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Experimental and Theoretical Studies of the Relaxation Processes Associated with Adsorption/Desorption Hysteresis in Mesoporous Materials

R. Valiullin1, S. Naumov1, P. Galvosas1, J. Karger1, F. Porcheron2, H-J. Woo3, and P. a. Monson2. (1) Abteilung Granzflächenphysik, Fakultät für Physik und Geowissenschaften, Universität Leipzig, Leipzig, Germany, (2) Univ. of Massachusetts, Dept. of Chem. Eng., Amherst, MA 01003, (3) Chemistry, University of Nevada, Reno, NV 89557

In recent years, progress in the development of novel synthesis strategies has led to the discovery of a large number of porous materials with controlled architectures and pore sizes in the mesoporous range. The pore spaces in these materials are sufficiently large that they can accommodate assemblies of molecules in condensed (liquid-like or solid-like) states at low temperature. An important feature of these materials is the phenomenon of hysteresis frequently observed in studies of gas adsorption/desorption at low temperatures (e.g. nitrogen at its boiling point, 77K). Here the amount of a gas contained by the material at a given bulk pressure is higher on desorption than on adsorption and this indicates a failure of the system to equilibrate. While this phenomenon has been known for over a century, the internal dynamics underlying it remain poorly understood. Here we present an experimental study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and further analyzed using computer simulations of molecular models. Using NMR techniques and Vycor porous glass as a model system, we have explored the relationship between microscopic translational mobility (i.e. molecular self-diffusion) and global uptake dynamics. For states outside the hysteresis region the relaxation process is essentially diffusive in character. However within this region the relaxation dynamics is dominated by activated rearrangement of the adsorbate density within the host material, an intrinsically slower process. The experiments and molecular model yield a consistent picture of the behavior.