The Stöber process is a physical chemistry process for the generation of monodisperse particles of silica. The process was discovered in 1968 by Werner Stöber et al. building on earlier work by G. Kolbe in published in 1956. The topic has since been widely researched. Tetraethyl orthosilicate is added to an excess of water containing a low molar-mass alcohol such as ethanol and containing ammonia. The resulting solution is then stirred. The resulting silica particles have diameters between 50 and 2000 nanometers depending on type of silicate ester used, type of alcohol used and volume ratios. A particle size up to 1000 micrometres has been reported in a modified emulsion technique. The reactions taking place are hydrolysis of the silyl ether to a silanol followed by condensation reactions. The particles have been analysed by light scattering.
The process is believed to take place via a LaMer model (monomer addition) in which nucleation is a fast process, followed by a particle growth process without further nucleation. In an alternative model called controlled aggregation, the particles grow by aggregation of smaller particles. This model is supported by microgravity experiments and by SAXS analysis. Kinetics have been investigated with variation in pH.
Reported morphological variations are a PEG graft and core-shell morphologies based on cyclen, polyamines and polystyrene.
Microporous silica particles can be obtained through organic templating followed by calcination. Reported organic additives for this process are cetyltrimethylammonium bromide and glycerol
Carbon spheres
The synthesis of carbon-based spheres has been reported in a similar process with resorcinol and formaldehyde.
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