Mesoporous Materials Obtained Using Fluorinated Surfactants

The above results demonstrate that partially fluorinated nonionic surfactants, having a fluorinated hydrophobic chain and a nonfluorinated poly(ethylene oxide) polar chain, can be successfully used to obtain mesoporous materials with controlled morphology and pore size. The specific surface areas can be high, similar to MCM-41 materials obtained using hydrocarbon surfactants. 

In the conditions reported by the authors [52], very well-ordered 2D hexagonal mesoporous silicas are obtained with a surfactant concentration lower than 25 

Antonietti et al. described the preparation of mesoporous silica in systems containing mixtures of nonionic fluorinated surfactants and nonionic hydrocarbon block copolymer surfactants [60], The fluorinated surfactant was CF3(CF2)6-i6 C2H4(E0)4 5 and the hydrocarbon-based copolymer was poly(ethylene-co-bulylene)-block-poly(ethylene oxide). 

The preparation of mesoporous materials with bimodal pore-size distribution has also been described by Stebe and coworkers, using mixtures of F(CF2) C2H4 (OC2H4)mOH nonionic ethoxylated surfactants and conventional hydrocarbon ethoxylated surfactants [61]. 

The use of fluorinated cationic surfactants has been studied by Rankin and coworkers [62, 63]. They described the preparation of hexagonally ordered mesoporous silica with rather small pore sizes (pore diameters approximately from 2.0nm to 2.6nm), and therefore they have similar properties to silica prepared with conventional hydrocarbon cationic surfactants (such as MCM-41). Thin pore walls could be a disadvantage, since it may lead to low mechanical and hydrothermal stability. 

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Figure 11.19 clearly shows that the mesostructure of MCM-41, with thirmer pore walls, collapses during the hydrothermal treatment. However, the mesoporous material obtained using the fluorinated surfactant is more robust and remains practically unmodified after the hydrothermal treatment. 

Table 11.2 Comparison between the main morphological characteristics of a mesoporous material prepared using the fluorinated surfactant and the typical values obtained for MCM-41 and SBA-15 materials.

Nowadays, partially or fuUy fluorinated surfactants are available with a variety of chemical structure and chain lengths. Therefore hydrophilic-lipophilic properties can be controlled and these new surfactant molecules may provide a rich phase behavior, which could be used to obtain well-organized mesoporous materials. 

It should be pointed out that although most of the research works used silica as a model material, fluorinated surfactants can be used for the preparation of a very wide variety of different mesoporous materials, including TiOj [70], aluminophos-phates and Fe- aluminophosphates [71], and Fe-Si oxides [72]. Organic functionalizations can also be obtained in direct synthesis processes. An example is the preparation in a one-step precipitation process, by Rankin et al., of vinyl-functionalized porous silica materials using mixtures ofTEOS and vinyltriethox-ysilane in aqueous solutions of cationic fluorinated surfactants [73]. 

In this context, silicone and fluorinated surfactants, which can impart peculiar properties to mesoporous materials, constitutes a research field that may be exploited to obtain new nanostructured materials for novel applications and many future developments may still arise in this field. The literature on the use of nonhydrocarbon surfactants for the preparation of mesostructured materials remains relatively scarce and many aspects are still unexplored. 

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