AS&T Article Highlight: First-order consumption of heptane to form carbon black – a case for chemical nucleation

By: Sarah Petters, Assistant Research Engineer, University of California Riverside

Authors of the Manuscript Featured: Arash Fakharnezhad, Dimitri M. Saad, Georgios A. Kelesidis & Eirini Goudeli

Soot – an annoyance, detrimental to health and the environment, or a valuable material produced en masse? 

Why not both? Soot is generally defined as the byproduct of incomplete combustion – a mixture of elemental and organic carbon. Carbon black is a manufactured pyrolysis product. Both annoying and useful, soot has been in production since Prometheus; new applications involve batteries and capacitors.

Fakharnezhad et al. have modeled the nucleation of soot clusters by the condensation of polycyclic aromatic hydrocarbon (PAH) molecules under conditions mimicking a gasoline engine. The question addressed is whether incipient soot clusters form by physical or chemical nucleation. Physical nucleation is the traditional assumption, and it implies that the molecules are held together by van der Waals forces after they encounter each other and stick; chemical nucleation is a more recent theory implying that a radical electron aids in forming a more robust bond upon collision. Remember, the violent rearrangement of the fuel molecule, in this case a C7 straight-chain alkane, has already occurred. The question, therefore, addresses the mechanism of particle nucleation, which controls the downstream polydispersity.

Center for Aerosol Science and Engineering

Here, the consumption of n-heptane under an inert atmosphere (pyrolysis) at 2200–2600 Kelvin and the rate of the subsequent formation and growth of PAH clusters is investigated using a reactive molecular dynamics simulation. During the rapid depletion of fuel, dimers grow in, followed by trimers, then tetramers, and so forth, as the population of lower clusters peaks and diminishes in favor of the larger clusters. The nucleation rate, J, for each n-mer is calculated as the rising slope of the n-mer concentration before it levels off and trends back downward. 

Nucleation and subsequent dissociation is common; only particles exceeding their critical size are able to stabilize and undergo thermodynamically-favored growth. The authors used the simulation to calculate the difference in Gibbs free energy between the gaseous and condensed PAH molecules as a function of particle size.

The findings reveal that the critical cluster size drops with higher temperatures, indicating that the PAH molecules are bound not purely by van der Waals but rather a reaction enhanced by temperature. The stability of these clusters seems independent of the fuel concentration, again arguing for chemical nucleation. Physical nucleation tends to be enhanced by raising the vapor pressure of the condensing molecule (i.e. its concentration). The nucleation rate is directly proportional to initial fuel concentration, suggesting the fuel ‘decays’ into particles at elevated temperature and pressure. An Arrhenius-like nucleation rate constant is reported. This simplifies computations because the one-step particle formation rate obviates the need to implement a set of reactions in a kinetic model.

Further reading:

Fakharnezhad, A., Saad, D. M., Kelesidis, G. A., & Goudeli, E. (2025). Nucleation rate of carbonaceous nanoparticles by n-heptane pyrolysis at high pressure and temperature via molecular dynamics simulations. Aerosol Science and Technology, 1–14. https://doi.org/10.1080/02786826.2025.2480625