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Mesoporous Materials, Synthesis and Properties 371

portantroleintheorganizationofthesurfactantmolecules.
In general, the interaction (electrostatic, Van der Waals,
hydrogen bonding) between the inorganic species and sur-
factant molecules determines the morphology and pore
geometry of the resulting mesophase. In order to under-
stand the necessary reaction conditions for obtaining a
FIGURE 1 Three structure types observed for silica–surfactant
particular mesoporous structure, it is useful to look at the
mesophases: (a) hexagonal; (b) cubic bicontinuous, Ia3d; (c)
lamellar. behavior of surfactant molecules in an aqueous solution.
indirect correlation of the shape and size of the organic
1. Behavior of Surfactants in Solutions
molecule is obtained in the structure and volume of the
cavity created in the final inorganic framework. In con- In aqueous solution, surfactant molecules exist as very
trast, in mesoporous mesophases the organic supramolec- active components with variable structures in accordance
ular array clearly acts as template, and there is a direct with increasing concentrations. At low concentrations,
correlation of the surfactant array size (and shape) to the surfactant molecules exist as monomolecules, but with in-
final pore size and geometry in the mesophase. The inti- creasing concentration the surfactant molecules aggregate
mate template–framework association is facilitated by the together to form micelles, which has the effect of decreas-
flexibility and compliance of the precursor inorganic net- ing the entropy of the solution. The driving force for the
works, which are built up of relatively small oligomers and aggregation of surfactant molecules to micelles is to min-
by the large radius of curvature of the organic template. imize repulsive interactions between their hydrophobic
The initial reports on the synthesis of mesoporous tails and water. The resulting micelles are either spheri-
molecular sieves described two types of materials, the cal or rod-shaped. The initial concentration at which the
M41SfamilyandFSM-16typematerials.Sincethenmuch surfactant molecules begin to aggregate into micelles is
scientific interest has been focused on the three groups of called the critical micelle concentration, CMC. As the sur-
M41S materials—the hexagonal (designated MCM-41), factant concentration increases further, hexagonal close
cubic (MCM-48) and lamellar (MCM-50) phases—which packed arrays of micelle rods appear, producing hexag-
are depicted in Fig. 1. The similarity between the differ- onal phases (such as those that lead to the formation of
ent M41S phases and known liquid-crystal phases hinted MCM-41). The next step, as the surfactant concentration
at a link between the two. For this reason it was ini- increases, is the formation of a lamellar phase, which is
tially suggested that so-called liquid-crystal templating sometimes but not always preceded, by the formation of
(LCT) mechanisms were responsible for formation of a cubic phase. The changes are illustrated in Fig. 2. The
M41S mesoporous molecular sieves. As discussed later, particular phase of the surfactant does not only depend
recent developments in synthesis methodologies have on its concentration, but also on the nature of the surfac-
added new varieties of mesoporous molecular sieves and tant molecules and their environment. Important environ-
expanded/clarified the formation mechanisms. mental factors include variables such as the length of the
surfactant hydrophobic (alkyl) carbon chain, the nature
of the surfactants’ hydrophilic head group, the properties
II. SYNTHESIS of the counterion, pH, temperature, ionic strength, and the
presence of other additives/dopants. It has been found that
In general, the procedures used for the preparation of in most cases the CMC decreases with increase of chain
mesoporous materials are similar to those utilized in the
synthesis of zeolites except that a surfactant instead of an
inorganic cation or base is used as the template. Meso-
porous materials (e.g., mesoporous silica) are prepared
under room temperature or mild hydrothermal condi-
tions (typically below 150 C) in the presence of anionic,
◦
cationic, or neutral surfactants, in either basic or acidic
conditions.

A. Assembly of the Surfactant System FIGURE 2 Phase sequence of a surfactant/water system. The
scheme shows the formation of micelles after the critical micelle
Surfactant chemistry is key to the formation of meso- concentration (CMC) and the sequence of phases as the concen-
porous materials. The inorganic species also play an im- tration of surfactant increases beyond the CMC.