Pressure-enhanced molecular self-diffusion in microporous solids

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