Peri-condensed aromatics with aliphatic chains as key intermediates for the nucleation of aromatic hydrocarbons

Soot nucleation bridges the transition from gaseous hydrocarbons to macromolecular building blocks (nanoparticles) that eventually turn into soot. Polycyclic aromatic hydrocarbons (PAH) have often been invoked as important compounds of this process but their role has not been clearly identified. In this paper we report on a detailed analysis of the physical interactions between PAH in the range 200–450 amu using Molecular Dynamics simulations. In particular, we identified a pool of nine aromatics and studied their clustering behaviors in systems composed of thousands of homo-molecular and hetero-molecular molecules to understand the influence of molecular mass, morphology and temperature on the nucleation process. At temperatures higher than 1000 K, small clusters of PAH (2–5 molecules) are detected but they are not stable enough to accommodate the further growth into larger particles. This result raises doubts on the ability of these molecules to become soot nuclei. Molecular morphology is another important parameter for the nucleation process. Aromatics with attached aliphatic chains show considerably faster nucleation rates than the corresponding polycyclic aromatic hydrocarbons of similar mass without any chain. The collision efficiency is not increased by the aliphatic chain attachments, which may indicate that a faster nucleation process for these systems is due to the ability of these molecules to accommodate the collision energy into additional internal vibrational modes of the aliphatic chains. The results of this study provide information on the clustering behavior of PAH and can lead to the development of a more complex model to describe the physical nucleation of PAH that includes molecular masses, morphologies and temperature as main parameters to describe the transition from gas-phase species to macromolecular structures.