Abstract
It is well know that the properties of nano-objects differ from those of their macroscopic analogs. 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-volume are located at the object surface and their cohesive energy is smaller than for the atoms in the bulk. As a consequence, the density of the nanoobjects is not homogeneous, and may decrease close to the object boundary. Here we show that when a fluid is confined in nano-volume, delimited by non-interacting pore walls, its density is on average smaller than the bulk density. The heterogeneous distribution of fluid density results from the nano-confinement, and progressively weakens when the pore size increases: it disappears for pores larger than 5 nm. On the other side, the fluid density approaches the ideal gas values in the limit of very small pores. This effect should be distinguished from the well know heterogeneity of density of fluids adsorbed in nanopores, driven by the difference between the strength of fluid-fluid and fluid-pore wall interactions. The reported observation has non-trivial influence on evaluation of excess/total adsorption in nanopores, as these two quantities are calculated assuming the known – and homogeneous – bulk density of gas in the pore. Additionally, the gas density in the pores depends on the definition of the pore volume which is neither straightforward nor unique. We analyze this phenomenon on an example of five gases: H2, CH4, the two intensively studied energy vectors, and N2, Ar, and Kr, commonly used for characterization of porous structures. Two model pore geometries with not adsorbing soft walls are analyzed (slit-shaped and cylindrical). For H2, the distributions of densities of gas confined in adsorbing and not adsorbing pores are compared and commented.