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Encyclopedia of Physical Science and Technology EN009I-420 July 10, 2001 15:8

Mesoporous Materials, Synthesis and Properties 373

LCT mechanism. At present, there is a broad consensus c. Cooperative charge density matching model.
that the formation of M41S materials (in the presence of This model involves
charged surfactants) takes place via a cooperative mecha-
nism where the interactions between the inorganic and sur- 1. Cooperative nucleation of inorganic and organic
factant ions play a key role in determining the morphology (surfactant) species.
of the resulting mesophase. Pathway 2 is therefore now 2. Liquid-crystal formation with molecular inorganics.
largely accepted as the most likely formation mechanism. 3. Inorganic polymerization and condensation.
Liquid crystal mesophases (according to pathway 1) are
not expected to exist at the low surfactant concentrations The model proposes that when charged surfactants are
usually used for synthesis of M41S materials. However, used, the initial step is preferential ion exchange of the
if a much higher concentration of surfactant is present, surfactant counterions with polycharged oligomeric inor-
pathway 1 can be employed to prepare mesoporous ma- ganic (i.e., silicate) species. These silicate species serve
terials. Indeed, pathway 1 is operative when surfactant as multidentate moieties, and can bind several surfactant
concentrations greater than 30 wt% are employed. In this molecules and screen the repulsive forces between the
case, a liquid crystal phase is formed which then acts as a headgroups within a surfactant aggregate. This can re-
cast or mold in which the inorganic network polymerizes duce the local curvature and allow the aggregate to grow
throughout the aqueous regions of the liquid-crystalline in size. The charge screening afforded by the silicate ions
phase. The formation of the ordered mesophase is there- can reduce the thickness of the double layer that keeps the
forelargelyindependentofsurfactant/inorganicinterfacial micelles separated. At the appropriate concentration, this
interactions. This pathway is now dubbed “true liquid- canallowattractiveinterfacialforcestodominatetheinter-
crystal templating.” aggregate repulsive forces and can induce self-assembly
Three LCT-based formation models have been sug- into a new ordered (hexagonal) morphology.
gested. A common feature of all the models is that sur- Before concluding the discussion on the LCT models,
factants in solution conduct the ordering of the materials, it is worth pointing out that under certain conditions a true
but the type of interactions between the surfactants and in- cooperativeself-assemblyofinorganicions(e.g.,silicates)
organic species are different. The three models discussed and surfactants is possible. For example, at low temper-
hereare(i)thepuckeringlayeredmodel,(ii)thesilicaterod ature and high pH, conditions that prevent condensation
assembly model, and (iii) the cooperative charge density of silicate ions, the formation mechanism schematically
matching model. For simplicity, the following discussion represented in Fig. 5 can be operative. According to this
on the LCT models assumes that the inorganic species are mechanism,theformationofmesoporousmaterialsoccurs
silicate ions and that a hexagonal mesophase is formed. via so-called “silicatrophic liquid crystals” (SLC). Three
steps are involved in the synthesis pathway:
a. Puckering layered model. This model is based
1. Assembly of the surfactant system and the formation
on the assumption that the hexagonal surfactant phases
ofionpairsbetweensurfactantmoleculesthatinteractwith
are present and that the inorganic (i.e., silicate) source
polydentate and polycharged inorganic species.
dissolves into the aqueous regions around the surfactant
2. Self-organization of ion pairs into a mesophase with
arrays. The dissolved silicates ions are organized into lay-
usually a liquid-crystal structure (hexagonal, lamellar, or
ers (or sheets) with cylindrical surfactant micellar rods
cubic), the nature of which depends on the composition
intercalated between the layers. These silicate sheets and
of the mixture, the pH, the temperature, and the reaction
hexagonal surfactant phases then give rise to hexagonal
time.
silica-surfactant mesophases via puckering of the silicate
3. Polycondensation of the inorganic (e.g., silicate)
sheets.
species leading to the formation of a rigid inorganic frame-
work.
b. Silicate rod assembly model. This model pro-
poses that randomly ordered, rod-like micelles form ini- Finally, to conclude the description of LCT mecha-
tially and interact with inorganic (i.e., silicate) species to nisms, it is worth noting that no clear and universally ac-
form surfactant rods encapsulated by 2–3 monolayers of cepted mechanism has emerged. However as mentioned
silica; these species then spontaneously assemble into a above, it is now generally accepted that pathway 2 of
hexagonal structure that has long-range order. Since the the LCT mechanism is responsible for the formation of
hexagonal phase does not form until addition of the inor- a majority of mesoporous materials currently available.
ganic species, this model is consistent with pathway 2 of It is, however, also important to point out that none of
the LCT mechanism described above. the models described above explicitly address mesoporous