Research projects

Lattice Monte Carlo Simulations of TMM Synthesis (MCF & SBA-15)

MCF and SBA-15

Mesostructured silicas MCF [1] and SBA-15 [2] can be synthesized by using triblock surfactants as templates. Structures of both the materials are shown in Fig. 1. MCF consists of spherical pores (cells) of diameters 20 to 50 nm interconnected by windows with diameters around 10 nm. SBA-15 consists of hexagonally arranged cylindrical pores (diameter 5-30 nm) and micropores (diameter <1 nm).

(a)                                                (b)

Fig. 1. Schematics ; (a) MCF , (b) SBA-15

The main driving force behind TMM synthesis is the ordering of surfactant molecules in liquid solution, also known as surfactant self assembly. Due to large system size and slow dynamics, it is difficult (if not impossible) to study self assembly using atomistic simulations. However, it is possible to bypass the drawbacks of atomistic simulations by using mesoscale simulations.

Our model

We have employed coarse-grained lattice models of the surfactants and the solvents based on previous work by Larson and coworkers [3,4]. In our system, the surfactant is modeled as a chain of adjacent lattice units, where each unit represents several monomer units of the experimental surfactant (shown in Fig. 2). The solvents are modeled as single lattice units. The system is equilibrated using standard Monte Carlo moves shown in Fig. 3.

Fig. 2. Surfactant lattice model

                   Reptation                     Twist              Configurational Bias

Fig. 3. Lattice Monte Carlo moves

Results

When optimized properly, the simple lattice model can provide valuable information regarding TMM synthesis and pore structures of mesoporous silicas. In MCF synthesis, increments in oil concentration in the template solution bring about changes in the pore structure in the following sequence: cylinder -> undulating cylinder -> mesocell. Fig. 4 shows a comparison between transitions observed experimentally and those observed in our simulation studies [5].

Simulation

           (a) cylindrical                      (b) lamellar                    (c) mesocellular

Experiment

           (a) cylindrical                  (b) undulating cylinder       (c) mesocellular

Fig. 4. Effect of oil concentration on silica structure

 [1] J. S. Lettow, Y. J. Han, P. Schmidt-Winkel, D. Zhao, G. D. Stucky and J. Y. Ying, Langmuir 16 (2000) 8291.

[2] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science 279 (1998) 548.

[3] R. G. Larson, L. E. Scriven and H. T. Davis, J. Chem. Phys. 83 (1985) 2411.

[4] R. G. Larson, J. Phys. II France 6 (1996) 1441.

[5] S. Bhattacharya and Keith E. Gubbins, J Chem Phys. 123 (2005) 134907.

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