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

摘要

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