Diffusion in Mesoporous Materials During Melting and Freezing

1. Introduction Porous solids are in the focus of scientific research due to their importance in various applications such as catalysis, mass separation, pharmaceutics, and chromatography. To date, large variety of porous materials containing both micro-and mesopores have been synthesized. Such a rapid growth of various types of porous materials consisting of several structural units with different internal porosities and pore morphologies, different spatial arrangements of which may itself give rise to quite different textural polymorphs, demands an appropriate development of experimental techniques for their structural characterization on different length scales. One of the important properties of porous materials is connectivity of the pore space. Some attempts for its quantification have earlier been undertaken based on an analysis of gas sorption and, especially, desorption scanning behavior [1]. Alternatively, some information on the interpore connectivity may be extracted from the tortuosity factor, a parameter accessible by pulsed field gradient (PFG) NMR. Because tortuosity is assessed by tracing the molecular displacements much exceeding typical pore size of the porous medium, it does provide the degree of interconnectivity averaged over the whole structure. In this respect, new experimental approaches providing more local information, e.g. a degree in which different modes of a pore size distribution function are spatially connected to each other, are highly welcome.