetic resonance chemical shifts for the monomers and dimers of the three model compounds of this series synthesized to date are in excellent agreement with experimental results for dilute and concentrated solutions in chloroform, respectively (Water enhances the aggregation of model asphaltenes in solution via hydrogen bonding Tan X. Fenniri H. Gray M. R. Tan X. Fenniri H. Gray M. R. Energy Fuels2009233687). The ΔH and ΔG 298 results show that hydrogen bonding is as important as π–π interactions for asphaltene aggregation.

Asphaltene adsorption on quartz sand in the presence of pre-adsorbed water

“…Self-aggregation, whether at interfaces or in solution, is understood to result from - stacking of the asphaltene polycyclic aromatic regions [9], although electrostatic and Hbonding contributions involving water have also been identified in model asphaltene systems [10][11][12]. This latter view supports other results from simulation and experiment, that low water concentrations promote asphaltene nanoaggregation in toluene solution [13], which is suggested to involve intermolecular H-bonded water bridges and asphaltene heteroatoms, e.g.…”

Fluorine tracers for the identification of molecular interaction with porous asphaltene aggregates in crude oil

“…It is known from experiment as well as DFT computations that mono-and polyaromatic rings interact strongly with each other and assume intermediate or even stable structures by a process called p-stacking. This process is responsible for asphaltene aggregation and precipitation [9][10][11], but also for the selective adsorption on ordered surfaces, and the generation of graphitic structures. Intermediate dimerization of benzene rings is another example of this interaction [12].…”

Effect of Water on Deposition, Aggregate Size, and Viscosity of Asphaltenes

“…7 Their analysis showed the formation of two water bridges composed of one, two, or three water molecules per bridge between two asphaltene monomers, which provided additional stabilization of asphaltene aggregates due to hydrogen bonding. Their enthalpy and Gibbs free energy calculations proved hydrogen bonding to be as important as π−π interactions in asphaltene aggregation.…”