Diffusion of Fluids in Mesoporous Host Materials

1. Introduction Since the first report on the synthesis of ordered mesoporous MCM-41 in 1992 [1], the interest to these materials having pore sizes in the range from 2 to 50 nm has continuously been growing. Due to a fascinating match of their structural properties and the adjustability of the surface chemistry, mesoporous materials provide very flexible options for their use in diverse applications in medicine [2], chemistry [3], optics [4], etc. On the other hand, the availability of mesoscopic voids with well-defined geometries and boundary conditions, where tiny molecular or atomic ensembles can be contained, has enabled studying various fundamental processes occurring on the mesoscale. These properties are often found to differ appreciably or even to be absent in the macroscopic systems [5]. Concerning both applications and fundamental studies, dynamics of confined ensembles and molecular diffusion, in particular, may play a decisive role in determining their properties. Therefore, better understanding of the diffusive dynamics of confined fluids is of crucial importance. Among a variety of techniques adopted to probe translational dynamics in pore spaces, pulsed field gradient nuclear magnetic resonance (PFG NMR) is proven to be especially suitable [6]. In particular, it has recently been demonstrated that by accessing simultaneously both microscopic dynamics, namely the molecular diffusivities, and phase equilibrium, namely the phase composition, the fluid behaviour in porous materials can be addressed in most informative way [7-9].