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.     

纳米粒子的界面自组装

近年来,随着纳米技术的发展及Pickering乳液在食品、化妆品、医药等领域中的应用,纳米粒子的界面自组装现象引起了人们的广泛关注。界面能的降低是纳米粒子液液界面自组装的主要驱动力。通过改变纳米粒子的尺寸和表面配体的化学性质,可控制纳米粒子的界面自组装行为。本文综述了不同类型纳米粒子实现界面自组装的研究工作,包括均质纳米粒子、Janus纳米粒子、棒状纳米粒子以及生物纳米粒子。最后,对纳米粒子的界面组装这一领域的可能发展做了展望。     

Particle laden fluid interfaces: Dynamics and interfacial rheology

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.     

Tuning properties of silver particle monolayers via controlled adsorption-desorption processes

Nanoparticle self-assembly at fluid-fluid interfaces has been traditionally exploited in emulsification, encapsulation and oil recovery, and more recently in emerging applications including functional nanomaterials and biphasic catalysis. We provide a review of the literature focusing on the open challenges that still hamper the broader applicability of this potentially transformative technology, and we outline strategies to achieve improved control over interfacial self-assembly of nanoparticles. First, we discuss means to promote spontaneous adsorption by tuning the interfacial energies of the nanoparticles with the fluids using capping ligands, and the occurrence of energy barriers. We then examine the interactions between interfacial nanoparticles and how they affect the formation of equilibrium interfacial suspensions versus non-equilibrium two-dimensional phases, such as weakly attractive glasses and gels. Important differences with colloidal interactions in a bulk suspension arise due to the discontinuity in solvent properties at the interface. For instance, ligand brushes rearrange in asymmetric configurations, and thus play a significant role in determining interparticle interactions. Finally, we briefly discuss the link between interfacial microstructure and the dynamic response of particle-laden interfaces, including interfacial rheology and the fate of nanoparticle monolayers upon out-of-plane deformation.     

The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

pH‐responsive microgels are unique stabilizers for stimuli‐sensitive emulsions that can be broken on demand by changing the pH value. However, recent experiments have indicated that electrostatic interactions play a different role to that in conventional Pickering emulsions. The influence of charges on the interactions between microgels at the oil–water interface is now described. Compression isotherms of microgels with different charge density and architecture were determined in a Langmuir trough, and counter‐intuitive results were obtained: Charged microgels can be compressed more easily than uncharged microgels. The compressibility of microgels is thus not determined by direct Coulomb repulsion. Instead, the different swelling of the microgels in the charged and the uncharged states is proposed to be the key parameter.     

Polyelectrolyte/surfactant mixtures in the bulk and at water/oil interfaces

Stabilization of emulsions by mixed polyelectrolyte/surfactant systems is a prominent example for the application in modern technologies. The formation of complexes between the polymers and the surfactants depends on the type of surfactant (ionic, non-ionic) and the mixing ratio. The surface activity (hydrophilic–lipophilic balance) of the resulting complexes is an important quantity for its efficiency in stabilizing emulsions. The interfacial adsorption properties observed at liquid/oil interfaces are more or less equivalent to those observed at the aqueous solution/air interface, however, the corresponding interfacial dilational and shear rheology parameters differ quite significantly. The interfacial properties are directly linked to bulk properties, which support the picture for the complex formation of polyelectrolyte/surfactant mixtures, which is the result of electrostatic and hydrophobic interactions. For long alkyl chain surfactants the interfacial behavior is strongly influenced by hydrophobic interactions while the complex formation with short chain surfactants is mainly governed by electrostatic interactions.     

Mechanical properties of hexadecane-water interfaces with adsorbed hydrophobic bacteria

This study focuses on how intact, hydrophobic bacteria in their stationary (i.e., non-dividing) phase could adsorb onto the hexadecane–water interface and alter its mechanical properties. The two strains of bacteria used in forming the interfacial films were Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c. Using the dynamic pendant drop technique, the film interfacial tension was monitored as the surface area was made to undergo transient changes. Under static conditions, both types of bacteria had no effect on the interfacial tension. When subjected to transient excitations, however, the two bacterial films exhibited clear and qualitatively similar rheological properties: they responded as two-dimensional Maxwellian materials when the interfacial areas were dilated suddenly, but appeared to be purely elastic upon rapid area compression. Such rheological behaviours are “non-linear” in that the responses of the tension to area dilation and contraction are not mirror images of one another. Despite their qualitative similarities, the two types of film had very distinct film elasticities and relaxation times. The most striking difference between the two bacterial films was revealed under continuous reduction of area, when the A. venetianus RAG-1 system displayed a “paper-like” interface, whereas the interface of the R. erythropolis 20S-E1-c system was “soap film-like”. These macroscopic observations could be explained by the surface ultrastructures of the two cell strains determined using transmission electron microscopy.     

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...     

纳米粒子稳定乳液及其在纳米结构合成中的应用*

本文在介绍常规乳状液、微乳液和固体稳定乳液的基础上,着重综述了纳米粒子稳定乳液的特点及其在纳米结构合成中的应用进展,并对目前该研究领域亟待解决的问题进行了分析。纳米粒子稳定乳液具有独特的油、水、固三相环境和水油、水固、油固三个相界面,分散相液滴尺寸可以在微米、亚微米乃至纳米尺度调节,因而可以作为合成组成、结构和性能极为丰富多样的纳米结构的介质。纳米粒子对乳液稳定作用的机理,以及纳米粒子稳定乳液中化学反应的特殊规律还有待深入研究。本文在介绍固体稳定乳液的基础上,着重综述了纳米粒子稳定乳液的特点及其在纳米结构合成中的应用进展,并对目前该研究领域亟待解决的问题进行了分析。纳米粒子稳定乳液具有独特的油、水、固三相环境和水油、水固、油固三个相界面,分散相液滴尺寸可以在微米、亚微米乃至纳米尺度调节,因而可以作为合成组成、结构和性能极为丰富多样的纳米结构的介质。纳米粒子对乳液稳定作用的机理,以及纳米粒子稳定乳液中化学反应的特殊规律还有待深入研究。     

Modeling electrified metal/water interfaces from ab initio molecular dynamics: Structure and Helmholtz capacitance

The structure and potential distribution of electric double layers (EDLs) are of close relevance to the performance of electrode materials. In the past years, despite tremendous efforts devoted to this topic, an atomistic picture of the EDL is still lacking, let alone understanding on how the EDL structure is related to the dielectric property of interface water. In this article, we briefly review the recent progress in modeling electrified metal/water interfaces using ab initio molecular dynamics (AIMD). The ab initio methods for EDL modeling is firstly summarized, and then we discuss the structures of interface water on metal electrodes at different potential conditions. Moreover, we illustrate the potential-dependent behavior of chemisorbed water on Pt(111) surface and its relationship with the peak of the differential Helmholtz capacitance observed by experiment. At last, we give some perspective for future development in ab initio modeling of electrochemical interfaces.     

Mechanical properties of protein adsorption layers at the air/water and oil/water interface: A comparison in light of the thermodynamical stability of proteins

Over the last decades numerous studies on the interfacial rheological response of protein adsorption layers have been published. The comparison of these studies and the retrieval of a common parameter to compare protein interfacial activity are hampered by the fact that different boundary conditions (e.g. physico-chemical, instrumental, interfacial) were used. In the present work we review previous studies and attempt a unifying approach for the comparison between bulk protein properties and their adsorption films. Among many common food grade proteins we chose bovine serum albumin, β-lactoglobulin and lysozyme for their difference in thermodynamic stability and studied their adsorption at the air/water and limonene/water interface. In order to achieve this we have i) systematically analyzed protein adsorption kinetics in terms of surface pressure rise using a drop profile analysis tensiometer and ii) we addressed the interfacial layer properties under shear stress using an interfacial shear rheometer under the same experimental conditions. We could show that thermodynamically less stable proteins adsorb generally faster and yield films with higher shear rheological properties at air/water interface. The same proteins showed an analog behavior when adsorbing at the limonene/water interface but at slower rates.     

颗粒乳化剂的研究及应用

近年来,颗粒乳化剂因其在食品、采油、化妆品、医药、催化以及功能纳米材料制备等领域具有潜在应用前景而备受关注。本文综述了近来颗粒乳化剂的研究进展,归纳了颗粒乳化剂的种类,包括:无机纳米粒子、表面改性或杂化的无机粒子、有机纳米粒子以及特殊的颗粒乳化剂Janus粒子;并对颗粒乳化剂能够在油水界面稳定吸附的热力学机理和动力学行为进行了阐述,颗粒乳化剂在油水界面接触角以及粒径大小是其在界面稳定吸附的关键参数,而颗粒在油水界面的排布方式则主要受粒子之间相互作用的影响。重点介绍了颗粒乳化剂的热点应用,包括:(1)利用颗粒乳化剂制备Pickering乳液,以及通过对颗粒乳化剂的功能化,使得Pickering乳液具备环境响应性(即pH、盐浓度、温度、紫外光、磁场敏感响应性);(2)以颗粒乳化剂为构筑基元、以Pickering乳液为模板制备Janus颗粒、Colloidosome、具有多级结构的粒子或膜,以及多孔结构材料;(3) Janus粒子在催化领域的应用。     

支状嵌段聚醚的界面聚集行为及对原油乳状液的破乳作用

合成了三种不同聚氧丙烯/聚氧乙烯(PPO/PEO)比例的含苯环支状嵌段聚醚, 通过界面张力、界面流变、表面压以及对原油乳状液的破乳脱水效果的测定, 考察了其界面聚集行为和破乳作用对PEO含量和分子量的依赖性, 并且对比研究了三种支状聚醚分子交联前后的破乳性能. 结果表明, PEO含量高且分子量大者,其单分子界面占据面积大, 在油/水界面达到吸附平衡的时间短, 其油/水界面扩张模量及扩张弹性均高于PEO含量较少者. 但是对原油乳状液的破乳脱水效果则是PEO含量居中的聚醚最好. 温度影响和交联与否的研究表明, 交联并不能提高分子量较大的聚醚对原油乳状液的破乳效果, 温度对聚醚分子交联前后的破乳效果有不同的影响规律. 本研究可为原油集输过程中化学品的选择与应用提供一定的依据.     

Spreading of oil from protein stabilised emulsions at air/water interfaces

Spreading of a drop of an emulsion made with milk proteins on air/water interfaces was studied. From an unheated emulsion, all oil molecules could spread onto the air/water interface, indicating that the protein layers around the oil globules in the emulsion droplet were not coherent enough to withstand the forces involved in spreading. Heat treatment (90 °C) of emulsions made with whey protein concentrate (WPC) or skim milk powder reduced the spreadability, probably because polymerisation of whey protein at the oil/water interface increased the coherence of the protein layer. Heat treatment of emulsions made with WPC and monoglycerides did not reduce spreadability, presumably because the presence of the monoglycerides at the oil/water interface prevented a substantial increase of coherence of the protein layer. Heat treatment of caseinate-stabilised emulsions had no effect on the spreadability. If proteins were already present at the air/water interface, oil did not spread if the surface tension (γ) was <60 mN/m. We introduced a new method to measure the rate at which oil molecules spread from the oil globules in the emulsion droplet by monitoring changes in γ at various positions in a ‘trough’. The spreading rates observed for the various systems agree very well with the values predicted by the theory. Spreading from oil globules in a drop of emulsion was faster than spreading from a single oil drop, possibly due to the greater surface tension gradient between the oil globule and the air/water interface or to the increased oil surface area. Heat treatment of an emulsion made with WPC did not affect the spreading rate. The method was not suitable for measuring the spreading rate at interfaces where surface active material is already present, because changes in γ then were caused by compression of the interfacial layer rather than by the spreading oil.     

Stabilization of emulsions by heterocoagulation of clay minerals and layered double hydroxides

 Paraffin/water emulsions were stabilized by colloidal particles without surface active agents. Mixtures of two types of particles with opposite signs of charge were used: a layered double hydroxide (the hydroxide layers carry positive charges) and the clay mineral montmorillonite (the silicate layers carry negative charges). The emulsions were very stable and did not separate a coherent oil phase. The stability of the emulsion (no oil coalescence after centrifugation) was independent of the mixing ratio of both the compounds when the total solid content was >0.5%. Solid contents up to 2.0% were optimal. In contrast to stability, flow behavior of the emulsion was dependent on the mass fraction χ of the hydroxide. The maxima of viscosity and yield value were observed at χ≈0.2–0.3; the emulsion showed pronounced antithixotropic behavior. Weak thixotropic properties, smaller viscosities and yield values were observed at χ≤0.2 and ≥0.5. In the absence of the clay mineral, the double hydroxide particles stabilize by forming envelopes around the oil droplets. Addition of bentonites creates a three-dimensional network of particles with high elasticity which impedes coalescence of the oil droplets. Received: 9 March 1998 Accepted: 6 April 1998     

Tuneable Control of Interfacial Rheology and Emulsion Coalescence

Breaking point: Switchable peptide surfactants are used to demonstrate that the extent of cross‐linking in an interfacial surfactant layer can control the rate of emulsion coalescence. Pictured is the rupture of an aqueous thin film where the peptide layer lacks sufficient strength to prevent hole formation, but nonetheless dramatically slows the rate of hole expansion.

     

Molecular simulation of protein adsorption and conformation at gas-liquid,liquid–liquid and solid–liquid interfaces

The adsorption of proteins at surfaces and interfaces is important in a wide range of industries. Understanding and controlling the conformation of adsorbed proteins at surfaces is critical to stability and function in many technological applications including foods and biomedical testing kits or sensors. Studying adsorbed protein conformation is difficult experimentally and so over the past few decades researchers have turned to computer simulation methods to give information at the atomic level on this important area. In this review we summarize some of the significant simulation work over the past four years at both fluid (liquid–liquid and gas–liquid interfaces) and solid–liquid interfaces. Of particular significance is the work on surfactant proteins such as fungal hydrophobins, ranspumin-2 from the túngara frog and the bacteria protein BslA. These have evolved unique structures impart very high surface-active properties to the molecules. A highlight is the elucidation of the clam-shell unhinging mechanism of ranspumin-2 adsorption to the gas–liquid interface that is responsible for its adsorption to and stabilization of the air bubbles in túngara frog foam nests.