Calculations of the dimerization of aromatic hydrocarbons: Implications for soot formation

Polynuclear aromatic hydrocarbons (PAH) are found in all sooting, hydrocarbon flames. These species are ideally suited to be chemical precursors and building blocks in soot formation, yet their possible role has not been elucidated. From a knowledge of the magnitude of the van der Waals interaction between pairs of PAH the equilibrium constants for dimer formation have been calculated. These values have been used with experimentally measured PAH concentrations to compute dimer concentrations, which were then compared to soot nuclei number densities to determine whether or not the dimers are numerous enough to serve as nucleation sites. The dimers of benzene, coronene (7 rings), and circumcoronene (19 rings), as well as mixed dimer pairs, have been examined. Despite choosing monomer concentrations and theoretical approaches which favor dimer formation, the dimerization of PAH does not yield a sufficient number of nucleation sites to account for soot formation in a homogeneous nucleation mechanism. If PAH do participate in the early stages of soot formation, irreversible chemical steps must be important.