The influence of the wet gels processing on the structure and properties of silica xerogels

The post-synthesis processing of silica alcogels was investigated, with the aim of optimizing the production of monolithic silica xerogels by evaporative drying. The influence of the ageing, washing and drying conditions on the structure and properties of the final xerogels was studied. The initial chemical composition and the remaining processing parameters were pre-determined: the silica precursor (tetraethoxysilane, TEOS), the molar ratios TEOS:H2O:Solvent (1:4:4), the pre-hydrolysis conditions (HCl:TEOS molar ratio of 0.003, stirring at 333 K for 60 min), and the condensation conditions (333 K, with NH4OH:HCl molar ratio of 1). The ageing solvent was varied from the mother liquor to a mixture of TEOS/water/i-propanol with the initial sol composition, the ageing period ranged from 1 to 96 h, the rehearsed washing solvents were n-heptane, n-propanol and i-propanol, and the drying temperatures ranged from 303 to 353 K. The xerogels were characterized by their envelope density, total porosity, specific surface area, pore morphology and molecular structure. It was shown that the best washing solvent to obtain silica monoliths is i-propanol, and that the total porosity and pore morphology of the xerogels are determined by the ageing conditions and by the drying temperature.