Hydration Forces Alter Elasticity of Nanoporous Solids

Sorption of water in nanoporous and nanostructured materials is ubiquitous. Sorption takes place due to the favorable interactions between the water molecules and the surface of the adsorbent. These strong intermolecular forces not only drive adsorption but also induce mechanical effects such as adsorption-induced deformation [1]. For example, when water vapor adsorbs in nanoporous Vycor glass, the sample expands by about 0.1%. Given the bulk modulus of porous glass (~8 GPa), this small strain corresponds to internal stresses of several hundred atmospheres -- an expression of hydration forces acting within the nanopores.

By combining vapor sorption experiments with ultrasonic measurements, we probe how water adsorption affects the elastic properties of the solid [2]. Although elastic moduli are generally treated as constants for a given solid material, our experiments reveal that adsorption of a monolayer of water increases the bulk and shear moduli of Vycor by nearly 3%. Such stiffening implies that the solid framework experiences an effective compressive load of hundreds of atmospheres, providing another experimental manifestation of hydration forces at the nanoscale.

[1] G. Y. Gor et al., Appl. Phys. Rev., 4, 011303 (2017).

[2] J. Ogbebor et al., Phys. Rev. E, 108, 024802 (2023).