Pressure-enhanced molecular self-diffusion in microporous solids

The H2O self-diffusion in hydrated fibrous zeolites (natrolite, mesolite, and scolecite) at hydrostatic pressure up to 1.5 GPa was investigated using nuclear magnetic resonance. It has been found that the effect of pressure on the diffusive mobility of water molecules arranged in the zeolite channels is different in various pressure-transmitting media. The compression in water enhances molecular mobility corresponding to a negative activation Volume of diffusion, which ranges from -5 to -12 cm(3) mol(-1) for the zeolites studied. The unusual pressure course of the activation process is shown to be associated with implanting exterior water molecules into the crystals which results in an increase of the concentration of steady-state defects responsible for the molecular diffusion. (C) 2001 Elsevier Science B.V. All rights reserved.