Adsorption under nanoconfinement: a theoretical–computational study revealing significant enhancement beyond the Langmuirian levels

Abstract

The principal goal of this work is to predict characteristics unique to equilibrated adsorption of a small number of molecules on atomic sites located inside a closed nanoscale space. Compared to the thermodynamic limit of macroscopic systems, significantly enlarged adsorbate coverage under nanoconfinement constitutes a major finding of the modeling. Concomitantly, nanoconfined adsorbates are expected to exhibit extra thermal stability against desorption. These effects on adsorption are explored using canonical partition-functions as well as an original relationship between coverage variations and the Langmuir constant, both in the frameworks of the ideal gas and lattice-gas models. With reported DFT adsorption-energies as input, adsorption isotherms are derived numerically for H₂ on Ti-doped graphene-like nanostructures. Remarkable deviations from the classical Langmuir isotherm are predicted for the first time, namely, system-size dependent enhanced H₂ adsorbate coverage. The effects are computed also for CO₂ inside MOF single-molecule traps, including their relationships to adsorption-energy, specific-heat and to coverage fluctuations. According to preliminary modeling, nanoconfinement effects are anticipated also for adsorption in nanopores undergoing molecular exchange with the external environment, and for impurity segregation in nanoparticle and nanocrystalline solids. The entropic origin of the nanoconfinement effect on equilibrium adsorption (NCEEA) is demonstrated analogously to the nanoconfinement effect on equilibrated chemical reactions studied by us previously. Besides unraveling some basic theoretical issues in physical nanochemistry, this study is expected to be pertinent to nanotechnological applications, such as gas storage and separation in nanoporous materials and other solid adsorbents.