Fat crystals: A tool to inhibit molecular transport in W/O/W double emulsions

Water-in-oil-in-water (W/O/W) double emulsions are a promising technology for encapsulation applications of water soluble compounds with respect to functional food systems. Yet molecular transport through the oil phase is a well-known problem for liquid oil-based double emulsions. The influence of network crystallization in the oil phase of W/O/W globules was evaluated by NMR and laser light scattering experiments on both a liquid oil-based double emulsion and a solid fat-based double emulsion. Water transport was assessed by low-resolution NMR diffusometry and by an osmotically induced swelling or shrinking experiment, whereas manganese ion permeation was followed by means of T₂-relaxometry. The solid fat-based W/O/W globules contained a crystal network with about 80% solid fat. This W/O/W emulsion showed a reduced molecular water exchange and a slower manganese ion influx in the considered time frame, whereas its globule size remained stable under the applied osmotic gradients. The reduced permeability of the oil phase is assumed to be caused by the increased tortuosity of the diffusive path imposed by the crystal network. This solid network also provided mechanical strength to the W/O/W globules to counteract the applied osmotic forces.     

Fat crystals and water-in-oil emulsion stability

Products such as cosmetics, pharmaceuticals, and crude oil often exist as water-in-oil (W/O) emulsions during their processing or in final form. In many cases, their dispersed aqueous phase is encased in a crystal network and/or by interfacially-adsorbed (‘Pickering’) particles [paraffins, triacylglycerols, polymers, etc.] that promote emulsion kinetic stability by hindering droplet–droplet contact, coalescence and macroscopic phase separation. In processed foods, important questions remain regarding whether a continuous phase fat crystal network or Pickering crystal provides better stabilization. This review explores the following factors related to crystal-stabilized W/O emulsions: i) the key properties dictating fat crystal spatial distribution (at the interface or in the continuous phase); ii) how temperature and freeze–thaw emulsion destabilization are intimately linked with fat crystal spatial distribution, and; iii) why oil-soluble surfactant interactions with the continuous oil phase influence fat crystal wettability and emulsifier efficacy. It is shown that these parameters strongly govern W/O emulsion formation and stability.     

High‐Internal‐Phase Pickering Emulsions Stabilized Solely by Peanut‐Protein‐Isolate Microgel Particles with Multiple Potential Applications

High‐internal‐phase Pickering emulsions have various applications in materials science. However, the biocompatibility and biodegradability of inorganic or synthetic stabilizers limit their applications. Herein, we describe high‐internal‐phase Pickering emulsions with 87 % edible oil or 88 % n‐hexane in water stabilized by peanut‐protein‐isolate microgel particles. These dispersed phase fractions are the highest in all known food‐grade Pickering emulsions. The protein‐based microgel particles are in different aggregate states depending on the pH value. The emulsions can be utilized for multiple potential applications simply by changing the internal‐phase composition. A substitute for partially hydrogenated vegetable oils is obtained when the internal phase is an edible oil. If the internal phase is n‐hexane, the emulsion can be used as a template to produce porous materials, which are advantageous for tissue engineering.     

Pickering乳液的制备和应用研究进展

Pickering乳液是一种由固体粒子代替传统有机表面活性剂稳定乳液体系的新型乳液。与传统乳液相比,Pickering乳液具有强界面稳定性、减少泡沫出现、可再生、低毒、低成本等优势,在化妆品、食品、制药、石油和废水处理等行业具有广阔的应用前景,受到越来越多研究者们的关注。本文综述了近年来Pickering乳液的研究进展,先介绍Pickering乳液相对于表面活性剂乳液的特色与优势,然后介绍Pickering乳液的制备研究进展,最后介绍Pickering乳液的应用研究进展。     

Extraction of Cr (VI) by pickering emulsion liquid membrane using amphiphilic silica nanowires (ASNWs) as a surfactant

Pickering emulsion is the replacement of surfactants with solid, often nano-sized particles. The particle-stabilized emulsions have good thermodynamic and kinetic stability. Pickering emulsion liquid membrane (PELM) was prepared using mahua oil as a diluent, aliquat 336 (Trioctyl methylammonium chloride) as a carrier and amphiphilic silica nanowires (ASNWs) (10–40?ml ethanol addition) as a surfactant. Sodium hydroxide (NaOH) was used as stripping phase in the concentration range from 0.1 to 0.5?M for the extraction of hexavalent chromium [Cr (VI)] from aqueous solution. The variety of edible and non-edible oils was investigated for the stability of water in oil emulsion. Factors that influence silica-stabilized Pickering emulsions are pH, agitation speed, stripping phase concentration, the volume ratio of membrane to stripping phase (M/S), initial feed concentration, treat ratio(feed to emulsion volume ratio) and surfactant concentration for better PELM stability. And also, the extraction efficiency of Cr (VI) was investigated using aliquat as a carrier. The physicochemical properties of ASNWs were studied using Scanning Electron Microscopy (SEM), Fourier Transforms Infrared Spectroscopy (FTIR) and Dynamic Light Scattering (DLS) techniques. At an optimum condition, 99.69% of Cr (VI) removal from aqueous solution was obtained.     

Pickering Emulsions as Vehicles for Bioactive Compounds from Essential Oils

Pickering emulsions are emulsion systems stabilized by solid particles at the interface of oil and water. Pickering emulsions are considered to be natural, biodegradable, and safe, so their applications in various fields—such as food, cosmetics, biomedicine, etc.—are very promising, including as a vehicle for essential oils (EOs). These oils contain volatile and aromatic compounds and have excellent properties, such as antifungal, antibacterial, antiviral, and antioxidant activities. Despite their superior properties, EOs are prone to evaporation, decompose when exposed to light and oxygen, and have low solubility, limiting their industrial applications. Several studies have shown that EOs in Pickering emulsions displays less sensitivity to evaporation and oxidation, stronger antibacterial activity, and increased solubility. In brief, the application of Pickering emulsions for EOs is interesting to explore. This review discusses recent progress in the application of Pickering emulsions, particularly as EO carriers, drug carriers, antioxidant and antimicrobial carriers, and in active packaging.     

A novel formulation of the pickering emulsion stabilized with silica nanoparticles and its thermal resistance at high temperatures

Stabilization of emulsions with solid particles can be used in several fields of oil and gas industry because of their higher stability. Solid particles should be amphiphilic to be able to make Pickering emulsions. This goal is achieved by using surfactants at low concentrations. Oil-in-water (o/w) emulsions are usually stabilized by surfactant but show poor thermal stability. This problem limits their applications at high-temperature conditions. In this study, a novel formulation for o/w stabilized emulsion by using silica nanoparticles and the nonionic surfactant is investigated for the formulation of thermally stable Pickering emulsion. The experiments performed on this Pickering emulsion formula showed higher thermal stability than conventional emulsions. The optimum wettability was found for DME surfactant and silica nanoparticles, consequently, in that region; Pickering emulsion showed the highest stability. Rheological changes were evaluated versus variation in surfactant concentration, silica concentration and pH. Scanning electron microscopy images approved the existence of a rigid layer of nanoparticle at the oil-water interface. Finally, the results show this type of emulsion remains stable in harsh conditions and allows the system to reach its optimum rheology without adding any further additives.     

Influence of microgel architecture and oil polarity on stabilization of emulsions by stimuli-sensitive core-shell poly(N-isopropylacrylamide-co-methacrylic acid) microgels: Mickering versus Pickering behavior?

Charged poly(N-isopropylacrylamide-co-methacrylic acid) [P(NiPAM-co-MAA)] microgels can stabilize thermo- and pH-sensitive emulsions. By placing charged units at different locations in the microgels and comparing the emulsion properties, we demonstrate that their behaviors as emulsion stabilizers are very different from molecular surfactants and rigid Pickering stabilizers. The results show that the stabilization of the emulsions is independent of electrostatic repulsion although the presence and location of charges are relevant. Apparently, the charges facilitate emulsion stabilization via the extent of swelling and deformability of the microgels. The stabilization of these emulsions is linked to the swelling and structure of the microgels at the oil-water interface, which depends not only on the presence of charged moieties and on solvent polarity but also on the microgel (core-shell) morphology. Therefore, the internal soft and porous structure of microgels is important, and these features make microgel-stabilized emulsions characteristically different from classical, rigid-particle-stabilized Pickering emulsions, the stability of which depends on the surface properties of the particles.     

Engineering hybrid microgels as particulate emulsifiers for reversible Pickering emulsions

Thermo-responsive microgels are unique stabilizers for stimuli-sensitive Pickering emulsions that can be switched between the state of emulsification and demulsification by changing the temperature. However, directly temperature-triggering the phase inversion of microgel-stabilized emulsions remains a great challenge. Here, a hybrid poly(N-isopropylacrylamide)-based microgel has now been successfully fabricated with tunable wettability from hydrophilicity to hydrophobicity in a controlled manner. Engineered microgels are synthesized from an inverse emulsion stabilized with hydrophobic silica nanoparticles, and the swelling-induced feature can make the resultant microgel behave like either hydrophilic or hydrophobic colloids. Remarkably, the phase inversion of such microgel-stabilized Pickering emulsions can be in situ regulated by temperature change. Moreover, the engineered microgels were capable of stabilizing water-in-oil Pickering emulsions and encapsulation of enzymes for interfacial bio-catalysis, as well as rapid cargo release triggered by phase inversion.

Hybrid poly(N-isopropylacrylamide)-based microgels are templated from inverse Pickering emulsions, and the tunable wettability renders as-prepared emulsions with reversible feature.     

Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

Fabrication of macroporous foam and microspheres of polystyrene by Pickering emulsion polymerization

Poly(styrene-co-methacrylic acid) (PS-co-MAA) particles were synthesized via surfactant-free emulsion polymerization and then used as particulate emulsifiers for preparation of Pickering emulsions. Our results showed that adjusting the solution pH can tune the wettability of PS-co-MAA particles to stabilize either water-in-oil (W/O) or oil-in-water (O/W) Pickering emulsions. Stable W/O emulsions were obtained with PS-co-MAA particles at low pH values due to their better affinity to the dispersed oil phase. In contrast, increasing the pH value significantly changed the stabilizing behavior of the PS-co-MAA particles, leading to the phase inversion and formation of stable O/W emulsions. We found that the oil/water ratio had a significant influence on pH value of the phase inversion. It decreased with decreasing the oil/water ratio, and no phase inversion occurred when the styrene volume fraction reduced to 10 %. Additionally, macroporous polystyrene (PS) foam and PS microspheres were obtained via polymerization of Pickering high internal phase emulsion (Pickering HIPE) and O/W Pickering emulsion, respectively.     

Structure of microparticles in solid-stabilized emulsions

Emulsions of oil and water stabilized by adsorbed solid particles are known as solid-stabilized emulsions (often referred to as Pickering emulsions). Using confocal microscopy, we have studied the assembly of colloidal-sized polystyrene particles in poly(dimethylsiloxane)-in-water solid-stabilized emulsions. Monodisperse polystyrene particles, when included in the emulsions at low concentrations, were found to form small patches with local "hexagonal" order, separated by other particle-free domains. Polystyrene particles with different sizes (1 and 4 microm) and different wettability could simultaneously segregate to the emulsion interface; even mixtures of hydrophobic and hydrophilic solid particles were found to simultaneously segregate to the same interface.     

Review of Functional Aspects of Nanocellulose-Based Pickering Emulsifier for Non-Toxic Application and Its Colloid Stabilization Mechanism

In the past few years, the research on particle-stabilized emulsion (Pickering emulsion) has mainly focused on the usage of inorganic particles with well-defined shapes, narrow size distributions, and chemical tunability of the surfaces such as silica, alumina, and clay. However, the presence of incompatibility of some inorganic particles that are non-safe to humans and the ecosystem and their poor sustainability has led to a shift towards the development of materials of biological origin. For this reason, nano-dimensional cellulose (nanocellulose) derived from natural plants is suitable for use as a Pickering material for liquid interface stabilization for various non-toxic product formulations (e.g., the food and beverage, cosmetic, personal care, hygiene, pharmaceutical, and biomedical fields). However, the current understanding of nanocellulose-stabilized Pickering emulsion still lacks consistency in terms of the structural, self-assembly, and physio-chemical properties of nanocellulose towards the stabilization between liquid and oil interfaces. Thus, this review aims to provide a comprehensive study of the behavior of nanocellulose-based particles and their ability as a Pickering functionality to stabilize emulsion droplets. Extensive discussion on the characteristics of nanocelluloses, morphology, and preparation methods that can potentially be applied as Pickering emulsifiers in a different range of emulsions is provided. Nanocellulose’s surface modification for the purpose of altering its characteristics and provoking multifunctional roles for high-grade non-toxic applications is discussed. Subsequently, the water–oil stabilization mechanism and the criteria for effective emulsion stabilization are summarized in this review. Lastly, we discuss the toxicity profile and risk assessment guidelines for the whole life cycle of nanocellulose from the fresh feedstock to the end-life of the product.     

Particle self-assembly at ionic liquid-based interfaces

This review presents an overview of the nature of ionic liquid (IL)-based interfaces and self-assembled particle morphologies of IL-in-water, oil- and water-in-IL, and novel IL-in-IL Pickering emulsions with emphasis on their unique phenomena, by means of experimental and computational studies. In IL-in-water Pickering emulsions, particles formed monolayers at ionic liquid–water interfaces and were close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. Interestingly, other than equilibrating at the ionic liquid–water interfaces, microparticles with certain surface chemistries were extracted into the ionic liquid phase with a high efficiency. These experimental findings were supported by potential of mean force calculations, which showed large energy drops as hydrophobic particles crossed the interface into the IL phase. In the oil- and water-in-IL Pickering emulsions, microparticles with acidic surface chemistries formed monolayer bridges between the internal phase droplets rather than residing at the oil/water–ionic liquid interfaces, a significant deviation from traditional Pickering emulsion morphology. Molecular dynamics simulations revealed aspects of the mechanism behind this bridging phenomenon, including the role of the droplet phase, surface chemistry, and inter-particle film. Novel IL-in-IL Pickering emulsions exhibited an array of self-assembled morphologies including the previously observed particle absorption and bridging phenomena. The appearance of these morphologies depended on the particle surface chemistry as well as the ILs used. The incorporation of particle self-assembly with ionic liquid science allows for new applications at the intersection of these two fields, and have the potential to be numerous due to the tunability of the ionic liquids and particles incorporated, as well as the particle morphology by combining certain groups of particle surface chemistry, IL type (protic or aprotic), and whether oil or water is incorporated.     

Synergetic Effect of Reactive Surfactants and Clay Particles on Stabilization of Nonaqueous Oil-in-Oil (o/o) Emulsions

Although surfactants and particles are often used together in stabilization of aqueous emulsions, the contribution of each species to such stabilization at the oil-water interface is poorly understood. The situation becomes more complicated if we consider the nonaqueous oil-oil interface, i.e, the stabilization of nonaqueous oil-in-oil (o/o) emulsions by solid particles and reactive surfactants which, to our knowledge, has not been studied before. We have prepared Pickering nonaqueous simple (o/o) emulsions stabilized by a combination of kaolinite particles and a nonionic polymerizable surfactant Noigen RN10 (polyoxyethylene alkylphenyl ether). Different pairs of immiscible oils were used which gave different emulsion stabilities. Using kaolinite with equal volumes of paraffin oil/formamide system gave no stable emulsions at all concentrations while the addition of Noigen RN10 enhanced the emulsion stability. In contrast, addition of Noigen RN10 surfactant to silicon oil-in-glycerin emulsions stabilized by kaolinite resulted in destabilization of the system at all concentrations. For all systems studied here, no phase inversion in simple emulsion was observed by altering the volume fraction of the dispersed phase as compared to the known water-based simple Pickering emulsions.      

Simple, reversible emulsion system switched by pH on the basis of chitosan without any hydrophobic modification

Chitosan without hydrophobic modification is not a good emulsifier itself. However, it has a pH-tunable sol-gel transition due to free amino groups along its backbone. In the present work, a simple reversible Pickering emulsion system based on the pH-tunable sol-gel transition of chitosan was developed. At pH > 6.0, as adjusted by NaOH, chitosan was insoluble in water. Chitosan nanoparticles or micrometer-sized floccular precipitates were formed in situ. These chitosan aggregates could adsorb at the interface of oil and water to stabilize the o/w emulsions, so-called Pickering emulsions. At pH < 6.0, as adjusted by HCl, chitosan was soluble in water. Demulsification happened. Four organic solvents (liquid paraffin, n-hexane, toluene, and dichloromethane) were chosen as the oil phase. Reversible emulsions were formed for all four oils. Chitosan-based Pickering emulsions could undergo five cycles of emulsification-demulsification with only a slight increase in the emulsion droplet size. They also had good long-term stability for more than 2 months. Herein, we give an example of chitosan without any hydrophobic modification to act as an effective emulsifier for various oil-water systems. From the results, we have determined that natural polymers with a stimulus-responsive sol-gel transition should be a good particulate emulsifier. The method for in situ formation of pH-responsive Pickering emulsions based on chitosan will open up a new route to the preparation of a wide range of reversible emulsions.     

Surfactant-Free Multiple Pickering Emulsions Stabilized by Combining Hydrophobic and Hydrophilic Nanoparticles

A series of W/O/W or O/W/O emulsion stabilized solely by two different types of solid nanoparticles were prepared by a two-step method. We explored the option of particular emulsifiers for the multiple Pickering emulsions, and a variety of nanoparticles (silica, iron oxide, and clay) only differing in their wettability was used. The primary W/O emulsion was obtained by the hydrophobic nanoparticles, and then the hydrophilic nanoparticles were used as emulsifier in the secondary emulsification to prepare the W/O/W emulsion. In a similar way, the primary O/W emulsion of the O/W/O emulsion was stabilized by the hydrophilic nanoparticles, while the secondary emulsification to prepare the O/W/O emulsion was effected with the hydrophobic nanoparticles. The resultant multiple Pickering emulsion was stable to coalescence for more than 3 months, except the W/O/W emulsions of which the secondary emulsion stabilized by clay nanoparticles became a simple O/W emulsion in a day after preparation. Moreover, the temperature and pH sensitive poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAm-co-MAA)) microgels were introduced as an emulsifier for the secondary emulsification to obtain the stimulus-responsive multiple W/O/W emulsion. Such microgel-stabilized multiple emulsions could realize the efficient controlled release of water-soluble dye, Rhodamine B (RB) on demand in a multiple-emulsion delivery system.      

碳纳米复合物作为催化剂和乳化剂用于水/有机两相体系反应(英文)

This mini-review summarizes some novel aspects of reactions conducted in aqueous/organic emulsions stabilized by carbon nanohybrids functionalized with catalytic species. Carbon nanohybrids represent a family of solid catalysts that not only can stabilize water-oil emulsions in the same fashion as Pickering emulsions, but also catalyze reactions at the liquid/liquid interface. Several exam-ples are discussed in this mini-review. They include (a) aldol condensation-hydrodeoxygenation tandem reactions catalyzed by basic (MgO) and metal (Pd) catalysts, respectively; (b) Fischer-Tropsch synthesis catalyzed by carbon-nanotube-supported Ru; and (c) emulsion polymerization of styrene for the production of conductive polymer composites. Conducting these reactions in emul-sion generates important advantages, such as increased liquid/liquid interfacial area that consequently means faster mass transfer rates of molecules between the two phases, effective separation of products from the reaction mixture by differences in the water-oil solubility, and significant changes in product selectivity that can be adjusted by modifying the emulsion characteristics.     

Sodium caseinate as a particulate emulsifier for making indefinitely recycled pH-responsive emulsions

pH-responsive emulsions are one of the simplest and most readily implementable stimuli-responsive systems. However, their practical uses have been greatly hindered by cyclability. Here, we report a robust pH-responsive emulsion prepared by utilizing pure sodium caseinate (NaCas) as the sole emulsifier. We demonstrate that the emulsification/demulsification of the obtained NaCas-stabilized emulsion can be triggered by simply changing the pH value over 100 cycles, which has never been observed in any protein-stabilized emulsion system. The NaCas-stabilized emulsion maintains its pH-responsive properties even in a saturated salt solution (NaCl ∼ 6.1 M) or seawater. We illustrate how NaCas functions in pH-responsive emulsions and show that when conventional nanoparticles such as zein protein or bare SiO₂ particles were coated with a layer of NaCas, the resulting formulated emulsions could be switched on and off over 10 cycles. The unique properties of NaCas thus enable the engineering of conventional Pickering emulsions to pH-responsive Pickering emulsions. Finally, we have integrated catalytically active gold (Au) nanoclusters (NCs) into the NaCas protein and then utilized them to produce emulsions. Remarkably, these NaCas–Au NCs assembled at the oil–water interface exhibited excellent catalytic activity and cyclability, not only in aqueous solution, but also in complicated seawater environments.

An unprecedented pH-responsive emulsion is shaped by utilizing pure sodium caseinate (NaCas) as the sole emulsifier for recyclable interfacial catalysis. This emulsion could be reversibly switched on and off over 100 cycles.     

Particle self-assembly in oil-in-ionic liquid Pickering emulsions

We have studied polydimethylsiloxane (PDMS)-in-1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) Pickering emulsions stabilized by polystyrene microparticles with different surface chemistry. Surprisingly, in contrast to the consensus originating from oil/water Pickering emulsions in which the solid particles equilibrate at the oil-water droplet interfaces and provide effective stabilization, here the polystyrene microparticles treated with sulfate, aldehyde sulfate, or carboxylate dissociable groups mostly formed monolayer bridges among the oil droplets rather than residing at the oil-ionic liquid interfaces. The bridge formation inhibited individual droplet-droplet coalescence; however, due to low density and large volume (thus the buoyant effect), the aggregated oil droplets actually promoted oil/ionic liquid phase separation and distressed emulsion stability. Systems with binary heterogeneous polystyrene microparticles exhibited similar, even enhanced (in terms of surface chemistry dependence), bridging phenomenon in the PDMS-in-[BMIM][PF(6)] Pickering emulsions.