Xerogels, pore diameters

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

Solvent removal from the gels has a strong influence on the structure of the materials. Simple evaporation of solvent gives the xerogel, whereas extraction with supercritical C02 gives the aerogel with much larger pore volume and pore diameter (Miller et al., 1994 Hutter et al., 1995). 

One major advantage of silica xerogel over other support materials is the ease of adjustment and control of mean pore diameter, specific surface area and the specific pore volume. Supports for catalytic applica-... 

Figure 5 Textural properties (pore volume and pore diameter) of dried xerogels containing different amounts of fumed silica (aerosil 0X50). (From Toki et al. [59].)...

In (2.6), )l is the surface tension of the liquid, d is the pore diameter, and 9 is the contact angle at the liquid-vapor interface. Water at 25° C leads to capillary forces of around 1,500 atm. Not many systems can sustain this kind of force. After complete solvent removal, the dry gel will have a porosity of approximately 50% and be termed a xerogel. 

Despite the small pore diameters of the xerogels, their Brunauer-Emmett-Teller (BET) surface areas are still quite high, at 50% of those of the corresponding native samples. On the other hand, despite their minimal shrinkage relative to the molds. 

In order to characterize the structure of RF and carbon xerogels, a combination of nitrogen adsorption (for micro- and meso-pores) and mercury porosimetry (for pore diameters from 7.5 to 150 nm) was used to obtain the BET surface area and pore volume (microporous and total) helium and mercury pycnometry were applied to determine the skeletal and bulk density. 

Pore-size distributions (normalized to lOQo/o at the modal pore diameter) and bulk densities of xerogels LpH5 (made at pH 2.5 with 5 moles water/mole TEOS) and HpHI2 (made at pH 9.2 with 12 moles water/mole TEOS), and LpH5 aged one week at 50°C in 3 M NH4OH prior to drying. From Brinker and Scherer (23J. 

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

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