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Realistic MCM-41 synthesis
The starting molecules were placed randomly in the simulation box at a silica/surfactant ratio of 3 and temperature and pressure equal to 300 K and 1 atm, respectively. Different crucial stages have been observed during the simulation; i) spherical micelle formation; ii) sphere-to-rod transition (through micelle fusion in the presence of anionic silica monomers); iii) twisted rods agregation (in the presence of cubic octamers; iv) rod arrangement to the hexagonal material in the presence of silica cubic octamers. Color code is as follows: hydrophobic surfactant tails in green, hydrophilic surfactant head in purple, anionic silica monomers in yellow, silica cubic octamers in red. Water molecules were removed for clarity.
HLC formation with 100% dimers
System with 6 weight percent of C16TA+ surfactant concentration and 100% of double charged silica dimers. The simulation started with all of molecules randomly placed at the beginning of the simulation. In this film it can been seen how surfactant prolate-shaped aggregates are formed rapidly to form small surfactant rods and then arrange into a twisted rods aggregate. After some simulation time, the disordered twisted rods form an ordered hexagonal arrangement. The consideration of silica reaction in this stage will form the silica scaffold which maintains the hexagonal shape of the template. Color code is as follows: green shows the hydrophobic surfactant tails, hydrophilic surfactant heads in purple and anionic silica dimers in red. Water molecules were removed for clarity.
Sphere-to-rod transition
Micelle fusion processes after addition of anionic silica monomer into a low surfactant (0.1 M of C16TA+) concentration water solution with spherical micelles in equilibrium. Anionic silica induces fusion between micelles instead of surfactant monomer aggregation. Color code is as follows: The surfactant hydrophobic micelle core in green, the hydrophilic surfactant heads (micelle surface) in purple and anionic silica monomers in red. Water molecules were removed for clarity.
Monomer-to-dimer exchange
Branched rods system with anionic silica monomers in equilibrium and 6wt% of C16TA+ of surfactant concentration. The exchange of anionic monomers by double charged dimers (maintaining the same amount of silica atoms) promoted a phase transition from branched rods to a hexagonal phase. The increase of density charge produces a strong dimer-to-surfactant head interaction, binding two rods at the same time, acting as a bridge between them and promoting a hexagonal ordered arrangement. Color code is as follows: green shows the hydrophobic surfactant tails, hydrophilic surfactant heads in purple and anionic silica dimers in red. Water molecules were removed for clarity.
Hexagonal-to-lamellar transition after removing a co-solvent
Hexagonal phase system with 30wt % surfactant (C16TA+) concentration and fully charged silica dimers (in red). Water molecules were removed for clarity. The addition of a co-solvent (benzene molecules, in orange) promotes a hexagonal-to-lamellar phase transition. Benzene molecules were arranged well inside the hydrophobic core of the surfactant molecules (in green) originating a more energetically favorable lamellar phase. Water molecules were removed for clarity.
Lamellar-to-hexagonal transition after addition of a co-solvent
The process observed in the film on the left was found to be reversible after removing the benzene molecules, thereby the system re-arranged into the original hexagonal phase. Color code is the same as in the film on the left.
