Aspects of Asphaltene Aggregation Obtained from Coarse-Grained Molecular Modeling

We have performed a molecular-simulation-based
study to explore
some of the underlying mechanisms of asphaltene aggregation. The daunting
complexity of the crude oil + asphaltene system precludes any type
of meaningful molecular simulation unless some assumptions are made
with respect to the key physical and chemical properties that must
be explicitly described. In the present work, we focus on molecular
simulations of a coarse-grained model of asphaltene molecules in pure
solvents, which are based on the assumption that the general size
asymmetry and asphaltene morphology play a key role in the aggregation
process. We use simple single isotropic Lennard-Jones sites to represent
paraffinic and aromatic C6 segments, which are used as
building blocks for the description of continental asphaltene models
and solvent moieties. The energy and size parameters for the intermolecular
models (ε and σ) for solute and solvent molecules are
chosen to reproduce the experimental density of the liquid phase for
different mixtures. An explicit pure solvent is considered, and the
relationship between the aggregation mechanism and the solvent nature
is investigated through direct simulation of the aggregation process.
The results reproduce accurately expected trends observed for more-complex
models as well as experiments, for example, strong aggregation of
asphaltene molecules in n-heptane and high solubility
in toluene. Different asphaltene models based on different geometries
reveal that even at this level of simplification the topology of the
molecules (number and position of aliphatic branches) does affect
the way molecules aggregate.