Light Absorption Coefficient and Hydrogen Content as Key Properties for Inferring Structural Features of Soot

The relationship between the light absorption coefficient of flame-formed soot and its hydrogen content was investigated by coupling UV-Visible spectroscopy with elemental analysis and FT-IR quantification of hydrogen tethered to soot sampled along the axis of a premixed ethylene flame. Just after soot inception, the preferential depletion of aliphatic hydrogen was observed moving from 0.2 to 0.1 hydrogen mole fraction and in correspondence of a relatively slight increase of the absorption coefficient. Soot formation rate decreased just after the disappearance of aliphatic hydrogen due to the consequent decrease of active sites on soot particles necessary for further carbon addition and growth. Crossing the threshold hydrogen mole fraction value of about 0.1, aromatic hydrogen resulted to survive to dehydrogenation producing a highly light-absorbing carbon. Thus, the high absorption coefficient appeared to be due to coagulation and thermal annealing marking the end of the soot formation process.