Pickering Emulsions Stabilized By Nanoparticles Of The Metal Hydroxide And Their Stimuli-responsiveness

An emulsion is a system of dispersed droplets of one immiscible liquid in another. Although it has been studied for over a thousand years, only when Pickering and Ramsdon reported the role of particles in the stabilization of oil/water interface one hundred year ago, did people start to realize that particles can totally or partially replace surfactant. As emulsifiers, particles have lots of natural advantages, for example,(1) cut down on the amount of surfactant and save cost;(2)lower toxicity to creature than surfactant; (3)friendly to environment;(4)strong emulsion stability, especially to coalescence and little effect by environment conditions, so they have been widely applied to industries like food, pharmaceutical, oil recovery, agrochemical industries and coatings etc.. what is more, with the rise of nanotechnology, particle stabilized emulsions have also been applied to the assembly of nanoparticles, the preparation of hollow structure, capsule and some other structures and materials with specific functions. Thus, the adsorption property of nanoparticles at the liquid-liquid interface and its related emulsion stabilization mechanism has always been a hot issue internationally. In industrial applications or scientific research, particle stabilized emulsions may be temporarily desirable or undesirable in cases like viscous oil transportation, emulsion and micro-suspension polymerization, drug controlling release etc.. Therefore, to design a stimulus-responsive particulate emulsifier is necessary as well as urgent.In recent years, systematical studies have been conducted on the particle stabilized emulsions. Some laws have been summarized about co-emulsifiers, particle concentration, pH, salinity and the natural features of the particles. And lots of comprehensive studies have been carried out on the role of these factors in stabilizing the emulsions. At the same time, a number of stimulus-responsive emulsion systems have been developed. However, there are still lots of problems which need further study:(1) the assessment of pure inorganic nanoparticles on emulsion stability; (2) the effect of nanoparticles with different structures and shapes on Pickering emulsion; (3) the key point for particles used as stimulus-responsive emulsifiers is the surface wettability, is there any other manners? (4) more new characterization methods should be employed to deepen the study on the mechanism of not only Pickering emulsion stability but also some other phenomenon of emulsions, like Pickering non-spherical transformation.Based on the study on emulsion field above, this dissertation studied two types of hydrophilic metal hydroxide nanoparticles, magnesium hydroxide nanoparticles (MPs) and aluminum hydroxide nanoparticles (APs). Initially, we studied Mps and APs to serve as a particulate emulsifier on paraffin/water system. Then, we studied the non-spherical emulsion droplets transformation at different aging temperatures. The adsorption behaviors of particles at the Pickering emulsion surfaces, and the emulsion properties including emulsion stability and emulsion types are investigated systematically using visual observation, contact angle, optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and Laser-induced fluorescent confocal micrograph (LFCM) experiments. In addition, by total organic carbon (TOC) measurement, zeta potential analysis, interfacial intension measurement and Fourier transform infrared (FTIR) spectrometer, the interactions between particles and surfactants are further investigated in detail. The main contents of this dissertation are as follows:1. The assessment of Mps particles on the stability of emulsions and its pH sensitivityIn the past, to produce particulate emulsifiers were directly put the pH sensitive organic molecules or polymers with inorganic nanoparticles together. Through these processes, the particulate emulsifier would be pH sensitive; the principle behind it is that the emulsion stability as well as its type could be controlled by adjusting the particle wettability in the water phase. By adjusting the degree of dissociation or dissolubility of the organic molecules, particle wettability could be well controlled. However, is there any other approach to achieve such pH sensitivity? Guided by this issue, we studied in situ formed magnesium hydroxide nanoparticles as a stabilizer for paraffin/water system and evaluated its pH sensitivity. By studying the effects of particle concentration and pH on emulsion stability to resist creaming and the droplet size, it was demonstrated that in situ formed magnesium hydroxide nanoparticles are of superb emulsification ability and the emulsions produced strongly rely on the pH in the emulsion system. Through the observation of emulsion droplets with the laser scanning confocal microscope and transmission electron microscope, it was confirmed that Mps nanoparticles could affect the stability of emulsions and particle layers do exist in the system. Besides, it was also shown through a series of pH switch experiments that nanoparticles have excellent switchability, and the emulsion stability would hardly be affected by several switchable cycles. Studies have also been performed on the properties of Mps nanoparticles, including the effect of wettability on the interfacial tension, the zeta potential etc. The results proved that the pH sensibility of Mps nanoparticles was resulted from the pH of aqueous phase which affects the particle concentration of Mps nanoparticles. Other factors had minimum effect on its pH sensibility. The mechanism of emulsion pH sensibility could be concluded from the study as follows:(1) When the pH value was lower than the precipitation limit for Mps, the magnesium element existing in the water phase in the form of ion could not protect the interface of the oil-water system. (2) Over the precipitation condition for Mps, stable O/W emulsion could be produced due to the existence of Mps particles in the water phase. (3) Switching the pH value under the precipitation limit of Mps again, the emulsion would soon be demulsified and complete oil/water separation as the Mps particles in the stable emulsion would be completely dissolved to be ions again.2. APs particles as an emulsion stabilizer and its pH sensibilityPreviously we have studied the emulsion stabilized by MPs and its pH switchability, the key point is to control the form of the element of magnesium, being ion or solid, by controlling the pH in the water phase. However, the Mps are amphoteric, to be exact, it could be easily dissolved under both basic and acidic solution. Presumably it would be very interesting to apply Mps to the particle stabilized emulsion system, so we studied in situ formed APs used as an emulsion stabilizer for paraffin/water system and found that Mps particles alone can hardly produce stable emulsion either by controlling the particle concentration or the pH value in the system. The results may relate to the strong hydrophilic property of Mps particles and little interaction with oil phase. In order to find out the reason, we added a certain amount of surfactant to make the particles to be more hydrophobic and we observed three different types of surfactant, sodium dodecyl sulfate, sodium oleate and Brij30 separately. The results showed that the particle adsorption ability could be improved when the surfactant concentration was 10-5M, so does the stability of creaming and coalescence. With the change of pH, specific pH range would occur in the emulsion system. When surfactant concentration was 10-4M, the pH area, which could obtain stable emulsion would cover the whole pH range that were investigated, and the emulsion sensibility would disappear. By testing the interfacial tension, the change of particle wettability and the change of the adsorption of surfactant etc., we believe that the pH sensibility of mixture of APs and surfactant was mainly related to the synergy of the amphotericity of Mps particles and surfactant. The pH sensibility would disappear if excessive surfactant was added into the system. The primary reason for this phenomenon was that the emulsion could be stabilized by the extra surfactant in the whole pH range and therefore the pH sensibility could not be observed.3. The change of the stability of emulsion stabilized by Mps particles under different surrounding temperaturesIn our research on the emulsion stabilized by Mps particles, we considered the influence of temperature on the emulsion and surprisingly found that the Mps emulsion presented change from spherical droplets to non-spherical ones. In our experiment, the Mps particles prepared under room temperature could perfectly keep being spherical, even being placed for several months. However, when the newly prepared emulsion was placed under certain aging temperature for a period, the droplet form would change from being originally spherical to non-spherical. In typical experiments, we studied several factors that may affect the non-spherical degree, including aging temperature, particle concentration, oil/water ratio and aging time. The results showed that aging temperature working as an important driving force, could significantly affect the non-spherical degree. Too low particle concentration is adverse to the stabilization of the whole emulsion system, while too high particle concentration is adverse to the formation of non-spherical droplets. The effect of oil/water ratio on the formation of non-spherical droplets is similar to that of particle concentration; droplets with high non-spherical degree could be produced when the oil/water ratio was controlled in a certain range. Extending the aging time would have a positive effect on the formation of non-spherical droplets only on condition that the aging temperature would be higher than 80℃. Below this aging temperature, the effect would be significantly decreased. Besides, when aging time reached a certain point, the influence of it on the formation of non-spherical droplets would become less significant. Through the observation of MHps with the XRD and transmission electron microscope, it was found that the particles experienced obvious growth after the aging process. Moreover, we used the laser scanning confocal microscope and microscope to observe of the particles on the surface emulsion droplets, the results showed that at low particle concentration, the particles adopted on the surface of emulsion droplets are mainly in the form of monolayer or multiple layer, while at high particle concentration, most of the particles on the interface are in small aggregates. Same results could be got when oil phase was replaced by styrene. The oil phase is polymerized through bulk polymerization at aging temperature, and then the appearance of particle layer and non-spherical droplets could be clearly observed with scanning electron microscopy (SEM). Based on the results above, it could be concluded that three key processes are necessary for Mps particle stabilized emulsion droplets to undergo non-spherical transformation. (1) Spherical emulsion droplets would be formed when in situ formed particles are adsorbed on the surface of the emulsion droplets. (2) The MHps on the surface droplets would experience Ostwald ripening at the aging temperature which results for particle size increase and the total surface area decrease, so does the emulsion coverage. In this case,'vacant area'takes on. (3) Partial coalescence occurs when droplets collide with each other and form non-spherical droplets. (4) If the vacant area is too large, the oil phase would release from the particle shell and particle layer would break up.