Aggregation Behavior of Model Asphaltenes Revealed from Large-Scale Coarse-Grained Molecular Simulations

Abstract

Fully atomistic simulations of models of asphaltenes in simple solvents have allowed the study of trends in aggregation phenomena and the understanding of the role that molecular structure plays therein. However, the detail included at this scale of molecular modeling is at odds with the required spatial and temporal resolution needed to fully understand the asphaltene aggregation. The computational cost required to explore the relevant scales can be reduced by employing coarse-grained (CG) models, which consist of lumping a few atoms into a single segment that is characterised by effective interactions. In this work CG force fields developed via the SAFT-γ [Müller, E.A., Jackson, G. (2014) Annu. Rev. Chem. Biomolec. Eng., 5, 405-427] equation of state (EoS) provide a reliable pathway to link the molecular description with macroscopic thermophysical data. A recent modification of the SAFT-VR EoS [Müller, E.A. and Mejía, A. (2017) Langmuir, 33, 11518-11529], that allows parametrizing homonuclear rings, is selected as the starting point to propose CG models for polycyclic aromatic hydrocarbons (PAHs). The new aromatic-core parameters, along with others published for simpler organic molecules, are adopted for the construction of asphaltene models by combining different chemical moieties in a group-contribution fashion. We apply the procedure to two previously reported asphaltene models and perform Molecular Dynamics simulations to validate the coarse-grained representation against benchmark systems of 27 asphaltenes in pure solvent (toluene or heptane) described in a fully atomistic fashion. An excellent match between both levels of description is observed for cluster size, radii of gyration, and relative-shape-anisotropy-factor distributions. We exploit the advantages of the CG representation by simulating systems containing up to 2000 asphaltene molecules in explicit solvent investigating the effect of asphaltene concentration, solvent composition, and temperature on aggregation. Upon employing large systems facilitated by the CG models, we observe stable continuous distributions of molecular aggregates at conditions away from the two-phase precipitation point. As a further example application, a widely accepted interpretation of cluster-size distributions in asphaltenic systems is challenged by performing system-size tests, reversibility proofs and time-dependence analysis. The coarse-graining procedure proposed is seen to be general and predictive, hence, can be applied to other asphaltenic molecular structures.

High resolution nanoscale chemical analysis of bitumen surface microstructures

“…In the past, the structural impact of asphaltene molecules has been unravelled with Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM) 28 and molecular agglomeration has been compared to modelling studies 29 – 31 . High bulk resolution of bitumen has been achieved with high-field Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry (MS) 32 .…”

Surrogate Models for Studying the Wettability of Nanoscale Natural Rough Surfaces Using Molecular Dynamics

“…This connection facilitates the direct estimation of effective force field parameters by fitting the resulting equation of state to a wide a range of thermophysical properties without the need to perform extensive simulations. To date, the SAFT force field has been used successfully in the prediction of the behavior of crude oils in bulk [81,82] and confined reservoirs [83], high pressure oil/water interfacial tensions [84], adsorption [85], and the wetting behavior of surfactant solutions on surfaces [56,86], amongst others.…”

Expanding the Applications of the SAFT-γ Mie Group-Contribution Equation of State: Prediction of Thermodynamic Properties and Phase Behavior of Mixtures

“…A heteronuclear model is implemented in SAFT-γ Mie, and a Mie (generalized Lennard-Jones) potential of variable repulsive and attractive ranges is used to represent the segment–segment group interactions. As evidenced by the growing number of published works − in which SAFT-γ Mie has been employed, the use of the approach is becoming widespread, not only in the traditional sense of GC modeling, but also as a basis for the development of coarse-grained force fields that are finding application in molecular simulations. − ,,,,,,,− ,,, …”