Ultrasonic—Sorption characterization of water-sorbing carbon xerogel

Adsorption of water vapor in nanoporous carbons is rather complex due to an interplay between their pore structure and surface chemistry. Deciphering the mechanism of adsorption requires the knowledge of the spatial distribution and the filling fraction of the adsorbed water. Sending an ultrasonic wave through a nanoporous sample allows to retrieve a wealth of information, such as properties and spatial distribution of fluids confined in the pores. Here, we studied the adsorption of vapor water on monolithic carbon xerogel, a porous material with a bimodal pore size distribution consisting of micropores(1 nm) and mesopores (8 nm). A novel adsorption-ultrasonicexperimental setup was employed to record the ultrasonic waveforms propagated through the water-sorbing xerogel sample while measuring its water sorption isotherm. Analysis of the elastic moduli evolution suggested that confined water shows nearly bulk-like properties in mesopores, while in micropores its modulus noticeably differs from that of bulk water. Furthermore, the observed increase in ultrasonic attenuation during micropore filling indicated the spatial heterogeneity of the water-filled pore space within the overall sample volume. This study demonstrates the utilization of nondestructive ultrasonic testing to probe both the fluid adsorption mechanism in a nanoporous medium and the properties of the adsorbed phase.