(400n) Control of Particle Structure and Size Distribution By Humidity

Georgios A. Kelesidis, Florian M. Furrer, Eirini Goudeli, Maximilian L. Eggersdorfer, Karsten Wegner, Sotiris E. Pratsinis

Particle Technology Laboratory, Institute of Process Engineering,

Department of Mechanical & Process Engineering, ETH Zurich, Switzerland

Nanoparticles with compact structure are attractive for biomedical applications, such as drug delivery and theranostics, due to enhanced bioavailability and light absorption in the near infrared spectrum (Sotiriou et al., 2014). Flame-made nanoparticles, however, typically are ramified agglomerates of primary particles with open structure and mass-mobility exponent, D_fm, of about 2.17 (Sorensen, 2011). Such agglomerates can restructure in the presence of humidity to smaller and more compact entities, as has been observed for soot (Ma et al., 2013). Here, this effect of agglomerate restructuring is investigated for flame-made silica aiming at small and compact nanoparticles with well-defined size distributions.

Water vapor produced by an evaporator is introduced into a diluted aerosol stream sampled above the flame, resulting in controlled saturation ratios, S, of 1.15 to 1.55. Water initially condenses on the particle surface and then is largely removed, as the humid aerosol flow passes through a series of two diffusion dryers. Combined Differential Mobility Analyser and Aerosol Particle Mass measurements are employed to determine particle structure and anisotropy by the D_fm and the mass-mobility prefactor, k_fm, respectively.

Silica agglomerates restructure more with increasing S resulting in more spherical and compact structures, corresponding to larger D_fm and smaller k_fm, respectively. The asymptotic morphology obtained after complete agglomerate collapse for S = 1.55 is characterized by a D_fm of 3.02 ± 0.11 and a k_fm of 0.26 ± 0.06, in good agreement with the D_fm of 2.79 and km of 0.63 measured from soot agglomerates restructured after water condensation and evaporation (Ma et al., 2013). The packing density, θ, is gradually increasing up to 19 % as the agglomerates become slightly more compact for S = 1.35. Further increase of S up to 1.55 results in silica agglomerate restructuring into compact, spherical entities with average θ = 0.28 ± 0.01, invariant of mobility diameter. The average θ is lower compared to that of soot (Zangmeister et al., 2014) due to the larger polydispersity of silica primary particles.

The compact structures attained with increasing S have smaller size, decreasing the average d_m from 107.6 nm in ambient conditions to 91.4 nm for S = 1.55. Larger agglomerates restructure more than smaller ones, resulting in narrower size distributions with smaller mobility-based geometric standard deviation, σ_g,m. A gradual σ_g,m reduction is obtained for increasing S, reaching a maximum 22.4 % decrease for S = 1.55. Thus, particle processing with humidity is a promising method to finely tune their morphology and size distribution.

References:

Ma, X.F., Zangmeister, C.D., Gigault, J., Mulholland, G.W., Zachariah, M.R., (2013) J. Aerosol Sci. 66 209-219.

Sorensen, C.M. (2011) Aerosol Sci. Technol. 45, 765-779.

Sotiriou, G.A., Starsich, F., Dasargyri, A., Wurnig, M.C., Krumeich, F., Boss, A., Leroux, J.C., Pratsinis, S.E. (2014) Adv. Funct. Mater. 24, 2818-2827.

Zangmeister, C.D., Radney, J.G., Dockery, L.T., Young, J.T., Ma, X.F., You, R.A., Zachariah, M.R. (2014) PNAS 111, 9037-9041.