Effects of aging and drying conditions on the structural and textural properties of silica gels

Sol–gel derived materials are widely used as porous matrices for preparing optical chemical sensors or biosensors. The porosity of the support matrix is a key variable that affects the sensitivity and response time of the sensors. The aim of this research is to study the effect of different aging and drying conditions on the structure and porosity of silica gels to obtain materials of tailored porosity. The gels were aged in ethanol or NH3(aq) (0.5 M and 2.0 M), and dried under atmospheric (xerogels) or supercritical conditions for ethanol (aerogels). Aging and drying conditions strongly affected the porous texture of silica gels. The surface area and micropore volume was higher in xerogels than in aerogels. Xerogel aged in ethanol was mainly microporous, while xerogels aged in ammonia were mesoporous due to cross-linking reactions in alkaline media. The maximum from the BJH distribution appeared at 4.0 nm for the xerogel aged in 0.5 M NH3(aq) and at 5.4 nm for the xerogel aged in 2 M NH3(aq). Aerogels were macroporous materials, and macropore volume constituted over 92% of the total pore volume, independently of the aging media. Esterification reactions inside the reactor promoted cross-linking, which resulted in a higher skeletal density and an increase in the absorbance of Si–O related IR bands. Cross-linking strengthens the gel network and reduces the amount of shrinkage under atmospheric conditions. SEM and TEM micrographs confirmed the textural properties of xero- and aerogels deduced by gas adsorption, Hg porosimetry and density measurements.