Designing novel colloidal materials from polyphenol-polymer complexes

This thesis explores the utilization of physico-chemical interactions between natural polyphenols and biocompatible polymers to design novel materials and complex colloidal dispersions and emulsions. These usually unwanted interactions and the resulting insoluble complexes are important in the creation of structures that could potentially be used in the design of novel foodstuffs. The theme of this thesis is therefore the interactions of polyphenols with polymers and emulsion stability and properties. The results are as follows. Chapter 3 presents a study on the interaction of polymer-polyphenol and the fabrication of new colloidal particles using the self-association of polymer-polyphenol complexes. The phase diagram of the physical appearance of mixtures of polyvinylpyrrolidone (PVP) with catechins and tannic acid has been explored and we have characterized the different structures by using a combination of light microscopy, UV-Vis and IR spectroscopy and dynamic light scattering techniques. The molar ratio between polymers and polyphenols has been found to play an important role in defining the structure of the complexes and their stability. By further optimizing the reaction conditions it is possible to obtain various structures resulting in colloids, microgels, and macroscopically gelled interfaces, which find applications in emulsion stabilization as explored in the following chapters. In Chapter 4 emulsification techniques and consideration on emulsion stability and formulation are investigated. Novel biodegradable surfactant free emulsions stabilized by non-covalently associated micro-gel particles made from PVP and TA have been successfully prepared. The strong interaction between PVP and TA, driven by hydrophobic interactions and hydrogen bonding between phenolic and pyrrolidinone rings, leads to formation of microgels during the emulsification process. The molar ratio between TA and PVP was found to play an important role in determining the mechanism of emulsion stabilisation. At low molar ratio a shell of PVP-TA microgels is formed and over time wrinkles have been observed on these shells, while at high molar ratio [TA]/[PVP] we observe Pickering emulsions stabilised by microgel particles and the continuous phase is a dispersion of microgel particles. The structure and stability of the systems have been investigated by light scattering, confocal and light transmission microscopy. The presence of TA leads to halochromism of the emulsions at high pH and the absence of surfactants makes these emulsions particularly desirable in terms of biodegradability. Chapter 5 focuses on the rheological properties of emulsions stabilised by gel particles and on the relationship between oil volume fraction and rheological behaviour. The rheological properties of emulsions and complex fluids are of pivotal importance in relation to their technical applications. These properties depend strongly on the emulsions composition, oil-phase volume fraction, microscopic droplet structure, and interfacial interactions. Our rheological tests showed that PVP-TA particles stabilised emulsions behave like flocculated dispersions and polymeric gels confirming the qualitative observation on emulsion gelation observed in the experiments carried out in the previous chapter. Chapter 6 introduces a new technique, microfluidics. Microfluidics devices were designed and built with the purpose of making monodisperse emulsions stabilized by PVP-TA complexes. The emulsions produced with microfluidics devices were found to be monodisperse rather than the polydisperse ones resulting from bulk emulsification processes. The basic theory behind the effect of microfluidic scaling affects the hydrodynamics is outlined. Experimental details on the design process and on how to use photo-lithography and soft-lithography to build microfluidics devices are given. Finally we have demonstrated how our microfluidics devices can be successfully used to make monodisperse emulsions stabilised by PVP-TA and double emulsions (oil in water in oil emulsions).