| Microemulsions are defined as optically isotropic, transparent, and thermodynamically stable dispersions of two or more immiscible liquids containing surfactants and possibly cosurfactants. Microemulsions have received much attention because of their applications in material preparation, reaction engineering, separation, and environmental governance. Surfactants are often thought necessary to stabilize microemulsions. However, it was reported that some ternary systems composed of oil, water and alcohol might form microemulsions in the absence of surfactant, which was considered as surfactant-free microemulsion (SFME). So far, all SFMEs reported in literatures are water-in-oil (W/O) systems, and the understanding for SFMEs is very shallow. The law and mechanism of SFME formation remains unclear, and the research about the application of SFME are rarely reported.In the paper, a series of SFMEs were synthesized and characterized using aliphatic compounds (oleic acid), aromatic compounds (toluene) or hydrophobic ionic liquids (bmimPF6) as oil phase, water or hydrophilic ionic liquid (bmimBF4) as water phase and amphi-solvent as the third group. The influence of temperature, pH and electrolytes was investigated. Nanoparticles such as layered double hydroxides (LDH), noble metals, and metallic oxides have been synthesized in SFME, respectively, and the morphology of nanoparticles prepared in ME and SFME was compared and contrasted. These works may lead us to get a better insight into the SFME and provide the basis for applications in nanomaterials preparation.The main contents of this paper are as follows.1. SFME of oleic acid, water and short-chain alcohols(1) Three surfactant-free microemulsions were synthesized using oleic acid as oil phase, water as water phase, and n-propanol, isopropanol or ethonal as the third group (amphi-solvent), respectively. The phase behavior of the ternary system was investigated, showing that there were a single-phase microemulsion region and a multiphase region in the ternary phase diagram. short-chain alcohols had obvious influence on the area of single-phase region and the ternary system containing n-propanol as amphi-solvent had the largest single-phase region. Temperature and salt concentration have no obvious influence on phase behavior(2) The microstructures and structural transitions of the microemulsion system were investigated by electrical conductivity measurement, and it was found that the microemulsions could exhibit water-in-oleic acid (W/O), bicontinuous (BC), and oleic acid-in-water (O/W) microstructures, similarly to traditional surfactant-based microemulsions. According to the results, the single-phase microemulsion region in phase diagram was divided into three corresponding microregions. The three microstructures in oleic acid/n-propanol/water SFME were further confirmed by freeze-fracture and cryogenic transmission electron microscopy (FF-TEM and Cryo-TEM) observations, and droplets were observed both in W/O or O/W microregions while a network-like structure was observed in the BC microemulsion. The results of electrical conductivity and TEM could confirm each other.(3) The pH values and zeta potential of ternary system were measured, respectively. compared with electrical conductivity measurements, results showed that the charged microemulsion droplets arise from the interfacial adsorption of OH-anions at the oil/water surface, which was the basis of SFME conductive.2. SFME of bmimPF6, water and DMF(1) The phase behavior of the ternary system consisting of1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), a hydrophobic ionic liquid (IL), DMF and water was investigated. Single-phase microemulsion and multiphase regions were observed in the ternary phase diagram and the single-phase microemulsion region occupies about66%of the total phase diagram. The effect of temperature and pH on the phase behavior of the ternary system was examined, and no change in phase diagram was observed.(2) The microstructures and structural transitions of the microemulsion system were investigated by cyclic voltammetry, electrical conductivity, UV-visible spectroscopy, and fluorescence spectroscopy measurements. It was found that the microemulsions could exhibit water-in-bmimPF6(W/IL), bicontinuous (BC), and bmimPF6-in-water (IL/W) microstructures. Accordingly, the single-phase microemulsion region in the phase diagram could be divided into three microregions. The boundaries between three microregions defined from different methods were consistent, indicating that these methods canbe used for the study of microstructures and structural transitions in the IL-SFME system.(3) The microstructures were further confirmed by freeze-fracture and cryogenic transmission electron microscopy (FF-TEM and Cryo-TEM) observations, and droplets were observed both in IL/W or W/IL microregions while a sponge-like structure was observed in the BC microemulsion, which is consistent with the the divide of microregions.(4) Interfacial tension of the microemulsion was measured and it revealed that ultra-low interfacial tension may not is a necessary condition for the formation of microemulsion.3. SFME of bmimBF4, ethonal and toluene(1) Surfactant-free microemulsions were synthesized using hydrophilic ionic liquid (IL)1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) as the polar phase, ethonal, n-propanol, or n-butanol as amphi-solvent, and hexane, cyclohexane, or toluene as oil phase, respectively. The phase behavior of the ternary system were investigated, showing that there were a single-phase microemulsion region and a multiphase region in all ternary phase diagrams, and only the area of single-phase region in bmimBF4/ethonal/toluene and bmimBF4/n-propanol/toluene system is larger.(2) The multiphase region of bmimBF4/ethanol/toluene system was investigated by temperature and salinity scanning method respectively, showing that there were a upper phase microemulsion and a lower phase microemulsion under different conditions.(3) Pulse field gradient spin-echo nuclear magnetic resonance (PGSE NMR) was employed to determine the self-diffusion coefficient for each component in the ternary system with high tuluene content, and results suggested that the studied system was ionic liquid-in-toluene (IL/O) microemusion indicting the formation of nonaqueous IL-SFME.4. Nanomaterials preparation in SFME(1) An aqueous Mg2Al-C1LDH nanosheet dispersion was obtained by a double-microemulsion technique in a surfactant-free reverse microemulsion composed of toluene, isopropanol and water. The LDH nanosheets consisting of a single brucite layer without any loading of organic molecules canbe used as building blocks for LDH-based functional materials.(2) Flowerlike Mg2Al-LDHs microspheres were successfully synthesized in a SFME consisting of hexane, isopropanol and water. The flowerlike Mg2Al-LDHs microspheres have a specific surface area of140m2·g-1and possess micro and mesoporous multilevel hierarchy. The calcined LDHs could be reconstructed to flowerlike microspheres when rehydrated.(3) Noble metals such as gold and silver nanoparticles were synthesized in different microregion, i.e., W/O, BC and O/W with a SFME containing oleic acid, water and n-propanol. It was found that the monodispersed noble metal particles can only be obtained in the W/O microregion and those nanoparticles can be stably dispersed in the non-polar phase.(4) TiO2hollow spheres with single crystalline and thin shells were synthesized in a nonaqueous SFME contaning of bmimBF4, ethonal and toluene. The growth mechanism canbe explained by the theory of reversed crystal growth. |
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