Tracing pore connectivity and architecture in nanostructured silica SBA-15

We record the probability distribution of molecular displacements (i.e., the distribution of the lengths of the diffusion paths) of guest molecules in nanostructured SBA-15 silica by means of pulsed field gradient (PFG) NMR. Two different morphologies of SBA-15 were studied, namely isolated fibers and bundles. The guest molecules (nitrobenzene) are applied in excess so that, by measurement below the bulk freezing point but above the freezing point in the nanopores, the space between the host particles becomes inaccessible for notable mass transfer, and the range of molecular displacements, as observed in the experiments, is determined by the nanopores and the shape of the individual particles. The measurements clearly reflect the 1-D channel architecture of SBA-15. Over the displacements traced in the experiment (5 μm) there is a distinct difference in the time dependence of the coefficients characterizing diffusion anisotropy. Diffusion measurements with the isolated fibers (embedded in the frozen guest phase surrounding the host particles) reveal increasing transport resistance with increasing observation time, acting both parallel and perpendicular to the channel direction. By contrast, in the bundles diffusivity in the channel direction remains constant while the rate of displacements perpendicular to this direction increases. These dependencies may be rationalized by the different sample morphologies.