Multiscale Model for the Templated Synthesis of Mesoporous Silica: The Essential Role of Silica Oligomers

A detailed theoretical understanding
of the synthesis mechanism
of periodic mesoporous silica has not yet been achieved. We present
results of a multiscale simulation strategy that, for the first time,
describes the molecular-level processes behind the formation of silica/surfactant
mesophases in the synthesis of templated MCM-41 materials. The parameters
of a new coarse-grained explicit-solvent model for the synthesis solution
are calibrated with reference to a detailed atomistic model, which
itself is based on quantum mechanical calculations. This approach
allows us to reach the necessary time and length scales to explicitly
simulate the spontaneous formation of mesophase structures while maintaining
a level of realism that allows for direct comparison with experimental
systems. Our model shows that silica oligomers are a necessary component
in the formation of hexagonal liquid crystals from low-concentration
surfactant solutions. Because they are multiply charged, silica oligomers
are able to bridge adjacent micelles, thus allowing them to overcome
their mutual repulsion and form aggregates. This leads the system
to phase separate into a dilute solution and a silica/surfactant-rich
mesophase, which leads to MCM-41 formation. Before extensive silica
condensation takes place, the mesophase structure can be controlled
by manipulation of the synthesis conditions. Our modeling results
are in close agreement with experimental observations and strongly
support a cooperative mechanism for synthesis of this class of materials.
This work paves the way for tailored design of nanoporous materials
using computational models.