Studies of Soot Formation and Soot in Artists’ Materials Using Electronic and Spectroscopic Methods
Open Access DepositedDuring soot formation small aromatic molecules form in the flame. These grow chemically into larger planar polycyclic aromatic hydrocarbon molecules (PAH). Because soot is three-dimensional, at a certain point the PAH molecules stop growing as planar molecules and form small PAH stacks. The PAH stacks then agglomerate into primary particles and ultimately, fractal aggregates. Changing the combustion conditions influences the soot particles formed. The Miller group has used two techniques, atom-pair minimization (APM) and Raman spectroscopy (RS) to study various effects as PAH molecules begin to stack and cluster. This work evaluates multiple APM methods, expanding APM to a greater range of molecules with non-circular shapes, heteroatoms, ions, and radicals. In addition, density functional theory is used to identify changes in the HOMO LUMO gap (HLG) of oxygen-containing PAH molecules (oxy-PAH). This work shows that oxygen containing moieties have little effect on the HLG of PAH molecules; only when the oxygen bonds disrupt the π cloud of the molecule does the HLG decrease. APM studies determined PAH cations were more strongly bound into PAH stacks than neutral molecules of the same size. RS of soot varies depending on the size of the PAH molecules in soot. However, the peak fitting routines used in RS vary widely between different research groups and goals. This work determined that the peak fitting routine significantly affects the interpretation of the RS results. Comparisons using RS of soots are only suitable if identical peak fitting routines are used. This was further applied to studying soots in artists’ materials. In East Asia, inks are traditionally made from two different soots, pine soot and lampblack. This work shows that RS discriminates between these two soots using peak dispersion. This work could not discriminate between the ink in four 18th-century books but showed that within one book different inks were used for discrete purposes.
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