Abstract
Molecular dynamics simulations of a coarse-grained model are used to study the formation mechanism of periodic mesoporous silica over a wide range of cationic surfactant concentrations. This follows up on an earlier study of systems with low surfactant concentrations. We started by studying the phase diagram of the surfactant–water system and found that our model shows good qualitative agreement with experiments with respect to the surfactant concentrations where various phases appear. We then considered the impact of silicate species upon the morphologies formed. We have found that even in concentrated surfactant systems—in the concentration range where pure surfactant solutions yield a liquid crystal phase—the liquid-crystal templating mechanism is not viable because the preformed liquid crystal collapses as silica monomers are added into the solution. Upon the addition of silica dimers, a new phase-separated hexagonal array is formed. The preformed liquid crystals were found to be unstable in the prese...
Highlights
Periodic mesoporous silicas (PMSs) are widely used in many applications such as catalysis, membrane separation, and drug vehicles.[1]
The results agree with the findings presented by Chen et al.,[45] who carried out the MCM-41 synthesis at surfactant concentrations of 38 wt % and characterized the selfassembly process using nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD) spectroscopy. 14N NMR showed that the hexagonal liquid crystal (HLC) phase was found at 22 °C before adding a silica source
We have investigated whether or not the liquid crystal templating (LCT) formation mechanism is consistent with our simulation data, even in systems where hexagonal phases form before silica addition
Summary
Periodic mesoporous silicas (PMSs) are widely used in many applications such as catalysis, membrane separation, and drug vehicles.[1]. The authors found that silica monomers induced micelle fusion, promoting a sphere-to-rod transition in the early stages of PMS formation We recently extended this CG model to describe different silica oligomers in MCM-41 precursor solutions.[22] Silica oligomers were found to promote the formation of hexagonal structures during the MCM-41 formation in systems with a very low surfactant concentration. In the original liquid crystal templating hypothesis[9] it is assumed that silicates would accumulate at the interfaces in a preassembled surfactant−water mesophase structure We investigate this possibility by performing molecular dynamics simulations with our CG model at conditions under which the surfactant alone can already form structured mesophases in aqueous solution. Our model is able to describe the formation of surfactant− silica mesostructures that are analogous to other PMS materials, such as MCM-48 and MCM-50.9 The remainder of this article is organized as follows: section 2 describes the model and simulation methods used section 3 details the results and discussion, and section 4 summarizes the work and offers concluding remarks
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