Molecular mobility of confined phases in model mesoporous (MCM-41) and microporous (AlPO(4)-5 zeolite) host materials.

The considered host materials are well suited to confine quasi-(1d) molecular phases, seeing that their porosities are composed of parallel unconnected cylindrical pores. For such a simple geometry, confinement effects can be simply described by a single parameter, the pore diameter phi. Our study concerns medium and ultra confinement ranges ( 40 A >or= Ø >or= 7.3 A). The primary effect of such confinements is the decrease of the molecular interactions within the confined phase. As a consequence, we have observed strong triple point depressure Delta T(3t) effects for hydrogen and water confined phases in MCM-41 samples. In the limit case of (1d) phase (the neopentane/AlPO(4)-5 system) it seems that a molecular mobility is observed even at very low temperature T=5 K. The secondary confinement effect is an increase of the interactions between the host inner surface and the confined molecular assembly induced by the pore diameter decreasing. Such host material influence gives rise, for medium range confinement to the physisorption of a curved solid film on the inner surface before the capillary phase condensation (hydrogen/MCM-41 (24 A)) and for ultra confinement to the solidification of the confined phase when the molecular species are commensurate with the inner surface sites (methane/AlPO(4)-5).