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.

We study the intrinsic optical spectroscopy (UV-vis-NIR absorption, Raman and photoluminescence) signatures of single wall carbon nanotubes (SWNT) dispersed in degassed water without additives, so called ‘‘eau de nanotubes” (EdN). They are found to be very close to those of SWNT dispersed in aqueous suspensions stabilized with surfactants. Absorption peaks appear to be even slightly better resolved for EdN, suggesting sharper excitonic resonances, which is also supported by the Raman data. On the other hand, the photoluminescence signal is significantly weaker. These signatures suggest that SWNT are dispersed as individuals in degassed water, in a similar way single layer graphene was recently shown to be readily dispersable in degassed water [1-3].References[1]G. Bepete et al, Nat. Chem. 2016, DOI 10.1038/NCHEM2669[2]G. Bepete et al, J. Phys. Chem. C 2016, 120 (49), 28204–28214.[3]G. Bepte et al, Phys. Stat. Solidi 2016, 10 (12), 895-899.

We discuss the intrinsic optical spectroscopy (UV-vis-NIR absorption, Raman and photoluminescence)signatures of single layer graphene (SLG) and single wall carbon nanotubes (SWNT) dispersed indegassed water without additives, so called "eau de graphene" (EdN) [1-3] and ‘‘eau de nanotubes”(EdN) [4].The most characteristic signature of SLG in EdG is a narrow and symmetric 2D band with a widthdepending on processing conditions [2,3]. The position of the G and 2D bands indicate moderate biaxialcompressive strain and weak n doping. The intensity of the D band and the width of the G band arediscussed in terms of point-defect density and flake size. We show that point defects can be easily curedby preparing thin films from EdG and annealing at 800°C.The signatures of SWNT in EdN are found to be very close to those of SWNT dispersed in aqueoussuspensions stabilized with surfactants [4]. Absorption peaks appear to be even slightly better resolvedfor EdN, suggesting sharper excitonic resonances, which is also supported by the Raman data. Thesesignatures support that SWNT are dispersed as individuals in EdN, in a similar way SLG aredispersable in EdG [1-3].[1] G. Bepete et al, Nat. Chem. 2016, DOI 10.1038/NCHEM2669[2] G. Bepete et al, J. Phys. Chem. C 2016, 120 (49), 28204–28214.[3] G. Bepte et al, Phys. Stat. Solidi 2016, 10 (12), 895-899.[4] G. Bepete et al, to be published

The adequate choice of the interaction model is essential to reproduce qualitatively and estimate quantitatively the experimentally observed characteristics of materials or phenomena in computer simulations. Here we present the results of a benchmarking of density-functional theory calculations of rigid and flexible metal-organic frameworks (MOFs). The stability of these systems depends on the dispersion interactions. We compare the performance of two functionals, Perdew-Burke-Ernzerhof (PBE) and PBE designed for solids, with and without the dispersion corrections (D2 and TS), in reproducing the high-accuracy low-temperature X-ray and neutron diffraction data for both groups of MOFs. We focus our analysis on the key structural parameters: the lattice parameters, bond lengths, and angles. We show that the dispersion long range correction is essential to stabilize the structures and, in some cases, to converge the system to a geometry that is in line with the experimentally observed structure, especially for breathing MIL-53 structures or zeolitic imidazolate frameworks. We find that for all structures and all analyzed parameters, the D2-corrected PBE functional performs the best, except for bonds involving the metal ions; however, even for these bonds the difference between the experimentally observed and calculated lengths is small. Therefore, we recommend the use of the PBE-D2 functional in further numerical analyses of rigid and flexible nanoporous MOFs.