Acoustic response of molecular adsorption and sound propagation in nanoporous materials

L Didier and A Sam and R Venegas and B Coasne, PHYSICAL REVIEW MATERIALS, 9, 056001 (2025).

DOI: 10.1103/PhysRevMaterials.9.056001

Nanoporous materials are expected to be game changers in the energy and environmental crisis, with central applications in fluid adsorption, separation, and catalysis. Yet, besides the abundant literature on fluids nanoconfined in their extreme porosity, the acoustic behavior of nanoporous solids subjected to fluid adsorption remains to be explored. In fact, while sound testing is a routine method used to probe fluid- filled porous materials- typically for geoscience applications, the molecular origin of the acoustic signature of fluid confinement at the nanoscale has yet to be unveiled. In particular, a molecular formalism describing how nanoconfinement and surface interactions affect longitudinal and transverse wave propagation is still missing. Here, using different fluids (CH4, CO2) in a prototypical nanoporous material (zeolite), we conduct a molecular simulation study to identify the acoustic modes (velocity and attenuation) from the system's dynamic structure factor. First, by varying the fluid nature and temperature as well as the fluid-solid interaction strength, we decipher the parameters that drive acoustic propagation and derive a simple kinematic model that predicts accurately the linear decay observed in the sound velocity upon increasing the fluid mass density (regardless of the fluid description level, e.g., flexible versus rigid, coarse-grained versus all-atom, etc.). Second, from the broadening of the acoustic modes in the dynamic structure factor, we show that attenuation increases with the fluid adsorbed amount and with the solid-fluid interaction strength due to phonon scattering at the fluid-solid interface. We establish that all data can be quantitatively rationalized by considering the change in the phonon lifetime through an additional relaxation time arising from the interaction between fluid molecules and the zeolite lattice. This simple stochastic model, which provides a robust framework to describe the acoustics of fluid-solid composites based on physical quantities amenable to simple experiments, offers new perspectives for the design of novel applications involving the acoustics of fluids in interaction with nanoporous solids.

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