Preparation Of Silica Particles By Quaternary Ammonium Compounds Assisted St(?)ber Method

Silica（SiO₂）particles are one of the most used colloidal particles and the regulation of their size,morphology,and microstructure are vital important for their applications in the fields of catalysis,biomedicine,chromatography,photonics,semiconductor polishing,etc.In the traditional preparation method,i.e.,St（?）ber method,the Si O2 particles were generated by hydrolysis/condensation of silicate esters by using ammonia as a catalyst in an alcohol/water mixed solvent.When the ammonia concentration was elevated from 0.2 to 1.5 m M,size of the resulting Si O2 particles increased from20 to 500 nm.The as-prepared Si O2 particles are microporous in character and spherical-like in shape.In the St（?）ber reaction,under the catalysis of ammonia,firstly the silicate esters undergo hydrolysis to form the silanol monomers,which undergo condensation（dehydration or dealcoholization）to form the oligomers,and then expansion of the siloxane networks in the oligomers and aggregation of the oligomers result in the formation of “nuclei”.Finally,the “nuclei” grow into the Si O2 particles according to the “monomer addition” or “aggregation-only” fashion.In previous study,our group found that when tetramylammonium hydroxide（TMAOH）was used as catalyst instead of ammonia in the St（?）ber reaction,on the one hand,the anionic group（OH-）dissociated from TMAOH may affect the hydrolysis/condensation balance of silicate esters,and on the other hand,the cationic group（TMA+）dissociated may affect the nucleation/growth balance via its interaction with the negatively charged siloxane network.Thus,it is assumed that the introduction of quaternary ammonium compounds into the St（?）ber reaction may provide us new opportunities in regulating size,morphology,microstructures and surface properties of Si O2 particles.Firstly,the effect of tetrabutylammonium hydroxide（TBAOH）,which is introduced into the St（?）ber reaction as a co-catalyst of ammonia,on hydrolysis/condensation rate of silicate ester and nucleation/growth process of Si O2 particles under the ammonia concentration low were investigated.It was identified that the anionic OH-dissociated from TBAOH was effective in promoting the hydrolysis rate of the silicate ester.Meanwhile,the cationic TBA+ dissociated from TBAOH is effective in lowering stability of the primary silica particles via neutralizing the negative charge on the siloxane surface,making growth of the silica particles well follow the aggregation-only mode along the whole reaction.With the increased amount of TBAOH added,the resulting Si O2 particles evolved spherical-like into fractal-like in shape.Compared with the spherical-like particles,the fractal ones bear higher isoelectric point and larger specific surface area,and thus show improved adsorption capacity towards bovine serum albumin.Secondly,the effect of tetrabutylammonium bromide（TBABr）on the hydrolysis/condensation of silicate ester and the nucleation/growth of Si O2 particles in the St（?）ber method were studied.It was identified that condensation rate of the silicate ester was suppressed to some extent due to the direct electrostatic interaction among the TBA+ dissociated from TBABr and the silicon hydroxyl group（-Si OH）in the siloxane networks.As a result,the introduction of TBABr resulted in the delayed nucleation and formation of Si O2 particles larger in size.When the concentration of TBABr was elevated from 0 m M to 20.0 m M,size of the resulting Si O2 particles,with polydispersity lower than 5%,increased from 0.39 μm to 1.08 μm.This TBABr assisted St（?）ber method represents a new approach for the preparation of high quality submicrometer Si O2 particles.When tetratoctyl ammonium bromide（TOABr）,a quaternary ammonium compound bearing longer carbon chains than TBABr,used to prepare the submicrometer Si O2 particles,surface of the resulting Si O2 particles became more hydrophobic in character due to stronger adsorption capacity of TOABr on the particle surface.By changing the type and amount of quaternary ammonium salt used in St（?）ber method,not only sizes of the Si O2 particles but also their surface properties could be well regulated.Finally,the effect of cetyltrimethylammonium bromide（CTAB）,a quaternary ammonium compound bearing long carbon chain,on the St（?）ber reaction was explored.It was identified,similar to the quaternary ammonium salt with shorter carbon chain,CTAB can also delay the nucleation/growth process of Si O2 particles via its direct electrostatic interaction with the siloxane network,resulting in the formation of Si O2 particles with size up to 2.0 μm.More interestingly,CTAB can act as a template to induce the formation of micrometer Si O2 particles with unique ultra-microporous structure（about 1.9 nm）under appropriate alcohol/water ratio around its critical micellar concentration.This work provides new insight into the preparation of Si O2 particles with different microstructures based on the St（?）ber method.