Ions and soot in flames

Abstract

An appreciation of the work of Sir Morris Sugden is given. In this study the positive and negative ions in five oxyacetylene flames were observed mass spectrometrically. The flames ranged from one which was fairly fuelrich (C/O = 0.80) to ones with a feather (including sooting flames) with C/O = 1.09. A great variety of ions was found. The positive ions could be divided into four groups: (i) those containing oxygen; (ii) polyacetylenic ions, i.e. C_xH⁺₃(x= 3–13); (iii) more hydrogenated, but aliphatic C_xH⁺_y ions (with y > 3); and (iv) aromatic C_xH⁺_y ions. It is concluded that ions in these very fuel-rich flames originate from CH^*+C₂H₂→ C₃H⁺₃+ e^– as well as CH + O → CHO⁺+ e^–. Many of the positive ions are subsequently formed in effect by protonation of those molecules and radicals which are either intermediates or products of combustion. There is evidence that C₂H₂ in these flames forms a wide variety of hydrocarbon radicals and polyacetylene and aromatic molecules. As for oxygen-containing species, the protonated versions of ketene, hydroxyketene, ethanol, formic acid, carbon monoxide and water appear, particularly in the less fuel-rich flames.

The observed negative ions fall into three groups: (i) H₃CO^–₃ and H₃CO^–₄, which are the only ones present throughout a flame; (ii) hydrocarbon ones which are ‘relatives’ of the positive ions in that they appear to derive from the same uncharged radicals and molecules; (iii) short-lived oxygen-containing ions, e.g. CHO^–₂, O^–₂, C₂HO^–(deprotonated ketene), HO^–₂, OH^– and O^–. Their reactions are discussed along with those of the free electrons, which are also present in these flames.

An additional ion-producing mechanism is found in the richest flame just before soot particles visibly appear in it. This seems to be a chemiionization reaction between an aromatic molecule and a hydrocarbon radical, an illustrative example being: C₁₆H₁₀+ C₆H → C₂₂H⁺₁₁+ e^–. Soot production can then proceed by rapid growth processes involving C₂H₂ in ion–molecule reactions of the type: C_xH⁺_y+ C₂H₂→ C_x+2H⁺_y+ H₂. The resulting large clusters grow to particles or semi-liquid droplets which either coagulate or coalesce to yield soot. In turn these particles are observed to ionize thermally. Thus the production of soot appears to derive, at least partly, from fairly small aromatic molecules. Their production from C₂H₂etc. in these flames is discussed, as also are the structures of some of these flame ions.