Particles at fluid interfaces : behaviour and derived materials

Rocher, Anais

Abstract

The objectives of this thesis are to enhance the understanding of the particle behaviour at fluid interfaces using novel and stimuli responsive particles, and how their adsorption at these interfaces affect the emulsions, foams or other materials that they are stabilising. Materials stabilised solely by particles are of great interest due to long-term stability, generally low emulsifier content and also in order to replace surfactant molecules, which are often potentially harmful with relatively inert solid materials. The adsorption/desorption of a particle from an interface depends on the particle wettability, which can be affected by the temperature, the pH or the liquid type used, to cite only a few examples. This is investigated through six different sections encompassing particle-stabilised emulsions, particle-stabilised foams and dry liquids.

The synthesis of stimuli-responsive particles and their use for production of stimuli-responsive materials is a recent area of interest particularly for bio-medical applications. It is shown in this thesis that temperature has a strong effect on the stability of water-in-oil emulsions stabilised by microwax particles. Separation of wax-stabilised emulsions can be controlled by changing the storing temperature of these emulsions: increasing the temperature results in melting of the wax, destabilising the emulsions. Conversely, the same wax particles give really stable emulsions at elevated temperature, due to potential release from the particles of surface-active molecules. Although it is observed more as a time than as a temperature effect, emulsions stabilised with biodegradable polymer particles undergo analogous separation. The initially high stability oil-in-water emulsions destabilise over time, most likely because of degradation of the polymer particles. It is observed that modification of the polymer particle surface, by grafting pHsensitive groups on their surface, hinders emulsion separation. It is also shown that sporopollenin particles, originated from natural Lycopodium clavatum spores, show a change in charge and wettability with pH. This leads to emulsion inversion from oil-in-water at their high natural pH to water-in-oil at low pH. Interestingly, the sporopollenin particles also exhibit preferred orientation around water droplets: the anisotropic sporopollenins orientate with their hemispherical side toward the oil either for a better packing geometry or due to a wettability difference.

The production of new particle-stabilised materials is another concern for this study. The production of novel emulsion drop architectures by using emulsion heteroaggregation has been attempted. Although aggregation of opposite-charge emulsion drops has been found difficult to obtain, the importance of pH, method used for mixing and excess of free particle in the continuous phase is discussed. It is also shown that the number ratio of small to large drops affects the drop aggregation. Another new material produced in this study is particle-stabilised non-aqueous foam. Fluoroethylene microparticles are observed to disperse in low surface tension oil, to stabilise air bubbles when aerated with intermediate surface tension oil, and to form a powder like material with high surface tension liquids. The effect of particle type, oil type and particle concentration on these foams are described, and freeze fracture electron microscopy is used in order to observe the close-packed arrangement of particles at the air-oil surface. Finally, production of a powdered emulsion is attempted in order to encapsulate low volume fraction of oils in a dry material. For this purpose, particle-stabilised oil-in-water emulsions were produced, before being blended with hydrophobic particles, resulting in an encapsulation of emulsion drops into particles. It is shown that the particle type, both for the initial emulsions and production of the powdered emulsions, the particle concentration, the blending time and the oil volume fraction affect the nature of the material obtained.

Citation

Rocher, A. (2011). Particles at fluid interfaces : behaviour and derived materials. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4212793

• Thesis Type: Thesis
• Deposit Date: Aug 16, 2012
• Publicly Available Date: Feb 22, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4212793
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Apr 1, 2011