Formation And Application Of Microemulsions And Nanoemulsions

Nanoemulsions are transparent or translucent emulsions with droplets about 50-500 nm. Nanoemulsions are significant in practical applications including chemical, pharmaceutical, cosmetic, agrochemical, and oil recovery fields, etc. However, nanoemulsions are not thermodynamically stable systems. Hence, nanoemulsions will be unstable during storage. Recently, to improve the stability of nanoemulsions, the most common method is to prepare nanoemulsions with long chain oil. However, there are still some problems in the following aspects.(1) The interfacial tension of the system with long chain oil is high. Hence, nanoemulsions with long chain oil were generally formed by high energy emulsification method. However, the energy efficiency of high energy emulsification method is very low, even less than 1%, which increases the cost of preparation of nanoemulsions. Hence, formation of nanoemulsions by low energy emulsification is an important development direction of nanoemulsions.(2) To prepare nanoemulsions by microemulsion dilution method is a very simple method. More importantly, due to the thermodynamic stability of microemulsion, microemulsion is an alternative to nanoemulsion to overcome the instability of nanoemulsion. However, the inter phase concentration of nanoemulsions formed by microemulsion dilution method is very low (not more than 10 wt%), which limit the application of microemulsion dilution method. More importantly, the mechanism of microemulsion dilution is not very clear. More methods should be explored to investigate the the mechanism of microemulsion dilution.(3) Nowadays, nanoemulsions have important application in drilling fluid. However, nanoemulsions were not thermodynamically stable systems. When paraffin nanoemulsions were used in Russia or sea where temperature is usually below 0℃, nanoemulsions would became unstable rapidly, losing the original excellent performance of nanoemulsions.Based on the above background in nanoemulsions research, the preparation of nanoemulsions with long chain oil paraffin by W/O microemulsion dilution method was firstly studied in this paper. We studied the effect of dilution temperature and dilution water amount on the droplet diameter of nanoemulsions. The mechanism of the formation of nanoemulsions was examined and illustrated by small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) in detail. Formation of nanoemulsion by W/O microemulsion dilution method is an excellent method to overcome the instability of nanoemulsions during storage. However, W/O microemulsions would be freeze at low temperature. Hence, we studied the formation of nanoemulsions which can remain stable at-30℃ by phase inversion composition method. The applications of these nanoemulsions in water-based drilling fluid were studied in detail. Finally, after drilling, oil-based drilling fluid remained on the casing and borehole wall will become thick filter cakes, which seriously affected the quality of drilling and well completion, especially well cementing. Hence, based on the theoretical study of microemulsion and nanoemulsion, we prepared a surfactant solution which can clean oil-based drilling fluid and studied the cleaning mechanism. This research was very important in practical application in oil-based drilling fluid.The present dissertation includes four topics.1. Formation of paraffin nanoemulsions by W/O microemulsion dilution methodThe preparation of nanoemulsions using long chain oil (C20 to C33) with remarkably small droplet size by microemulsion dilution method is generally difficult. In this work, a simple, W/O microemulsion dilution method was used to prepare O/W nanoemulsions with long chain oil in water/Span 80-Tween 80/paraffin system. With the increase of dilution temperature from 40 to 80℃, the emulsion droplet diameter decreased from 1.2 μm to 61 nm. The increase in the amount of dilution water led to the increase of the droplet diameter of nanoemulsions. Meanwhile, nanoemulsions with different charge, even positive charge, were also formed by adding various concentration of Jeffamine (D230) or cetyltrimethylammonium bromide (CTAB) in the W/O microemulsions. More importantly, paraffin nanoemulsions, formed in situ when W/O microemulsion was added to water-based drilling fluids, have effective lubrication and permeability plugging ability. Hence, formation of nanoemulsion by microemulsion dilution method demonstrated here is of great importance for practical applications.2. Formation of concentrated biodiesel nanoemulsions by W/O microemulsion dilution methodGenerally, dispersed phase mass fraction of nanoemulsions formed by microemulsion dilution method was not over 10 wt%. In this work, we show the formation of concentrated biodiesel O/W nanoemulsion (dispersed phase mass fraction was up to 0.5) by diluting W/O microemulsion in the water/Tween 80/biodiesel system. The mechanism of the formation of nanoemulsions was examined and illustrated by small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At high temperature, nano-sized droplets formed spontaneously due to the surfactant migration and inversion upon dilution of W/O microemulsions, but these droplets were highly unstable. When cooled to room temperature, their stability was highly enhanced due to the decrease of collision frequency rate and the enhancement of stabilization of the oil/water interface. Even though, the Ostwald ripening still results in growth of droplets of the nanoemulsions after long-term storage, which would limits the practical applications of nanoemulsions. W/O microemulsions were thermodynamically systems. Hence, W/O microemulsions which can form nanoemulsions by simple dilution of water can be used as an alternative to O/W nanoemulsion during storage and transport. Furthermore, biodiesel nanoemulsions could meet the requirements of green chemistry and engineering and be used as new green lubricants in water-based drilling fluid.3. Formation and application of low temperature resistant nanoemulsionsIn previous study, we found that W/O microemulsions, which can form nanoemulsions on demand, could be an alternative to nanoemulsions during storage. However, W/O microemulsions still freeze at low temperature (<-20℃). Hence, we prepared low temperature (-30℃) resistant nanoemulsions with paraffin and wax as oil phase by glycerin water solution. We studied the effect of HLB of surfactants, oil/ surfactant ratio and water mass fraction of nanoemulsions on the droplet diameter of nanoemulsions. Through theoretical calculation, we found that low temperature resistant nanoemulsions had effective permeability plugging ability which can prevent differential pressure sticking. This work has important theoretical significance and application value in water-based drilling fluid.4. Application of surfactant solution in cleaning oil-based drilling fluidThe filtrate of oil-based drilling fluid is oil, so it can avoid the water sensitivity of oil layer. Hence, oil-based drilling fluid is often used in well areas with high sensitivity to reservoir water. However, oil-based drilling fluid remained on the casing and borehole wall will become thick filter cakes, which seriously affected the quality of drilling and well completion, especially well cementing. In this work, we prepared a surfactant solution using alkyl polyglucoside APG and dodecyl polyoxyethylene ether compound surfactants. When the surfactant solution encountered liquid paraffin which was the oil phase of oil-based drilling fluid, the O/W microemulsion was formed spontaneously, resulting in the effective clean of the oil-based drilling fluid. The cleaning process can be divided into two parts:(1) Due to the low interfacial tension, O/W microemulsions were formed spontaneously when the surfactant solution encountered oil droplets on the borehole wall. At the same time, the solid particles on the borehole wall also fall off due to the scour of the surfactant solution. (2) The hydrophobic group of excess surfactants would adsorb on the original hydrophobic wall, the hydrophilic head base is exposed outside. Hence, the wettability of the wall is changed from hydrophobic to hydrophilic. This research is of great importance for practical applications.