How dense is the gas adsorbed in nanopores?

Abstract

Abstract Unlike macroscopic objects, any system of nanometric size shows characteristics that strongly depend on its size and geometric form. It is mainly because the major part of atoms (or molecules) of nano-object is located at its surface, and their cohesive energy is smaller than for the atoms in the bulk. Here we show that when a fluid is confined in nano-volume, delimited by non-attractive pore walls, its density is heterogeneous, in particular close to the pore wall, and, on average, smaller than the density of bulk fluid. This effect progressively weakens when the pore size increases, and totally disappears for pores larger than 5 nm. The reported observation has non-trivial influence on evaluation of total and excess amount of fluid adsorbed in nanopores, as these quantities are traditionally calculated assuming the known – and homogeneous –density of the bulk fluid. Additionally, we propose a new method of the estimations of the accessible pore volume, based on the analysis of the density of confined fluid. The right estimation of both: pore volume and gas density is essential for quantitative interpretation of experimental adsorption isotherms: evaluation of pore size distribution and of the adsorbed amount. Although we analyze these problems taking an example of hydrogen at 77 K, our conclusions are general and apply to any fluid confined in nanopores.