Studies On Aliphatic Acid Ester/N-Propanol/Water Surfactant-free Microemulsions

Traditional microemulsion is defined as an optically isotropic, transparent, and thermodynamically stable dispersion formed by two or more immiscible liquids containing appropriate amounts of surfactants and possibly cosurfactants. Generally, it is believed that surfactants are necessary components for stabilizing a microemulsion. However, in 1970 s it was reported that some ternary systems composed of oil(hydrocarbon), water and alcohol might form microemulsions in the absence of surfactant, which was considered as surfactant-free microemulsion(SFME), and the properties of SFME are similar to the traditional microemulsion(containing surfactant microemulsion). More and more people have focused their attention on SFME because of its simple component and surfactant-free structure, and the important potential applications in many fields such as enzyme catalyzed reaction, preparation of nanomaterials and drug delivery. But to date, the basic law and mechanism of SFME formation remains unclear, and needs further study.In the paper,the phase behavior of five ternary systems consisting of long-chain aliphatic acid ester(ethyl laurate and ethyl oleate) or short-chain aliphatic acid ester(butyl acetate, isopentyl acetate, and glyceryl triacetate), water and amphi-solvent(n-propanol) was studied, as well as the possibility, type and influence of various factors on the formation of SFME. It was found tha the hydrogen bond and electrostatic repulsion play an important role in the formation and stability of SFME. These works may lead us to extend the range of SFME system and get a better insight into the SFME.The main contents of this paper are as follows.1. phase behavior of long-chain aliphatic acid ester(ethyl laurate and ethyl oleate)/n-propanol/water(1) The phase behavior of ethyl laurate/n-propanol/water and ethyl oleate/n-propanol/water system were investigated. A single-phase microemulsion region and a multiphase region were observed in the ternary phase diagram. The single-phase microemulsion region was transparent and the multiphase was the microemulsion region in equilibrium with an excess water or oil phase, in which the system was turbid under stirring.(2) The microstructures and the structural transitions of the microemulsion system were examined by electrical conductivity, UV–visible spectroscopy, and fluorescence spectroscopy measurements. It was found that the microemulsion system could present three types, that is, water-in-oil(W/O), bicontinuous(BC), and oil-in-water(O/W), similar to traditional surfactant-based microemulsion systems. The three microstructures of the microemulsion system were further confirmed by cryogenic transmission electron microscopy( Cryo-TEM) observations.(3) DLS measurements were carried out along the water dilute line in the W/O region and results showed that the swelling phenomenon of droplets is similar to the traditional surfanctant-based microemulsion.(4) According to the experimental results of electrical conductivity, the presence of trace electrolytes(impurities) in the oil phase may be the main cause of the droplet charge, which may play an important role in the formation and stability of SFMEs.2. phase behavior of short-chain aliphatic acid ester(butyl acetate, isopentyl acetate, and glyceryl triacetate)/n-propanol/water(1) The phase behavior of three ternary systems including butyl acetate/n-propanol/water, isopentyl acetate/n-propanol/water and glyceryl triacetate/n-propanol/water were investigated. A single-phase microemulsion region and a multiphase region were observed in the ternary phase diagram, similar to the system containing long-chain aliphatic acid ester. The single-phase microemulsion region was transparent and the multiphase was the microemulsion region in equilibrium with an excess water or oil phase, in which the system was turbid under stirring.(2) The microstructures of the single-phase system were examined by electrical conductivity, UV–visible spectroscopy, and DLS measurements. It was found that the butyl acetate/n-propanol/water and isopentyl acetate/n-propanol/water could form the microemulsion system, which exhibit water-in-oil(W/O), bicontinuous(BC), and oil-in-water(O/W) microstructures. However, there was no microemulsion structures in the single phase region of glyceryl triacetate/n-propanol/water ternary system, but some aggregates with a particle size of less than 10 nm was found, and the internal reason is not clear.(3) Comparing the experimental results of electrical conductivity in butyl acetate/n-propanol/water and isopentyl acetate/n-propanol/water ternary system, it was believed that the charge of the droplet is mainly derived from the electrolyte ionization.(4)The effects of additives on droplet size of butyl acetate/n-propanol/water SFME was investigated by DLS measurements. It was found that the particle size increased after adding CaCl2, HCl, and NaOH to the system, which indicated that the hydrogen bond and electrostatic repulsion play an important role in the formation and stability of SFME.