Inorganic Salts-Tuned Phase Inversion Temperature Method For Preparation Of O/W Nano-emulsions

Nano-emulsions with small droplet size (between 20 and 500 nm), often appear transparent or translucent to the naked eye and possess long-term physical stability, showing properties superior to the conventional emulsions. Compared with microemulsions, nanoemulsions can be prepared using moderate surfactant concentrations, which has great attraction for practical applications in many fields, such as cosmetics, pharmaceuticals, miniemulsion polymerization, and so forth. Conventionally, nano-emulsions are prepared by high-energy emulsification methods using high-pressure homogenizers or ultrasound generators to generate high mechanical energy and produce fine droplets. Since the 1980s, several low-energy emulsification methods (phase inversion temperature, PIT method and emulsion inversion point, EIP method etc.) have been proposed and been applied to the preparation of nano-emulsions. These methods take advantage of the chemical energy stored in the ingredients and produce the nano-emulsions almost spontaneously, thus have great attraction both in theoretical study and practical application.O/W nano-emulsions prepared in nonionic surfactant systems by low-energy emulsification methods have been researched extensively. However, they are usually negatively charged and the zeta potential cannot be controlled. Since most materials in nature provide negatively charged surfaces, positively charged nano-emulsions would be advantageous to adsorb on their surfaces rather than negatively charged nano-emulsions, and, therefore, are potentially significant in the areas where negatively charged surfaces are involved. Nano-emulsions stabilized by mixed ionic and nonionic surfactants have been widely studied by high-energy emulsification methods,which would generate large costs for industrial implementation, and only a few studies have been published using low-energy emulsification methods. But the electrophoretic properties and effects of additive such as inorganic salts (including the special silicate) were not investigated in these reports. However, electrophoretic properties are very important to the nano-emulsion stability and applications, and salts are frequently added into emulsions for various purposes or occur as impurities. Thus, study of salts effect on O/W nano-emulsions and preparation of positively charged nano-emulsions in mixed ionic-nonionic surfactant systems using one-step cross-PIT or sub-PIT method are interesting and significant to both theoretical and practical researches.Liquid paraffin has been applied in many fields as a component of O/W emulsions, mostly as macro-emulsions. However, the formation of liquid paraffin emulsions with submicron droplets using the PIT method has occasionally been reported. Tween 80 and Span 80 are Polyoxyethylene (20) sorbitan monooleate and sorbitan monooleate, respectively. They are generally regarded as non-toxic and non-irritating and widely used in food products, oral pharmaceuticals etc. It is well-known that certain mixtures of surfactants can provide better performance than a single surfactant for a wide variety of applications. In this dissertation, we used a mixture of Tween 80 and Span 80 that show synergism in the dispersion of paraffin oil in O/W nano-emulsions.In this context, liquid paraffin O/W nano-emulsions are prepared in nonionic surfactant and cationic-nonionic mixed surfactant systems using the PIT method. First, O/W nano-emulsions stabilized by Tween 80/Span 80 were prepared and the influence of different kinds of inorganic salts on the formation and properties of the emulsions was studied. Second, positively charged O/W nano-emulsions were prepared by one-step sub-PIT method in Tween 80-Span 80-CTAB systems. The mechanism of the emulsification is proposed. In addition, this method can be simply extended to other ethoxylated nonionic and cationic/anionic surfactants.The present dissertation includes three topics.1. Preparation of O/W nano-emulsions stabilized by nonionic surfactants using the inorganic salts-tuned PIT methodParaffin oil/water nano-emulsions stabilized by Tween 80/Span 80 were prepared using the PIT method in the presence of inorganic salts. The influence of different kinds of inorganic salts on the PIT, electrophoretic properties and long-term stability of the nano-emulsions was studied by conductivity measurements, zeta potential measurement and dynamic light scattering. It was found that the salts can tune the PIT of the systems, and that ability is related to the ionic strength, type and valence of the salts. The anions Cl-, SO42- and monovalent cations Na+, K+ reduce the PIT of the systems, but the di- and trivalent cations Ca2+, Al3+ raise the PIT of the emulsions, and this ability becomes strong with increasing the ionic strength. The zeta potential of the original nano-emulsion droplets is negatively charged (about -36 mV) due to the specific adsorption of the hydroxyl ions. The addition of salts reduced the absolute value of zeta potential of the nano-emulsion droplets due to their counterion binding effect, and, particularly, AICl3 brought about a charge reversal. The corporate salting-out/in effect and reduction of zeta potential by the salts lead to various emulsion droplet size evolution with time, and the mainly instability mechanisms of the nano-emulsions were found to be Ostwald ripening and/or coalescence. It was found that, for a system with high PIT, an optimum PIT can be obtained in the presence of salting-out salts, and thus a stable emulsion can be prepared. This is significant to the practical application.2. Preparation of O/W nano-emulsions stabilized by nonionic surfactants using the silicate-tuned PIT methodFirst, the liquid paraffin O/W nano-emulsions were prepared by the PIT method. Then, the effects of water dissoluble silicate (Na2SiO3 and K2SiO3) on the PIT, initial stability, viscosity, electrophoretic properties of the nano-emulsions were studied by conductivity measurements, zeta potential measurement, rheology measurements and dynamic light scattering. To the best of our knowledge, no studies have been reported about the effects of silicate on the formation of the nano-emulsions using the PIT method and their properties. Under alkaline conditions, the silicates raised the PIT of the systems, due to their different structural formula at different pH value. FIIR analysis confirmed the strong hydrogen bond interactions or reaction between the silicate and the Tween 80 molecule, this lead to the adsorption of silicate on the droplet surface, thus raising the PIT of the systems. Compared to the FIIR analysis of Brij30 and Na2SiO3, it was found that their hydrogen bond interactions were very weak, thus the PIT of the Brij30 systems was depressed by the addition of Na2SiO3. At very low concentration of alkaline silicate, the droplet size distributions were narrow and the droplet sizes were small, accordingly, the viscosities of the system were big. This showed that small amount of silicate was helpful to the stability of the nano-emulsions. Fixed the concentration of silicate, the PIT of the system was reduced with the decrease of pH value. The viscosity of the system was biggest under about neutral condition. However, the droplet size distributions were broad and the droplet sizes were big because of polymerization of the neutral silicate. The zeta potential was reduced with the decrease of the pH value, then charge reversal occurred and ultimately positive charges were taken on. This demonstrated that the origin of the negative surface charges result from the adsorption of hydroxyl ions.3. Preparation of positively charged O/W nano-emulsions stabilized by mixed cationic-nonionic surfactants using the inorganic salt-tuned sub-PIT methodBased on the previous studies about O/W nano-emulsions with tunable PIT induced by inorganic salts, we explore a simple low-energy method-one-step sub-PIT method to prepare positively charged nano-emulsions in a mixed cationic-nonionic surfactant system for the first time. Positively charged oil/water nano-emulsions were prepared by adding a cationic surfactant to the system. However, the cationic molecules change the spontaneous curvature of the surfactant layers and raise the PIT above 100?. The PIT can be depressed by addition of NaBr, as shown by conductivity measurements and equilibrium phase behavior. Therefore, these nano-emulsions can be prepared by the PIT method. We found that the formation of the nano-emulsions did not require a cross-PIT cycle. Cryo-TEM observation confirmed the mechanism of the emulsification was the formation of mixed swollen micelles that can solubilize all the oil above a "clearing boundary", followed by a stir-quench to a temperature where these droplets become metastable emulsions. The zeta potential measurements showed that the original nano-emulsion droplets were negatively charged due to the specific adsorption of the hydroxyl ions. The zeta potential of the emulsion droplets can be easily tuned by varying the cationic surfactant concentrations. Due to electrosteric stabilization, the resulting nano-emulsions are highly stable, thus could find significant applications in areas such as pharmaceutical, cosmetics and food industries.