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As Fig. 2 shows, starting from nutrients, hydrolysis, silica oligomerization and condensation reactions catalyzed by hydroxyls lead to entities of different size (and hence stability) and order. Depending on the synthesis conditions (pH, temperature,and presence of structure-directing agents) an amorphous phase can be obtained as either intermediate phase (which would either transform into a crystalline phase or dissolve to be incorporated into a more stable product) or final product. In this scheme, the role of hydroxyl ions has been well described in breaking and remaking Si,Al¨CSi,Al bonds in the corresponding condensation and rearranging reactions leading to pre-nuclei and then to nuclei particles; the concept of hydrophobic hydration has been introduced to explain how some organic molecules can organize silica and how others cannot ; and finally subcolloidal or precursor nanoparticles which could be the previously mentioned pre-nuclei have been identified by means of NMR, TEM, DLS and SAXS techniques . In fact, some of these particles, called nanoslabs (slab shaped particles), have been claimed to be of defined sizes of 1.3 nm ¡Á 4 nm ¡Á 4 nm, each one containing a TPA+ cation , even if this finding has been the subject of serious controversy .Another aspect recently introduced is the concept of molecular recognition to describe the early interaction between silica complexes and template molecules to form clusters from which ordered structures may grow . |
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