ABSTRACT
II . Porosity of Silica Gels from Isobars 169 A. Temperature-programmed desorption 169
III. Porosity of Various Silicas by TG Method 182 A. Nuclear membranes as a model of porous solids 182 B. Porosity of silica gels with bonded alkyl grafts 186 c. Porosity of mixed silica samples 192 D. Pore size distribution of mesoporous silica molecular sieves 193 E. Porosity of thermally treated silica gels 196
IV. Conclusions 200
References 201
In contrast to crystalline particles of neutral silica, amorphous silica is characterized by its porosity, introducing a large surfaces area inside its particles. Amorphous silica gels exhibit a large diversity in structural properties depending on their preparation. The most popular and documented is the sol-gel method of preparation of silica gels. In this process Si(OH)4 molecules condense to form a siloxane network. The precursor solutions are usually soluble alkali-metal silicates
oralkoxysilanes.HydrolysisofthosecomponentsandcondensationofSi(OH)4 entitiesmaybecontrolledinvariousways,producingsilicawithvarietyofstructuralcharacteristics[1-10].Alotofworkhasbeendirectedtowardthemodelingof silicatextureduringgellinganddryingprocesses.Polycondensationoccursbetween oligomericsilicaspecies,resultinginlargerparticles(colloidal,aggregates,networks,sediments).Largeparticlesofsphericalshapewhichareformedduring condensationdeterminethespecificsurface,andtheirpackingdensitydetermines mainlythevolumeandporedimensions.Thevariationofphysicalproperties betweenvarioussilicasiscausedbychangingthepH,additionofvarioussubstancesasinorganicsaltsandsurfactantstosol-gelreactionmixture,temperature, agingtime,anddryingprocess.
