Soot with 1013 cm-3 high concentration and 25 Å radius of gyration as detected by small-angle X-ray scattering in a premixed ethylene-air flame at sooting threshold

SAXS experiments on a premixed ethylene-air flame burning at the so-called sooting threshold, i.e., a flame with Carbon-to-Oxygen molecular ratio between fuel C2H4 and air equal to [C]/[O] = 0.65 (?Air=25 cm3/min, ?C2H4=11.7 cm3/min) is the object of this work. Measurements are performed at synchrotron SOLEIL / beamline SWING under the approved proposal #20130463. The height above burner considered for this case study is z=21 mm (?res about 10ms). The background is made by considering an identical lean flame with [C]/[O]=0.19. The data processing approach consisting in subtracting to the signal S, the X-ray scattering from the air molecules surrounding the flame as background BCKG, is here revealed to be incorrect, owing to the fact that sometimes the S-BCKG evaluated as above is a negative quantity. This is caused by the much higher cool gas molecule number density at ambient conditions (300 K) with respect to the lower gas molecule density inside the flame at hot temperatures (about 1800-2100 K). Vice versa, by using a lean flame one obtains as BCKG the scattering contribute from the gas molecules alone in the absence of soot at about the identical premixed flame under test, which results in appreciating even the low signal from ultrafine incipient soot nanoparticles. The main results from the fit of SAXS data at larger q's are two: first, the demonstration of ultrafine nanoparticles, with gyration radius Rg=25±10 Å, as inferred by the Guinier regime fit, and overall size D=45±10 Å, as evaluated through Kratky plots; second, the fact that these entities are present in the premixed flame with a huge concentration, about N=1013 cm-3. Such nanoparticles with D=45 Å (Rg=25 Å), do coexist contemporarily to isolated 20 nm primaries and 160 nm nanoparticle aggregates, which are observed with a number concentrations 6 and 8 order of magnitudes smaller with respect to the one of ultrafine nanoparticles. Our SAXS results do accord very well previous findings on size of premixed flame soot nanoparticles reported both by Wersborg et al. (1973) from molecular beam sampling and TEM analysis and, with concern to the number concentration, the results by Minutolo et al. (1998) and Sgro et al. (2009).