Understanding RF gel formation : modelling fundamental nanoscale processes for enhanced material development

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2022
Thesis identifier
  • T16248
Person Identifier (Local)
  • 201779841
Qualification Level
Qualification Name
Department, School or Faculty
  • The wide-ranging application potential for porous materials has been of significant interest over the years, with a particular focus on those which possess attractive properties, such as low densities and high surface areas. Materials such as these have proven to be effective in a wide range of applications, many of which are imperative in reducing or eradicating detrimental environmental impacts of industry, heightening their pertinence to recent research. This work focuses on one such class of organic porous materials—resorcinol–formaldehyde (RF) gels—which are formed via a sol–gel process and subsequently dried, producing the lightweight, nanoporous structure of the final gel. Despite extensive research into these materials in recent years, a number of questions still remain around their formation mechanism and the impact of various parameters associated with their synthesis. As a result of this, their application potential is yet to be fully realised, especially given the wide range of properties that can be achieved through fine tuning and optimisation of the RF gel synthesis process. In this work, the formation mechanism of RF gels is explored through both experimental and computational means. Through experimental analysis of the final textural properties of synthesised gels, the impact of variations in catalyst concentration and catalyst species are investigated, aiming to elucidate the specific role the catalyst compound plays within the RF reaction – something that, to date, has been widely debated. The importance of the metal cation within the catalyst is highlighted through the results presented here, its concentration decoupled with the initial solution pH, and the significance of both discussed in detail. The comparative efficacy of different solvents used within the solvent exchange step of gel synthesis is also investigated, measured in their ability to preserve the structure during drying, minimising the pore shrinkage and collapse that takes place. The implications of the results obtained are discussed in relation to process optimisation to achieve desirable properties applicable to specific uses. The synthesis and analysis of RF gels is time consuming, therefore, simulating these processes computationally in an efficient manner could be pivotal to facilitating their widespread use. In this work, a three-dimensional model is developed which captures the formation and growth of RF gels using lattice-based kinetic Monte Carlo theory, accounting for varying catalyst concentration and solids content – two parameters proven to control gel properties in experimental work. The textural properties of the resulting simulated materials are analysed, including the accessible surface area and accessible porosity, the values of which reflect the increased structural density and inter-connected complexity associated with increasing solids content and catalyst concentration. Furthermore, the fractal properties of these materials are analysed through correlation dimension and Hurst exponent calculations, the results demonstrating that while fractal properties are not typically observed in scattering experiments for RF gels, they are possible to achieve with sufficiently low solids content and catalyst concentration. As the most commonly employed method of assessing properties of porous materials experimentally, adsorption analysis was carried out computationally for the simulated RF gel structures. The results indicated that both low catalyst concentrations and low solids contents resulted in structures with open transport pores that were larger in width, while high catalyst concentrations and solids contents resulted in structures with bottleneck pores that were narrower. Importantly, the computational isotherm data and pore size distributions were also compared to those obtained experimentally, showing a promising agreement in trends between the two for varying catalyst concentrations, providing validation for the kinetic Monte Carlo model developed. Finally, the performance of RF gels in a specific application is tested, assessed in their ability to remove an endocrine disrupting pollutant from water through UV-Vis concentration measurements. Using the results obtained from both the experimental and computational analysis of the materials, the comparative efficacy of two RF gels synthesised under different catalyst concentrations is predicted and subsequently explored, and the properties required for optimal performance determined. This work not only highlights the potential for RF gels to be used for vital environmental applications, but also introduces the potential way in which a computational model could be used to predict and tailor the properties of these materials for maximum effectiveness in a given application.
Advisor / supervisor
  • Fletcher, Ashleigh
  • Mulheran, Paul
Resource Type
Date Created
  • 2021