Precursor-surfactant interactions

The use of ionic precursors in template-directed hydrolysis-polycondensation reactions clearly indicated a strong contribution of the precursor toward the formation of structured mesophases, based on ionic precursor-surfactant interactions. The work of Che and coworkers beautifully illustrates that templating methods involving surfactant-precursor ion pairs allow accessing highly structured siuface-functionalized silica materials. However, this work exclusively focused on the use of monosilylated costructure-directing agents and the formation of siuface-fimctionalized mesoporous silica phases. 

Scheme 16.9 Ionic precursor surfactant interactions in the formation of hybrid imidazolium-containing ionosilica.

Surfactant S or N Inorganic precursor 1 Interaction Examples (structure) (hex = hexagonal and lam = lamellar)... 

F S , where negatively eharged surfactant interact with positively morganie oxide precursors via coulombic interactions. 

I S, where neutral template molecules such as amine and block copolymer surfactants interact with inorganic oxide precursors via hydrogen bonds or coordination bonds. 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

In our attempts to synthesize nano structured or even nanoporous chromium (III) oxide we used a great variety of surfactants (amines, polyethylen oxides, carboxylates, sulfates, phosphates and quartemary alkylammonium salts) and Cr3+ precursors (nitrate, acetate, acetylacetonate and iso-propyl ate) to obtain the desired materials [19]. Different kinds of interaction are established throughout the structure-directing process such as electrostatic (S+... 

Figure 16.2 Possible mechanisms of the formation of mesoporous silica materials, by generation of hexagonal silica-surfactant mesophases horn the interaction of silica precursor with (a) spherical micelles, (b) rod-like micelles, or (c) a preformed liquid crystalline phase of the surfactant.

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