Theoretical Investigation of Soot Nanoparticle Inception via Polycyclic Aromatic Hydrocarbon Coagulation (Condensation): Energetic, Structural, and Electronic Features

Carbon nanoparticles, generated during combustion at relatively low [02], or under pyrolysis conditions, can be seen both as, soot precursors and as primary pollutants in themselves, since they are also directly emitted in the troposphere by vehicles. Soot particle inception (transition of relatively low-mass molecular systems from the gaseous phase to a solid nature) occurs at least in part via polycyclic aromatic hydrocarbon (PAR) coagulation/condensation. Complexes of different PAR systems, bound only by dispersion/multipolar forces, are investigated here by density functional theory, and their structural, and energetic features discussed. The energetic features of the complexes allow to define an interplane interaction energy per C atom which compares satisfactorily with published experimental data on graphite exfoliation (i.e., removal of a single layer from the top of its bulk). The temperature dependence of the equilibrium K for these systems is then calculated to estimate the importance of PAH coagulation (condensation) in carbon nanoparticle generation. Energy alone would suggest that the larger interacting systems will be better stabilized by dispersion forces, but the trends in free energies are affected also by the entropy factor. This implies that beyond some temperatures the components of the largest systems will be more prone to fly apart than those of smaller systems, thus limiting the Sin of crystallites beyond some temperature. On the basis of our computational results, at high temperatures sheer stacking via van der Waals interaction can hardly be a major factor in causing soot nanoparticle inception.