Morphology and surface properties of fumed silicas

Several series of fumed silicas and mixed fumed oxides produced and treated under different conditions were studied in gaseous and liquid media using nitrogen and water adsorption–desorption, mass spectrometry, FTIR, NMR, thermally stimulated depolarization current (TSDC), photon correlation spectroscopy (PCS), zeta potential, potentiometric titration, and Auger electron spectroscopy methods. Aggregation of primary particles and adsorption capacity (V_p) decrease and hysteresis loops of nitrogen adsorption–desorption isotherms becomes shorter with decreasing specific surface area (S_BET). However, the shape of nitrogen adsorption–desorption isotherms can be assigned to the same type independent of S_BET value. The main maximum of pore size distribution (gaps between primary nonporous particles in aggregates and agglomerates) shifts toward larger pore size and its intensity decreases with decreasing S_BET value. The water adsorption increases with increasing S_BET value; however, the opposite effect is observed for the content of surface hydroxyls (in mmol/m²). Associative desorption of water (2(SiOH) → SiOSi + H₂O) depends on both the morphology and synthesis conditions of fumed silica. The silica dissolution rate increases with increasing S_BET and pH values. However, surface charge density and the modulus of zeta-potential increase with decreasing S_BET value. The PCS, ¹H NMR, and TSDC spectra demonstrate rearrangement of the fumed silica dispersion depending on the S_BET value and the silica concentration (C_SiO2) in the aqueous suspensions. A specific state of the dispersion is observed at the $C_{{\mathrm{SiO}}_2}$ values corresponding to the bulk density of the initial silica powder.