Exploring functionalization of colloidal silica for nanoparticle-stabilized emulsions

Doctoral thesis, 2019

The main objective of this thesis was to evaluate how surface functionalized colloidal silica can be utilized in emulsions stabilized solely by particles, so called Pickering emulsions. To achieve this, water-dispersed silica nanoparticles were functionalized with hydrophilic and hydrophobic groups. The surface coverage of the functional groups was studied using NMR spectroscopy, including diffusometry. To further explore the attained properties of the modified particles, these have been studied using zeta potential and surface charge measurements. In addition, studies of how pH affects the flocculation of the functionalized silica sols have been performed. Hydrophilic functionalization of the silica sols was achieved by attaching methyl poly(ethylene glycol) silane (mPEG silane) to the silica particle surface. This resulted in an efficient reduction of surface charge density, a pH dependent and controllable flocculation behavior and surface active particles. These properties are beneficial for emulsion formulation. In addition, temperature-dependent phase-separation of the silica suspensions was attained. The observed cloud-points were influenced by interparticle interactions and conformational changes of the grafted PEG-chains, and these could be controlled by electrolyte concentration and pH. To increase the particle-oil interaction, hydrophobic functionalization of the silica particles was performed by attaching organosilanes containing methyl, propyl and octyl groups.

Emulsification performance was evaluated by preparing emulsions using particles, functionalized with varying degrees and combinations of hydrophilic and hydrophobic groups, as stabilizers. It was found that colloidal silica functionalized with hydrophobic groups produced emulsions with smaller emulsion droplets compared to using unmodified silica. The emulsification performance was further improved by attachment of both mPEG silane and hydrophobic organosilanes such as a propyl silane. The balance between hydrophilic and hydrophobic groups is important, 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 median emulsion droplet size, due to the larger surface area available for stabilization. The pH and the salt concentration are important for efficient emulsion droplet formation. Low pH conditions provide flocculated particles owing to the mPEG silane functionalization and the PEG-silica interactions. Pickering emulsions obtained display a high stability towards coalescence over a long period of time (from five weeks to at least five years).

By exploiting the clouding behavior observed for mPEG-grafted particles, phase inversions of butanol emulsions were observed, triggered through changing the temperature during emulsion preparation. Inversions were achieved in emulsions stabilized by particles modified with both mPEG and propyl silane, and the reversibility of the system was also studied. Electrolyte concentration and pH affect the phase inversion temperature, e.g. through salting-out effects and surface charge reductions, which decrease the inversion temperature. Understanding the phase inversion conditions of particle-stabilized emulsions could expand the use of these surfactant-free emulsions in industrial applications and facilitate emulsion fabrication.