The Design, Preparation and Characterisation of Light-Responsive Pickering Emulsions

Change log
Richards, Kieran 

Light-responsive particle-stabilised (Pickering) emulsions can, in principle, be selectively emulsified or demulsified on-demand through the remote application of light. However, despite their wide-ranging potential in applications such as drug delivery and biphasic catalysis, their rational design is extremely challenging and there are very few examples to date. In this thesis, we investigate a model system based on silica particles functionalised with azobenzene-derived photoswitches to understand the key factors that determine the characteristics of light-responsive Pickering emulsions. We address their design on three distinct length scales: the photoswitch at the molecular level, the photoswitchable particle at the sub-micron scale, and light-responsive emulsion at the tens of micron scale.

Key to the creation of light-responsive Pickering emulsions is the design of photo-switchable molecules that can impart light-responsive behaviour. These molecules are attached to the particle-stabilisers and upon switching change the hydrophobicity of the particle, resulting in a change in the emulsion’s stability. Two groups of photoswitches are investigated in this work, which belong to the azobenzene and arylazopyrazole families. When irradiated with either UV or blue light, these molecules isomerise from a more hydrophobic trans state to a less hydrophobic cis state. Both groups are derivatised with hydrophobic or hydrophilic terminal modifications and by the addition of a carbon chain spacer. The arylazopyrazoles are also inherently more hydrophilic. Investigation of the optical properties of both families show successful photoswitching and, for arylazopyrazoles, an exceptionally high photostationary state (>90 %) and half-lives as long as 24 days. The photoswitchable molecules are then appended to fumed silica particle at three different grafting concentrations.

The effect of the grafting density and type of photoswitch on the hydrophobicity of the particle is first analysed by surface energy analysis, before and after trans-cis photoisomerisation. It is found that the length of the carbon spacer is the most important factor in controlling particle hydrophobicity and the azobenzene-modified particles show a greater difference in hydrophobicity when isomerised compared to the arylazopyrazoles. Emulsions are then produced using oils of different polarity and their stability and morphology assessed by optical microscopy. A computer-vision application is produced to help extract droplet size information from microscopy images using the circle Hough-transform. This method is benchmarked against the most commonly used alternative, a region growing technique, and it is found to have higher accuracy, recall and precision. The light-responsive behaviour of the emulsions is also assessed and, for the first time, a reversible transition between emulsified water-in-oil droplets and demulsified water and oil phases is observed with the application of either UV or blue light, which can be repeatedly cycled. Using the observed trends and data from the surface energy analysis of the particles, a set of design rules are presented which will help facilitate the rational design and, therefore, more widespread application of light-responsive Pickering emulsions.

Evans, Rachel
Computer Vision, Emulsions, Light-Responsive Materials, Particle-Stabilised Emulsions, Photochemistry, Photoswitches, Pickering Emulsions, Soft Matter, Stimuli-Responsive Materials
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
EPSRC (2111141)
EPSRC (EP/R513180/1) Postgraduate Studentship