Nanoparticle growth in thermally diffusive sublimation-condensation systems with low vapor pressure solids

Engineered, condensational growth of aerosol particles is a well-established technique incorporated into particle detection and sampling systems. It is typically accomplished through passage of particles through controlled temperature, supersaturated systems with moderate-to-high vapor pressure compounds, which are liquid at room temperature. However, there are instances where it is advantageous to intentionally grow particles with room temperature, low vapor pressure (<10⁻³ Pa) solid compounds, either as part of particle collection or ionization systems. Sublimation-condensation, though demonstrated previously, is much less examined, particularly for nanoparticle growth. In this study, we examine a thermally diffusive, laminar flow sublimation-condensation system, wherein solid organic powders (all ionization facilitating matrices) are sublimated at moderate temperature (70–130 °C) and subsequent cooling results in vapor supersaturation, promoting growth of particles. The extent of particle growth was characterized via tandem differential mobility analyzer (TDMA) measurements, with 30 nm–200 nm particles. The grown diameter distributions were monitored for three different low vapor pressure organic solids at variable sublimator temperature settings. We found optimized operating conditions enabled particle growth above diameters of 500 nm. Growth experiments with 7 nm protein (transferrin) particles using ferulic acid as the working solid additionally yielded grown particles in excess of 200 nm in diameter. However, we also found heterogeneous growth occurred simultaneously with homogeneous nucleation of the working solid, which could not be avoided if growth to diameters >500 nm was desired. Homogeneously nucleated particles were broadly distributed in size, and generally smaller than the test particles grown by condensation, but still with diameters in excess of 100 nm. While SEM images of grown particles revealed smooth surfaces not indicative of agglomeration with homogeneously nucleated particles, we cannot rule out the contribution to growth of homogeneously nucleated particle coagulation with sampled particles in the growth system. By comparing to numerical simulations, we use TDMA measurements to estimate effective vapor pressures for the tested sublimated solids, assuming growth occurred solely by vapor condensation. Simulations suggest that saturation ratios in excess of 10² are needed for nanoparticle growth.