Compressibility of Nitrogen in Nanopores
New Jersey Institute of Technology
Nitrogen adsorption is one of the main characterization techniques for nanoporous materials. An experimental adsorption isotherm provides information about the surface area and pore size distribution (PSD) for a sample. In this presentation I will show that additional insight into the pore sizes can be gained when in the course of nitrogen adsorption experiment, the speed of sound propagation through a sample is measured.
I will start with the analysis of the literature data on ultrasound propagation through a nanoporous Vycor glass sample during nitrogen adsorption experiment , to show that such data provide the change of the longitudinal and shear moduli of the sample as a function of relative vapor pressure. It appears that the shear modulus of the sample does not change upon filling the pores, evidencing that adsorbed nitrogen at 77 K has zero shear modulus, similarly to a bulk liquid. The longitudinal modulus of the sample behaves differently: it changes abruptly at the capillary condensation and keeps gradually increasing thereafter. Then I will present the predictions for the longitudinal modulus of the nitrogen-saturated Vycor glass based on the Gassmann equation , where the compressibility of adsorbed nitrogen is calculated from Monte Carlo molecular simulation . Such theoretical predictions match the longitudinal modulus derived from the experimental data. Additionally, I will show the results of molecular simulations to model nitrogen adsorbed in silica pores of sizes ranging from 2 to 8 nm, suggesting that the isothermal elastic modulus of adsorbed nitrogen depends linearly on the inverse pore size. This dependence, along with the proposed recipe for probing the modulus of adsorbed nitrogen, sets up the grounds for extracting additional information about the porous samples, when the nitrogen adsorption is combined with ultrasonic experiments. K. L. Warner, J. R. Beamish, J. Appl. Phys., 1988, 63(9), 4372.  G. Y. Gor, B. Gurevich, Geophys. Res. Lett., 2018, 45, 146-155.  M. A. Maximov, G. Y. Gor, Langmuir, 2018, 34, 15650.