Structure and permeability of gels

The textures of silica gels made by two-step acid/base and acid/acid catalysis of TEOS have been examined by thermoporometry (TPM) and NMR, and their permeabilities (D) have been measured by a thermal expansion technique. Using the pore size distribution given by TPM, which includes a large proportion of macropores (30 nm), calculated values of D are seriously overestimated. We conclude that, consistent with a theoretical prediction, compliant materials such as gels undergo contraction during freezing in the calorimeter, so that most of the macropore volume reported by TPM is actually extracted from mesopores. The mesopore radius reported by TPM is underestimated by only 20%, even if 50% of the pore liquid is drained during crystallization, assuming that the change in pore radius is related to the cube root of the volume change. NMR does not distinguish macropores, because of diffusional averaging, but provides an apparent distribution that permits an accurate estimate of the permeability.