Pereira, José Carlos G.; (1997) Atomistic modelling of the structure and kinetics of silica-based sol-gel processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis concerns the study of the structural and kinetic aspects of silica-based sol-gel processes, which we investigate using ab-initio and molecular mechanics computer modelling techniques. It is divided into five parts. The first is a general introduction to sol-gel processes. It contains four chapters, which describe the historical evolution of sol-gel processes, their technological importance, the current scientific background in the chemistry of silica-based processes and a guide to the principal sol-gel bibliography. The second part is a general introduction to atomistic modelling. It is divided in four chapters. The first is a brief historical overview focusing on the crescent motivation to apply computing methods. The second reviews ab-initio theory. The third is a review of molecular mechanics theory and the fourth is a guide on the main atomistic modelling bibliography. The third part contains all density functional and Hartree-Fock studies, aiming to elucidate the structural growth and reaction mechanisms occurring in sol-gel processes. It contains seven chapters. Small species are analysed in chapter one to investigate the accuracy of the methods. The role of different silicate conformations is analysed in chapter two. In chapter three, the study of the silicate clusters is completed. All information is gathered in chapter four to create a growth kinetic model. The influence of the alkoxy groups is studied in chapter five. Solvation effects are investigated in chapter six. Hydrolysis and condensation reaction mechanisms are studied in chapter seven. The fourth part contains all molecular dynamics studies, aiming to create a realistic model to simulate sol-gel processes in solution. It contains four chapters. In the first, a model is developed to simulate liquids in a large range of thermodynamic conditions. This model is used in chapter two to study mixture and aggregation effects in diluted sol-gel solutions. The methodology developed in chapter one is enhanced in chapter three. This model is applied in chapter four to study aggregation and solvation effects in more realistic sol-gel solutions. The fifth part is an overall discussion with two sections. The first summarises the implications of this work and considers possible directions for future studies. The second contains general considerations about the present and future of computer modelling. The thesis concludes with an alphabetical list of bibliography covering all references cited in the work.

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