SELF AND TRANSPORT DIFFUSION OF FLUIDS IN SIO2 ALCOGELS STUDIED BY NMR PULSED GRADIENT SPIN-ECHO AND NMR IMAGING

Preparation of SiO2 aerogels generally includes a fluid exchange step either to chemically modify the material or to allow an optimized dryi ng of the alcogel. As the degree of fluid already exchanged affects th e properties of the resulting aerogel or xerogel and the fluid replace ment is a time consuming process, it is a big advantage if the exchang e rate can be predicted for a certain type of gel. An autoclave for pr essures up to 10 MPa and temperatures up to 80 degrees C, that can be used as probehead for a standard NMR system, has been designed. This d evice allows in situ pulsed gradient spin echo (PGSE) and NMR imaging. Self diffusion coefficients for methanol in TMOS gels of different na nostructure have been determined by PGSE. NMR imaging was applied to o bserve online the fluid exchange of methanol for liquid CO2 at a press ure of about 7 MPa at 15 degrees C. Alcogel structures have been inves tigated by small angle X-ray scattering (SAXS). (C) 1998 Elsevier Scie nce B.V. All rights reserved.