Ultrasound Propagation in Water-Sorbing Carbon Xerogel

Adsorption of water vapor in nanoporous carbons is rather complex due to an interplay between the pore structure and surface chemistry of these materials. Deciphering the mechanism of adsorption requires the knowledge of the spatial distribution and the filling fraction of the adsorbed water. Characteristics of ultrasonic wave propagation through a nanoporous sample provides a wealth of information such as properties of fluids confined in the pores and spatial distribution of adsorbate in the pores. Here we studied water vapor adsorption on carbon xerogel, with a bimodal pore size distribution consisting of micropores (1 nm) and mesopores (8 nm) and overall porosity 68%. The relative humidity was increased in steps and the amount of water adsorbed by the sample was measured gravimetrically at each step, producing a type V isotherm, characteristic of water adsorption to weakly interacting carbon nanopores. Concurrently, we recorded the waveforms for longitudinal and transverse ultrasonic waves that passed through the sample at each saturation step. These measurements provided the wave amplitudes and speeds as a function of relative humidity, and these data were used to calculate the elastic moduli and ultrasonic attenuation of the xerogel-water composite. 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, consistent with theoretical predictions. 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. Overall, this study demonstrates the successful utilization of nondestructive ultrasonic testing to probe the fluid adsorption mechanism in a nanoporous medium and the properties of adsorbed phase.