In Situ Pore Structure Studies of Xerogel Drying

Low-field spin-lattice relaxation (20 MHz proton NMR) measurements of ethanol contained in two base-catalyzed silica xerogels aged under different conditions have been applied as an in situ probe of pore structure development during xerogel drying. Two base-catalyzed silica gels were prepared and aged under different conditions. One sample (designated E) was aged in pure ethanol for 1 week and the second (designated B) in an ethanol-KOH mixture for the same time period prior to drying. 180 degrees-tau-90 degrees relaxation experiments were performed at 20 MHz and 303 K as each gel dried. In addition, gel shrinkage and weight loss were monitored. After complete drying, the surface area and pore size distribution of each sample were obtained from nitrogen adsorption/condensation. During drying, a gradual pore shrinkage for both samples was observed until the structures stiffened, causing the fluid meniscus to penetrate the pores. For sample E, this caused the surface area to decrease from approximate to 1000 to 600 m(2)/g and microporosity development in the pore size distribution. For sample B, the surface area decreased from approximate to 1500 to 200 m(2)/g and microporosity disappeared. The final NMR-derived pore size distributions were in good agreement with that obtained from nitrogen condensation.