Nanoemulsions Prepared By The Phase Inversion Composition Method At Elevated Temperature

Emulsions with droplet size in the nanomettic scale are often referred to as nanoemulsions. Due to their characteristic size, nanoemulsions appear transparent or translucent to the naked eye and possess stability against sedimentation or creaming. These properties make nanoemulsions of interest for fundamental studies and for practical applications (e.g. pharmaceutical, cosmetic, food, etc. fields). Since emulsions are thermodynamically unstable systems, energy input is required for the formation of emulsions. Two main approaches are currently used for the preparation of nanoemulsions:high energy and low energy. For the high-energy methods, intense mechanical energy input is carried out by extreme shear stirring, high-pressure homogenizers, or ultrasounds. For the low-energy methods, the chemical energy stored in the components is used by changing the spontaneous curvature of the surfactants. For nonionic surfactant system, this can be achieved by changing the temperature at constant composition, i.e., phase inversion temperature, PIT method, or changing the composition at constant temperature, i.e., phase inversion composition, PIC method. Studies of nanoemulsion formation by low-energy method are receiving an increased interest because less energy input and simple apparatus is required.Currently, there are many systematic works on nanoemulsion preparation by various methods and the effects of formulation and process parameters were discussed thoroughly. Nevertheless, we note that there are still many problems in low-energy methods need further study.(1) In the previous work the oil phase of nanoemulsions is mainly constituted of short-chain alkanes (C8-C16) with low viscosity. Due to the significant solubility in the water phase Ostwald ripening rate is rapid for these oils. The droplet size of nanoemulsions increased to micron scale within a few weeks. The emulsification of viscous oils makes nanoemulsions stable for months. However, nanoemulsions with long-chain alkanes prepared by low-energy methods have been rarely reported.(2) In the mechanism discussions in the low-energy methods it has been proposed that double emulsions exist during the emulsification process. But direct experimental evidence has not yet been found.(3) Positively charged nanoemulsions with a wide range of application prospects were prepared by high-energy methods. But most of nanoemulsions produced by low-energy methods were negatively charged. There are few studies on the adjustment of the electrical properties of nanoemulsion.Based on the above background in nanoemulsions research, in this dissertation, two kinds of nonionic surfactants Span80and Tween80are selected as the emulsifers and liquid paraffin is used as the oil phase. All the emulsions are prepared by the PIC method. First, the effect of emulsification temperature on nanoemulsion properties is investigated. Monodisperse nanoemulsions with droplet diameter of51nm could be obtained at70℃and the droplet size remain unchanged after5months of storage. Therefore, the temperature of preparation is fixed at70℃for further investigation. Afterwards, the W/O/W structure of emulsions produced by the PIC method is characterized by optical microscopy, small angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and UV-vis spectrum. Moreover, by the addition of cationic surfactant the electric property of nanoemulsions is controlled without changing the emulsion size or stability.The present dissertation includes three topics.1. Highly stable concentrated nanoemulsions by the phase inversion composition method at elevated temperatureThe PIC method has a great potential for scale-up applications because of the ease of formation and relatively low energy costs. A phase transition is produced by stepwise addition of water to a mixture of the surfactant and oil for the formation of oil-in-water (O/W) nanoemulsions. This process is well-known for the emulsification process at25℃. In this dissertation the liquid paraffin is mainly constituted of long-chain isoalkanes (C20to C33), which makes the dispersed phase hard to be emulsified by the PIC method below30℃. The droplet diameter decreases from10.3μm to51nm with the increase of emulsification temperature at a constant composition, indicating the significant influence of temperature on droplet size. The reason is that the interfacial tension decreases with the increase of temperature, leading to the gradual increase of the amount of surfactant molecules adsorbed at the O/W interface.In low-energy methods most nanoemulsions are prepared at relatively low volume fractions of the dispersed phase (φ). Since nanoemulsions are thermodynamically unstable systems, the droplet radii increase dramatically with time. The effect of φ on the droplet size distribution has been studied less frequently because of the disadvantage on storage stability especially when the droplets are concentrated. The size distributions of nanoemulsion (φ=0.1) have not changed over5months due to the high viscosity of the oil phase. So we investigate the effect of the droplet volume fraction on emulsions properties. The droplet diameter remains less than58nm when the volume fraction of the dispersed phase is varied from0.1to0.62. A free oil phase was present above the emulsion with φ=0.62after50days. The droplet diameters of other samples with φ less than0.62remain unchanged after5months of storage. This is the first report about the formation of stable concentrated nanoemulsions by low-energy methods.2. Water-in-oil-in-water nanoemulsions by the phase inversion composition method at elevated temperatureThe structure of nanoemulsion droplets prepared by the PIC method at elevated temperature is investigated. Nanoemulsions are generally classified as water-in-oil (W/O) or oil-in-water (O/W) based on which phase constitutes the dispersed phase. The droplet size of nanoemulsions lies in the nanometer range and is even smaller than the innermost dimension of common double emulsions. As a result, the droplet morphology of nanoemulsions was rarely taken into account years ago. Nevertheless, the structure of nanoscale double emulsions was published for the first time in Nature in2008. With the development of characterization methods, determining the mesoscopic structure of nanoemulsions is attracting particular interest. As is shown in the optical photomicrograph, the micronscale oil droplets prepared by the PIC method at25℃contain several small internal droplets, indicating the multiple structure of this microscale emulsion. Then the structure of nanoemulsions droplets prepared by the PIC method at70℃is characterized by cryo-TEM, SAXS, and UV-vis spectrum, demonstrating the double structure of these nanoemulsions. These results indicate that with the increase in emulsification temperature the droplet size of emulsions decreases, whereas the droplet structure remains unchanged. To the best of our knowledge, this is the first experimental confirmation of W/O/W nanoemulsions prepared by low-energy methods. Formation of double nanoemulsions is attractive for both a theoretical and practical point of view.3. Positively charged nanoemulsions by the phase inversion composition method at elevated temperatureThe electric property of nanoemulsions prepared by the PIC method at elevated temperature is explored. For emulsions stabilized by nonionic surfactants, the droplets are found to be negatively charged. The probable reason for this negative surface charge is suggested to be the specific adsorption of hydroxyl ions at the O/W interface and the presence of trace fatty acids impurities dissolved in the oil phase. So far, intensive research efforts have been concentrated on the applications of positively charged nanoemulsions in petroleum, pharmaceutics, agriculture, cosmetics, and other areas. Therefore, a small amount of the cationic surfactant CTAB is added to the negatively charged system to control the electric property of nanoemulsions droplets. It is found that the addition of CTAB could adjust the charge of emulsions without changing the droplet size or stability.Afterwards, three special characteristics of the positively charged nanoemulsions prepared by the PIC method at elevated temperature are examined.(1) The thermostability of nanoemulsions. The positively charged nanoemulsions remain stable after10days of storage at70℃. Both the appearance and the droplet size keep unchangeable during the process at elevated temperature. To the best of our knowledge, this is the first report about thermostable nanoemulsions.(2) The modification at the solid surface. The coverslips are immersed in nanoemulsions with different charge for24hours. After that the coverslips are dried and the contact angle measurements of water at the glass surface are carried out. The contact angle of the coverslip, modified by the positively charged nanoemulsion, increases dramatically. In contrast, the contact angle of the coverslip, modified by the negatively charged nanoemulsion, remains unchanged. This result demonstrates the electrostatic adsorption of the positively charged droplets on the solid surface. This property makes positively charged nanoemulsions have potential applications in papermaking, metal processing, textile and other fields.(3) The non-coalescence of oppositely charged nanoemulsions. By the addition of CTAB or SDS, we prepare nanoemulsions that consist of oppositely charged droplets. Mixed nanoemulsions are obtained by combining equal volumes of these oppositely charged emulsions. The appearance and droplet size of the mixed emulsion remain stable after stored at room temperature for20days. The non-coalescence of oppositely charged droplets proves that electrostatic interactions between droplets do not determine the emulsion stability.