| An ordinary emulsion is a dispersion system with limited kinetic stability rather than thermodynamic stability, which has been widely used in pharmaceutics, food. cosmetic, painting, coating, paper and petroleum industries. Nano-emulsion. possessing the characteristics of small size, narrow size distribution and kinetic stability, has better properties and good application foreground compared to other general emulsions. However, the knowledge about the formation mechanism and the stability modification of nano-emulsion is scarcely even now. The study on the formation and stability of nano-emulsion has attracted much interest of researchers. The O/W nano-emulsion of paraffin oil/Span20-Tween20/water was prepared with the method of emulsion inversion phase (EIP method) at constant temperature in the present dissertation. The phase behavior during the formation of nano-emulsion was investigated here. Besides, the effects of emulsifier concentrations and added ionic surfactants (CTAB, SDS), polymers (HEC, HMHEC) on droplet size and the stability of nano-emulsion were studied. With the measurements of interfacial tension. Zeta potential, surface pressure and interfacial rheology, the stabilizing mechanism of nano-emulsion were demonstrated. The aggregation behavior of HPAM and CTAB at O/W interface was studied with the method of interfacial dilational rheology. At the last, the application of CTAB in the treatment of oily wastewater from oil field was investigated. The dissertation is composed of six sections as below.In the first section, the importance of the study on emulsion stability and the progress on the formation and stability of emulsion and nano-emulsion were summarized.In the second section, the results of conductivity, observation using the polarized microscopy and rheology measurement were combined to study the phase behavior. With the measurements of droplet size. Zeta potential, the water removal after centrifugation and the photos taken on the settled samples, the effect of emulsifier concentration on the stability of nano-emulsion was investigated. Besides, the effects of the temperature change and the addition of electrolytes on the stability were studied to reveal the dominant mechanism of the stability. It was found that the system experienced the phase states of W/O micro-emulsion, lamellar liquid crystal or the bi-continuous phase in sequent as increasing the dosage of water untill the presence of O/W nano-emulsion with kinetic stability. As the dosage of emulsifier is above 3 wt%. the prepared nano-emulsions were settled stably and the droplet sizes were kept unchanged 35 days which indicating a weak Ostwald ripening effect of the nano-emulsions. As increasing the dosage of emulsifier. the droplet size decreased and the coalescene stability increased, accompany with the increase of the ability to resist the influence of temperature. That means the increase of the steric repulsion of adsorbed layer with the increase of emulsifier dosage. Coupled with the results of above, it can be deduced that the stability of nano-emulsion is influenced by not only the steric repulsion of adsorbed layer but also the electrostatic repulsion and the dominant one is steric repulsion.In the third section, the solutions of CTAB and SDS were used as water phase to prepare the nano-emulsions. respectively. The change of stability, droplet size and Zeta potential with the added ionic surfactants were compared. With the measurements of interfacial tension, the surface pressure and interfacial dilational rheology, the influence of CTAB and SDS on the stability of nano-emulsions were studied. From the results, it can be seen that the addition of CTAB and SDS can both reduce the droplet size due to the decrease of interfacial tensions and the increase of the micellar solubilization on the oil drops. The stability of nano-emulsion decreased with the increase of CTAB concentration while increased with SDS. Combined with the measurements of interfacial tension and surface pressure, it can be concluded that the adsorbed layer at oil/water interface is consisted mainly of Span20 and Tween20. The CTAB or SDS present in the adsorbed layer can increase the expanding property of interfacial film from the measurement of surface pressure. Compared to that of SDS, the increase of CTAB on the expanding property of interfacial film is more significant, that is because the longer hydrophobic chain of CTAB can increase the molecular area of the film. Coupled with the measurement of Zeta potential, the conclusion can be drawn that the adsorbed CTAB at interface can displace some non-ionic surfactants, neutralize with the negative charge, and reduce the effect of electrostatic repulsion and steric repulsion, eventusly the stability of nano-emulsion. However, the present SDS at interface increased the electrostatic and steric repulsive effects on the drops and thus the stability of nano-emulsion. The results of interfacial rheological measurements show that both CTAB and SDS can reduce the dilational modulus and the dilational modulus of system with CTAB added is smaller than that with SDS added. Therefore, the droplet size of nano-emulsions with CTAB or SDS added will be expected smaller than that without CTAB or SDS. The droplet size with CTAB added will be smaller than that with SDS added, which is well corresponding to the results of the droplet size.In the fourth chapter, the solutions of hydroxyethyl cellulose (HEC) and hydrophobically hydroxyethyl cellulose (HMHEC) were used as water phase to prepare nano-emulsions. The variations of stability and droplet size of nano-emulsions on the concentration of added polymers were studied. At the same time, the interfacial tension and rheology were measured to demonstrate the mechanism of the polymers on the stability of nano-emulsions. The experimental results show that both the two polymers can increase the stability of nano-emulsions. As the dosage of HEC is above 20mg/L. the coalescence stability has slight change. The water removal can fall down to about 0.2% with the increase of HMHEC dosage, indicating the significant increase of HMHEC on the stability. The droplet size decreased first and then increased with the increase of polymer dosage, the reason can be attributed to the influence of polymers on the film strength and the viscosity of system during the formation of the nano-emulsions. For the system with surface active HMHEC added, the oil/water interfacial tension will be reduced which enhanced the disruption of oil droplets and the decrease of droplet size on the one hand. On the other hand, the adsorbed HMHEC at interface increase the bending modulus of interfacial film, which is unfavorably to the change of the spontaneous curvature of the film and increase the droplet size. Besides, the viscosity increase of the system makes against the shearing dispersion because of the cross-linking between HMHEC molecules in the water phase, which is also unfavorable to the decrease of the droplet size. However, the added HEC exists mainly in the water core of W/O micro-emulsion with little adsorbed at oil/water interface, which promotes the change of spontaneous curvature of surfactant film and eventually the disruption of oil droplets. In addition, the thickening effect of HEC can also increase the droplet size. The rheological measurements show that HEC increased the viscosity of nano-emulsions without significant viscoelastic property, indicating the absence of network structures in the system. As the concentration of added HMHEC is at 500mg/L or 1000mg/L, the system exhibited significant viscoelastic property due to the formation of network structure, which can increase the stability of nano-emulsion. The results of interfacial tensions show that HEC in the bulk can decrese the amount of Tween20 and increase that of Span20 at interface through the formation of hydrogen bonds. However, the HMHEC molecules are ready to adsorb at interface, favoring to the formation of interfacial film with higher strength. That is one important reason for the stability increase of nano-emulsion.In the fifth chapter, the aggregation between CTAB and HPAM at the paraffin oil/water interface was studied via the measurements of interface tension and dilational viscoelasticity. The results indicate that the dilational modulus increases with the increase of frequency and decreases with the increased CTAB concentration. HPAM can increase the elastic property of the interface film and the dilational modulus remarkably due to the relaxation process of the complex at interface. Compared the adsorption and Gibbs elasticity of CTAB at different interfaces, the adsorption of CTAB at oil/water interface is smaller than that at air/water interface. The Gibbs elasticity at oil/water interface is bigger than that at air/water interface. The reason can be ascribed to the different state of two interfaces and the difference is given in schematic diagram. With the comparison between Gibbs elasticityεg and the measured dilational modulus, the conclusion can be drawn that the Gibbs elasticity reflects the equilibrium property of adsorbed layer while the dilational modulus considers more factors of the dynamic property. Besides, the choice of appropriate model is very important to calculate the Gibbs elasticity.In the sixth chapter, the application of CTAB and the influences of Al2(SO4)3 and HPAM in the treatment of oily wastewater from in oil field were studied. Together with the measurements of interfacial (surface) tension and Zeta potential, the clarifying mechanism of CTAB was demonstrated. From the clarification experiments, it can be seen that CTAB has good performance in the treatment of such wastewater. The addition of Al2(SO4)3 can increase the clarifying ability of CTAB, that is the promotion of A12(SO4)3 on the adsorption of CTAB to interface and the increase of the amount of CTAB micelle in bulk. With the results of turbidity and Zeta potential, the micellar flocculation mechanism of CTAB was demonstrated. That is. as the dosage of CTAB is below CMC, the positive-charged CTAB monomers can counteract the surface charge of the oil droplets and decrease the repulsive forces between the droplets. As the dosage is above CMC, the CTAB micelles can flocculate the adjacent droplets as bridge. With the further addition of CTAB. the symbol of surface charge of oil droplet is reversed, which increases the repulsive forces between the oil droplets again. Therefore, the transmittance reaches a plateau when CTAB dosage exceeds a certain value. The effect of HPAM on the performance of clarifying reagents is related to the adding sequence and amount. As added into the wastewater ahead to reagents. HPAM can increase the turbidity of system. On the contrary, a little HPAM can decrease the turbidity greatly as added behind the addition of other clarifying reagents.The character and novelty of this dissertation:1. The variations of nano-emulsion stability on emulsifier and added different surfactants (CTAB, SDS) and polymers (HEC, HMHEC) were systematically investigated. Through the measurements of droplet size. Zeta potential, rheology and the property of interfacial film of nano-emulsion, the stability mechanism of nano-emulsion and the effects of various factors on the stability were proposed. The dissertation is a good complement to the study on nano-emulsion and guidance to the preparation and stability modification of nano-emulsion.2. The interfacial dilational rheology and interfacial tensions were measured to investigate the aggregation behavior of CTAB and HPAM at oil and water interface. With the comparison of adsorption and Gibbs elasticity of CTAB at different interfaces, the influence of the arrangement of surfactant molecule at different interface on the interfacial dialational rheology was revealed. The difference between experimental dilational modulus and caculated Gibbs elasticity reflects the different natures of such two quantities.3. The interfacial dilational rheology and the droplet size of prepared nano-emulsion were linked in the dissertation, which is a helpful guidance to the modification on droplet size of emulsion.The shortage of this dissertation:The instrument of small angle X-ray scatter (SAXS) should be used in the study of phase behavior for the further verification with the observation using polarized microscopy and rheology measurement. In the study of influence of added surfactants on the property of interfacial film, the surface pressure was measured. That reflects only the spreading behavior of air/water interface and is limited for corresponding to the stability of oil/water interfacial films. The investigation on the treatment of oily wastewater was not sufficiently for the limitation of samples from oilfield.
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