Nitrogen sorption in aerogels

Proper equilibration upon nitrogen sorption at 77.4 K is the key for a reliable characterization of porous materials with respect to their mesopores. In the past, aerogels were often insufficiently equilibrated, thus yielding misleading isotherms and highly erroneous pore size distributions (PSDs). To allow for the extraction of the mean mesopore size with an accuracy better than 10% total run times up to 150 h are required for 2 mm sized samples with porosities above 90%. The total run time needed to measure a well equilibrated isotherm is increasing with mesoporosity, sample size and compliancy of the sample. If highly porous aerogels are well equilibrated and their deformation due to capillary forces upon sorption is taken into account, both, adsorption and desorption branches of the isotherm yield the same mean pore size when calculated via the Kelvin equation for cylindrical pores.