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 drying of the alcogel. As the degree of fluid already exchanged affects the properties of the resulting aerogel or xerogel and the fluid replacement is a time consuming process, it is a big advantage if the exchange rate can be predicted for a certain type of gel. An autoclave for pressures 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 device allows in situ pulsed gradient spin echo (PGSE) and NMR imaging. Self diffusion coefficients for methanol in TMOS gels of different nanostructure have been determined by PGSE. NMR imaging was applied to observe online the fluid exchange of methanol for liquid CO2 at a pressure of about 7 MPa at 15 degrees C. Alcogel structures have been investigated by small angle X-ray scattering (SAXS). (C) 1998 Elsevier Science B.V. All rights reserved.