Dynamic polarity of curved aromatic soot precursors

Abstract

In this paper, we answer the question of whether polar curved aromatics are persistently polar at flame temperatures. We find, using electronic structure calculations and transition state theory, that the inversion barriers of curved aromatics (cPAH) of 0.9-1.2 nm in diameter are high and that they are not able to invert over the timescales and at the high temperatures found in sooting flames. We find a transition for smaller curved aromatics between 11-15 (≈ 0.8 nm) rings where the increasing strain introduced from the pentagonal ring increases the inversion barrier leading to rigidity. We then performed ab initio quantum molecular dynamics to find the molecular dipole fluctuations of a nanometre sized cPAH at 1500 K. We found the bending mode of the bowl shaped molecule gave rise to the largest fluctuations on the dipole moment by ±0.5-1 debye about the equilibrium value of 5.00 debye, indicating persistent polarity. We also observed binding of a chemiion at 1500 K over 2 ps, suggesting the molecular dipole of cPAH will be an important consideration in soot formation mechanisms.

The impact of cyclic fuels on the formation and structure of soot

“…Furthermore, the dimerisation energy of cPAHs with one or two pentagonal rings were found to be comparable to that of similar-sized planar PAHs [54]. These curved structures are stable at flame temperature [56]. Moreover, it has been demonstrated that the reaction leading to the conversion of a planar PAH to a curved one through ring addition at an armchair site is very fast at temperatures below 2000 K [73].…”

Kinetic Monte Carlo statistics of curvature integration by HACA growth and bay closure reactions for PAH growth in a counterflow diffusion flame

“…Curvature induced during the growth of polycyclic aromatic hydrocarbons (PAHs) has important consequences for carbon materials. In the case of soot, it has been shown that curved PAHs possess a dipole moment due to the flexoelectric effect [1] that persists at flame temperatures [2]. Such PAHs have been observed in premixed [3,4] and non-premixed flames [5], and it has been hypothesised that their dipole moments may influence the formation of the first particles [6,7].…”

Nanostructural Disorder and Reactivity Comparison of Flame Soot and Engine Soot Using Diesel and Jatropha Biodiesel/Diesel Blend as Fuels

“…The differences in the crystallite structure parameters of engine and flame soots (DE-DF or JE-JF) could well be due to excess oxygen and higher pressure and temperature in engines that may be supporting planar PAH formation over curved ones or would be eliminating the curved ones that are more reactive than planar ones . The effect of combustor operating conditions on curved PAH chemistry is not well understood, and it is still an active area of research. − …”