Preprint 77 published
Preprint 77, "Modelling the Internal Structure of Nascent Soot Particles", has been published on the CoMo Group website.
In this paper we present studies into clusters of polycyclic aromatic hydrocarbon (PAH) molecules similar in size to small soot particles. The clusters studied were comprised of coronene (C24H12) or pyrene (C16H10) molecules and represent the types of soot precursor molecule typically found in flame environments. A stochastic 'basin-hopping' optimisation scheme was used to locate local minima on the potential energy surface (PES) of the molecular clusters. TEM-style projections of the resulting geometries show similarities with those observed experimentally in TEM images of soot particles. The mass densities of these clusters have also been calculated and are lower than bulk values of the pure crystalline PAH structures, and are also significantly lower than the standard value of 1.8 g/cm3 used in our soot models. Consequently we have varied the mass density between 1.0 g/cm3 and 1.8 g/cm3 to examine the effects of varying soot density on our soot model. The resulting particle size distribution showed that increasing the density by less than a factor of two caused an order of magnitude reduction in the peak aggregate number density. Based on similarities between nascent soot particles and PAH clusters a more accurate soot density is likely to be significantly lower than 1.8 g/cm3. As such, for modelling purposes, we recommend that the density of soot should be taken to be the value obtained for our coronene cluster of 1.12 g/cm3