Nitrogen adsorption in compliant materials

The adsorption of nitrogen at 77.4 K, a standard tool for the characterization of porous materials, has been carefully reexamined for compliant mesoporous materials. Simultaneous measurements of nitrogen sorption and linear length change of silica aerogels with bulk moduli between 1.8 and 16 MPa reveal a characteristic shape of the isotherm that is due to contraction and reexpansion of the sample in different phases of adsorption and desorption. The capillary pressure upon filling and emptying of mesopores results in a volumetric contraction of up to 50%. Once the mesopores in the bulk of the contracted porous backbone are completely filled, the capillary pressure is released and the material slowly reexpands, accompanied by further adsorption until the sample is fully relaxed. This is the first time that the reexpansion close to saturation has been observed. The lack of this feature in previous experiments is due to insufficient equilibration of the sample upon adsorption. Analysis of the isotherm indicates that, even for compliant samples, a breakthrough radius can be extracted that is in excellent agreement with values derived from other methods. In addition, the isotherm of compliant samples allows for a calculation of bulk modulus.