Mass transfer in nanoporous materials

AuszugThe mass transfer, i. e. diffusion and surface permeation, of guest molecules in nanoporous materials is studied by analysing intracrystalline concentration profiles, which evolve during uptake and release experiments recorded by interference microscopy. Several ways were investigated to determine the transport parameters, i. e. the diffusivity and the surface permeability, from one-dimensional concentration profiles directly, namely without further assumption on the mass transfer. The three-dimensional mass transfer, where the recorded profiles are nontrivial integrals of the concentration, is investigated by introducing an algorithm which recalculates the intracrystalline concentration profiles. Moreover, analytical techniques are derived which allow a straightforward determination of the limiting transport resistance and an estimation of the molecular pathways, i. e. the fraction of molecules entering/leaving through a particular host face. Furthermore, IR micro-imaging was applied for the first time to record intracrystalline concentration profiles. These unprecedented profiles enable the calculation of the self-exchange transport parameters in a single nanoporous crystal. It is apparent that these experimental techniques combined with the derived analytical methods offer unique access to information that cannot be easily obtained by other techniques and, therefore, has the potential to lead to further important advances in our understanding of the mass transfer of guest molecules within the pores of nanoporous materials.