Density of fluids confined in nanopores

Unlike macroscopic objects, any system of nanometric size shows characteristics that strongly depend on its size and geometric form.Here we focus on the influence of confinement on properties of fluids, when the confining volumes are of nanometric size.Using Grand Canonical Monte Carlo simulations we show that when a fluid is confined in nano-volume, delimited by non-attracting pore walls,its density is heterogeneous close to the pore wall, and, on average over the whole pore volume, it is smaller than the density of bulk fluid. Thisfluid heterogeneity, resulting from the nano-confinement, progressively weakens when the pore size increases, and totally disappears inside thepores larger than 5 nm. On the other side, in the limit of very small pores, the fluid density approaches the ideal gas value. This effect should bedistinguished from the well know heterogeneity of density of fluids adsorbed in nanopores, driven by the difference between the strength offluid-fluid and fluid-pore wall interactions, that varies with the distance from the pore wall.We show that the density distribution inside the non-attracting reservoir strongly depends the distance from the pore walls, and the pore size andshape. Such a behavior, although non-intuitive in macroscopic sample, has a simple physical explanation. The energy of a given gas moleculestrongly depends on the number of its nearest neighbors. In the nanopores this number decreases when the molecule is closer to the pore wall.Less intuitive is the observation that at higher pressures (p > 100 bar) the density of the gas close to the wall may be higher than in the middle ofthe pore, and even higher than the one of the bulk gas at the same thermodynamic conditions