Silica xerogel structure

Adsorption of hard sphere fluid mixtures in disordered hard sphere matrices has not been studied profoundly and the accuracy of the ROZ-type theory in the description of the structure and thermodynamics of simple mixtures is difficult to discuss. Adsorption of mixtures consisting of argon with ethane and methane in a matrix mimicking silica xerogel has been simulated by Kaminsky and Monson [42,43] in the framework of the Lennard-Jones model. A comparison with experimentally measured properties has also been performed. However, we are not aware of similar studies for simpler hard sphere mixtures, but the work from our laboratory has focused on a two-dimensional partly quenched model of hard discs [44]. That makes it impossible to judge the accuracy of theoretical approaches even for simple binary mixtures in disordered microporous media. 

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

A. Fidalgo and L. M. Ilharco, Chemical Tailoring of Porous Silica Xerogels Local Structure by Vibrational Spectroscopy, Chem. A. Eur. J., 2004, 10, 392. 

Figure 6. Globular structure of a silica xerogel magnification x240 000.

Until the availability of high-resolution electron microscopy, the structure of silica xerogels remained a matter of conjecture. Now, there is no longer any doubt that the amorphous framework is made up of very small globular units (see Figure 10.6). These primary particles are isotropic and have fairly uniform size of 1-2 nm (with a molar mass 2000 g mol 1). In some xerogels the primary particles are densely packed within secondary particles, whereas in other systems there is a more open arrangement (Barby, 1976). 

In view of the complexity of the structure of most silica xerogels, it is to be expected that their adsorptive behaviour would be equally complex. The following... 

Colomer, M.T., and Anderson, M.A. 2001. High porosity silica xerogels prepared by a particulate sol-gel route Pore structure and proton conductivity. Journal of Non-Crystalline Solids 290, 93-104. 

The effect of mercury intrusion analysis on structure was examined for a series of silica xerogels with different pore size distributions. This analysis was achieved by applying nitrogen sorption analysis to the silicas both before and after mercury intrusion analysis. The study required the development of a method for the removal of mercury from a sample after the initial intrusion measurement that does not damage the structure. The results show the potential for an elastic deformation of the structure during compression as well as irreversible compression during mercury intrusion. 

Davison silica gel is an excellent example of a high area small pore xerogel structure. Its adsorption isotherm is a Type I curve according... 

Adsorption-desorption isotherms demonstrate the sharp contrast between a small pore silica xerogel and a large pore aerogel structure. 

Orcel, G., Phalippou, J., Hench, L. L. (1986). Structural changes of silica xerogels during low temperature dehydration. J. Non-Cryst. Sohds 88,114-130. 

Figure 1-3. Relation among X-ray structural parameters for different silica xerogels (o) and glasses (d 800° C-treated silica gels silica glass).

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