Adsorption-Induced Deformation of a Nanoporous Material: Influence of the Fluid-Adsorbent Interaction and Surface Freezing on the Pore-Load Modulus Measurement

TL;DR

This study uses molecular simulations to analyze how fluid-solid interactions and surface freezing influence the measurement of pore-load modulus in nanoporous materials, revealing conditions where surface freezing affects deformation readings.

Contribution

It demonstrates the validity of the pore-load protocol in simulations and highlights the impact of surface freezing on deformation measurements in nanoporous materials.

Findings

Pore-load protocol is valid when the fluid remains liquid.

Surface freezing significantly alters pore-load measurements.

Ordered fluid layers can form above freezing point due to surface interactions.

Abstract

Liquid adsorption in nanoporous materials induces their deformation due to strong capillary forces. The linear relationship between the liquid pressure and the solid strain (pore-load modulus) provides an experimental technique to determine the mechanical properties of nanosized solids. Puzzling experimental results have often been reported, leading to a severe reconsideration of the mechanical properties of the thin walls, the introduction of surface stresses, and the suggestion of a mutual influence of fluid adsorption and matrix deformation. This work presents a molecular simulation examination of the fundamentals of the pore-load measurement technique. The pore-load protocol is reproduced as in experiments by measuring the solid deformation in presence of the liquid ("numerical experiment"), and the result is compared to the expected mechanical response of the solid. Focusing on a single nanoplatelet mimicking silicon stiffness, we show that the pore-load protocol is valid as long as the liquid in the pores remains liquid. However, when an ordered layer can form at the solid surface, it significantly affects the pore-load measurement. It is shown that this may happen above the freezing point even for moderately strong fluid-solid interactions. This observation could help for the interpretation of experimental data, in particular in porous silicon, where the expected presence of atomically smooth surfaces could favor the formation of highly ordered fluid layers.