Characterization of aerogels

Characterization of the pore size distribution in aerogels is complicated by their extreme compressibility. Techniques such as mercury porosimetry, thermoporometry, and even nitrogen adsorption apply capillary pressures on the aerogel network that can cause large volumetric compression, leading to false values for the pore volume and pore size. Since the bulk modulus of aerogels is known to exhibit a power-law dependence on density, it is possible to estimate the magnitude of the compression; however, it is not possible to calculate the true pore size distribution in cases where substantial compression has occurred. Even when the gel is rigid enough to withstand the capillary pressure during condensation of nitrogen, the standard interpretation of the adsorption isotherm can be seriously in error for networks with low density. When the spacing of the nodes of the network is large compared to the thickness of the links (i.e. polymers or chains of particles joining the nodes), then the adsorbate can adopt a surface of zero curvature while the pore space remains largely empty. Since the condensation stops when the negative curvature disappears, the isotherm would appear to indicate that the pores are large (>50 nm), even when all of the pores are actually in the mesopore range.