A Transient Nano-Dense Molecular State in Nanoparticle Inception: Bridging Physical Clustering and Chemical Stabilization

The formation of nanoparticles in combustion systems, including soot and selected inorganic materials, involves coupled physical and chemical processes that remain incompletely understood. Current descriptions tend to emphasise either gas-phase chemical growth pathways or physically driven clustering mechanisms. This work proposes a testable, physically grounded hypothesis in which the inception of nanoparticles proceeds through the formation of a transient, non-equilibrium, nano-dense molecular state (NDMS), which is composed of precursor species. In this state, intermolecular interactions arising primarily from quantum-mechanical dispersion forces and configurational effects promote local densification and increase encounter lifetimes. Meanwhile, retained molecular mobility enables coalescence and enhances reactive stabilisation. Subsequent chemical processes reduce volatility and progressively stabilise the evolving structure, leading to the formation of stable particles. This hypothesis provides a continuous description of inception that links gas-phase chemistry and condensed-phase particle formation without the need for a strictly discrete nucleation event. It is consistent with several recent experimental and computational observations in soot formation. However, the relative contributions of physical clustering and chemical stabilisation remain a matter of active debate. The applicability of this hypothesis to inorganic systems is discussed in the context of cluster-mediated particle formation. The hypothesis also yields predictions that can be tested experimentally, concerning cluster lifetimes, spectroscopic signatures and deviations from classical nucleation behaviour. Further quantitative experimental and computational studies are required to investigate the existence, properties, and kinetic relevance of the transient nano-dense molecular state (NDMS).