Functionalization and characterization of aqueous silica sols and their application in Pickering emulsions

Licentiatavhandling, 2016

The main objective of this thesis is to create an understanding of how surface functionalized colloidal silica can be utilized in emulsions stabilized solely by particles, so called Pickering emulsions. To achieve this, water-based silica sols have been functionalized with hydrophilic and hydrophobic groups. The amounts of attached surface functional groups on the particles have been characterized using NMR spectroscopy, including NMR diffusometry. To further explore the attained properties of the modified particles, they have been studied using zeta potential and surface charge measurements as well as studies of how pH affects the flocculation of the functionalized silica sols. The emulsification abilities of the modified particles were evaluated by preparing Pickering emulsions using particles with varying degrees and combinations of hydrophilic and hydrophobic groups. In emulsion systems the wettability of the particles and the interfacial tension at the oil/water interface are relevant parameters, why surface and interfacial tension at the silica suspension/air and silica suspension/oil interfaces have been analyzed. Hydrophilic functionalization of the silica sols was achieved by attaching methyl poly(ethylene glycol) silane (mPEG silane) to the silica particle surface. This way provides an efficient reduction of surface charge density, a pH dependent and controllable flocculation behavior is achieved and surface activity of the particles is reached. These properties are all beneficial for emulsion formulation. The hydrophobic functionalization of the silica sols was accomplished by attaching organosilanes containing propyl, methyl and octyl groups. It was found that colloidal silica functionalized with hydrophobic groups produced emulsions with smaller emulsion droplets compared to using i.e. unmodified silica. The emulsification performance was however further improved by attachment of both mPEG silane and propyl silane, preferably in combination with one more organosilane. The balance between hydrophilic and hydrophobic groups is of high importance, where a high degree of mPEG silane renders particles too hydrophilic to be efficient as emulsifiers. When studying the effect of silica particle size, it was found that smaller particles reduce the emulsion droplet size distribution due to the larger surface area available for stabilization. Also the pH and the salt concentration are important for efficient emulsion droplet formation. A pH below 6 to 4, depending on the particle used, is beneficial and in some cases required for emulsion formation. The low pH provides bridge-flocculated particles owing to the mPEG silane functionalization. The Pickering emulsions obtained display a high stability towards coalescence over a long period of time (from five weeks to 1.5 years).