Coalescence of Two Carbonaceous Nanoparticles: A Steered Molecular Dynamics Study of the First Steps of the Soot Aggregate Formation

Soot nanoparticles, resulting from incomplete combustion processes of fossil fuel or biomass, play a central role in many environmental and industrial phenomena, while posing major challenges for public health and global warming estimate. A better quantification of all the soot impact is thus strongly needed, which requires a better understanding of their formation and ageing processes. Experiments carried out on the structure of soot nanoparticles highlighted that their morphology is characterized by aggregation of spherical primary soot grains (carbonaceous spherules of a few to tens of nanometers in size), the resulting aggregates being of submicrometer size [1,2]. In the present work, we have used the steered molecular dynamics (MD) method [3] to investigate, at the atomic level, the coalescence of two carbonaceous spherules, aiming at modeling thus the very first steps of the aggregation process. Computations have been performed with the molecular dynamics software LAMMPS, and the AIREBO interaction potential model has been used to calculate the carbon-carbon interactions in the corresponding systems. Because it is the first time (as far as we know) that such an approach is used in this context, a thorough investigation of the influence of the intrinsic parameters of the steered MD on the results has been performed, by varying the temperature, the duration of the simulations, the spring constant values, and the thermostat. This work, which can thus be viewed as a mandatory stage for our studies on spherule aggregation, emphasizes that using steered MD is a promising approach for accurately modeling, at the atomic scale, structural changes resulting from soot aging. [1] H. Michelsen, Probing soot formation, chemical and physical evolution, and oxidation: A review of in situ diagnostic techniques and needs, Proceedings of the Combustion Institute, 36 (2017) 717–735 [2] P. Parent et al., Nanoscale characterization of aircraft soot: A high-resolution transmission electron microscopy, raman spectroscopy, X-ray photoelectron and near-edge X-ray absorption spectroscopy study, Carbon, 101 (2016) 86–100. [3] M. Lbadaoui-Darvas et al., Molecular-scale description of interfacial mass transfer in phase-separated aqueous secondary organic aerosol, Atmos. Chem. Phys., 21 (2021) 17687-17714