Effect of drying on properties of silica gel

Viscoelastic properties and permeability of a silica gel have been followed during the course of drying. The modulus and viscosity of the network rise by several orders of magnitude, and exhibit power-law dependence on the density of the gel. An aerogel was prepared from an undried gel, and its modulus was found by hydrostatic compression in a mercury porosimeter. Pore size distribution of the aerogel was determined by nitrogen sorption before and after compression. The bulk modulus of the aerogel was very close to that of the wet gel at low densities, but at higher densities the wet gels were more rigid that the compressed aerogels. This difference is attributed to aging of the wet gels in the pore liquid (water). The final shrinkage of the wet gels was greater than expected, based on the viscoelastic properties and the calculated capillary pressure, and the difference is attributed to accelerated viscoelastic relaxation of the network under high capillary stresses. The permeability decreases by almost four orders of magnitude as the gel contracts and pore size inferred from the permeability (r_w) decreases in direct proportion to the pore volume. The pore size measured by nitrogen desorption (r_BT) agrees closely with r_w, as expected on theoretical grounds. For compliant aerogels, the nitrogen desorption process causes substantial compression of the gel, because of the capillary pressure exerted by liquid nitrogen. In such cases, the true pore size and pore volume can be estimated, given knowledge of the elastic modulus of the network; the corrected value of r_BT agrees well with r_w. At high densities r_w was about 0.5 nm smaller than r_BT, and this discrepancy is probably caused by neglect of the existence of a relatively immobile layer of water of that thickness on the surface of the network.