Synergistic Effect Of Nanoparticles And Surface Active Materials On The Stabilization Of Pickering Emulsions

Emulsions are widely used in many fields such as road paving, foods, paints, cosmetics and pharmacy etc. The investigation of surfactant and/or polymer stabilized-emulsions is more mature than that of particle stabilized-emulsions. In the beginning of 20's century, Pickering found that, the colloid particles can be used to stabize emulsions. Masliyah et al. had found that, the clay and SiO₂ particles, being separated from crude oil emulsion system, can be used to stabilize emulsion with high coalescence stability. Recently, the mechanisms that a nanoparticle in the stabilization of Pickering emulsions are well understood. In some fields, they can be used to replace the common surfactants or reduce the surfactant dosage in a formula. In the same time, the detailed study in Pickering emulsions can be used as the basic theory of breakage of crude oil emulsions.The field of emulsions stabilized by surfactants and polymers are more mature than that stabilized solely by particles (so-called Pickering emulsions), and the contribution of particle combining with surfactants is still un-known by far. The silica and latex sphere particles are commonly used as model particle emulsifiers in preparation of Pickering emulsions. The properties such as preparation techniques, modification of particle surface and the control of uniform nature are investigated in details. The effect factors such as ion strength, dispersion pH, oil/water ration, contact angles, surface charge density are investigated detailedly for the silica particles solely stabilized-emulsions, and the effect of mixed particles with different surface hydrophobicity on the stabilization is studied by Dai et al. By now, the synergistic effect of surfactants (and/or polymer) together with particles are studied scarcely in system. Environment sensitive Pickering emulsion is also a kind of new material, which is potentially applied in many fields such as the control of drug release, cell populations, and mineral flotation. A new kind of environment sensitive particle emulsifier is synthesized successfully as oleic acid bilayer coated Fe₃O₄ nanoparticles, and firstly used in preparation of Pickering emulsion. Emulsion stability and phase inverse are controlled by adjusting simply the ion strength and dispersion ,. The physical properties of A200 particles are changed by physical adsorption of cationic surfactants, nonionic surfactant (block copolymer) and non-active short chain amine TEA (Triethylamine), and the synergistic effect of these species are investigated detailedly. Fe₃O₄ nanoparticles coated with oleic acid bilayer (a diameter about 12 nm) were synthesized. The structure and composition of the particles were characterized by TEM, FTIR and TGA. The structure and properties of modified Fe₃O₄ nanoparticles are sensitive to pH and ion strength of aqueous dispersion. So that, the Pickering emulsions stabilized by modified Fe₃O₄ particles are also sensitive to pH and ion strength.The experiment methods are as following: the coalescence and creaming stability of emulsions are determined by visual inspection of the oil and water creaming fractions; the long-term stability of emulsions can be comfirmed by the two methods of placing aside and super-centrifugation; the adsorption kinetics is studied by determing surface tension and interface tension of surface active materials and nanoparticles at the oil-water interface; the adsorption isothermals of nanoparticles and surface active materials are by depletion method; the adsorbed status of nanoparticles at the surface of emulsion droplets is studied by TEM; the size distribution of nanoparticles before and after modification, and the emulsion droplets can be determined by LDS and microscopic technologies; the wettability of nanoparticles can be expressed as the three-phase contact angle determined by modified table method; the surface charges of emulsion droplets and dispersed particles aggregates are determined by potentiometer analysizer to relate to the emulsion stability.So, the paper mainly contains four parts:1. Synergistic effect of CTAB and A200 particles in stability of Pickering emulsionsThe basic mechanisms underlying the stabilizing effect of nanoparticles together with surfactants are not completely understood yet. According to Binks, the increased stability is related to the presence of particles on the interface of emulsion droplets. In fact, particle engulfment exerts a remarkable resistance. Other mechanisms may relate to the formation of liquid bridges between particles in a configuration that may be particularly stable which is related to the interfacial viscoelasticity. The interfacial configuration of nanoparticles on the surface is strongly affected by the attachment of particles, which are affected by the CTAB adsorption significantly. To further investigate the synergistic effects of particles and cationic surfactants on the stability of emulsions, we prepare the emulsions by A200 particles together with CTAB and investigate the emulsions stability by the interaction between particles and surfactants under different environments (e.g. different dispersion pH and ion strength) on the oil-water interface with the methods of interfacial tension, adsorption and TEM (Transmission Electron Microscopy) imaging.The preparation and stability of the emulsions stabilized by A200 particles and cationic surfactant CTAB are investigated. Effect factors including NaCl concentration, aqueous dispersion pH, A200 particles concentration and CTAB concentration on the stability of emulsions are also investigated. The preliminary conclusions can be drawn: (1) The synergistic area of CTAB and A200 particles on the stability of emulsions is found: 10-3～3.0 mmol·g^-1 for CTAB and 0.1～5.0 wt% for A200. (2) The synergistic effect comes from three sources: CTAB can change the surface of A200 particles from being full hydrophilic to being appropriately hydrophobic; promote A200 particles into aggregates with appropriate size; reduce the oil-water interfacial tension to facilitate the formation of emulsions. (3) The interfacial adsorption of A200 particles increases with increasing CTAB concentration under [CTAB] < 1.0 mmol·g(-1), but decreases by desorption in the form of large flocs with further increasing CTAB concentration. The interfacial adsorption of A200 particles is also promoted with appropriate addition of NaCl, but not affected by aqueous dispersion pH. From TEM images, the interfacial structure of A200 aggregate is mixtures of monolayer and multilayers. (4) In synergistic area, the super-high coalescence stability of emulsions comes from the high elasticity of dense A200 particle layers around the surface of emulsion droplets, while the high creaming stability comes from the increase of viscosity induced by the networks formed by the particles around emulsion droplets and the particle aggregates in continuous phase. 2. Synergistic effect of block Copolymer F68 and A200 particles in stability of Pickering emulsionsIn this paper, a combination of a PEO-PPO-PEO copolymer F68 and colloidal silica should be a strong candidate for an effective emulsifying agent, which in turn might introduce the possibility of reducing the amount of costly copolymers employed in emulsification processes. The Synergistic effect in emulsion stability can be addressed here. As a result, a combination of copolymer and colloidal silica should be a strong candidate for an effective emulsifying agent, which in turn might introduce the possibility of reducing the amount of copolymers employed in emulsification processes. Also a study of the oil-water-silica nanoparticle-copolymer system may give insights into the relationship of interaction of nanoparticles and copolymers on the curved surface of emulsion drops and emulsion stability. In this paper, the stability of emulsions stabilized by combination of copolymer and silica nanoparticle demonstrates a clear synergy effects. The results should be useful in drug release, crude oil separation, and cosmetics.The preparation and stability of the emulsions stabilized by A200 particles and cationic surfactant F68 are investigated. Effect factors including NaCl concentration, aqueous dispersion pH, A200 particles concentration and F68 concentration on the stability of emulsions are also investigated. The preliminary conclusions can be drawn: (1) The synergistic area of F68 and A200 particles on the stability of emulsions is found: 0.01～1.0 g/g for F68 and 0.1～5.0 wt% for A200 particles. (2) The synergistic effect comes from three sources: F68 can promote A200 particles into aggregates with appropriate size; reduce the surface charge of particles to promote the interfacial adsorption of particle aggregates; reduce the oil-water interface tension to facilitate the formation of emulsions. (3) The interfacial adsorption of A200 particles increases with increasing F68 concentration under [F68] < 1.0 g/g, but decreases by desorption in the form of large flocs with further increasing F68 concentration. The interfacial adsorption of A200 particles is also promoted with appropriate addition of NaCl and reduction of aqueous dispersion pH (higher than ZPC). (4) The adsorption of F68 on the oil-water interface and particle surface is enhanced with increasing temperature, which facilitates the stabilization of emulsions. (5) In Synergistic area, the super-high coalescence stability of emulsions comes from the high elasticity of dense A200 particle layers around the surface of emulsion droplets, while the high creaming stability comes from the increase of viscosity induced by the networks formed by the particles around emulsion droplets and the particle aggregates in continuous phase.3. Synergistic effect of TEA and A200 particles in stability of Pickering emulsions The effect of non-active material TEA and A200 nanoparticles on Pickeringemulsions is first studied here. The stabilization mechanisms are elucidated in many aspects. The wettability of A200 particles is changed rather small by TEA than that in CTAB system.The Synergistic effect of TEA and A200 particles in stability of Pickering emulsions is investigated systemically. Preliminary conclusions can be drawn: (1) the stability to coalescence and creaming of emulsions decreases when there are redundant TEA molecules in bulk solution; while the stability greatly enhanced with removing the free TEA molecules in bulk solution; (2) when in lower TEA concentrations, the produced emulsion volume is so little and the emulsion stability is so weak, the stability increases appreciably with increasing A200 particles concentration due to the reduction of collision probability induced by dense particle bulk concentration. The weak stability can be attributed to the adsorbed amount of TEA molecules is so low that the particles keep in low hydrobicity; (3) when the TEA concentration is higher than 0.1 wt%, the emulsion stability is greatly enhanced for the flocculation and hydrophobicity of particles are properly modification. But the stability decreases with further increasing TEA concentration for the excessive free TEA molecules; (4) it can be found from the interface tension experiments that, the adsorption of TEA molecules and modified particles onto the oil-water interface behavior as a competitive relationship. The adsorption configuration and the adsorbed amount of A200 particles are affected greatly by the excessive interface adsorption of TEA molecules, which accordingly affect highly the stability of Pickering emulsions.4. Stability and phase inverse of Pickering emulsions prepared by bilayer oleic acid coated Fe₃O₄ nanoparticles The control of emulsion types is necessary for many practical applications. The emulsion type can commonly be tuned by changing the wettabilty of particles in situ with certain surfactants and W/O ratio. But these methods are limited to normal conditions. Recently, a new simple method has been developed to control phase inversion of emulsions by adjusting dispersion pH, ion strength and temperature. Binks et al. first studied phase inversion of emulsions stabilized by ionizable nanoparticles by adjusting aqueous dispersion pH and inorganic salt concentrations. Fujii et al. prepared a new kind of stimulus-responsive particle emulsifier by co-polymerizing 4-vinylpyridine in the presence of ultra-fine silica particles, and then cross-linked by ethylene glycol dimethacrylate (EGDMA). In addition, To Ngai et al. developed a new kind of PNIPAM microgel particle emulsifier. The stability of all emulsions mentioned above is sensitive to aqueous dispersion pH and ion strength. The stability of emulsions prepared by paramagnetic particles under an external magnetic field has been detailedly studied by S. Melle et al. But up to now, stable magnetic nanoparticles coated with surfactant bilayer have not been used to stabilize emulsions. In the present work, emulsification, stability and phase inversions of emulsions prepared by Fe₃O₄ particles coated with oleic acid bilayer in a mixture of paraffin oil and pure water at different aqueous dispersion pH and ion strength were studied. The results may be significant for the control of drug release, cell populations, and mineral flotation. The Fe₃O₄ nanoparticles coated with oleic acid bilayer (a diameter of about 12 nm) have been synthesized. The structure and composition have been analyzed by TEM, FTIR and TGA. The bilayer has been characterized by TGA experiments by its distinct two-stage mass loss. The packing densities of primary and second layer surfactant are both as high as that at the surface of pure solution. The partition of modified Fe₃O₄ nanoparticles is affected by aqueous dispersion pH values and ion strength. Accordingly, Pickering emulsions are also sensitive to aqueous dispersion pH values and ion strength. The phase inversion occurs at 1.00 < pH < 12.05, no phase inversion can be realized after complete desorption of the second layer surfactants at pH > 13.50. The ion strength can change the hydrophilicity -hydrophobicity, so as to adjust the phase inversion by NaCl concentration in aqueous dispersion. The hydrophilicity of particles can be further increased after entering oil phase by desorption of partial or full second layer surfactants. In the interfacial adsorption experiments, the hydrophobic Fe₃O₄ nanoparticles form particle clusters rather than uniformly distribute on the curved surface, while the hydrophilic particles form uneven multilayers on the curved surface in the central thick black spot and sparsely distribute in the other areas with a few particle islands.