PAH dimerization as the first step to soot particle inception

摘要

Particle inception, or the transition from the gas-phase to solid-phase, is a complex process that influences the growth of combustion-emitted particles, especially their size distribution and morphology. Unfortunately, although it is widely believed that nanoparticles form via polycyclic aromatic hydrocarbons (PAHs), little is known about the exact molecular mechanisms involved in the process. In this paper, we investigate the effects of molecular structure on the tendency of PAHs to dimerize. We measured the thermodynamic stability of aliphatic-substituted PAHs by calculating the free energy surfaces of monomer-monomer interactions. Specifically, we varied the geometry and chain lengths of these PAHs. Free energy was chosen as the best measure of thermodynamic stability because it takes into account stabilization due to both potential and entropic effects. The results show that the stability of aliphatic-substituted PAH dimers increases with the length and saturation of the aliphatic chains, resulting in lower free energy minima than their unsaturated analogs. The results can be used to expand the current soot models to include various species for the particle inception process.