soot particle structure: insights from molecular dynamics ... · soot particles made of polycyclic...
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Soot particle structure:
Insights from molecular
dynamics and Monte Carlo
simulations
Kimberly Bowal, Jacob Martin, Peter Grancic, Markus Kraft
Computational Modelling Group
Department of Chemical Engineering and Biotechnology
University of Cambridge
Kimberly Bowal
Why study soot?
1Guarnieri and Balmes, The Lancet 383 (2014)2Hansen and Nazarenko, Proc. Natl. Acad. Sci. 101 (2004)3IARC Monographs, 35, (1985)
Contributes to air pollution1
Poses significant health problems3
Affects the climate2
Reduce soot emissions from
combustion devices
2
Kimberly Bowal
[email protected], P. et al. Carbon, 109 (2016)
Soot nanostructure
Graphitic shell
Amorphous core
Soot particles made of polycyclic
aromatic hydrocarbons (PAHs).
Experiments show many different
PAH sizes form a core-shell
structure.
3
To date, molecular modelling
studies represent soot particles as
homogeneous PAH clusters.
More work is required to
understand the behaviour of
heterogeneous PAH clusters as it
relates to soot morphology.
Kimberly Bowal
Aim
Use molecular modelling methods to investigate the stable
configurations of soot-sized mixed PAH clusters to
understand whether partitioning of molecule sizes could be
due to physical interactions
4
Kimberly Bowal
Simulation set up
Randomly mixed Janus Core-shell Core-shell
ovalene
C32H14
pyrene
C16H10
5
circumcoronene
C54H18
coronene
C24H12
Replica exchange molecular dynamics
Kimberly Bowal
REMD simulation videos
Low temperature replica (400K) High temperature replica (1500K)
6
Kimberly Bowal
REMD radial distance over time
Average radial
distance of each
molecule type
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Kimberly Bowal
REMD cluster snapshotsCircumcoronene & Coronene Ovalene & Pyrene
Final configurations
independent of initial
arrangement.
Stacked structure
dominant.
Partitioning seen:
larger molecules stack
in core and smaller
molecules cap the
ends of those stacks
and form parallel outer
stacks.
8
Bowal, K. et al. Carbon 143 (2019)
Kimberly Bowal
Basin-hopping Monte Carlo method
Challenge: many invalid
iterations because PAHs
often interlock during
random movement
Uses minimisations
between random
translation and rotation
steps to search potential
energy surface5
5Wales and Doye, J. Phys. Chem. A, 28 (1997)
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Kimberly Bowal
Evaluation of SEMC method
[6]
[7]
[8]
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6Rapacioli, M. et al. J. Phys. Chem. A (2005)
7Bartolomei, M. et al., J. Phys. Chem. C (2017)
8Hernández-Rojas, J. et al., J. Phys.Chem.Chem.Phys. (2016)
Kimberly Bowal
Circumcoronene & Coronene
REMD REMDSEMC SEMC
Ovalene & Pyrene
REMD and SEMC results12
SEMC energies show <5% mean
difference of the minimum energy with
10-fold speed up compared to REMD.
Kimberly Bowal
Relating to experimental work
Experiments say:
small molecules in core,
large molecules in shell
Simulations say:
large molecules in core,
small molecules in shell
What does
this mean?
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Graphitic shell
Amorphous core
Kimberly Bowal
Subsequent experimental work
9Botero, M. et al. Carbon, 141 (2018)
Modelling led to experimental work using high resolution electron
microscopy which found that young soot particles contain larger, more
stacked molecules in the core compared to the shell
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Kimberly Bowal
Conclusions
Replica exchange MD and newly developed Sphere Encapsulated MC
methods used to explore heterogeneous PAH clusters
Stable structures present stacked motif with core-shell arrangement, with
larger molecules in core
Further experimental work confirms this trend for young soot particles,
suggesting that intermolecular interactions dominate in arrangement of
young particles
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