Access to Reliable Pore Size Distributions of Compliant Mesoporous Materials from N2 Adsorption Analysis

Abstract

N2 adsorption analysis is an established method for the analysis of micro- and mesoporosity of open-porous solids. Thus, all researchers developing open-porous materials such as aerogels apply this commercially available technique. However, due to their extraordinary properties, i.e., huge specific mesopore volumes combined with low mechanical stiffness, aerogels question this method. A prerequisite for N2 adsorption analysis is the determination of a well-equilibrated isotherm. This study investigates experimental parameters and discusses their impact on the resulting isotherms. The findings yield recommendations for the reliable determination of adsorption and desorption isotherms of aerogels. In the case of aerogels, a second factor affecting the shape of the isotherms comes into play, i.e., the superposition of the isotherm and deformation effects resulting from capillary pressure upon mesopore filling. This yields erroneous bimodal pore size distributions (PSD) with their mean values differing from the true ones by up to a factor of 2, thus strongly compromising the strategies for targeted materials synthesis for a given application. This paper uses silica aerogel as a model system and provides, for the first time, an approach for the correction of deformation artifacts upon N2 adsorption analysis from the recorded isotherm only, and thus allows reliable determination of PSDs for compliant mesoporous materials.