Adsorption under Nano-Confinement: Predicting Remarkably Higher Levels of Surface Coverage

Remarkable Nano-Confinement Entropic effects on equilibrated Adsorption in a system consisting of small numbers of molecules and adsorption-sites inside a nanoscale space are revealed for the first time by employing statistical-mechanical principles in the combined framework of the ideal-gas and lattice-gas models. The novel effect (termed NCEA) is reflected in computed adsorption isotherms exhibiting considerable positive deviations from the classical Langmuir isotherm of macroscopic systems, namely a system-size dependent significant increase of coverage (Fig.1). Likewise, nano-confined coverage variations with temperature at constant volume exhibit heightened adsorption levels. In particular, the NCEA is found to induce selectivity enhancement for H₂O vs. CO as well as for CO₂ vs. N₂, when the respective two molecular species are co-physisorbed on N-doped carbon materials. Similarly to nano-confinement effects on chemical-equilibrium reported by us previously [1,2], the NCEA origin is associated with distinct intrinsic variations in mixing-entropy and pressure. The present introduction of nano-confined adsorption phenomena indicates the emergence of a new subdiscipline of nanoscience, which can be pertinent also to nanotechnological applications such as gas storage and gas-mixture separation.

[1] "Nanochemical equilibrium involving a small number of molecules: A prediction of a distinct confinement effect"‏ M. Polak, L. Rubinovich - Nano Letters 2008.

[2] "The intrinsic role of nanoconfinement in chemical equilibrium: Evidence from DNA hybridization‏" L. Rubinovich, M. Polak - Nano Letters 2013.‏

Figure 1. Isotherms at 300 K for n molecules of H₂ physisorbed on n Ti-sites in doped graphene under nanoconfinement (varying volume V). Insets: illustration of the preadsorption and predesorption configurations and pressures for the smallest system.