Preparation and characterization of silica aquasols

Abstract

Aquasols containing silica nanoparticles with diameters of 75 to 95nm were obtained directly by hydrolysis of 2wt.% tetraethoxysilane (TEOS) in water in the presence of a non-ionic surfactant. The reaction was catalyzed by hydrochloric acid, ammonia, or sodium hydroxide. The particle size, which mainly depends on the concentration of TEOS in water, was determined by dynamic light scattering (DLS). Whereas the catalysts have almost no influence on the particle size, they very strongly affect the morphology of the silica particles formed. The dried SiO2 particles obtained via the HCl-catalyzed reaction have film-forming properties and show no measurable BET surface area. SiO2 particles prepared with ammonia as catalyst form nanoporous films on glass, and the BET surface area of the freeze-dried particles is 540m2/g. Using sodium hydroxide as catalyst results in some agglomeration of uniform spherical particles with a BET surface area of 237m2/g. 29Si MAS NMR investigations of the freeze-dried particles provide information about the degree of condensation and the ratio of “free” hydroxyl groups. The silica aquasols described have a surprisingly high hydrophilizing effect on hydrophobic fibers (PP, PET). Silica nanoparticles of comparable diameters, prepared by the “Stöber method”, dispersed in alcohol do not show any hydrophilizing properties worth to mention.