Visual Study On Stability And Ion Transport Of Emulsified Liquid Membrane

Emulsified liquid membrane, also called double emulsion, has attracted a lot of research interests due to their broad applications in various fields including industry, agriculture, food, cosmetics, pharmaceuticals and so on. However, the stability of emulsified liquid membrane and material transport between the phases are still not well understood, hence limiting its transportation, storage and the development of more commercial products. Therefore, it is necessary to investigate the stability of emulsified liquid membrane and the matter transfer between the interfaces through the liquid film.In this work, using the advanced capillary video-microscopy technique, the ion transport through a surfactant-containing emulsified liquid membrane and the coalescence between the internal droplets were visually studied by preparing emulsified liquid membrane system within the confined space of a thin-walled, transparent, cylindrical microtube. Therefore, the stability of double emulsion was unarguable investigated, and this visual study can also distinguish and provide unambiguous evidence for ion transport mechanisms. This visual technology eliminates the disadvantages of traditional methods. Additionally, benefited from this visual method supported by capillary video-microscopy technique, the formation of zirconia precursors with the two reverse-emulsions precipitation method was also visually studied. Through repeated experiments, the different mechanism from that reported in the previous studies was obtained, which is helpful for the practical application of the two reverse-emulsion precipitation method. On the theoretical side, two mathematical models on the interactions between different phases of emulsified liquid membrane were established to analyze the stability.Some major findings obtained in this work are as follows:1. Using a capillary video microscopy technique, the effects of surfactants with polyoxyethylene chains on the coalescence between the internal aqueous droplets of water-in-oil-in-water (W₁/O/W₂) emulsified liquid membrane system were investigated. By varying the concentrations of the above-mentioned surfactants, either water-soluble or oil-soluble, in the internal W₁ phase or in the oil membrane and visually observing the occurrence of the internal coalescence, it was found that all these surfactants with polyoxyethylene chains could lead to internal coalescence and the polyoxyethylene chain length was a key factor: surfactants with long polyoxyethylene chains could easily cause internal coalescence, while the one with shorter polyoxyethylene chains needed a higher surfactant concentration to achieve the similar coalescence. In addition, the presence of such water-soluble surfactants in the external aqueous phase was found to have no significant effects on the internal coalescence, but internal coalescence rapidly happened if the water-soluble surfactants dissolved in the internal water droplets, which was in agreement with the reported hole-nucleation theory.2. To theoretically study the stability of emulsified liquid membrane, a mathematical model for analyzing the van der Waals interaction between the internal aqueous droplets (W₁) and the external aqueous phase (W₂) has been established. Combined with the electrostatic interaction energy reported in the previous study, the interaction energy between the internal water droplets and the external aqueous phase of emulsion liquid membrane has been calculated. Variations of the interaction energy between the two aqueous phases were analyzed in different cases when there was no absorbed surfactant layer, only a single absorbed layer or there were two different absorbed layers at the W₁/O and O/W₂ interfaces.3. A simple theoretical model for analyzing the steric repulsion energy between the internal aqueous droplets and the external aqueous phase of emulsified liquid membrane, which is resulted from the steric interaction between the surfactant molecules adsorbed at the two interfaces, has been established. The steric interaction is dependent on the separation distance between the internal aqueous droplets and the external aqueous phase. the thicknesses of the two adsorbed surfactant layers, the size of the internal aqueous droplets and the oil globules. The thickness of each of the two surfactant layers have the same effects on the steric repulsion and stronger steric interaction can be achieved with thicker adsorbed layers, which can effectively prevent the coalescence between the internal aqueous droplets and the external aqueous phase. Increasing the internal aqueous droplet size can produce stronger steric repulsion; however, larger oil globules will weaken the steric repulsion, indicating that a more stable double emulsion system can be achieved by preparing the system with smaller oil globules and bigger internal aqueous droplets.4. Using the capillary video-microscopy technique, the ion transport at liquid-liquid interfaces and through a surfactant-containing emulsion liquid membrane was also visually studied. NaCl and AgNO₃ were selected as the model reactants and were prepared to form a NaCl/AgNO₃ pair across the oil film. By observing and measuring the formed AgCl deposition, it was found that both Cl^- and Ag⁺ could transport through a thick oil film and Ag⁺ was transported faster than Cl^-. Interestingly, the ion transport was significantly retarded when the oil film became extremely thin (< 1μm). The results suggested that the transport of ions mainly depends on the "reverse micelle transport" mechanism, in which reverse micelles with entrapped ions and water molecules can be formed in a thick oil film and their construction will get impeded if the oil film becomes extremely thin, leading to different ion transport rates in these two cases. The direction of the ion transport depends on the direction of the osmotic pressure gradient across the oil film and the ion transport is independent on the oil film thickness in the investigated thick range. Ions with smaller Pauling radius are easier to be entrapped into the formed reverse micelles and, therefore, will be transported faster through the oil film than bigger ions. Oil-soluble surfactant facilitates the ion transport;however, too much surfactant in the oil film will slow down the ion migration.5. The TBP-Span80-kerosene emulsified liquid membrane (ELM) system was adopted to treat wastewater containing Cr(VI) and the efficiency of ELM for removing Cr(Vl) was studied. The effects of pH, Span80, TBP and NaOH on both the stability and Cr (VI) extraction efficiency were analyzed.6. Finally, the formation of ZrO₂ precursors with the two reverse-emulsion precipitation method was visually studied using the capillary video-microscopy technique. Two types of aqueous droplets, i.e., one contained ZrO(NO₃)₂ solution and the other contained ammonia solution, were prepared in the oil phase to produce ZrO₂ precursors. It was found that the ZrO₂ precursors could hardly be formed by the coalescence of the emulsion droplets, even if they were in contact with each other. Instead, the formation of ZrO₂ precursors was mainly induced by ammonia diffusion across the oil phase. Interestingly, the growth of ZrO₂ precursors did not occur within any of the prepared aqueous droplets, which are generously considered as the space-limiting microreactors, as expected by many previous researches. The ZrO₂ precursors were mostly formed by the distortion and split of the aqueous droplets containing ZrO(NO₃)₂ solution. Increased surfactant content in the oil phase slowed down the formation of ZrO₂ precursors, probably due to the increased viscosity, which can hinder the distortion and split of the aqueous droplets. In addition, the growth of ZrO₂ precursors was also retarded by the increase of the size of the aqueous droplets.