Molecular dynamics simulation of the early stages of the synthesis of periodic mesoporous silica

We present results of detailed atomistic modeling of the early stages of the synthesis of periodic mesoporous silica using molecular dynamics. Our simulations lead to the proposal of a mechanism that validates several previous experimental and modeling studies and answers many controversial issues regarding the synthesis of mesoporous silicas. In particular, we show that anionic silicates interact very strongly with cationic surfactants and, significantly adsorb on the surface of micelles, displacing a fraction of previously bound bromide counterions. This induces an increase in micelle size and also enhances silica condensation at the micelle surface. The presence of larger silica aggregates in solution further promotes the growth of micelles and, by binding to surfactant molecules in different micelles, their aggregation. This work demonstrates the crucial role played by silica in influencing, by way of a cooperative templating mechanism, the structure of the eventual liquid-crystal phase, which in turn determines the structure of the porous material

Role of Lyotropic Liquid Crystals in Templating Mesosilica Materials

Here we consider the lyotropic liquid crystals and their role in templating the scaffolds of mesoporous silica materials. It was in 1992 that a Mobil Research group disclosed a method to produce silica particles having a regular network of pores with hexagonal and cubic symmetries. The method was proposed as a liquid-crystal 'templating' mechanism. Since the symmetries resulting in the silica scaffolds are those observed in the mesophases of lyotropic liquid crystals, the Mobil Research group supposed the presence of mesophases directly in the templating process. Here we discuss the method as it was reported in 1992 and what is today defined as the true or direct liquid-crystal templating LCT approach. It will be stressed that, in any case, LCT is a surfactant-assisted method that can be better defined as a supramolecular templating method. The template mainly happens in the form of a modified Stöber process. In this framework, the role of the curvature of silica-surfactant interfaces will be considered, the cubic phases of lyotropic liquid crystals analyzed in depth and the related surfaces with zero mean curvature discussed in detail

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

Engineering mixed surfactant systems to template hierarchical nanoporous materials

The mixing of the hydrocarbon surfactant cetyltrimethyl ammonium bromide (CTAB) and the fluorocarbon surfactant, Zonyl-FSN-100 with the average chemical structure of C8F17C2H4 (OC2H4)9OH, is quantified. The critical micelle concentration (CMC), the size and shape of the micelles and their composition have been investigated by surface tension, fluorescence, small-angle neutron scattering (SANS), electron paramagnetic resonance (EPR), pulsed-gradient spin-eco NMR spectroscopy (PGSE-NMR), 1H-NMR and 19F-NMR. The pure surfactant aqueous solutions and the mixtures have also been studied in the presence of hydrocarbon oil (hexane) and fluorocarbon oil (perfluorohexane, PFH) in order to investigate the swollen micelle shape and structure. The surfactants mix nonideally except for a degree of ideality at some CTAB mole fraction (0.5 > αCTAB > 0.7). The pure surfactant FSN-100 forms disc-like micelles with a small aggregation number

Lyotropic liquid crystals as templates for mesoporous silica materials

The paper considers the lyotropic liquid crystals and the role of their mesophases as templates to create the scaffold of mesoporous silica materials. It was in 1992 that a Mobil Research group disclosed a method to produce silica particles having a regular network of pores with hexagonal and cubic symmetries. The method was proposed as based on a liquid-crystal 'templating' mechanism. Since the symmetries resulting in the silica scaffolds are those observed in the mesophases of lyotropic liquid crystals, the Mobil Research group supposed the presence of a mesophase directly in a stage of the templating mechanism. Here we discuss the method as it was reported in 1992 and what is today defined as a true or direct liquid-crystal templating approach. It will be stressed that, in any case, the templating is a surfactant-assisted method, that can be better defined as supramolecular templating method. The template mainly happens in the form of a modified Stöber process. In this framework, the role of the curvature of silica-surfactant interfaces will be considered, the cubic phases of lyotropic liquid crystals analyzed in depth and the related surfaces with zero mean curvature discussed in detail

Formation of mesoporous SBA-15 from a colloidal perspective

The formation of the hexagonal ordered mesoporous SBA-15 material has been invetsigated. The evolution from free spherical micelles into an ordered material was studied with time-resolved in-situ SAXS, USAXS and SANS measurements. Both the mesoscopic region and the colloidal region were monitored. Temperature, precursor and salt additions have been varied. From the time-resolved measurements the events taking place in the formation of SBA-15 was identified and put on a time line. An additional step was found in the formation of plate-like particles, 7 smaller primary particles build up a larger secondary particle. The aggregation of primary particles was highly specific, an oriented aggregation. It was possible to hinder the oriented aggregation by diluting the synthesis reaction at a specific time. Moreover, a salt addition at this specific time give rise to particles with a very large diameter while the thickness was constant. A model to explain the morphology of the mesoporous particles was proposed. The model is based on colloid and surface chemistry arguments and stresses the importance of the surface free energy of the respective interfaces

Modelling the self-assembly of silica-based mesoporous materials

Periodic Mesoporous Silicas (PMS) are one of the prime examples of templated porous materials – there is a clear connection between the porous network structure and the supramolecular assemblies formed by surfactant templates. This opens the door for a high degree of control over the material properties by tuning the synthesis conditions, and has led to their application in a wide range of fields, from gas separation and catalysis to drug delivery. However, such control has not yet come to full fruition, largely because a detailed understanding of the synthesis mechanism of these materials remains elusive. In this context, molecular modelling studies of the self-assembly of silica/surfactant mesophases have arisen at the turn of the century. In this paper, we present a comprehensive review of simulation studies devoted to the synthesis of PMS materials and their hybrid organic-inorganic counterparts. As those studies span a wide range of time and length scales, a holistic view of the field affords some interesting new insight into the synthesis mechanisms. We expect simulation studies of this complex but fascinating topic to increase significantly as computer architectures become increasingly powerful, and we present our view to the future of this field of research

INVESTIGATION OF THE ASSEMBLY OF SURFACTANTS AT THE SOLID-LIQID INTERFACE FOR ADSORPTION AND MATERIALS APPLICATIONS

This dissertation addresses two topics associated with the assembly of surfactants at the solid-liquid interface for adsorption and materials synthesis. The first is the adsorption of an anionic fluorinated surfactant, tetraethylammonium perfluorooctylsulfonate (TEA-FOS), at the solid/liquid interface. Attenuated total reflection Fourier transform infrared spectroscopy is used to study the adsorption kinetics and average orientation of surfactants at the hydroxylated germanium surface. Atomic force microscopy provides complementary images of the adsorbed layer structure on mica. The adsorption follows unusual three-stage kinetics in which the rate of adsorption starts fast, slows as the surface becomes crowded, and then (surprisingly) accelerates due to nucleation of a heterogeneous multilayer structure. These fast-slow-fast three stage adsorption kinetics are observed for a wide range of concentrations at pH 6, and the rates of the three stages are modulated by pH and salt by tuning electrostatic interactions among surfactants, counterions, and the surface. The results suggest that tetraethylammonium mediates interactions between surfactants and with negatively charged surfaces. The dichroism measurements and AFM are consistent with a mechanism in which TEA-FOS first forms an incomplete layer with chains oriented randomly or somewhat parallel to the surface, followed by formation of flattened multilayer clusters with the chains oriented somewhat normal to the substrate. The second topic is the sol-gel synthesis of mesoporous silica materials using dual surfactant templates. Studies of templating with mixed cetyltrimethylammonium bromide and octyl-beta-D-glucopyranoside surfactants shows that the ternary phase diagram of surfactants in water can be used to predict mesoporous materials structure, and that vapor-phase ammonia treatments can either stabilize the structure or induce swelling by the Maillard reaction. Studies of sol-gel reaction-induced precipitation with demixed hydrocarbon and fluorocarbon cationic surfactant micelles show a wide variety of pore structures. A number of synthesis parameters are adjusted to tune the pore structure, for instance to adjust the size and populations of bimodal mesopores. Selective swelling of the two surfactants by liphophilic and fluorophilic solvents is observed. Finally, proteinaccessible hollow spherical silica particles with mesoporous shells are reported. The methods for engineering mesoporous materials reported here have potential applications in adsorption, controlled drug delivery and for catalysis