Stober silica colloids, monodisperse

Segmented gas-liquid (Taylor) flow was used for particle synthesis within the liquid slugs. Tetraethylorthosilicate in ethanol was hydrolyzed by a solution of ammonia, water and ethanol (Stober synthesis) [329]. The resulting silicic acid monomer Si (OH)4 is then converted by polycondensation to colloidal monodisperse silica nanoparticles. These particles have industrial application, for example, in pigments, catalysts, sensors, health care, antireflective coatings and chromatography. 

Stober, W., Fink, A., and Bohn, E.J. (1968) Controlled growth of monodisperse silica spheres in the micron size range. Colloid Interface Sci. 26, 62-69. 

Dense silica nanoparticles have received considerable attention since monodisperse colloidal silica spheres, obtained liom aimnoniacal TEOS solution, were reported in the late 1960s by Stober et aL [1]. These dense particles, more often being monodispersed with controlled particle size, a well-defined morphology, and a surface with silanol groups by which they could be functionalized, are used for a variety of commercial applications including colmants, fillers and pigments. 

Sol-gel synthesis is the process of formation of porous, three-dimensional, integrated solid network (gel) of either discrete particles or network potymers from the conversion of monomers into stable suspension of colloidal solid particles or pol miers (sol) in a continuous liquid phase. Most popular precursors for the synthesis of colloids are metal alkoxides and alkoxysilanes. Tetramethoxysilane (TMOS) and Tetraethoxysilane (TEOS) are commonly used alkojq silanes, which form silica gel. The remarkable property of these silanes is that they readity react with water in the presence of shorter chain alcohol such as ethanol and ammonia to form monodispersed silica particles [7]. The size of silica particles formed is between 50 and 200 nm and depends on the silica ester used, type of alcohol, and molar ratios of water and alkoxysilane [32]. In this process, alcohol acts as a homogenizing solvent between alkoxides and water as both are immiscible but can be easily dissolved in alcohol. With the presence of this homogenizing agent, hydrolysis can be facilitated [33] due to the complete miscibility. However, aluminates borates and titanates often mixed with TEOS or TMOS are commonly used in sol-gel process. The hydrolysis of alkoxysilane proceeds according to Stober s process (Fig. 18.6). 

Stober W, Fink A, Bohn E Controlled growth of monodisperse silica particles in the micron size range, J Colloid Interface Sd 26 62-69, 1968. 

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