(631c) Pore Size Estimation from Ultrasonic Measurements during Nitrogen Adsorption Experiment

Max A. Maximov¹ and Gennady Gor¹

¹ Otto H. York Department of Chemical and Materials Engineering, New Jersey Institute of Technology,
323 Dr Martin Luther King Jr Blvd, Newark, NJ 07029

Nitrogen adsorption is one of the main techniques for characterization of micro- and mesoporous materials This method can give such information as the BET surface area and pore size distribution. The traditional approaches are based on analysis of the adsorption isotherm obtained by volumetric or gravimetric methods [1]. However, Warner and Beamish showed that the adsorption isotherms can be extracted from measurements of sound wave propagation through the sample during an adsorption experiment [2].

We show that from the analysis of data from the ultrasonic experiments one can extract information about the shear and longitudinal moduli of the fluid adsorbed in the pores. The resulting shear modulus of the confined liquid nitrogen is zero, similarly to a bulk fluid. However, the longitudinal modulus changes abruptly at the point of capillary condensation, and gradually changes after that. To mimic this behavior, we performed grand canonical Monte Carlo simulations of this system and demonstrated a good match with the experimental data [3]. We also performed simulations for the pore sizes from 2 to 8 nm. The model illustrates that the dependence of the elastic modulus at saturation on the inverse pore size is linear so we can estimate the mean pore size. In addition to that, we suggest that the linear dependence of the elastic modulus on the solvation pressure in the pores can be used for the prediction of the pore size distribution.

Overall, the ultrasonic measurements provide a good piece of complementary information to the traditional adsorption methods, namely the shear and longitudinal modulus as a function of vapor pressure, and the data have a nice agreement with simulations. We also demonstrated that our work sets the ground for the pore size distribution prediction from the isotherms obtained using the ultrasonic method.

References:

1. J. Rouquerol, F. Rouquerol, P. Llewellyn, G. Maurin, K. S. Sing, Adsorption by powders and porous solids: principles, methodology and applications, Academic Press, 2013.
2. K. L. Warner, J. R. Beamish, Ultrasonic measurement of the surface area of porous materials, J. Appl. Phys. 63 (9) (1988) p. 4372–4376.
3. M. A. Maximov, G. Y. Gor, Molecular Simulations Shed Light on Potential Uses of Ultrasound in Nitrogen Adsorption Experiments, Langmuir, 34 (51), 2018, p. 15650-15657