In contrast to the recent progress achieved in the understanding of the adsorption mechanism of fluids in highly ordered mesoporous materials with simple pore geometries (e.g. M41S materials), there are still many open questions concerning the phase and sorption behavior of fluids in more complex pore systems. In order to address this problem, we have performed systematic adsorption studies of nitrogen (77 K) and argon (at 87 K and 77 K) on a series of novel micro/mesoporous silica materials with hierarchical pore structures (KLE and KLE/IL silicas) [1,2]. These materials consist of spherical pores (mean pore diameters ranging from below 10 nm up to 35 nm) connected through narrow micro- and mesopores. The hierarchical pore structure of these samples has been also confirmed by small-angle neutron scattering (SANS) in combination with in-situ nitrogen adsorption at 77 K. The nitrogen and argon adsorption isotherms obtained on these samples reveal wide hysteresis loops of types H1/H2. An analysis of these data confirms that pore condensation occurs delayed due to the existence of metastable adsorption films and hindered nucleation of liquid bridges, while evaporation of the condensed pore fluid from the spherical cavities occurs by cavitation if the width of the connecting pores is smaller than a critical value („T 5 nm) [2]. Our data reveal that the size of the cavities has an appreciable effect on the cavitation pressure if the cavity size is smaller than ca. 14 nm, but not for larger cavity diameters. It appears further that the size of the connecting pores does not effect the cavitation pressure. Summarizing, our results lead to a better understanding of the adsorption and phase behavior of fluids in micro/mesoporous materials, which is not only important for an accurate textural characterization of adsorbents consisting of complex micro/mesoporous pore networks but also for optimizing their use in practical applications (e.g., catalysis, separation).

References [1] O. Sel, D, Kuang, M. Thommes, B. Smarsly, Langmuir 22, (2006) 2311 [2] M. Thommes, B. Smarsly, M. Groenewolt, P.I, Ravikovitch, A.V. Neimark, Langmuir 22, (2006) 756 [3] O. Sel, A.Brandt, D. Wallacher, M. Thommes, and B. Smarsly, Langmuir 23, (2007), 4724

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