On-Line Characterization of Morphology and Water Adsorption on Fumed Silica Nanoparticles

• Published in: Aerosol science and technology Vol. 45; no. 12; pp. 1441 - 1447
• Main Authors: Keskinen, Helmi, Romakkaniemi, Sami, Jaatinen, Antti, Miettinen, Pasi, Saukko, Erkka, Jorma, Joutsensaari, Mäkelä, Jyrki M., Virtanen, Annele, Smith, James N., Laaksonen, Ari
• Format: Journal Article
• Language: English
• Published: Colchester Taylor & Francis Group 01.12.2011; Taylor & Francis; Taylor & Francis Ltd
• Subjects: Adsorption; Aerosols; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Measurement; Nanoparticles; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Silica; Solid-liquid interface; Surface physical chemistry; Water; Relative humidity; Measurement; Binary compound; Wetting; Nanoparticle; Density; Silica; Particle mobility; Solid; Characterization; Particle; Cavity; Structure; Diameter; Agglomerate; Fitting; Suspension; Electron microscopy; Differential analyzer; Adsorption; Fractal dimension; Atmosphere; Morphology; Aerosols; Isotherm; Technique; Growth factor; Particle number
• ISSN: 0278-6826; 1521-7388
• DOI: 10.1080/02786826.2011.597459

The first wetting layer on solid nanoparticles has direct implications on the roles these particles play in industrial processes and technological applications as well as in the atmosphere. We present a technique for online measurements of the adsorption of the first few water layers onto insoluble aerosol nanoparticles. Atomized fumed silica nanoparticles were dispersed from aqueous suspension and their hygroscopic growth factors (HGF) and number of the adsorbed water layers at subsaturated conditions were measured using a nanometer hygroscopic tandem differential mobility analyzer (HTDMA). Particle morphology was characterized by electron microscopy and particle density was determined by mobility analysis. The HGFs of the size-selected particles at mobility diameters from 10 to 50 nm at 90% relative humidity (RH) varied from 1.05 to 1.24, corresponding to 2-6 layers of adsorbed water. The morphology of the generated fumed silica nanoparticles varied from spheres at 8-10 nm to agglomerates at larger diameters with effective density from 1.7 to 0.8 g/cm 3 and fractal dimension of 2.6. The smallest spheres and agglomerates had the highest HGFs. The smallest particles with diameters of 8 and 10 nm adsorbed two to three water layers in subsaturated conditions, which agreed well with the Frenkel, Halsey, and Hill (FHH) isotherm fitting. In comparison to the small spheres or large agglomerates, the compact agglomerate structure containing a few primary particles increased the number of adsorbed water layers by a factor of ∼1.5. This was probably caused by the capillary effect on the small cavities between the primary particles in the agglomerate.