10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Polyaromatic Hydrocarbon Dimerization Rates: Results from ReaxFF Molecular Dynamics

EIRINI GOUDELI, Christopher Hogan Jr., University of Minnesota

     Abstract Number: 387
     Working Group: Combustion

Abstract
Collisions dynamics between carbonaceous molecules in combustion systems are what ultimately govern soot nanoparticle formation (nucleation) and growth. When collisions lead to binding/reaction between molecular monomers, the result is a nanocluster, which may in turn grow collisionally. Nanocluster collisional growth rate calculations are often based upon simplified approaches, in which the binding rate (collision and sticking rate) is calculated assuming hard-sphere potential interactions between colliding entities and assuming a sticking efficiency of 100%. However, this approach neglects both potential interactions between monomers and nanoclusters and does not accurately consider incomplete kinetic energy to internal energy exchange during collision.

In particular, reactions between polyaromatic hydrocarbons (PAHs) are prominent in the earliest stages of soot nanoparticle formation. Here, we examine the dimerization rate of common PAH molecules (e.g., naphthalene, pyrene, coronene), that lead to formation of nascent soot, via reactive molecular dynamics using ReaxFF C-H potentials. In simulations, PAH clusters are equilibrated (at a predefined temperature) and collide with another PAH cluster of same kind with a specified velocity and impact parameter. A map of the sticking (or reaction) probability between such molecules is obtained ab initio as function of velocity and impact parameter. In simulations, we find that high relative velocities for PAH clusters lead to non-sticking (bounce, dissociation). Integration over the reaction probability for the above impact parameters and velocities (in accordance with the Maxwell-Boltzmann distribution) yields accurate reaction rate coefficients of dimerization that are compared to experiments. Furthermore, we discuss how these rates vary with temperature and cluster size. An effective collision efficiency is proposed for PAH clusters of different mass.