Adsorption in aerogel networks

Pore volume of aerogels is often seriously underestimated by nitrogen adsorption measurements. Undetected pore volume has sometimes been attributed to macropores (i.e., with diameters > 50 nm), but permeability measurements indicate that the volume of macropores in gels with solids contents exceeding 5-10 vol.% is below the percolation threshold (∼ 20 vol.%). The discrepancy may be attributable to the use of the conventional adsorption model, which assumes that the pores are cylindrical, to interpret adsorption isotherms on networks. The adsorbate/vapor interface in a sparse network can adopt a surface of zero curvature while much of the pore space remains empty. The extent of this phenomenon depends on the density of the network, not its pore size, so adsorption can be quite limited in a sparse network, even if the pores are small. A model is presented that accounts for the adsorption/desorption isotherm of a silica aerogel containing ∼ 13% solids, in which almost half of the pore volume is undetected, even though the largest pore diameter is less than 35 nm.