In situ preparation of stabilizing units for Pickering emulsions and some applications

Abstract

Self-assembly is a very versatile tool for the fabrication of materials on the nanoscale and plays a predominant role in the formation and stabilization of both classical (surfactant stabilized) and Pickering (particle stabilized) emulsions which for their part find numerous applications in e. g. biomedicine, cosmetics and food industry. Furthermore, emulsion droplets can serve as templates for different materials as well as compartments for reactants or chemical reactions. One of the most important tasks with respect to both the preparation and the application of emulsions is to understand the underlying self-assembly process and gain control over it as this enables the rational design of emulsions with specific physical and chemical properties. The different projects covered within this thesis focus on the in situ preparation of stabilizing units for Pickering emulsions by different self-assembly processes at the oil-water interface. Subsequently, the as-prepared emulsion droplets are modified either physically or chemically. Although in any case the miniemulsion technique is applied in order to obtain emulsions with droplet sizes in the sub-μm range, two fundamentally different systems are investigated: on the one hand, commercially available hydrophilic silica nanoparticles are hydrophobized in situ by the self-assembly with neutral polymeric hydrophobizing agents and consequently used to stabilize aqueous droplets with a diameter between 180 and 450 nm in an oily continuous phase. In the first step, the adsorption behaviour of the hydrophobizing agent on silica nanoparticles as well as the influence of size and charge of the particles, kind and amount of hydrophobizing agent and the composition of the aqueous dispersed phase on emulsion characteristics is evaluated. In the second step, the initially milky-white emulsion is heated to evaporate water from the inside of the droplets and subsequently change the composition of the aqueous phase. As the stabilizing particles are small compared to the wavelength of visible light, one emulsion droplet and the corresponding stabilizing nanoparticles act as one single scattering object. This enables the fine-tuning of the refractive index (RI) of the emulsion droplets by evaporation of water and consequently the preparation of Pickering emulsions with a remarkable transmittance of up to 86 % across the visible spectrum without adjusting the RI of the particles. This property is unique in the field of Pickering emulsions as the only reports on highly transparent particle stabilized emulsions deal with polymeric particles whose RI is carefully matched with the one of both the continuous and the dispersed phase before preparing the emulsion. The second emulsion system is used to establish a completely new approach of stabilizing emulsions. Water-soluble organic dyes are shown to self-assemble into aggregates in situ at the oil-water interface and these dye aggregates act as very efficient stabilizers for oil-in-water emulsions. Thus, in contrast to conventional Pickering emulsions, the stabilizing particular units are formed in situ instead of being already employed in the form of nanoparticles. By comparing the stabilization behaviour of fluorescein as an example for water-soluble dyes with the one of the classical amphiphile sodium dodecyl sulfate (SDS) exemplarily, similarities and differences between surfactants and dye aggregates as stabilizers for direct miniemulsions are revealed. Different parameters such as interfacial tension, concentration of the stabilizer, salinity of the aqueous phase, pH-value and polarity of the organic oil are taken into account and discussed. With respect to potential applications, not only the stabilization but also the controlled destabilization of emulsions might be of special interest. For Pickering emulsions stabilized by dye aggregates, different methods of demulsification including heating, addition of electrolyte, change in pH-value and addition of solid adsorbents are investigated and show the great potential of this new kind of stabilizer regarding the controlled phase separation of emulsions which are typically stable for years when letting unaffected. Since it could be shown that styrene can be employed as the organic liquid in dye stabilized emulsions, the follow-up work focusses on the free radical polymerization of styrene in emulsion droplets stabilized by fluorescein and alizarin yellow. The system containing alizarin yellow is investigated in detail with respect to the influence of dye concentration and ultrasonication time on the resulting latex as well as the nucleation mechanism of the polymerization. In the last step, oil droplets containing the silica precursor tetraethyl orthosilicate (TEOS) are stabilized by different anionic and cationic dyes and the hydrolysis and condensation reaction at the oil-water interface is investigated. By the choice of the proper dye, either one silica capsule or many monodisperse sub-20 nm silica particles are obtained from one single emulsion droplet. The formation of small nanoparticles in emulsions stabilized by negatively charged dyes is consequently investigated as this reaction is unexpected while the formation of silica capsules has already been described in literature for emulsions stabilized by e. g. cetyltrimethylammonium bromide (CTAB). As the oil-water interface in miniemulsions is much bigger than macroscopic interfaces, the kinetics of hydrolysis and condensation reaction is increased significantly which enables the synthesis of silica particles at ambient temperature and pH-value under static conditions. As organic solvents can be abandoned completely and the stabilizing dye can be removed with the help of solid adsorbents easily, particle dispersions comprising only water at neutral pH, particles and traces of ethanol can be prepared in accordance with the idea of “green chemistry” which is in contrast to literature known synthesis routes to sub-20 nm silica nanoparticles. In order to get a better understanding of the reaction mechanism, several parameters such as concentration of the dye, salinity, pH-value, reaction temperature, reaction time and kind of the stabilizing dye are taken into account and varied systematically.

In summary, different self-assembly processes were investigated in order to understand their role in the formation of stabilizing units for Pickering emulsions with droplet sizes in the sub- μm range. The respective findings contribute to the subsequent rational physical or chemical modification of emulsion droplets. Hence, the preparation of highly transparent Pickering emulsions stabilized by commercially available silica nanoparticles in a very simple two-step process without adjusting the RI of the stabilizing particles could be implemented. On the other hand, water-soluble organic dyes which are frequently applied for colouring applications were introduced as new building blocks for the in situ preparation of dye aggregates which act as a kind of molecular scale Pickering stabilizer in direct miniemulsion systems. These emulsion droplets were successfully used as templates for i) the free radical polymerization of styrene inside the droplets which enables the preparation of surfactant free latex and ii) the interfacial hydrolysis and condensation reaction of TEOS which results in either silica capsules if positively charged dyes are employed as stabilizers or sub-20 nm silica particles in the case of anionic dyes. As the dyes are easily removable from the emulsion system by the addition of an appropriate solid adsorbent, these emulsions can be separated in a stabilizer-free oil and a stabilizer-free aqueous phase in a very controlled manner which is unique in the field of both Pickering and classical emulsions. Thus, dye stabilized emulsions offer a great potential for several applications including the compartmentalization of chemical reactions as they enable a very simple work-up.