(750d) Sticking Efficiency of Polyaromatic Hydrocarbons at High Temperatures By Reactive Molecular Dynamics

The early stages of soot formation and growth in the gas-phase involve collisions between Polyaromatic Hydrocarbon (PAH) molecules that eventually react and stick leading to a larger cluster. Simplified approaches are often adopted to model cluster collision growth rates, where the sticking rate is calculated assuming hard-sphere potential interactions between colliding entities that bind with a sticking efficiency of unity. This approach, however, neglects potential interactions between monomers and nanoclusters.

Dimerization of common PAH molecules, such as naphthalene, pyrene, coronene and pentacene, is investigated by reactive Molecular Dynamics simulations accounting for accurate potential interactions that increase the binding rate as compared to the hard-sphere approach. Furthermore, the kinetic-internal energy exchange during collision is accounted for in such simulations leading to decrease in the binding rate in comparison to conventional approaches. A framework is presented where the binding probability is systematically determined as function of relative velocity of PAH molecules, their impact parameter and temperature. The PAH dimerization rate is determined by quantifying their binding probability function and their sticking efficiency is obtained by ab initio trajectory calculations. Heavy PAH molecules (e.g. coronene) exhibit higher sticking efficiency compared to lighter ones (e.g. naphthalene), consistent with the literature.

The accurate determination of such sticking rates allows revisiting current coagulation rates of soot nanoparticles. The revised coagulation rates can be employed in mesoscale aerosol dynamics and soot formation models.