The study of solid particles residing at fluid–fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at ...interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil–water, air–water, oil–oil, air–oil, and water–water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.
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.
Fit for consumption: Peanut‐protein‐isolate microgel particles existing at the interface and in the continuous phase inhibited the destabilization of high‐internal‐phase (87 % edible oil or 88 % n‐hexane) oil/water Pickering emulsions (see picture). Such emulsions based entirely on natural and nontoxic raw materials could potentially be adapted for multiple applications in food and tissue engineering simply by changing the internal‐phase composition.
Pickering emulsions are an excellent platform for interfacial catalysis. However, developing simple and efficient strategies to achieve product separation and catalyst and emulsifier recovery is ...still a challenge. Herein, we report the reversible transition between emulsification and demulsification of a light‐responsive Pickering emulsion, triggered by alternating between UV and visible light irradiation. The Pickering emulsion is fabricated from Pd‐supported silica nanoparticles, azobenzene ionic liquid surfactant, n‐octane, and water. This phase behavior is attributed to the adsorption of azobenzene ionic liquid surfactant on the surface of the nanoparticles and the light‐responsive activity of ionic liquid surfactant. The Pickering emulsion can be used as a microreactor that enables catalytic reaction, product separation as well as emulsifier and catalyst recycling. Catalytic hydrogenation of unsaturated hydrocarbons at room temperature and atmospheric pressure has been performed in this system to demonstrate product separation and emulsifier and catalyst re‐use.
A light‐responsive Pickering emulsion was designed and prepared, which could be reversibly switched between stable and unstable upon exposure to visible and UV light, respectively. The reversible, light‐responsive phase behavior of the Pickering emulsion enabled its use as a microreactor, allowing highly efficient catalytic hydrogenation, followed by product separation and emulsifier and catalyst recycling.
It is a dream that future synthetic chemistry can mimic living systems to process multistep cascade reactions in a one-pot fashion. One of the key challenges is the mutual destruction of incompatible ...or opposing reagents, for example, acid and base, oxidants and reductants. A conceptually novel strategy is developed here to address this challenge. This strategy is based on a layered Pickering emulsion system, which is obtained through lamination of Pickering emulsions. In this working Pickering emulsion, the dispersed phase can separately compartmentalize the incompatible reagents to avoid their mutual destruction, while the continuous phase allows other reagent molecules to diffuse freely to access the compartmentalized reagents for chemical reactions. The compartmentalization effects and molecular transport ability of the Pickering emulsion were investigated. The deacetalization–reduction, deacetalization–Knoevenagel, deacetalization–Henry and diazotization–iodization cascade reactions demonstrate well the versatility and flexibility of our strategy in processing the one-pot cascade reactions involving mutually destructive reagents.
Particle-stabilized oil foams Binks, Bernard P.; Vishal, Badri
Advances in colloid and interface science,
20/May , Letnik:
291
Journal Article
Recenzirano
The area of oil foams although important industrially has received little academic attention until the last decade. The early work using molecular surfactants for stabilisation was limited and as ...such it is difficult to obtain general rules of thumb. Recently however, interest has grown in the area partly fuelled by the understanding gained in the general area of colloidal particles at fluid interfaces. We review the use of solid particles as foaming agents for oil foams in cases where particles (inorganic or polymer) are prepared ex situ and in cases where crystals of surfactant or fat are prepared in situ. There is considerable activity in the latter area which is particularly relevant to the food industry. Discussion of crude oil/lubricating oil foams is excluded from this review.
Display omitted
•Oil foams can be stabilized by suitable hydrophobic solid particles.•They can also be stabilized by surfactant/fat crystals prepared in situ.•Oleogels may be aerated to produce very stable oil foams.•Oil foams stabilized by surfactant/fat crystals are thermo-responsive destabilizing around the melting point of the crystals.•A summary of the various techniques being used to study oil foams is included.
Small particles attached to liquid surfaces arise in many products and processes, including crude-oil emulsions and food foams and in flotation, and there is a revival of interest in studying their ...behaviour. Colloidal particles of suitable wettability adsorb strongly to liquid-liquid and liquid-vapour interfaces, and can be sole stabilizers of emulsions and foams, respectively. New materials, including colloidosomes, anisotropic particles and porous solids, have been prepared by assembling particles at such interfaces. Phase inversion of particle-stabilized emulsions from oil in water to water in oil can be achieved either by variation of the particle hydrophobicity (transitional) or by variation of the oil/water ratio (catastrophic). Here we describe the phase inversion of particle-stabilized air-water systems, from air-in-water foams to water-in-air powders and vice versa. This inversion can be driven either by a progressive change in silica-particle hydrophobicity at constant air/water ratio or by changing the air/water ratio at fixed particle wettability, and has not been observed in the corresponding systems stabilized by surfactants. The simplicity of the work is that this novel inversion is achieved in a single system. The resultant materials in which either air or water become encapsulated have potential applications in the food, pharmaceutical and cosmetics industries.
To address the limitations of batch organic–aqueous biphasic catalysis, we develop a conceptually novel method termed Flow Pickering Emulsion, or FPE, to process biphasic reactions in a continuous ...flow fashion. This method involves the compartmentalization of bulk water into micron-sized droplets based on a water-in-oil Pickering emulsion, which are packed into a column reactor. The compartmentalized water droplets can confine water-soluble catalysts, thus “immobilizing” the catalyst in the column reactor, while the interstices between the droplets allow the organic (oil) phase to flow. Key fundamental principles underpinning this method such as the oil phase flow behavior, the stability of compartmentalized droplets and the confinement capability of these droplets toward water-soluble catalysts are experimentally and theoretically investigated. As a proof of this concept, case studies including a sulfuric acid-catalyzed addition reaction, a heteropolyacid-catalyzed ring opening reaction and an enzyme-catalyzed chiral reaction demonstrate the generality and versatility of the FPE method. Impressively, in addition to the excellent durability, the developed FPE reactions exhibit up to 10-fold reaction efficiency enhancement in comparison to the existing batch reactions, indicating a unique flow interface catalysis effect. This study opens up a new avenue to allow conventional biphasic catalysis reactions to access more sustainable and efficient flow chemistry using an innovative liquid–liquid interface protocol.
pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant ...as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles’ surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experimental evidence including adsorption isotherms, zeta potentials, microscopy, and three-phase contact angles is given to support the postulated mechanisms.
Display omitted
Can a mixture of sucrose ester surfactant in vegetable oil be aerated to yield stable oleofoams? Is foaming achievable from one-phase molecular solutions and/or two-phase crystal ...dispersions? Does cooling a foam after formation induce surfactant crystallisation and enhance foam stability?
Concentrating on extra virgin olive oil, we first study the effect of aeration temperature and surfactant concentration on foamability and foam stability of mixtures cooled from a one-phase oil solution. Based on this, we introduce a strategy to increase foam stability by rapidly cooling foam prepared at high temperature which induces surfactant crystallisation in situ. Differential scanning calorimetry, X-ray diffraction, infra-red spectroscopy, surface tension and rheology are used to elucidate the mechanisms.
Unlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temperature into the two-phase region containing surfactant crystals. At high temperature in the one-phase region, substantial foaming is achieved (over-run 170%) within minutes of whipping but foams ultimately collapse within a week. We show that surfactant molecules are surface-active at high temperature and that hydrogen bonds form between surfactant and oil molecules. Cooling these foams substantially increases foam stability due to both interfacial and bulk surfactant crystallisation. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a maximum over-run of 330% and the absence of drainage, coalescence and disproportionation being achievable.
PDF
