The use of bio-inspired methods for production of mesoporous silicas could lead to significant improvements in the synthetic conditions at which these materials are traditionally produced, removing the need for strong pH as well as high temperatures and pressures, and opening the way for milder treatments for template removal. However, due to the complexity of these systems, with many processes occurring simultaneously and high dependence on the specific synthetic conditions,obtaining a detailed description of their mechanism of formation based only on experimental methods is often very difficult. To overcome this difficulty, simulations methods, particularly molecular dynamics, have been developed and used to shed some light into this complex but fascinating problem.In the present thesis, the processes underlying the synthesis of bio-inspired silica materials are investigated at computational level by means of a multi-scale approach.This methodology has two major advantages: it enables to explore longer time and length scales, beyond the current limit of atomistic simulations, while allowing to maintain realism at the lower resolution levels, which are calibrated to match properties obtained at higher levels of theory.The work can be divided into two main parts. The first part aims to provide more insight into the synthesis of two early examples of bio-inspired materials(HMS and MSU-V), by means of a combination of atomistic and coarse-grained simulations. HMS and MSU-V materials share some common characteristics: they are both synthesised using amine surfactants as templates and a neutral templating route has been proposed to explain their formation. By simulating their synthesis at different pH values, it was possible to show that charged species are necessary to promote mesophase formation (disordered packing of rod-like micelles for HMS materials and lamellar structures for HMS).In both systems, in fact, neutral species produced phase separation of the templating materials into an unstructured and non-porous phase, and the lack of interactions with silicates indicates that these conditions cannot lead to structural organisation. Hence, molecular dynamics simulations reveal that, similarly to other mesoporous silicas and contrary to what has been previously hypothesised, charge matching interactions rather than hydrogen bonds are responsible for the self-assemble this class of materials. This knowledge is fundamental to provide further control over the properties of these solids and target their design for specific applications.In the second part, atomistic simulations are used to help elucidate the mechanism of template removal from a bio-inspired silica material by solvent extraction.This revealed that mild post-synthetic acid treatments allow to remove the templating additive by reducing, and eventually switching off, its interaction with the silica material. In agreement with experimental findings, which show that the majority of the additive is removed between pH 5 and 4, simulations indicate that at pH below 4.2 thermal fluctuations are sufficient to cause widespread release of the template. This result suggests that molecular simulations can be used as a simple and inexpensive tool for choosing appropriate solvents and experimental conditions in the material purification processes.
|Date of Award||7 May 2017|
- University Of Strathclyde
|Supervisor||Miguel Jorge (Supervisor) & Jan Sefcik (Supervisor)|