Aerogels structure

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

Similar methodology has also been applied to the preparation of monolayer coated aerogels, which are virtually impossible to make via conventional condensed phase solvent methods. In SCFs, monolayer deposition proceeds smoothly and results in the same high level of crosslinking observed in the MCM-41 materials. Once the monolayer is deposited, it serves to structurally reinforce the aerogel structure sufficiently to where it becomes stable... 

Before we discuss the aerogel structure itself, we give a brief survey of methods commonly used to reveal their structural features. Generally, three techniques can be distinguished. 

In particular, the effect of the resorcinol-to-catalyst (R/C) ratio on the final aerogel structure has been studied extensively. In aqueous media, this ratio has been typically varied from 50 to 300. FormatiOTi of particles connected with large necks was reported at low R/C ratios ( 50). CrossUnked microspheres were observed at very high R/C ratios ( 1,500). The final pore structure and the gelatirMi time depend strongly on the sol pH at low pHs, precipitation rather than gelation has been reported. 

In the case of SiO2-X-Au(0) aerogels, however, no surface-coating molecules are used to keep the Au(0) particles apart the aerogel structure and the growth mechanism play that role. The experimental question then becomes what happens if molecules that interact with Au(0) are present as they are grown photolytically in the aerogels ... 

Through this approach, X. Liu [26,27] was able to run a series of experiments in which a global ratio of 1 3 was maintained in each reaction and the Au(III) concentration was always the same, but the aerogel structure cmitrolled the diffusion such that the local [Au(III)] [L-cysteine] ratio was varied from 1 3 to 25 1. The result was that as the local concentration ratio in the nanodomains changed, the product changed from the white Au(I) product to the red Au(0) product, even though the stoichiometric ratio between reactants and the cmicentra-tion of Au(ni) were held constant globally. 

The sonogel route produces network aerogel structure close to that one of a bulk glass. 

Figure 23.1. Diagram of an aerogel structure and the heat transfer mechanisms involved. Heat is transferred via chains of primary particles forming the solid backbone (indicated by the red particle chain), by thermal radiation (wavy yellow arrows), and by gas molecules present in the porous structure of the aerogel blue dots).

Figure 24.3. DLCA aerogel structure at 0.5% gel volume fraction, prepared (by the author) according to the algorithm proposed by Hasmy et al. [61].

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