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Testing the validity of small angle and adsorption-based characterization techniques by atomic-scale simulation

R.J.M. Pellenq and B. Coasne
Centre de Recherche en Matière Condensée et Nanosciences, CNRS, FR

Keywords:
simulation, small angle scattering, adsorption, nanoporous

Abstract:
This work aims at testing small angle scattering and adsorption/desorption-based characterization techniques routinely used for nanoporous material characterization (specific surface and pore size distribution measurements). For that purpose, we report Grand Canonical Monte Carlo results of Ar adsorption at 77 K in various silica nanopores including regular cylinders, realistic MCM41 models and a vycor-like matrix. Both morphological and topological disorders are shown to significantly affect the capillary condensation phenomenon. Simulation data are then used to test BET and BJH characterization methods. Except for the Vycor-like matrix, the BET surface assessed from Ar adsorption isotherm at 77 K always significantly overestimates the intrinsic surface of the pore (even for a planar surface). The disagreement between the BET surface and the exact geometrical surface obtained from chord-length distribution is found to increase as the confinement increases and/or the pore shape becomes asymmetrical (relative proportions of negative and positive curvature regions). We further show that the BJH-pore size always underestimates the true pore size. Finally, we demonstrate that the departure from the Porod behaviour in small angle scattering experiments on nanoporous materials corresponds to surface roughness at the scale of a few nanometers with no requirement for considering the fractal concept.

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