Pickering emulsions: Versatility of colloidal particles and recent applications

The versatility of colloidal particles endows the particle stabilized or Pickering emulsions with unique features and can potentially enable the fabrication of a wide variety of derived materials. We review the evolution and breakthroughs in the research on the use of colloidal particles for the stabilization of Pickering emulsions in recent years for the particle categories of inorganic particles, polymer-based particles, and food-grade particles. Moreover, based on the latest works, several emulsions stabilized by the featured particles and their derived functional materials, including enzyme immobilized emulsifiers for interfacial catalysis, 2D colloidal materials stabilized emulsions as templates for porous materials, and Pickering emulsions as adjuvant formulations, are also summarized. Finally, we point out the gaps in the current research on the applications of Pickering emulsions and suggest future directions for the design of particulate stabilizers and preparation methods for Pickering emulsions and their derived materials.     

Aggregation in Pickering emulsions

For the first time, the particle distribution and aggregation in Pickering emulsions were made visible by transmission X-ray microscopy. Oil/water emulsions were stabilized by heterocoagulates of a clay mineral and magnesium aluminum hydroxide. Stability is optimum when the particles surround the oil droplets and also assemble to form a network extending through the coherent phase. Received: 18 September 1998 Accepted: 28 September 1998     

High internal phase Pickering emulsions

Macroemulsions rendered stable by adsorbed colloidal particles are termed Pickering emulsions. If the volume fraction of dispersed phase exceeds around 0.75, the emulsions are named high internal phase Pickering emulsions abbreviated to HIPPEs, which present new properties and potential applications. We review here the recent progress in preparing and studying HIPPEs of both oil-in-water and water-in-oil types. This includes discussion of the range of solid particle emulsifiers, the choice of the two immiscible liquids and methods for their preparation. As a result of their high interfacial area and long-term stability, HIPPEs are being put to use in many potential applications including drug delivery, catalysis, and in the production of novel porous materials.     

Protein nanocage-stabilized Pickering emulsions

The utilization of surface-active engineered protein nanocages as stabilizers for emulsions provides avenues for the design of new tailor-made functional materials in various fields including food, pharmaceutical, and biotechnology. They can be used to codeliver bioactive molecules of different polarities in a tailored manner to the body, act as a platform for screening cells or enzymes, or function as targeted drug delivery systems. Knowledge on the mechanisms that underlie the protein nanocage-driven stabilization of emulsions and their colloidal structure can have direct implications for the rational design of the new advanced functional colloids.This contribution summarizes the recent progress in protein nanocage-stabilized emulsions. It discusses the advances in the precision bioengineering of protein nanocages for emulsion design, highlights challenges in the characterization of structure and dynamics in these materials, and demonstrates selected applications in the field of functional food materials.     

Non-aqueous olive oil-in-glycerin (o/o) Pickering emulsions: Preparation,characterization and in vitro aspirin release

Non-aqueous olive oil-in-glycerin Pickering emulsions are successfully prepared and stabilized solely by hydrophobic silica nanoparticles possessing 50% silanol groups. Various aspects related to the preparation and physicochemical stability of such promising emulsions are investigated. The resulted emulsions exhibited excellent stability against coalescence for above one year. The apparent viscosity of the olive oil-in-glycerin emulsions is explored as a function of silica nanoparticle concentration and drop volume fraction for the first time. The flow behavior of these emulsions followed the non-Newtonian shear-thinning trend. Both simple o/o and multiple o/o/o emulsion types can be stabilized by one and the same silica nanoparticles during the catastrophic phase inversion, occurred at drop volume fraction between 0.4–0.5. Our findings are correlated with the widely accepted surfactant or solid-stabilized systems. The potential use of such unique emulsions as drug delivery vehicles is studied.     

Pickering emulsions stabilized by ultrashort nanotubes

We describe a simple method to prepare high-efficiency ultrashort nanotube Pickering emulsifiers. The polydivinylbenzene (PDVB) nanotubes with a slight degree of sulfonation, then interrupted to several microns in length, can stabilize hundred times their own mass of oil or water phase and form different Pickering emulsion types. The emulsion is very stable and can be stored for more than half a year without demulsification. A layer of magnetic Fe₃O₄ nanoparticles can be grown on the surface of the ultrashort sulfonated PDVB nanotubes. After being emulsified, oil-phase and magnetic nanotubes can be collected using a magnet, which have huge potential application for separation and recovery of organic solvents in environmental protection.     

Recent developments in intelligent biomedical polymers

Intelligent polymers or stimuli-responsive polymers may exhibit distinct transitions in physical-chemical properties, including conformation, polarity, phase structure and chemical composition in response to changes in environmental stimuli. Due to their unique 'intelligent' characteristics, stimuli-sensitive polymers have found a wide variety of applications in biomedical and nanotechnological fields. This review focuses on the recent developments in biomedical application of intelligent polymer systems, s...     

两亲性Janus粒子的合成及其在Pickering乳液中的应用

Janus粒子由于在光、电、力、磁及表面亲/疏水性等方面表现出各向异性,因此在稳定乳液、生物医药及功能涂层等方面展现出广阔的应用价值.两亲性Janus粒子是指一侧具有亲水性、另一侧具有疏水性的不对称材料,由于同时具有表面活性剂的性质和固体颗粒的效应,在稳定Pickenng乳液方面极具优势.基于此,本文对两亲性Janus...     

Recent developments in oriented polymers for biomedical and engineering applications

A review is presented of recent research at Leeds University which has been directed at devising novel methods for the production of oriented polymer structures. First, the new hot compaction process for oriented fibre and tapes is described, together with its applications to polyethylene and polypropylene where there are a number of practical developments. Secondly, there is the use of hydrostatic extrusion to make load bearing oriented products from hydroxyapatite filled polyethylenes. The production routes include the application of high pressure annealing prior to hydrostatic extrusion and the preparation of high modulus polyethylene fibre/hydroxyapatite billets as the starting point. Finally, recent progress on die-drawing as a means to producing oriented monofilaments and biaxially oriented tubes is described, where the applications include polymer ropes, pipes for gas and water distribution and transparent cans for packaging.     

Recent progress in conductive polymeric materials for biomedical applications

Advanced polymeric materials undoubtedly constitute one of the most promising classes of new materials due to their intriguing electronic, optical, and redox properties. The incredible progress in this area has been driven by the development of novel synthetic procedures owing to the emergence of nanotechnology and by the large array of applications. In particular, hybridization of polymeric materials with nanomaterials has allowed the production of promising functional materials with tailored properties and functionalities for targeted biomedical applications. Consequently, sufficient researchers have carried out imperative studies on these advanced polymeric materials over the last decade. Beyond scientific and fundamental interest, such advanced materials are conspicuous from technological perspectives as well. In this review, we accentuate the proliferation of advanced polymeric materials in diverse biomedical applications.     

Sustainable food-grade Pickering emulsions stabilized by plant-based particles

This review summarizes the major advances that have occurred over the last 5 years in the use of plant-based colloidal particles for the stabilization of oil-in-water and water-in-oil emulsions. We consider the characteristics of polysaccharide-based particles, protein-based particles and organic crystals (flavonoids) with respect to their particle size, degree of aggregation, anisotropy, hydrophobicity and electrical charge. Specific effects of processing on particle functionality are identified. Special emphasis is directed towards the issue of correctly defining the stabilization mechanism to distinguish those cases where the particles are acting as genuine Pickering stabilizers, through direct monolayer adsorption at the liquid–liquid interface, from those cases where the particles are predominantly behaving as ‘structuring agents’ between droplets without necessarily adsorbing at the interface, for example, in many so-called high internal phase Pickering emulsions. Finally, we consider the outlook for future research activity in the field of Pickering emulsions for food applications.     

Recent developments of temperature-responsive polymers for ophthalmic applications

Blindness is one of the most feared disabilities. From cataracts and glaucoma to age-related macular degeneration and retinal vascular diseases, ocular diseases have adverse impacts on patients and pose a huge burden to the healthcare system. The World Health Organization estimates that out of 2.2 billion people with visual impairment, almost half of the cases can be prevented or has yet to be addressed. This presents an urgent clinical and societal need to be met. Temperature-sensitive hydrogels are one of the most biocompatible materials, which can be applied into the eye. By exploiting physiological temperature as a stimulus for in situ gel formation, control of the mechanical properties, rate of drug release, and biomechanical interactions can be tuned. They are very versatile and have immense potential in ocular applications by acting as vitreous substitutes in retinal surgery or topical eye drops and lenses for ocular discomfort and inflammation. In this article, we provide a review of the recent developments in temperature-responsive polymers in ophthalmic therapy in the past 5 years including retinal detachment, retinal vascular diseases, dry eyes, cataracts, age-related macular degeneration, and glaucoma.     

Soft X-ray spectromicroscopy on solid-stabilized emulsions

Oil–water emulsions stabilized by solids have been imaged with sub-100 nm spatial resolution and analyzed spectroscopically using a scanning transmission X-ray microscope. The emulsions are stabilized by particle heterocoagulate cages surrounding the oil droplets. These cages form due to the interaction of negatively charged clay mineral particles (sodium montmorillonite, Wyoming) and positively charged particles of calcium/aluminum layered double hydroxide (LDH). The emulsions were studied at atmospheric pressure, without any pretreatment using carbon K and calcium L X-ray absorption edges. Oil- and calcium-rich LDH were separately mapped, and the clay mineral dispersions were also imaged. Applying X-ray absorption-edge contrast, oil could be distinguished from water in the emulsion near the carbon K absorption edge (284 eV, 4.4 nm). Spectromicroscopy near the calcium L absorption edge (346 eV, 3.6 nm) allowed the structural details of heterocoagulate formation to be revealed. Received: 11 December 1998 Accepted in revised form: 10 January 1999     

Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles: A New Effective Medium for Oxidation Reactions

Decyl‐, dodecyl‐, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW₁₂O₄₀]^3? anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic “Pickering” emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).     

Recent advances in separation applications of polymerized high internal phase emulsions

Polymerized high internal phase emulsions as highly porous adsorption materials have received increasing attention and wide applications in separation science in recent years due to their remarkable merits such as highly interconnected porosity, high permeability, good thermal and chemical stability, and tailorable chemistry. In this review, we attempt to introduce some strategies to utilize polymerized high internal phase emulsions for separation science, and highlight the recent advances made in the applications of polymerized high internal phase emulsions for diverse separation of small organic molecules, carbon dioxide, metal ions, proteins, and other interesting targets. Potential challenges and future perspectives for polymerized high internal phase emulsion research in the field of separation science are also speculated at the end of this review.     

Pickering乳液在功能高分子材料研究中的应用

功能高分子材料制备的瓶颈问题是如何解决多重材料的相容性问题,传统的物理共混技术和聚合添加技术无法保证材料的稳定性及均一性。 Pickering乳液具有成本低、毒性小、环境友好、稳定性好、制备的多重材料结构稳定等优点,在制备功能高分子材料的应用中越来越受到人们的重视。 本文详细介绍了Pickering乳液在功能性高分子材料制备中的应用研究进展,提出了Pickering乳液聚合制备功能高分子材料面临的一些问题,并结合本课题组的研究方向,对其发展前景进行了展望。     

Pickering乳状液的研究进展

随着纳米技术的发展以及Pickering乳液在食品、化妆品、医药等领域的潜在应用前景,人们对Pickering乳液给予了关注。本文全面总结了近些年来Pickering乳液的研究进展,对本课题组在Pickering乳液研究领域所取得的成果进行了介绍。本文主要分为4个部分:第一部分概述了Pickering乳液的研究现状;第二部分全面考察了Pickering乳液的各种影响因素;第三部分介绍了Pickering乳液的转相特性;最后综述了颗粒和表面活性物质协同稳定乳液的研究进展。这些研究成果促进了乳液基础理论的发展,并且拓展了乳液的应用范围。     

Continuous Flow Pickering Emulsion Catalysis in Droplet Microfluidics Studied with In Situ Raman Microscopy

Pickering emulsions (PEs), emulsions stabilized by solid particles, have shown to be a versatile tool for biphasic catalysis. Here, we report a droplet microfluidic approach for flow PE (FPE) catalysis, further expanding the possibilities for PE catalysis beyond standard batch PE reactions. This microreactor allowed for the inline analysis of the catalytic process with in situ Raman spectroscopy, as demonstrated for the acid-catalyzed deacetalization of benzaldehyde dimethyl acetal to form benzaldehyde. Furthermore, the use of the FPE system showed a nine fold improvement in yield compared to the simple biphasic flow system (FBS), highlighting the advantage of emulsification. Finally, FPE allowed an antagonistic set of reactions, the deacetalization–Knoevenagel condensation, which proved less efficient in FBS due to rapid acid-base quenching. The droplet microfluidic system thus offers a versatile new extension of PE catalysis.     

光交联Pickering乳液中的粒子稳定剂制备胶体体微胶囊

研究了光敏性聚合物刷修饰二氧化硅纳米粒子的制备及其乳化行为, 提供了一种制备新颖“胶体体”微胶囊的方法. 首先, 在二氧化硅纳米粒子表面引入原子转移自由基聚合(ATRP)引发剂分子, 通过表面引发ATRP得到含有香豆素侧基聚合物刷光敏性的纳米二氧化硅粒子(SiO₂-PMMA-PCMA); 然后, 以该粒子为乳化剂制备油包水型的皮克林乳液; 最后, 在紫外光的照射下引发香豆素侧基的交联反应从而得到“胶体体”微胶囊. 结果表明, 香豆素单体7-(2-甲基丙烯酰乙氧基)-4-甲基香豆素(CMA)和SiO₂-PMMA-PCMA在紫外光作用下都可以发生交联反应, 以SiO₂-PMMA- PCMA纳米粒子作乳化剂可以制备稳定的皮克林乳液, 光交联后得到的“胶体体”微胶囊的平均粒径约为170 μm.     

Recent developments in food foams

The scientific literature from 2015 onwards with respect to foams and thin films in the context of foods has been reviewed. Proteins are the dominant foaming agents in foods, and investigations of the classic, meringue-forming egg white protein still dominate the literature because the unique properties of this system are still not properly understood. The current drive of many studies is to find suitable replacers of egg proteins, driven by consumer trends for more plant-based alternatives. This has led to investigations of the stabilizing properties of various protein aggregates, ‘nanoparticles’ and microgel particles as Pickering-type stabilizers of foams (Pickering foams). At the same time, other work has sought to manipulate the surface properties of biopolymer- and nonbiopolymer–based particles by chemical means, to make the particles adsorb more strongly. Few, truly novel foam stabilizers have emerged, but two include saponin aggregates and bacteria as particle-type stabilizers.