A Surprising Drop of the Diffusivities of Benzene in a Mesoporous Material of Type MCM-41 at Medium Pore Filling Factors

Pulsed Field Gradient (PFG) NMR is applied to study molecular diffusion of benzene in a commercial sample of MCM-41. Over a very small concentration range at medium pore filling factors the diffusivity is found to sharply drop by up to one order of magnitude, while it remains essentially constant over the total remaining range from vanishing concentrations up to over-saturation. Similarly extreme deviations from monotonous concentration patterns have so far not been described in the literature. The observed effect may be rationalised as a consequence of the onset of capillary condensation in the transport pores in the hyper-structure of the MCM-41 particles under study.