Question

Silica nanoparticles are synthesized through the sol-gel method. The size of the nanoparticles is controlled by the amount of water. Why is the size of nanoparticles controlled by the amount of water?

Answer 1

For the synthesis of Silica nanoparticles through Sol-gel method,process involves hydrolysis and condensation of metal alkoxides (Si(OR)₄) such as tetraethylorthosilicate (TEOS, Si(OC₂H₅)₄) or inorganic salts such as sodium silicate (Na₂SiO₃) in the presence of mineral acid (e.g., HCl) or base (e.g., NH₃) as catalyst.The hydrolysis of TEOS molecules forms silanol groups. The condensation/polymerization between the silanol groups or between silanol groups and ethoxy groups creates siloxane bridges (Si–O–Si) that form entire silica structure.This condensation step decides the size of nuclei .So the removal of water is done by drying techniques which agglomerates the nuclei of nanoparticles formed.

Therefore,in aqueous system, the agglomeration behavior was caused by condensation reactions at the interparticle contacts during the drying process. In addition, Brownian motion, hydrodynamic effect, and capillary drag during the drying process can also contribute to the agglomeration behavior.Hence the water removed in the condensation step for the formation of Silicon alkoxide provides contact between the particles of the aqueous solution which induces an additional drag on the particle and favours them to agglomerate.So the size of nanoparticles formed is entirely dependent on the amountof water used for hydrolysis step and water removed by drying techniques in order to form dry nanoparticles.