Shrinkage of silica gels aged in TEOS

A model has been developed for predicting the shrinkage that occurs during drying, given knowledge of the initial pore size and modulus of the gel. The model is in good agreement with experimental results for silica gels given a variety of aging treatments. The initial pore size (used to predict the capillary pressure that drives shrinkage) can be found by making an aerogel and performing nitrogen desorption, or by calculating the effective pore size from the measured permeability; it is shown that those methods give similar results. Thus the drying shrinkage can be predicted using data obtained from the wet gel (viz. permeability and modulus). At the moment that the liquid/vapor meniscus enters a pore, the walls will be covered with a film of adsorbed liquid, which reduces the radius of the meniscus below the radius of the pore, and thereby increases the capillary pressure. The film thickness for the solvents used is not known so one assumes a thickness of 1 nm. If the presence of the layer is ignored, the theory underestimates the drying shrinkage.