Surfactant-free Microemulsion Research Articles

CO2-Responsive Surfactant-Free Microemulsion.

A surfactant-free microemulsion (SFME) with CO2 stimuli responsive properties was prepared. The oil and the water phases were N, N-dimethylcyclohexylamine (DMCHA) and deionized water, respectively, and N, N-dimethylethanolamine was used as an amphisolvent. The single-phase and multiphase zones were measured by the ternary-phase diagram, and the microstructure of the SFME was determined by measuring the change trend of the electrical conductivity of the system with increasing DMCHA content. While using methyl orange as a probe, the microstructure of the SFME was further confirmed by an UV-visible spectrometer. The microstructures of water-in-oil (SFME-I) and oil-in-water (SFME-II) microemulsions were obtained by changing the DMCHA content in the system. The SFME-I system has a significant phase separation after the action of CO2. However, with the continuous introduction of CO2, the upper phase of DMCHA is gradually protonated and dissolves in the aqueous phase, resulting in a gradual decrease in the volume of the upper phase, and eventually in an aqueous solution of ammonium bicarbonate. For SFME-II, CO2 does not directly cause phase separation, but eventually it becomes an aqueous solution of ammonium bicarbonate with the addition of CO2. Both the water-in-oil structure SFME-I and the oil-in-water structure SFME-II have excellent CO2 stimuli responsive performance.

A surfactant-free microemulsion consisting of water, ethanol, and dichloromethane and its template effect for silica synthesis

A new type of surfactant-free microemulsion (SFME) containing water/ethanol/dichloromethane was constructed, and three microregions water-in-dichloromethane (W/O), bicontinuous (B.C.) and dichloromethane-in-water (O/W) regions were identified. The polarity environment of the SFME was investigated. Solid silica nanoparticle (SSN) was selected as a model nanomaterial to investigate the feasibility of the water/ethanol/dichloromethane SFME for the preparation of nanomaterials. In the O/W SFME region of the microemulsions, uniform spherical solid silica nanoparticles (SSNs) were synthesized. Under the same experimental conditions, they are of smaller particle size and have narrower range of diameter distribution, than the SSNs synthesized from ethanol and water mixture. The effects of tetraethylorthosilicate concentration (CTEOS), ammonia hydroxide concentration (CNH3∙H2O) and dichloromethane content on the size and morphology of the SSNs were investigated. The average diameters of the SSNs increased with increasing CTEOS andCNH3∙H2O. However, the effect of increasing CNH3∙H2O on the particle size is more significant. The time evolution of the morphology and diameter of the SSNs were also investigated to elucidate the growth mechanism for the SSNs synthesized in the O/W SFMEs.

Salt effects in surfactant-free microemulsions.

The weakly associated micellar aggregates found in the so-called "pre-ouzo region" of the surfactant-free microemulsion water/ethanol/1-octanol are sensitive to changes in the system composition and also to the presence of additives like salt. In this work, we study the influence of two salts, sodium iodide and lithium chloride, on aggregates in water/ethanol/1-octanol by molecular dynamics simulations. In both cases, ethanol concentration in the nonpolar phase and at the interface is increased due to a salting out effect on ethanol in the aqueous pseudo-phase. In addition, minor charging of the interface as a consequence of differential adsorption of anions and cations occurs. However, this charge separation is overall weakened by the erratic surface of octanol aggregates, where polar hydroxyl groups and hydrophobic patches are both present. Furthermore, ethanol at the interface shields hydrophobic patches and reduces the preferential adsorption of iodide and lithium.

Synthesis and application of amino acid ionic liquid-based deep eutectic solvents for oil-carbonate mineral separation

A novel amino acid ionic liquid-based deep eutectic solvent (TrpBF4/U) was synthesized using l-tryptophan, fluoroboric acid and urea. The melting point of TrpBF4/U was over 100 K lower than that of TrpBF4. It is found that the TrpBF4/U could form non-aqueous surfactant-free micro-emulsion through mixing with ethanol and toluene. The size of the micro-emulsion droplets was less than 7 nm and was dependent on the concentration of toluene. This newly prepared TrpBF4/U-based micro-emulsion was applied to extract the extra-heavy oil from carbonate asphalt rocks. Results showed that the oil recovery was enhanced by at least 11% compared with that without the TrpBF4/U micro-emulsions. The enhancement in the oil-solid separation was mainly ascribed to the interfacial alteration at the oil and mineral surfaces by the TrpBF4/U, including lowering the interfacial tension and increasing solid wettability. Assisted by the TrpBF4/U micro-emulsion, the quality of oil product (less solids entrained) and residual solids (less solvents attached) were also improved compared with traditional ionic liquids (i.e., [Emim]BF4) at given conditions. It also demonstrated that the TrpBF4/U micro-emulsion could be recycled and reused at least 4 times without sacrificing its efficiency. The above findings suggested the potential applications of the amino acid ionic liquid-based deep eutectic solvent in oil-solid separations or other extractions.

A surfactant-free microemulsion composed of isopentyl acetate, n-propanol, and water

It has been demonstrated that in the absence of traditional surfactants, microemulsions can form from a ternary mixture of oil, water, and an amphi-solvent. These microemulsions are called surfactant-free microemulsions (SFMEs). To date, only a small number of SFME systems have been reported, and the current understanding of SFMEs is very limited. Herein, we report an SFME consisting of isopentyl acetate (IA), n-propanol, and water, in which IA (a simple ester compound) and n-propanol are used as the oil phase and amphi-solvent, respectively. The microstructures and structural transition of the SFME were investigated by cyclic voltammetry, fluorescence spectroscopy, and UV-visible spectroscopy techniques. Moreover, three kinds of microstructures, namely, oil-in-water (O/W), bicontinuous (BC), and water-in-oil (W/O), have been identified in the SFME, which are directly verified by cryo-TEM observations. A change in the composition of the SFME may lead to a structural transition from O/W through BC to W/O or vice versa, which is similar to the case of traditional surfactant-based microemulsions (SBMEs). To the best of our knowledge, this is the first time that the microstructures and structural transition of an SFME obtained using a simple ester compound as the oil phase have been identified.

Synthesis of hierarchical flower-like Mg2Al-Cl layered double hydroxide in a surfactant-free reverse microemulsion

Hierarchical flower-like Mg2Al-Cl LDH microspheres were synthesized using a reverse surfactant-free microemulsion (SFME) route at a low temperature (∼25°C), and characterized by XRD, TEM, SEM, FT-IR, TG, N2 adsorption/desorption, and elemental analyses. The SFME used consists of n-hexane, isopropanol, and water. The so-obtained flower-like microspheres are constructed with LDH nanosheets with ∼10nm thickness. After destroyed by calcination, their crystal structure and flower-like morphology can be reconstructed by rehydration. The flower-like LDH and its calcined product (CLDH) have large specific surface areas being ∼84.3 and 163.9m2g−1, respectively. The sorption of methyl orange (MO) on the LDH and CLDH were determined, showing high MO sorption capacities of ∼559 and 1112gg−1, respectively, which are obviously higher than those reported for conventional LDHs and CLDHs. Possible mechanisms for the flower-like microsphere formation and the MO sorption are discussed. Our results demonstrate that, like traditional surfactant-based microemulsions, SFMEs as microreactors or templates can be used to synthesize nanomaterials. The hierarchical flower-like LDH microspheres and their calcined product have potential applications such as in the treatment of organic waste water.

Synthesis of tin monosulfide (SnS) nanoparticles using surfactant free microemulsion (SFME) with the single microemulsion scheme

Synthesis of monomorphic, SnS nanoparticles without using a capping agent is a difficult task with chemical route of synthesis. This paper reports on synthesis of tin monosulfide (SnS) nanopartilces with dimension in the quantum-dot regime using surfactant free microemulsion with single microemulsion scheme. This has been achieved by reaction in microreactors in the CME (C: chlorobenzene, M: methanol and E: ethylene glycol) microemulsion system. This is an easy and controllable chemical route for synthesis of SnS nanoparticles. Nanoparticle diameter showed prominent dependence on microemulsion concentration and marginal dependence on microemulsion temperature in the temperature range studied. The SnS nanoparticles formed with this method form stable dispersion in Tolune.

Modular construction of single-component polymer nanocapsules through a one-step surfactant-free microemulsion templated synthesis.

Formation of O/W surfactant-free microemulsions from water/oil/acetone ternary systems is exploited to construct precisely-defined shell-functionalized core-loaded nanocapsules with tunable diameters (ranging from 50 to 190 nm) in one step.

Surfactant-free microemulsion electrokinetic chromatography (SF-MEEKC) with UV and MS detection - a novel approach for the separation and ESI-MS detection of neutral compounds.

Microemulsion electrokinetic chromatography (MEEKC) is a powerful tool to separate neutral species based on differences in their hydrophobic and hydrophilic properties. However, as a major drawback the conventionally used SDS based microemulsions are not compatible with electrospray ionization mass spectrometry (ESI-MS). In this work, a surfactant-free microemulsion (SFME) consisting of water, ethanol, and 1-octanol is used for surfactant-free microemulsion electrokinetic chromatography (SF-MEEKC). Ammonium acetate was added to the SFME enabling electrophoretic separations. The stability of SFMEs containing ammonium acetate was investigated using small-angle X-ray scattering and dynamic light scattering. A method for the separation of a model system of hydrophobic and hydrophilic neutral vitamins, namely the vitamins B2 and D3, and the cationic vitamin B1 was developed using UV/VIS detection. The influence of the ammonium acetate concentration on the separation performance was studied in detail. The method was characterized concerning reproducibility of migration times and peak areas and concerning the linearity of the calibration data. Furthermore, SF-MEEKC was coupled to ESI-MS investigating the compatibility between SFMEs and the ESI process. The signal intensities of ESI-MS measurements of the model analytes were comparable for SFMEs and aqueous systems. Finally, the vitamin D3 content of a drug treating vitamin D3 deficiency was determined by SF-MEEKC coupled to ESI-MS using 25-hydroxycholecalciferol as an internal standard. Graphical abstract The concept of surfactant-free microemulsion electrokinetic chromatography coupled to electrospray ionization mass spectrometry.

Ultrathin layered double hydroxide nanosheets prepared from a water-in-ionic liquid surfactant-free microemulsion for phosphate removal from aquatic systems

A water-in-(ionic liquid) surfactant-free reverse microemulsion were used for the controllable preparation of the small sized Mg2Al-layered double hydroxide (LDH) nanosheets. The metallic salts and alkali aqueous solutions were respectively employed as water phase for preparation of BmimPF6-dimethylformamide-water reverse microemulsions. Then the uncontaminated LDH sheets (LDH-M) have been afforded with a narrow diameter distribution (∼10–35nm). The LDH nanoplatelets present a uniform lateral dimension (∼31nm) and a small thickness (∼0.71nm), which may probably be composed of single brucite-like layer. They display larger surface areas (110–291m2/g) and smaller pore sizes (17.57nm and ∼3.83nm) than LDH based on traditional co-precipitation technique. The treatment of its calcined product (LDO) in water results in the flower-like LDH nanosheets with obvious aggregation and larger size. The phosphate adsorption experiments show that the small sized LDH nanosheets perform superior removal ability toward low concentration phosphate to the large sized LDH sheets. However, the LDO product exhibits further excellent adsorption efficiency due to losing the interaction of carbonate with metal hydroxide layers. After five recycle usage, the ultrathin LDH-M nanoflakes and the calcined product LDO keep about 88.7–93.5% uptake efficiency of the original adsorbents, indicating a good reusability.

Surfactant-free Microemulsion Reinforced Hollow-Fiber Liquid-Phase Microextraction Combined with Micellar Electrokinetic Capillary Chromatography for Detection of Phthalic Acid Esters in Beverage and Urine

For the first time, the surfactant-free microemulsion reinforced hollow-fiber liquid-phase microextraction (SFME-HF-LPME) was developed for the determination of phthalic acid esters (PAEs) followed by field-enhanced sample injection with reverse migrating micelles (FESI-RMM) in capillary electrophoresis. An SFME composed of toluene, isopropanol, and water was used to reinforce HF. The prepared SFME-HF was applied to extract PAEs. After extraction, analytes were desorbed and analyzed by capillary electrophoresis. Several important parameters influencing the extraction efficiency of SFME-HF-LPME such as the type of extraction solvent, SFME component, extraction time, desorption condition, pH value, stirring rate, and salting-out effects were optimized. The proposed method provided a good linearity, low limits of detection, and good repeatability of the extractions. This method was then utilized to analyze trace PAEs in beverage and urine. The results indicated that the developed method is an excellent alternative for the PAEs routine analysis in the food and biological field.

Consistent definitions of “the interface” in surfactant-free micellar aggregates

The theoretical description of systems with soft interfaces requires a precise definition of the interfaces that are present. Such definitions are common for interfaces of simple liquids and for surfactant assemblies like micelles, membranes, and microemulsions. Here, we focus on several possible rigorous definitions in a recently described surfactant-free microemulsion that consists of octanol, ethanol, and water, in which micelle-like aggregates are in dynamic equilibrium with the surrounding pseudo phase. We test different definitions that are based on the radial distribution functions of the components with respect to the center of these surfactant-free micelles. All definitions result in experimentally indistinguishable locations of the interface within the limits of resolution of light, X-ray, and neutron scattering experiments, since their locations differ by at most 0.4 nm, which corresponds to roughly 2-3 bond lengths.

From nanodroplets by the ouzo effect to interfacial nanolenses.

Polymerizing nanodroplets at solid-liquid interfaces is a facile solution-based approach to the functionalization of large surface areas with polymeric lens-shaped nanostructures. In this work, we have applied a one-pot approach to obtain polymeric nanolenses with controlled sizes and densities. We take advantage of the formation mechanism by the direct adsorption of nanodroplets from a surfactant-free microemulsion onto an immersed hydrophobic substrate. The interfacial nanodroplets were photopolymerized to produce polymeric nanolenses on the substrate surface. The surfactant-free microemulsion of the monomer nanodroplets was obtained through the spontaneous emulsification (i.e., ouzo effect) in the tertiary system of ethanol, water, and precusor monomer. The size of nanolenses on the surface was adjusted by the nanodroplet size, following a linear relationship with the ratio of the components in the microemulsion. This simple approach is applicable to produce nanolenses over the entire surface area or on any specific area at will by depositing a drop of the microemulsion. Possessing high optical transparency, the resulting substrates may have potential application as functional biomedical supporting materials or effective light-harvesting coatings.

Synthesis of Mg2Al-Cl layered double hydroxide nanosheets in a surfactant-free reverse microemulsion

The synthesis of Mg2Al-Cl layered double hydroxide (LDH) nanosheets in a surfactant-free reverse microemulsion is described. The microemulsion was composed of toluene, isopropanol, and an aqueous solution as the dispersed phase. An aqueous LDH nanosheet dispersion was obtained by a double-microemulsion technique. LDH nanosheets were characterized by X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and thermogravimetric and elemental analyses. The LDH nanosheets consisted of a single brucite layer without any loading of organic molecules. To the best of our knowledge, this is the first report of a naked LDH monolayer aqueous dispersion being directly obtained. The LDH monolayers can be used as building blocks for LDH-based functional materials.

Surfactant-Free Microemulsion Composed of Oleic Acid, n-Propanol, and H2O

Generally, a microemulsion consists of oil, water, surfactant, and sometimes cosurfactant. Herein, we report a surfactant-free microemulsion (denoted as SFME), consisting of oleic acid (oil phase), water, and n-propanol without the amphiphilic molecular structure of a traditional surfactant. 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. The electrical conductivity measurement was employed to investigate the microregions of the single-phase microemulsion region, and three different microregions, that is, water-in-oleic acid (W/O), a bicontinuous (B.C.) region, and oleic acid-in-water (O/W), were identified, which were further confirmed by freeze-fracture and cryogenic transmission electron microscopy (FF-TEM and Cryo-TEM) observations. The polarity and the salt solubility of water domains in the W/O SFME were investigated by UV-visible spectroscopy using methyl orange and potassium ferricyanide as probes, respectively. Experimental results showed that the water domains in the W/O microemulsion had a lower polarity than bulk water and a normal solubility for salt species, indicating that the SFMEs have much significance in the preparation of various nanomaterials.

Hierarchical MnO2 nanostructures: synthesis and their application in water treatment

Hierarchical MnO₂ nanostructures were prepared through the reaction between KMnO₄ and oleic acid at room temperature in the surfactant-free microemulsion system. The obtained samples were characterized by powder X-ray diffraction, N₂ adsorption, scanning electron microscopy, and transmission electron microscopy. The results indicated that the flowerlike nanospheres were three-dimensional (3D) porous microstructures consisting of nanoplates. The surface area of the sample was 171.5 m(2)/g and the distribution of pore diameter lay within the range of 5-15 nm. The prepared hierarchically structured MnO₂ showed excellent adsorption capacity and rapid adsorption rate for methylene blue ions in water. The maximum adsorption capacity of methylene blue was as high as 273.9 mg/g and 97.5% of the dye was removed within initial 5 min of contact time. Compared with other adsorbents, the synthesized hierarchical MnO₂ nanostructures displayed a faster adsorption rate and higher adsorption capacity, which implied potential application for removing dye pollutants from waste water.

Surfactant-free oil/water and bicontinuous microemulsion composed of benzene, ethanol and water

Generally, a microemulsion consists of oil, water, surfactant and sometimes cosurfactant. Herein, we report a novel surfactant-free microemulsion (denoted as SFME) composed of benzene, water and ethanol without the amphiphilic molecular structure of traditional surfactant. The phase behavior of the ternary system was investigated, finding that there were a single-phase region and a two-phase region in ternary phase diagram. The electrical conductivity measurement was employed to investigate the microregion of the single-phase region, and a bicontinuous microregion and a benzene-in-water (O/W) microemulsion microregion were identified, which was confirmed by freeze-fracture transmission electron microscopy (FF-TEM) observations. The sizes of the microemulsion droplets are in the range of 20–50 nm.

A Novel Surfactant‐free Microemulsion System: N,N‐Dimethyl Formamide/Furaldehyde/H2O

AbstractGenerally, a microemulsion consists of oil, water, surfactant and sometimes cosurfactant. Herein, for the first time to our knowledge, a novel surfactant‐free microemulsion (SFME), consisting of furaldehyde (oil phase), water and N,N‐dimethyl formamide (DMF) without the amphiphilic molecular structure of traditional surfactant is reported. The phase behavior of the ternary system was investigated, finding that a single‐phase microemulsion region and a two‐phase region were formed. The electrical conductivity measurement was employed to investigate the single‐phase microemulsion region. On the basis of the percolation theory, the single‐phase microemulsion region was identified to consist of three different microregions: furaldehyde‐in‐water (O/W), bicontinuous region and water‐in‐furaldehyde (W/O), which were further proved by freeze‐fracture transmission electron microscopy (FF‐TEM) observations. The diameter of the microemulsion spherical droplets is in the range of 40–70 nm.