Temperature-Induced Reversible-Phase Transition in a Surfactant-Free Microemulsion

Cosmetically Approved Short-Chain Alcohol/Triethyl Citrate/Water Surfactant-Free Microemulsions and Potential Application to Transdermal Penetration of α-Arbutin

Surfactant-free microemulsions (SFMEs) exhibited remarkable advantages and potential, attributed to their similarity to traditional surfactant-based microemulsions and the absence of surfactants. Herein, a novel SFME was developed utilizing cosmetically approved materials, such as short-chain alcohol as an amphi-solvent, triethyl citrate (TEC) as the nonpolar phase, and water as the polar phase. 1,2-Pentanediol (PtDO)/TEC/water combination can form the largest monophasic zone, accounting for ∼74% of the total phase diagram area, due to an optimal hydrophilic (water)-lipophilic (TEC) balance. Comparable to surfactant-based microemulsion, PtDO/TEC/water SFME can also be categorized into three types: water-in-oil, discontinuous, and oil-in-water. As TEC or water is increased, or PtDO is decreased, the nanoaggregates in PtDO/TEC/water SFME grow from <5 nm to tens of nanometers. The addition of α-arbutin (ABN) does not disrupt PtDO/TEC/water SFME, but rather enhances its formation, resulting in a larger monophasic area and consistent size (2.8-3.8 nm) through participating in interface assembly. Furthermore, ABN-loaded PtDO/TEC/water SFME exhibits remarkable resistance to dilution, exceptional stability, and minimal irritation. Notably, PtDO/TEC/water SFME significantly boosts ABN's solubility in water by 2 times, its percutaneous penetration rate by 3-4 times, and enables a slow-release DPPH• radical scavenging effect. This SFME serves as a safe and cosmetically suitable nanoplatform for the delivery of bioactive substances.

Sustainable Utilization of Surfactant-Free Microemulsion Regulated by CO2 for Treating Oily Wastes: A Interpretation of the Response Mechanism

Surfactant-free microemulsions (SFMEs) have been explored extensively to avoid the residual surfactant problem caused by traditional surfactant microemulsions. Many researchers focused on the SFMEs with tertiary amine, which exhibited the typical CO2 response behavior. In this study, the phase diagram of the SFMEs consisting of tripropylamine (TPA), ethanol, and water was readily prepared via the measurements of electrical conductivity. The CO2 response behavior of SFME was confirmed by determination of conductivity and measurement of the average diameter of SFME, which was mainly dependent on the protonation of TPA induced by the additional CO2. The transition of protonated TPA to a more hydrophilic nature from lipophilicity to hydrophilicity should be responsible for the variation of SFME average diameter. In addition, the SFMEs exhibited remarkable solubilizing capacity of crude oil, and three types of SFMEs achieved more than 80% oil removal rate in the washing process of oil sands. It was noted that both oil-in-water and bicontinuous SFMEs could be circularly utilized at least three times with a relatively high oil removal rate (%). Our work provided the insight perspective on the mechanism of SFMEs with a CO2 response behavior.

A perspective of microemulsions in critical metal separation and recovery: Implications for potential application of CO2-responsive microemulsions

Since the discovery of microemulsions, they have attracted great attention due to its unique properties, such as ultra-low interfacial tension and nanoscale droplets. During the past several decades, microemulsions have shown unparalleled advantages in critical metal separation and recovery, e.g., high separation rate, high recovery efficiency, and good selectivity. Therefore, fundamental understandings of such metal recovery behavior are of great significance for the continuous development of microemulsion-based separation technology in this field. Herein, we first systematically reviewed the application of regular microemulsion in the separation and recovery process of critical metals focusing on their separation mechanisms. Then, we summarized the recent progress of CO2-responsive microemulsions and highlighted their potential application in critical metal separation and recovery, aiming to provide some insights into alleviating the difficulties in demulsification during the stripping stage using regular microemulsions. In this section, the latest development of CO2-responsive microemulsions is introduced, and the relationship between their composition, microstructure and macroscopic properties is discussed. Discussion and future perspectives are provided highlighting the design of new microemulsions and potential application of CO2-responsive microemulsions for metal separation and recovery in the future.

Redox-responsive microemulsion: Fabrication and application to curcumin encapsulation

HYPOTHESIS

Stimulus-responsive microemulsions have aroused significant attention because of their versatile and reversible switchability between stable and unstable states. However, most stimuli-responsive microemulsions are based on stimuli-responsive surfactants. We posit that the change in the hydrophilicity of a selenium-containing alcohol triggered by a mild redox reaction could also influence the stability of microemulsions and provide a new nanoplatform for the delivery of bioactive substances.

EXPERIMENTS

A selenium-containing diol (3,3'-selenobis(propan-1-ol), PSeP) was designed and used as a co-surfactant in a microemulsion with ethoxylated hydrogenated castor oil (HCO40), diethylene glycol monohexyl ether (DGME), 2-n-octyl-1-dodecanol (ODD) and water. The redox-induced transition in PSeP was characterized by ¹H NMR, ⁷⁷Se NMR, and MS. The redox-responsiveness of the ODD/HCO40/DGME/PSeP/water microemulsion was investigated through determination of a pseudo-ternary phase diagram, analysis by dynamic light scattering, and electrical conductivity, and its encapsulation performance was evaluated by determination of the solubility, stability, antioxidant activity, and skin penetrability of encapsulated curcumin.

FINDINGS

The redox conversion of PSeP enabled efficient switching of ODD/HCO40/DGME/PSeP/water microemulsions. Addition of oxidant (H₂O₂), oxidized PSeP into more hydrophilic PSeP-Ox (selenoxide), disrupting the emulsifying capacity of the combination of HCO40/DGME/PSeP, markedly reducing the monophasic microemulsion region in the phase diagram, and inducing phase separation in some formulations. Addition of reductant (N₂H₄·H₂O), reduced PSeP-Ox and restored the emulsifying capacity of the combination of HCO40/DGME/PSeP. In addition, PSeP-based microemulsions can significantly enhance the solubility in oil (by 23 times), stability, antioxidant capacity (DPPH∙ radical scavenging by 91.74 %), and skin penetrability of curcumin, showing clear potential for encapsulation and delivery of curcumin and other bioactive substances.

Influencing of hydrogeochemical conditions and engineering parameters on phase behaviors and remediation performance of in-situ microemulsion for residual PCE in aquifers

In-situ microemulsion has great potential for remediation of chlorinated solvent contaminated aquifers due to its efficient solubilization, and the in-situ formation and phase behaviors of microemulsion is a key factor in determining the remediation performance. However, the role of aquifer properties and engineering parameters on microemulsion in-situ formation and phase transition has been rarely attended. In this work, the influences of hydrogeochemical conditions on in-situ microemulsion phase transition and solubilization ability for tetrachloroethylene (PCE) were explored, and the formation condition, phase transition and removal efficiency for in-situ microemulsion flushing under various flushing conditions were investigated. The results indicated that the cations (Na⁺, K⁺, Ca²⁺) all facilitated the microemulsion phase altering from Winsor I → III → II, whereas the anions (Cl^-, SO₄^2-, CO₃^2-) and pH variation (5-9) had no profound influence on phase transition. Besides, the solubilization capacity of microemulsion was enhanced by the pH variation and the cations, which was proportional to the cation concentration of groundwater. The column experiments demonstrated that PCE underwent the phase transition from emulsion to microemulsion and then to micellar solution during the flushing process. The formation and phase transition of microemulsion were mainly related to injection velocity and PCE residual saturation in aquifers. The slower injection velocity and higher residual saturation were profitable to the in-situ formation of microemulsion. In addition, the removal efficiency could achieve 99.29 % for residual PCE at 12 °C, enhancing with the finer porous medium, lower injection velocity and intermittent injection. Furthermore, the flushing system exhibited high biodegradability and weak reagent adsorption onto the aquifer media, presenting a low environmental risk. This study provides valuable information on the in-situ microemulsion phase behaviors and the optimal reagent parameters, facilitating the application of in-situ microemulsion flushing.

Multiple-Stimuli-Responsive Surfactant-Free Microemulsions Based on Hydrophobic Deep Eutectic Solvents

Hydrophobic deep eutectic solvents (HDESs) have been applied to colloidal systems such as microemulsions, despite the development of stimulus-responsive HDESs still being in a preliminary stage. Here, menthol and indole were hydrogen bonded to form CO₂-responsiveness HDES. A surfactant-free microemulsion constituted of HDES (menthol-indole) as the hydrophobic phase, water as the hydrophilic phase, and ethanol as the double solvent was demonstrated to be CO₂- and temperature-responsive. Dynamic light scattering (DLS) proved the single-phase region of the phase diagram, while conductivity and polarity probing techniques confirmed the kind of microemulsion. The ternary phase diagram and DLS methods were used to investigate the responsiveness of CO₂ and effect temperature on the microemulsion drop size and behavior of the phase of the HDES/water/ethanol microemulsion. The findings revealed that when temperature increased, the homogeneous phase region increased. The droplet size in the homogeneous phase region of the associated microemulsion may be reversibly and accurately adjusted by adjusting the temperature. Surprisingly, a slight temperature change can cause a significant phase inversion. Furthermore, in the system, there was no demulsification in time for the CO₂/N₂ responsiveness process but rather the production of a homogeneous and pellucid aqueous solution.

pH-Responsive Regulation of a Surfactant-Free Microemulsion Based on Hydrophobic Deep Eutectic Solvents

Microemulsions containing a responsive hydrophobic deep eutectic solvent (HDES) as the oil phase that can replace conventional organic solvents are considered to be a green strategy. It is anticipated that a pH-responsive HDES is synthesized to prepare rapid responsive surfactant-free microemulsions (SFMEs), which enable the transition from SFMEs to nanoemulsions. Menthol and n-octanoic acid (OA) were assembled into HDES by hydrogen bonding at a molar ratio of 1:2. The pH-responsive HDES as the oil phase and isopropyl alcohol (IPA) as the double solvent could form HDES/IPA/water SFMEs, which have unique responsiveness. Specifically, from the nuclear magnetic resonance hydrogen spectrum, pH, thermogravimetry, and Fourier transform infrared spectroscopy investigations, the excellent switchability and stability of menthol-OA were demonstrated. On the basis of these complexes, microemulsions were successfully prepared. Electrical conductivity and pH measurements were used to determine the structures of microemulsions and the phase inversion process. The effects of the contents of water and HDES, NaCl concentration, and pH of the system were investigated. Nanoemulsions were successfully prepared on the basis of the pH response of the microemulsions. In addition, the prepared nanoemulsion has a unique pH-responsive behavior that can be controllably regulated among nanoemulsions, microemulsions, and phase separation systems.

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Spontaneous Ouzo Emulsions Coexist with Pre-Ouzo Ultraflexible Microemulsions

Even in the absence of surfactants, polymers, or particles, spontaneous emulsions produced by dilution with water can be stable over days. This "Ouzo effect" used by the industry is obtained by rapid dilution from an identified "pre-Ouzo" domain of composition where weak aggregates are present: nanometer-sized clusters covered by a surface layer enriched in a hydrotrope such as ethanol. In these systems, Ostwald ripening is not an effective destabilizing mechanism. Using in situ autodilution small-angle X-ray scattering (SAXS), we follow the morphological transitions occurring in a ternary mixture of water/n-octanol/ethanol throughout the monophasic and biphasic regions. This allows for the first time an online characterization of the multiscale coexisting microstructures. Small-angle neutron scattering (SANS) profiles on metastable emulsions as well as phase-separated samples complete the SAXS data, taking advantage of contrast variation via isotopic substitution. After crossing the phase boundary into the two-phase region, coexisting phases are both ternary solutions structured at the nanometer scale when the emulsion is stable. The transition from single phase to two phases is asymmetric around the plait point. When the initial concentration of the hydrotrope is below the minimum hydrotrope concentration (MHC), emulsification failure occurs, i.e., emulsions cream within seconds. Beyond MHC, the low interfacial tension between coexisting ternary fluids results in a Laplace pressure below 100 Pa, explaining the puzzling resilience of spontaneous emulsion to the universal mechanism of Ostwald ripening.

CO2 and Temperature Control over Nanoaggregates in Surfactant-Free Microemulsion

Smart microemulsions (MEs) recently have attracted significant interests. However, MEs, especially surfactant-free MEs (SFMEs) that respond to more than one stimulus, are rarely reported to date. Here, we reported the first example of dual-responsive SFME in which a CO₂-sensitive hydroxyethylamine was used as an amphisolvent. This SFME was investigated utilizing ternary phase diagram, dynamic light scattering, and UV-visible spectrum techniques. It was found that three hydroxyethylamines could stabilize the octanol-water mixture to form transparent and isotropic SFMEs including nanoaggregates-rich pre-ouzo zone, regardless of the number of the hydroxyl group. Among them, 2-(dimethyl amino) ethanol (DMEA)-based SFME possesses the largest single-phase region and most sensitive to CO₂ and the changes in temperature. With bubbling of CO₂/N₂ or decreasing/increasing temperature, both the single-phase region and pre-ouzo zone reversibly shrink and expand, as well as with breathing. However, CO₂/N₂-induced change is more significant than that induced by temperature. The former is mainly ascribed to the reversible protonation and deprotonation of DMEA, while the latter is generally interpreted as the effects of temperature on hydrogen bond interaction. Note that CO₂ leads to a thorough demusification from Winsor IV ME to oil-rich and water-rich two phases without nanoaggregates, while cooling only causes to a particular phase separation, producing two new MEs phases, not typical Winsor I or II MEs. Such a unique dual-responsive SFME can not only be applied in the remediation of contaminated soil, drug delivery, and nanoparticles preparation but also opens a new door to switchable emulsion.

Oxidation of organosolv lignin in a novel surfactant-free microemulsion reactor

Lignin is considered as a promising substitute for fossil resources, but its efficient conversion remains a huge challenge due to the structural complexity and immiscibility with typical solvents. Herein, a series of surfactant-free microemulsion reactors comprised of n-octane, water and n-propanol were designed and their corresponding phase behaviors alongside their ability to intensify oxidative depolymerization of lignin were explored. Experimental results show that the phenolic monomer yield improves substantially (40-500 wt%) by comparison with processes performed in a single solvent. Detailed characterizations also suggest that the above intensification is rationalized by the solubilization effect of microemulsion system and directional aggregation of lignin at the microemulsion interface.

Temperature-Switchable Surfactant-Free Microemulsion

Stimuli-responsive microemulsions have recently attracted significant interest due to their unique properties. Here, we developed a novel surfactant-free microemulsion (SFME) in a nontoxic ternary mixture, in which dimethyl sulfoxide (DMSO) was used as an amphisolvent, n-butanol was used as a nonpolar phase, and water was used as a polar phase. The DLS results confirmed the presence of the preouzo zone, and the polarity experiment revealed that the single-phase region can be further divided into oil-in-water, bicontinuous, and water-in-oil subregions. The size of droplets increased upon increasing the water or n-butanol content but decreased with increasing DMSO content. With increasing temperature, the area of the single-phase region increased, accompanied by a decrease in the size of the droplets, and the critical point moved to the corner of n-butanol. No matter in what subregion the formulation was found, decreasing temperature to below the phase-transition temperature (PTT) will induce a transition from monophasic MEs to complete phase separation and vice versa. This is mainly attributed to the effect of temperature on the hydrogen-bond interaction. Ag nanoparticles (Ag NPs) can be prepared above the PTT and facilely separated below PTT. The Ag NPs obtained from the current SFME showed higher catalytic activity than that obtained from a common surfactant-based ME.