The transition from gas-phase species to condensed-phase particles plays an important role in many fields, including atmospheric, interstellar, combustion, wildfire, and climate-change science. These transitions are also crucial to a range of engineering applications, including nanoparticle synthesis, air purification, and film production by chemical vapor deposition. Measurements of particles as they are forming can provide insight into the fundamental chemistry and physics of these transitions and may also enable process monitoring and optimization for some applications.
“There are methods for measuring nanoparticle characteristics, but most require extracting the particles from the reaction mixture with a probe, which perturbs the chemistry we’re trying to study,” said Hope Michelsen, a professor at the University of Colorado Boulder and lead author of a paper describing a new methodology that overcomes this limitation.
Small-angle x-ray scattering (SAXS) enables measurements of particle characteristics without perturbing reaction conditions. However, a significant challenge to employing SAXS to measure particle formation in reacting aerosol flows involves distinguishing gas-phase reactants from newly formed particles.
Gas-phase precursors may be in the same size range as the newly formed particles, and the composition and temperature of the gas-phase reactants may be unknown and evolving. Particle concentrations may also initially be very low. As a result, SAXS signals from newly formed particles may be swamped by, and difficult to distinguish from, gas-phase scattering.
The new method exploits the temperature and composition dependence of the gas-phase SAXS signal. Temperature measurements using a laser-based spectroscopic technique called coherent anti-Stokes Raman spectroscopy (CARS) were combined with SAXS measurements at Advanced Light Source Beamline 7.3.3 to distinguish between the temperature-independent instrument background and the temperature-dependent gas-phase signal. The temperature measurements were also used to estimate the change in signal associated with gas-density changes.
The study demonstrated that, once these factors are taken into account, the gas-phase signal can be fit using an exponential functional form initially proposed by French physicist, André Guinier over 70 years ago. Furthermore, these fits can be performed, and the gas-phase component of the scattering can be isolated, without knowledge of the gas-phase composition, allowing the possibility of using SAXS to study particle formation as it occurs.
H.A. Michelsen, M.F. Campbell, I.C. Tran, K.O. Johansson, P.E. Schrader, R.P. Bambha, J.A. Hammons, E. Schaible, C. Zhu, and A. van Buuren, “Distinguishing Gas-Phase and Nanoparticle Contributions to Small-Angle X-ray Scattering in Reacting Aerosol Flows,” J. Phys. Chem. A 126, 3015 (2022), doi:10.1021/acs.jpca.2c00454.