Discriminating the molecular pathways during uptake and release on nanoporous host systems

The use of optical techniques, such as interference microscopy, has enabled the observation of transient concentration profiles generated by intracrystalline transport diffusion in nanoporous host materials. In this way, the relevant transport parameters become directly accessible by experiment. We demonstrate that this novel type of information allows one to determine the fraction of molecules which, during molecular uptake (or release), are entering (or leaving) the host crystal through its different faces. By means of numerical calculations of a large amount of different uptake processes, the fraction of the respective fluxes is found to be reasonably calculated by means of the analytical solution for constant (i.e., mean) transport parameters. A more straightforward procedure, based on the assumption that the fraction of the flux is inversely proportional to the time constant calculated with the mean transport parameters, also yields reasonable results. It is shown that even for anisotropic mass transfer with strong concentration dependencies, the divergence from the actual results is generally less than 8%. In most cases such deviations would be below the limits of the accuracy of the determined transport parameters.