Surface properties of heterogeneous polycyclic aromatic hydrocarbon clusters

In this paper we investigate the impact of molecular inhomogeneity on the surface properties of soot particles. Using replica exchange molecular dynamics and solvent-excluded surface analysis, we evaluate detailed surface properties directly from particles containing polycyclic aromatic hydrocarbon molecules of different sizes. The temperature-dependent behaviour of surface roughness and number densities of reactive sites are evaluated for particles from 1-5 nm in diameter. The percentage of carbon atoms and zig-zag sites on the particle surface are found to be independent of molecular composition, while molecule heterogeneity influences the accessible hydrogen atoms and free-edge sites. These relationships allow the prediction of surface composition for a given particle diameter. The surface densities of carbon and hydrogen atoms are explained by the morphological changes and molecule size contributions for solid-like and liquid-like configurations. Small molecules contribute significantly to the particle surface properties at low temperatures, regardless of the proportion of molecule sizes, which results in an increased density of accessible carbon atoms for heterogeneous particles. Interestingly, the surface density of edge carbon atoms and free-edge sites can be predicted from the average molecule size alone. The density of hydrogen atoms on the surface follows the average expected values from the constituent molecule sizes, suggesting that for particles containing many different molecule sizes the α parameter corresponding to the HACA mechanism converges to a linear temperature-dependent trend. This quantitative evaluation of the accessibility of reactive sites for heterogeneous particles provides important information for understanding soot particle growth and oxidation.