PORE STRUCTURE EVOLUTION IN SILICA-GEL DURING AGING DRYING .2. EFFECT OF PORE FLUIDS

A two-step acid-base-catalyzed silica gel has been aged in a series of alcohol and water baths and some chemical and physical structures of the gel were measured using several techniques (low-field NMR, Si-29 and C-13 MAS-NMR, IR, Raman, nitrogen adsorption) on gels in wet and dry states. When the gel was placed in alcohol, the surface area increased as a result of esterification and depolymerization to 1500-2000 m2/g in the wet state which then decreased to 1000 m2/g after drying. In water, hydrolysis and condensation decreased surface area to 1000 m2/g (wet) and 500 m2/g (dry). This process was reversible. Alcohol-aging led to small pore size distributions (in much shorter times than previous studies of aging in the gel's mother liquor, approximately 90% ethanol, 10% H2O). Surface areas calculated from Si-29 MAS-NMR-derived Q distributions were in good agreement with adsorption-derived values for the samples dried from alcohol but water-dried samples appeared to contain some surface area which was inaccessible to adsorbing nitrogen after drying. These results indicate that the pore structure of silica gels may be dramatically altered during the aging process and that their wet gel features may be preserved (at least partially) upon drying.