(95b) Coarse-Grained SAFT-? Force Fields for the Molecular Modelling of Resins and Asphaltenes

Asphaltenes and resins are the least characterized
fractions of crude oils. They include macromolecules made up of mixtures of
polynuclear aromatic segments decorated with alkane moieties which have a
tendency to precipitate out of solution at reservoir and plant operating conditions. In spite of their large molecular weight (ca. 1000 Da) and their low overall
concentration (typically 0.2 to 7 % w/w), 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
modelling is at odds with the required spatial and temporal resolution needed
to fully understand the asphaltene aggregation process. The computational cost
required to explore such scales can be reduced by using coarse-grained (CG)
models, which consist of lumping a few atoms into a single segment that is
characterized by effective interactions. We employ here the SAFT-γ force-field
[Müller, E.A., Jackson, G. (2014) Annu. Rev. Chem. Biomolec. Eng., 5,
405-427] which provides for a reliable pathway to link the molecular
description with macroscopic thermophysical data and has proven to be ideal to
describe reservoir fluids [C. Herdes, T. S. Totton, and E. A. Müller (2015) Fluid
Phase Equilibria, 406, 91–100.].

Figure
1. Snapshots
of the equilibrium states for atomistic (left) and CG (right) MD simulations.
The corresponding cluster-size distributions are plotted as an example of the
validation tests for the CG models.

In this work, we
explore trends in the force fields of small polynuclear aromatic hydrocarbons
(PAHs) and propose simple fittings of the parameters to develop CG models for
the aromatic cores in asphaltenes. The new aromatic-core parameters, along with
others published for simpler organic molecules, allow the construction of
asphaltene models by joining different chemical moieties, in a
group-contribution fashion. We apply the procedure to two asphaltene models and
study both fully atomistic benchmark systems and CG systems of 27 asphaltenes
in pure solvent (toluene or heptane) via Molecular Dynamics (MD) simulations.
An excellent match in both levels of description is observed for cluster size,
radii of gyration, and relative-shape-anisotropy-factor distributions (Fig. 1).
The effects that flexibility and unlike-interaction parameters have in the
clustering behaviour is tested and used to improve the models. Finally, we
exploit the advantages of the CG representation by simulating systems
containing up to 500 asphaltenes in explicit solvents, and prove how
system-size effects are crucial to appropriately characterize asphaltene
aggregation. The coarse-graining procedure is seen to be general and
predictive, hence can be employed to other asphaltenic molecular structures.