Nitrogen adsorption in compliant materials

The adsorption of nitrogen at 77.4 K. a standard tool for the characterizat ion of porous materials, has been carefully reexamined for compliant mesopo rous materials. Simultaneous measurements of nitrogen sorption and linear l ength change of silica aerogels with bulk moduli between 1.8 and 16 MPa rev eal a characteristic share of the isotherm that is due to contraction and r eexpansion 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 adsorpt ion until the sample is fully relaxed. This is the first time that the reex pansion close to saturation has been observed. The lack of this feature in previous experiments is due to insufficient equilibration of the sample upo n adsorption. Analysis of the isotherm indicates that, even for compliant s amples, a breakthrough radius can be extracted that is in excellent agreeme nt with values derived from other methods. In addition, the isotherm of com pliant samples allows for a calculation of bulk modulus. (C) 2000 Elsevier Science B.V. All rights reserved.