Pickering Emulsions Stabilized By Nanoparticles And Short-chain Amphiphiles

An emulsion is a thermodynamically unstable system which consists of dispersed droplets of one immiscible liquid in another. The system shows a tendency to break to reduce the interfacial area. Therefore a stabilizer is needed, including surfactants, surface-active polymers and solid particles. Emulsions stabilized by colloid particles, commonly named Pickering emulsions, have been studied extensively for their widespread use in practical applications, such as food, pharmaceutics, cosmetics, oil recovery and wastewater treatment since the beginning of20th century. In many of these applications, the stability of emulsions is necessary to keep their properties over long periods of time. Up to now, various theories about emulsions stabilized by solid particles have been developed and recently a thorough understanding of them has been achieved. According to these theories, particle wettability is a crucial parameter which determines the type and stability of the emulsions. Stable emulsions are prepared with particles of intermediate wettability.To meet the need of appropriate wettability for stability of emulsions, the particles are usually modified via addition of surfactants. There is strong synergy in most combinations of particles and surfactants in stabilizing emulsions. Cationic, anionic, and nonionic surfactants have all been applied successfully to tailor the wettability of particles in liquids and thus prepare stable emulsions. However, there is always competitive adsorption at the oil/water interfaces between the modified particles and the free surfactant molecules in the bulk solution. The free surfactant molecules can even dominate and determine the properties of emulsions. This will bring about many disadvantages in the practical applications of Pickering emulsions. The introduction of short-chain amphiphiles can resolve this problem effectively, due to the incapability of displacing particles at the oil/water interfaces. In addition, the short amphiphiles exhibit high solubility and critical micelle concentrations in water, enabling the surface modification of high concentrations of particles.Based on the study on emulsion field above, two kinds of hydrophilic particles with different shapes and opposite charges, layered double hydroxides (LDH) and Laponite, are investigated. First, liquid paraffin-in-water emulsions stabilized by Laponite particles modified with short-chain aliphatic amines (diethylamine, DEA or triethylamine, TEA) were prepared. Then, methyl orange molecules have been used as fluorescent amphiphiles in preparation of emulsions stabilized by LDH particles. The self-assembly of fluorescent particles at droplet surfaces is accomplished, which has been exploited to produce hollow colloidosomes. Nonspherical droplets in emulsions stabilized by LDH particles in situ modified with sodium alkyl carboxylates (sodium butyrate, C4Na or sodium benzoate, C7Na) can be obtained directly through emulsification. The presence of nonspherical droplets is mainly attributed to the partial coalescence of neighboring droplets.The present dissertation includes three topics.1. Pickering emulsions stabilized by Laponite particles modified with short-chain aliphatic aminesLiquid paraffin-in-water emulsions were successfully prepared by mixing oil with Laponite suspensions after in situ modification of the particles with short-chain aliphatic amines, DEA and TEA. The adsorption of DEA or TEA brings about enhancement of particle hydrophobicity and decrease of zeta potential, which make it easier for them to adsorb at oil-water interfaces. It was found that, at low clay concentration (0.5wt%), stable emulsions can be obtained when the clay-amine suspensions are flocculated. The emulsion stability improves with amine addition and reaches a plateau above a certain amine concentration. The variation of mean droplet size of emulsions is consistent with the emulsion stability. Emulsions of best stability are formed from the most unstable particle suspensions. The adsorption of the modified clay particles on the droplet surface results in emulsion stabilization, which was confirmed by laser-induced fluorescent confocal micrographs and SEM observations.Furthermore, extremely stable emulsions, where no oil and aqueous phase release within the observation time (6months after emulsification), were prepared in high concentrated suspensions (4.0wt%). The emulsion stability can be attributed to the particle adsorption onto the droplet surface, and more importantly, the gel structure of the suspension. This kind of emulsions with high particle loading and outstanding stability can be potential candidates for macroporous materials.2. Pickering emulsions stabilized by LDH particles and methyl orangeThe adsorption of colloidal particles on liquid droplets in Pickering emulsions has also been exploited to produce colloidosomes and macroporous materials. Emulsions stabilized by the functional particles could result in the materials with expected function. However, very few studies focused on the particle hydrophobization through the adsorption of functional amphiphiles, although it could offer a facile way to fabricate functional materials. Liquid paraffin-in-water emulsions have been prepared by mixing oil with LDH dispersions after in situ modification of the particles with MO. The roles of MO in the emulsions stabilized by LDH particles have been investigated in detail. MO can tailor the wettability of particles in liquids, confirmed by the increased contact angles and enhanced emulsion stability. Then the fluorescence of MO was exploited to offer the in situ microscopic images of particle adsorption onto droplet surface using confocal fluorescence microscopy. Finally, with the obtained emulsion as a soft template, hollow colloidosomes composed of MO-modified LDH with fluorescence were fabricated and observed with SEM.Owing to the fluorescence and amphiphilic property of MO, the LDH particles are rendered fluorescent and partially hydrophobic in a single-step process, leading to the self-assembly of fluorescent particles at oil/water interfaces. This could open further opportunities for the development of a facile way to fabricate fluorescent materials. Moreover, due to the fact that some dye molecules may affect the particles at liquid interfaces, we also offer a valuable warning for future investigations that fluorescent dyes should not be blindly employed as probes in Pickering emulsions.3. Nonspherical droplets of emulsions stabilized by layered double hydroxide particles modified with sodium alkyl carboxylatesNonspherical droplets of emulsions stabilized by layered double hydroxide (LDH) particles in situ modified with sodium alkyl carboxylates (sodium butyrate, C4Na or sodium benzoate, C7Na) were prepared directly through emulsification. The nonspherical degree of droplets as a function of sodium alkyl carboxylate concentration at different rotating speeds was discussed and the origin of nonspherical droplets was investigated.Based on the results of nonspherical degree, optical microscope images and releasing oil fraction, the generation of nonspherical droplets can be ascribed to partial coalescence of droplets with vacant holes in the particle layer induced by shear during emulsification. The increased contact angles of hydrophilic LDH particles (closer to90°) as a result of the increased sodium alkyl carboxylate concentration leads to the strong affinity of particles for the water/oil interface. Besides, the decreased zeta potential of LDH particles confers the decrease of the electrostatic repulsion among the particles. In this case, a denser particle layer will form at the oil/water interface. Besides, the decrease of particle surface charge density can also promote the attachment of particles at the droplet surface. These factors would facilitate the formation of a stronger structure composed of particles at the interface, corresponding to a lower value of deformation degree. SEM was employed to observe the morphology and surfaces of emulsion droplets. The nonspherical droplets were surrounded with the jammed plate-like particles, demonstrating that nonspherical shapes are preserved by the jammed particle shells resulted from the partial coalescence of neighboring droplets as well.