(376u) Investigating the Core-Shell Structure of Soot Using Molecular Modelling Methods

Soot particle
structure: Insights from molecular dynamics and Monte Carlo simulations

Kimberly
Bowal^a, Jacob W. Martin^a,b,
Peter Grančič^c, Markus Kraft^a,b,d

^aDepartment of
Chemical Engineering and Biotechnology, University of Cambridge, West Site Philippa Fawcett Drive,
Cambridge, CB3 0AS UK

^bCambridge Centre for Advanced Research and Education in Singapore
(CARES), CREATE Tower, 1 Create Way, 138602 Singapore

^cInstitute of Soil Research, University of
Natural Resources and Life Sciences, Peter-Jordan-Straße
82, 1190 Vienna

^dSchool of Chemical and Biomedical
Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459
Singapore

Incomplete combustion produces carbonaceous particulate
matter, known as soot, which negatively affects combustion devices, human
health and the environment. The development of soot prediction models and
mitigation strategies requires an accurate knowledge of soot particle formation
and structure. Electron microscopy has revealed that mature soot particles
often have a core-shell structure, in which the molecules near the centre of
the particle differ in size from those in the surface layers [1]. However, it
is unknown what interactions cause this partitioning and to what extent it is
present in nascent particles.

In this work, the behaviour of heterogeneous polycyclic
aromatic hydrocarbon (PAH) clusters was investigated
using two independent molecular modelling methods to understand the structure
of nascent soot particles. Clusters of up to 100 molecules containing
combinations of the different sized PAHs circumcoronene,
coronene, ovalene, or pyrene were evaluated. Replica
exchange molecular dynamics (REMD) simulations sampled many configurations at
high and low temperatures to determine stable low energy structures [2]. These
results were compared with a novel Sphere Encapsulated
Monte Carlo (SEMC) method, developed here to extend basin-hopping minimisation
methods for use with planar aromatic molecules.

Both the REMD and SEMC simulations showed that the most
stable cluster structures consist of parallel columns of stacked PAHs in a
core-shell structure, in which the larger PAHs are located in the core, and the
smaller PAHs are found in a surrounding shell, as seen
for a representative cluster in Figure 1. This is an inverse partitioning
compared to that seen in mature soot particles, proposing a unique structure
for nascent soot particles and suggesting that the molecular organisation
determined by intermolecular interactions is not responsible for the core-shell
structure of mature soot. These results present the first simulations of
soot-sized heterogeneous PAH clusters and can help further understanding of
soot formation and growth processes.

Figure
1: Radial distances of each molecule type shown for a cluster containing 16 coronene molecules (coloured red) and 16 circumcoronene molecules (coloured blue) using replica
exchange molecular dynamics simulations and the Sphere Encapsulated Monte Carlo
method. Initial and final cluster snapshots are also shown.

[1] T. Ishiguro, Y. Takatori,
and K. Akihama. Combust. Flame, (1997), 108,
231–234.

[2] K. Bowal, J.W. Martin, M. Kraft. Carbon,
(2019), 143, 247-256.