(223x) Towards the Development of a Model for Particle Nucleation

Towards the development of a model for particle nucleation

Jeffrey Lowe1, Paolo Elvati2, and Angela Violi3

1Department of Chemical Engineering, University of Michigan

2Department of Mechanical Engineering, University of Michigan

3Departments of Mechanical Engineering, Chemical Engineering, Biomedical Engineering, Macromolecular Science and Engineering, and Applied Physics, University of Michigan

The widespread emission of carbonaceous nanoparticles (CNPs) from combustion sources poses a significant health risk to humans; therefore, CNP formation has been studied in great detail. CNP formation consists of a number of steps involving both chemical and physical growth mechanisms. The particle nucleation step, or the transition from gas-phase species to solid-phase species, is not well-understood. Our current work invokes the physical dimerization of aliphatic-substituted polycyclic aromatic hydrocarbons (PAHs) as the initial nucleation step. We have quantified the effect of particle geometry on the free energy stability of dimerized compounds of PAHs. We employed molecular dynamics techniques coupled with the well-tempered Metadynamics algorithm to measure the free energy surfaces (FESs) of dimerization between compounds of interest. The FESs provided us with the propensity of sticking between molecules based solely on the structure of the molecules themselves, eliminating a dependence on empirical parameters. The sticking propensities obtained in this study can be used in conjunction with collision frequencies to predict nucleation rates in systems of interest. Molecules with saturated chains were found to display an increase in dimer stability with an increase in the number of substituted chains whereas molecules with unsaturated chains had dimer stabilities relatively unaffected by the number of chains. Further, we determined that the most preferential configurations of molecules within a dimer depend on the distance between each of the molecules involved in dimerization. Future incorporation of the findings in the current study into an overall particle nucleation model in the gas-phase will aid researchers in minimizing the environmental impact of CNP release into the atmosphere.
This research was funded by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract No. DE-SC0002619