Study on Freeze/Thaw Induced Demulsification of Water-in-Oil Emulsions
|School||Dalian University of Technology|
|Keywords||Emulsion Demulsification Stability Coalescence Crystallization|
Freeze/thaw is a new physical method for demulsifying water-in-oil （W/O） emulsions. It has a strong capacity for breaking emulsions with good stability, high viscous oil phase or micro-particles, and it can also control the morphology of particles to form a shell structure in the preparation process of inorganic micro/nano particles by water-in-oil-in-water emulsion liquid membrane （W/O/W ELM）. However, the study of freeze/thaw induced demulsification is still in its start stage now. The microscopic process and mechanism as well as the driving forces of the demulsification are still unclear, and the effects of emulsion properties and operation parameters on demulsification performance need to be further investigated.In this work, the influencing factors and mechanisms of freeze/thaw induced demulsification were studied for the W/O emulsions with high and low viscous oil phases, respectively, as well as the difference of oil phase being frozen before or after droplet phase. Moreover, the solidification temperature of droplet phase （STDP） was closely related to the determination of freezing condition, but supercooling of droplet phase appeared in a common cooling process, thereby the reasons and the factors causing the supercooling were investigated first.Emulsions with different oil phases, droplet phases, surfactants, droplet sizes or water contents were prepared, and then their STDPs were determined by a differential scanning calorimeter （DSC）. It was found that the supercooling degree was dependent on the compositions of oil and water phases as well as the structure and composition of surfactant, while independent of water content, surfactant concentration, and interfacial tension. The effect of oil phase on the supercooling degree of droplet phase was generally restricted by the composition of droplet phase and interface, and it was just perceptible for the emulsion with deionized water used as droplet and small molecular surfactant with high vibration of polar group used as emulsifier. The key factors influencing the supercooling degree were the restraint of electrolyte in crystallization due to hydration, the vibration and hydrogen bounding effect of polar group of surfactant molecular. Supercooling degree increased with the increase of concentration of brine and the decrease of droplet size and the vibration of surfactant molecular at oil-water interface. The supercooling degree of droplet phase of the emulsion stabilized by succinimide （T151, T155 or T158） was 36℃, larger than that of the emulsion stabilized by sorbitan monooleate （Span80） （12.5 27.5℃）. And then, model emulsions were frozen/thawed via DSC and thermostatic storage （freezing by refrigerator, cryogenic bath, dry-ice or liquid nitrogen; thawing by atmosphere or water bath） to determine the demulsification performance and to optimize the operating condition. To determine the microscopic demulsification mechanism and influencing factors, and the crystallization and thawing behaviors of emulsion were examined by DSC and temperature controlled microscope, and the changes of microstructure of emulsion in freeze/thaw process were monitored by an optical microscope equipped with a home-made freezing system. In addition, the characteristics of liquid-liquid or solid-liquid interfaces were also analyzed.The results showed that the freeze/thaw demulsification proceeded in a gradual way for emulsions with high viscous oil phases, and several freeze/thaw cycles were needed for breakage of the emulsions with small droplets （≤5.5μm）, since demulsification of this kind of emulsion was mainly due to the squeezing effect on oil film performed by ice crystal. As for the emulsions with low viscous oil phases and the droplet phases that can be frozen before the oil phases on cooling, their dewatering ratios were relatively low and greatly dependent on freezing conditions, because the demulsification was caused by droplet collision between the liquid and frozen droplets which was resulted from the volume expansion of frozen droplets and hindered settlement. While the emulsions whose oil phases were frozen prior to droplet phases were easily broken and their dewatering ratios were generally over 80%. For this case, the following processes can be described the demulsification. On cooling, oil phase froze first, and then the crystalline oil phase was broken with fine gaps/crevasses when droplets froze with volume expansions. The gaps/crevasses were impregnated with some unfrozen aqueous solution due to capillary action during the gradual process of freezing phase transition of droplet phase, thus forming a large network that bridged droplets. Subsequently, on heating, the network fused, leading to a coalescence of droplets or phase inversion due to interfacial tension.Freezing/thawing conditions （temperature and method） strongly affected the demulsification performances. The demulsification performance was high when freezing temperature was lower than the solidification temperature of droplet phase （STDP） which in turn led to the totally freeze of droplet. Freezing by cryogenic bath or dry-ice was usually superior to freezing by refrigerator and chilling by liquid nitrogen for demulsification. While, the demulsification performance resulted from being chilled in liquid nitrogen was largely dependent on emulsion properties. The paraffine emulsion with highly viscous oil phase was demulsified with over 80% of dewatering ratio by this method. As for the emulsion with low viscous oil phase, if the droplet size was larger than 10μm or water content was equal to or higher than 70%, this freezing method could lead to a similar high dewatering ratio as that obtained by cryogenic freezing and dry-ice freezing. As for the emulsion whose oil phase was frozen prior to droplet phase, it was greatly demulsified by this freezing method, and in most cases, the dewatering ratios reached the maximum, nearly 100%. Moreover, it was also found that thawing in ambient air was superior to that of using water bath for water removal.The investigation of the effects of emulsion system parameters on demulsification showed that the demulsification performances were obviously increased with water content and droplet size regardless of the characters of oil phases. Dewatering ratio increased almost linearly as water content was lower than 60%, and slowly as water content was over 60% where the dewatering ratio was over 80%.Finally, the effect of inorganic salt on demulsification performance was investigated. Sixteen kinds of neutral inorganic salts were dissolved in deionized water and used as droplet phases, and then their emulsions were frozen/thawed via themostatic storage in different conditions. It was found that the addition of salt changed the crystallization and thawing behaviors of droplet phase in terms of the volume expansion ratio of phase transition, crystallization rate and crystal configuration at interface, thus resulting obvious distinctions of demulsification performance between emulsions. The volume expansion ratios of phase transition of NH4Cl, KCl, NaCl, BaCl2, NH4NO3, KNO3, NaNO3, （NH4）2SO4, K2SO4, and CuSO4 aqueous solutions were 1.078-1.093, near to the 1.088 of deionized water. Their kerosene emulsions were demulsified by a single freeze/thaw cycle with dewatering ratio of over 30%. Therein, the addition of NH4Cl, KCl, BaCl2, K2SO4, CuSO4, NH4NO3 or （NH4）2SO4 promoted the crystallization rate fast, thus their kerosene emulsions obtained the higher dewatering ratio, over 70%. However, the emulsion with the MgCl2, CaCl2, FeCl3, Mg（NO3）2, Cu（NO3）2 or Fe（NO3）3 aqueous solution as droplet phase was hardly demulsified, the dewatering ratio was less than 10%, since the volume expansion ratios of phase transition of these aqueous solutions were small, less than 1.055. Moreover, it was found that 1.0 M CuSO4 aqueous solution-in-kerosene emulsion was demulsified easer than other emulsions, which might lie in the stronge cavitation on thawing.