The association of polyaromatic hydrocarbons (PAHs) in mixtures of organic solvents is central to a diverse range of contemporary engineering challenges including the fabrication of organic photovoltaics, design of high-performance discotic liquid crystals, and prevention of petroleum asphaltene aggregation and fouling. PAHs and their derivatives have a strong propensity to self-associate, which must be either carefully controlled to obtain optimum material properties or appropriately inhibited to avoid unwarranted behaviour. Molecular simulation of systems that exhibit complex behaviour like aggregation require spatial and temporal scales that are difficult to access by an all-atom description. 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 characterised by effective interactions. An accurate procedure to obtain CG force-fields is the use of statistical associating fluid theory (SAFT) [Müller, E.A., Jackson, G. (2014). Annual Rev. Chem. Biomolec. Eng., 5, 405-427]; this allows one to link the molecular description with thermodynamic data. A recent modification of the SAFT-VR Mie equation [Müller, E.A., Mejía, A. (2017) Langmuir submitted] explicitly considers non-linear connectivity in the description of planar ring-like molecules and is particularly suited for PAHs.
In our current work, we find trends in the force fields obtained for PAHs using the SAFT methodologies and propose simple correlations fittings of the parameters as a function of the ratio between the number of carbons not linked to a hydrogen and the total number of carbons. These fittings allow us to present CG Mie potentials for arbitrary PAHs. As an example of their application, we use the procedure to model two asphaltene molecules from the literature. We perform CG simulations of asphaltene molecules (in toluene or heptane) at the same thermodynamic conditions as benchmark atomistic simulations presented in [Headen, T.F., et al. (2017) Energy and Fuels, (31), 1108–1125]. Cluster-size, radii of gyration, and shape-factor distributions are calculated in the CG systems and compare favourably to the atomistic results. Finally, we exploit the advantages of the CG representation by simulating large systems containing 500 molecules of a model asphaltene in explicit toluene and heptane, and report the aggregation behaviour in these large systems. A key observation is the continuous growth of asphaltene clusters under unfavourable conditions, discarding the suggestion that there is an unequivocal “nanoaggregate” cluster size. We expect to apply the modelling procedure to other asphaltene molecular structures and identify the scopes and limitations of these CG models.
