Evidence of anisotropic self-diffusion of guest molecules in nanoporous materials of MCM-41 type

The pore architecture of MCM-41 was investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR) self-diffusion measurements of adsorbed water. Over diffusion length scales which exceed the radius of the one-dimensional channels of the MCM-41 by 2 orders of magnitude but which are smaller than the size of the individual MCM-41 particles, the molecular propagation of the guest molecules was found to be highly anisotropic. The PFG NMR experimental data are best represented by an axisymmetrical diffusion tensor rather than by pure one-dimensional diffusion along the channels. This suggests that the pore walls of the one-dimensional channels are permeable for water or that over the probed diffusion length the channels are disordered and bent. Both interpretations provide information on the pore structure of the nanoporous material which is not available from X-ray diffraction data.