Silica and ionisable latex particles as novel emulsifiers
This thesis is concerned with the use of silica and latex particles as novel emulsifiers III solid-stabilised emulsions. This project was sponsored by GlaxoSmithKline, a world leading pharmaceutical company interested in the research and development of new products for consumer healthcare. Solid-stabilised emulsions offer the advantage of longer stability in comparison to surfactant-stabilised emulsions, and therefore the company wants to apply this technology in their products. Three oils with different functional groups, such as triglyceride, silicone and hydrocarbon were used to prepare these emulsions that will be discussed in detail in this thesis. The characterisation of emulsions was systematically studied by optical microscopy, light scattering, rheology, conductivity, stability and drop test. The wettability of solid particles was also investigated by measuring the contact angle of water drops on solid surfaces coated by either silanising agent or latex particles. It will be shown that a novel method of emulsion inversion has been discovered by increasing the particle concentration. The type and inversion of emulsions stabilised by silica particles containing triglyceride oils are highly dependent on the location and concentration of particles. The dispersion of particles in oil prior to emulsification resulted in the inversion of emulsions from w/o/w to w/o with increasing particle concentration. Same particles but dispersed in water did not induce the emulsion inversion, instead all emulsions were either w/o/w or o/w at any particle concentration. The same behaviour is also found in silica-stabilised emulsions containing silicone oil. It is shown that particle hydrophobicity also affects the inversion of emulsions. Silica particles of intermediate hydrophobicity (57 and 71 % SiO~) are able to stabilise both types of emulsions, and therefore the inversion occurs from o/w to w/o with increasing particle concentration in oil. However, very hydrophilic (87% SiOH) or very hydrophobic (36% SiOH) only stabilise o/w and w/o emulsions respectively. It is proposed that the inversion of emulsions is induced by the reduction of effective silanol content in situ due to the formation of hydrogen bonds between silanol groups with increasing particle concentration in oil. For the first time, emulsions stabilised by polystyrene latex particles can be successfully inverted using the same particle. The type, inversion and stability of emulsions containing hexadecane are very dependent on the pH, salt concentration, particle concentration and surface charge density in the aqueous particle dispersion. Carboxyl polystyrene particles of low surface charge density (σ= 12.7 µC cm⁻²) are able to stabilise both types of emulsions and therefore induce the inversion from w/o to o/w by increasing pH, salt concentration or particle concentration. However, same particles but with higher surface charge density (σ= 101 µC cm⁻²) did not cause the inversion as they only stabilise o/w emulsions at any condition. The same behaviour is also observed for polystyrene latex particles containing amino groups (aliphatic or diblock copolymer), as they have a high density of hydrophilic groups on the particle surface. The wettability of solid surfaces of both silica and latex particles is also investigated. Firstly, the oil-water contact angle in silica surfaces is dependent on the protocol and hydrophobicity of the solid. The contact angle is higher when water contacted the solid surface first, whereas it is lower when oil contacted the solid first. The hysteresis of contact angles is shown to be related to the hysteresis in the preparation of emulsions in which the particle location determines the emulsion type. Secondly, it is shown that particle films of polystyrene latex particles with diblock copolymers are dependent on the pH in the particle film. The hydrophobicity of the film increased as the pH increased due to the deprotonation of amino groups on the particle surface. The increase of contact angle is related to the decrease of both creaming and coalescence in emulsions stabilised by these particles.