A study on effects of size and structure on hygroscopicity of nanoparticles using a tandem differential mobility analyzer and TEM

A hygroscopicity tandem differential mobility analyzer (HTDMA) technique is used to determine size-effect of nanoparticles (NaCl, (NH₄)₂SO₄, KCl, NH₄NO₃, MgCl₂, CaCl₂) on their hygroscopic properties (deliquescence relative humidity (DRH) and hygroscopic growth factor (GF)). The HTDMA system uses a combination of two nano DMAs and two regular DMAs to measure particle size change in a wide dynamic particle size range. Particles are subsequently analyzed with a transmission electron microscopy to investigate the potential effect of particle structure or morphology on the hygroscopic properties. We found that structural properties of NaCl and (NH₄)₂SO₄ particles also play an important role in determination of the DRH and GF and are more pronounced at smaller diameters. Data show that the DRH of NaCl nanoparticles increased from ~75% up to ~83% RH at 8 nm and that their GF decreased with decreasing size. The extent to which the GF of NaCl nanoparticles decreased with decreasing size was greater than theoretically predicted with the Kelvin correction. The GF of furnace-generated NaCl nanoparticles that have pores and aggregate shape was found to be smaller than that of atomizer-generated particles that are close to perfectly cubic. For the case of atomizer-generated (NH₄)₂SO₄ nanoparticles, we observed no significant size-effect on their DRH, and the measured GF agreed well with predicted values using the Kelvin correction. For furnace-generated (NH₄)₂SO₄ nanoparticles, a gradual growth at moderate RH without noticeable deliquescence behavior occurred. Their TEM images showed that contrary to atomizer-generated (NH₄)₂SO₄ nanoparticles the furnace-generated (NH₄)₂SO₄ nanoparticles are not perfectly spherical and are often aggregates having pores and holes, which may favor holding residual water even in the dried condition. For atomizer-generated KCl, MgCl₂, and CaCl₂ nanoparticles, we observed no significant size-effects on their DRH and GF for the mobility size as small as 20 nm.