Equilibrium nanostructure of primary soot particles

Abstract

This paper addresses the origin of the characteristic shell/core nanostructure of primary soot particles, in which a zone of concentrically oriented graphene layers surrounds a disordered core. In situ microscopy is used to examine the liquid crystalline optical textures on the external surfaces of droplets and pools of molten mesophase pitch, taken as a model system for investigating the self-organization of high-molecular weight polycyclic aromatic hydrocarbons in condensed phases. A general thermodynamic theory is then developed that governs the equilibrium configuration of ensembles of disk-like polyaromatic compounds with and without chemical reactivity at edge sites. The experiments and theory indicate that the shell/core nanostructure is the equilibrium configuration of nearly planar polyaromatic clusters under the high-temperature reactive conditions of soot formation. The boundary between the concentric shell and the disordered core is predicted to be the point where orientational elastic strain overcomes the free energy difference between bulk isotropic and ordered phases. The theory also yields a quantitative estimate of the size of the core regions, 4–6 nm, in agreement with the observed core diameters in many mature soot particles. Implications for the soot formation process are discussed.