Thermodynamics of poly-aromatic hydrocarbon clustering and the effects of substituted aliphatic chains

Over the last few decades, the understanding of the processes related to the formation of soot has progressed considerably. However, the mechanisms that are responsible for the nucleation of soot are still unclear. While there is consensus that the formation of soot nuclei can be related to two classes of mechanisms (physical and chemical growth), their relative importance is still under debate. In particular, the aggregation of polycyclic aromatic hydrocarbons (PAHs), especially pyrene, has been proposed as key step for soot formation but strong experimental or computational proofs are still missing. To shed light on this issue, we conducted a thermodynamic analysis of the physical growth of poly-aromatic hydrocarbons using atomistic models. Free energy profiles of dimerization and trimerization processes of several PAHs are computed using molecular dynamics simulations in conjunction with advanced sampling techniques. Our study focuses not only on the potential energy of the clustering processes, but it also addresses the entropic contributions that affect the dimerization and trimerization of PAHs. The results of these simulations show that even at 1000 K, only the formation of dimers of relative big species, such as ovalene or bigger, are favored over their corresponding free monomers, ruling out the simple stacking of pyrene as the main step for soot formation. The shapes of the free energy profiles also illustrate that there are no barriers in the exploration of the phase space and therefore, that the dimerization process itself is not kinetically controlled. Geometrical factors, such as the symmetry of the monomers, as well as the presence of substituted aliphatic chains play an important role in the physical agglomeration of PAH that can eventually lead to soot nuclei. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.