Modeling elastic properties of Vycor glass saturated with liquid and solid adsorbates

A combination of ultrasonic experiments with gas adsorption is a promising tool for improved characterization of nanoporous materials. The use of ultrasound requires understanding of the effects of adsorbates on the elastic properties of nanoporous medium. This issue is not trivial, because nanostructured materials, as well as nanoconfined matter, may exhibit physical properties that differ substantially from the properties of "normal" bulk materials. In this paper, we investigate the change of elastic properties of Vycor glass filled with adsorbed liquid and solid argon within the context of elasticity and compare the modeling results with the ultrasonic measurements. The modeling requires the knowledge of solid moduli of Vycor glass and the pore geometry, which cannot be measured directly. Instead, we estimate these parameters from the dry moduli using so-called Differential Effective Medium (DEM) theory, in which the pores are assumed to be of spheroidal shape characterized by a single aspect ratio. Predictions of the Gassmann equation give an excellent fit to the measured elastic moduli of Vycor glass completely filled with liquid argon at temperature 80 K. Estimates of the DEM show a reasonable agreement with ultrasonic measurements on the elastic moduli of Vycor glass fully saturated with solid argon at 74 K in shear modulus but a significant overestimate in bulk modulus. This might be due to the effects of the confinement on the moduli of argon in nanopores. Although the validation and generalization of this conclusion requires further laboratory experiments for a number of well characterized solid–fluid systems, our finding shed light on the understanding of elastic properties of nanoporous materials mixed with adsorbates in various phases. These results provide steps toward development of methods for ultrasonic characterization of confined fluid and solid phases.