Extracting the pore size distribution of compliant materials from nitrogen adsorption

Adsorption of nitrogen at 77.4 K is a well-established technique for the characterization of mesopores. However, this method has been questioned in the case of compliant materials, as the capillary pressures upon sorption can be as large as several MPaʹs and thus high deformations are expected upon analysis. Simultaneous recording of nitrogen sorption data and linear length changes for silica aerogels, a class of highly porous, compliant materials, reveals a new type of isotherm, that is characteristic for compliant materials in general. Comparison of dilatation and sorption data shows that the bulk modulus of a compliant material can be calculated from its nitrogen sorption isotherm alone. Using this information together with a model that describes the relationship between macroscopic deformation and changes in pore size and volume, the isotherm can be corrected for the contraction of the sample during analysis. The corresponding pore size distribution is found to be in excellent agreement with results deduced from small angle X-ray scattering.