A surfactant-free microemulsion containing diethyl malonate, ethanol, and water: Microstructure, micropolarity and solubilizations

Surfactant-Free Emulsion Polymerization of Styrene in Ethanol-Water Mixtures: The Role of Mesostructures in the Formation of Polystyrene

Although the synthesis of monodisperse polystyrene nanoparticles (MPSPs) in surfactant-free systems has been widely investigated, the role of mesostructures in forming MPSPs is still unclear. Herein, the styrene/ethanol-water (St/EtOH-H2O) ternary system with certain mesostructures was employed as a model to investigate the correlation between the mesostructure of systems and the properties of polystyrene products. The mesostructures of the ternary system, including styrene droplets, a sponge-like structure, and water droplets, were investigated by transmission electron microscope (TEM) with negative staining, dynamic light scattering (DLS), and nanoparticle tracking analysis (NTA). Classical molecular dynamics (MD) simulations showed the spontaneous aggregation of ethanol at the styrene-water interface, which may be one of the factors stabilizing the mesostructures. We demonstrated that the formation of MPSPs can only occur in the systems containing styrene droplets. Cooling the reactants of styrene droplet systems in the early stages of polymerization resulted in incomplete polystyrene nanospheres (PSNSs), indicating that the formation of MPSPs originated from the interface of styrene droplets. Subsequently, the MPSPs were gradually formed through self-templating polymerization. This study offers valuable insights into the preparation and understanding of the formation process of polymer nanoparticles in other similar surfactant-free systems.

Emulsion Polymerization of Styrene to Polystyrene Nanoparticles with Self-Emulsifying Nanodroplets as Nucleus

The mechanism of the emulsion polymerization of styrene to polystyrene nanoparticles (PSNPs) remains a subject of debate. Herein, a series of reaction parameters with different surfactant concentrations, monomer contents, temperatures, and equilibration times were investigated to understand the formation mechanism of PSNPs, which demonstrate a correlation between the properties of PSNPs and the mesostructure of the premix. Cooling the model systems with self-emulsifying nanodroplets (SENDs) in the early reaction stages resulted in the hollow polystyrene spheres (H-PSSs), ruptured PSNPs, and dandelion-like PSNPs, further indicating that the oil nanodroplets are the key sites for the formation of PSNPs. Therefore, the oligomer radicals generated in the medium at the early polymerization stages are preferentially captured by the oil-H2O interface, and once in the oil droplets, the oligomeric radical continues to grow until colliding with another radical or monomers run out. The self-emulsifying oil nanodroplets with a particle size of 100-300 nm are formed under high temperature and low surfactant concentration conditions and serve as nucleation sites. This study not only contributes to a profound understanding of the correlation between the mesostructure and nucleation pathways but also paves the way for the flexible regulation of the monodispersity and morphology of PSNPs.

Fabrication of tetraethyl orthosilicate/ethanol-water surfactant-free microemulsions and their applications in self-templating synthesis of monodispersed silica colloidal spheres

Surfactant-free microemulsions (SFMEs) composed of tetraethyl orthosilicate (TEOS), ethanol, and water have been successfully fabricated by visual titration and electrical conductivity methods. Three types of SFMEs, water in TEOS (W/O), bicontinuous (BC) and TEOS in water (O/W), were identified by dynamic light scattering and transmission electron microscopy with negative-staining methods. We demonstrated that there are significant differences in the properties of silica products synthesized with different types of SFMEs, and monodispersed silica colloidal spheres (MSCSs) can only be synthesized in the O/W type SFMEs. Moreover, we found that the particle size of MSCSs is closely related to the size of oil droplets. Cooling the O/W type SFMEs in the early reaction stages, results in the larger MSCSs with different condensation degrees. Furthermore, if the cooling temperature decreased to -20 °C, ring-like spheres could be observed. Based on our results and observations, a self-templating growth mechanism was proposed to explain the formation of silica spheres.

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.

Formation of Ethanolamine-Mediated Surfactant-Free Microemulsions Using Hydrophobic Deep Eutectic Solvents

Hydrophobic deep eutectic solvents (HDESs) are emerging as versatile, relatively benign, and inexpensive alternatives to conventional organic solvents in a diverse set of applications. In this context, the formation of microemulsions with HDES replacing the oil phase has become an area of active exploration. Because of recent reports on the undesirable toxicity of many common surfactants, efforts are under way to investigate the formation of surfactant-free microemulsions (SFMEs) using HDES as an oil phase. We present SFME formation using HDESs constituted of n-decanoic acid and five (5) structurally different terpenoids [thymol, l(-)-menthol, linalool, β-citronellol, and geraniol] at a 1:1 molar ratio as the oil phase and water as the hydrophilic phase. Ethanolamine (ETA) exhibited the best potential as a hydrotrope among several other similar small molecules. Results showed a drastic increase in water solubility within the HDESs in the presence of ETA. ETA exerted its hydrotropic action at different extent for each DES system via chemical interaction with the H-bond donor (HBD) constituent of the HDES. The optimum hydrotropic concentration (minimum hydrotrope and maximum water retention, XETAOPT) assigned for each DES/ETA/water system and water loading are reported, and the trends are discussed in detail. Ternary phase diagrams are constructed using visual observation and the dye staining method. The area under the single- and multiple-phase regions (assigned in ternary phase diagrams) was estimated. "Pre-Ouzo" enforced by ETA was investigated using dynamic light scattering (DLS) of the DES/ETA/water systems at XETAOPT. A systematic growth in nanoaggregates was observed with the subsequent addition of water in DES/ETA systems while continuously changing the existing microstructure. The presence of a core (oil)-shell (water)-like structure as indicated by the fluorescence response of Nile red in the "pre-Ouzo" region is speculated. We were able to prepare a homogeneous solution of [K3Fe(CN)6] salt in "pre-Ouzo" mixtures with no apparent deviation in the Beer-Lambert law.

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.

Microemulsion systems: from the design and architecture to the building of a new delivery system for multiple-route drug delivery

Poorly soluble active pharmaceutical ingredients (APIs) create major problems in drug dosage form formulation resulting in significant delays in drug pharmaceutical screening, impairing the drug dosage form production. Aiming to minimize the use of excipients for increasing drug apparent solubility and, as a result, its bioavailability, exploration of innovative approaches is an earnest need. Microemulsion is an alternative drug delivery system that emerged as a valuable tool to achieve safe formulations for insoluble compounds and to improve their biopharmaceutical properties and pharmacokinetics. This review aims to present the state of the art of microemulsion systems, bringing an overview about their origin and how they can be properly produced and thoroughly characterized by different approaches. Furthermore, comments on regulatory issues regarding stability assessment and toxicity evaluation are discussed. The review concludes with a current opinion on microemulsion systems. The overall objective of this work was to describe all the potentialities of microemulsion systems as a drug carrier for therapeutic purposes, highlighting the unique features of this nanotechnological platform. Display Image.