Active colloidal particles at fluid-fluid interfaces

This review explores the intersection between two important fields of colloid and interface science – that of active colloidal particles and of (passive) particles at fluid-fluid interfaces. The former uses energy input at the particle level to propel particle motions and direct dynamic assemblies. The latter relies on the spontaneous adsorption of particles at fluid interfaces to modify the interfacial energy, rheology, and permeability of biphasic materials. Here, we address two key questions that connect these otherwise distinct fields of study. How do liquid interfaces influence the dynamics of active or driven colloidal particles? How can particle activity influence the dynamics of liquid interfaces? These questions motivate the pursuit of active particle surfactants that move and organize at fluid interfaces to perform useful functions such as enhancing mass transport or modulating interfacial properties. Drawing examples from the literature, we discuss how fluid interfaces can provide a unique environment for the study of active colloids, how surface tension can be harnessed to propel particle motions, and how capillary interactions can be activated to achieve dynamically tunable emulsions and foams. We highlight opportunities for the future study and application of active particles at liquid interfaces.     

两亲性Janus粒子在蜂窝状多孔聚合物膜上的自组装性能

采用具有两亲性的两面体(Janus)粒子实现稳定的粒子界面组装与水滴模板法自组装过程相结合的方法获得了粒子在蜂窝状多孔聚合物薄膜内壁的高效定向修饰.通过与均质粒子组装形貌的对比,证明了Janus粒子因其特殊的界面自组装活性,可以获得高粒子加量条件下的规则多孔结构,解决了使用均质粒子时存在的结构有序性和粒子修饰密度之间的矛盾.而在较低粒子加量的条件下,Janus粒子也展示出与均质粒子极为不同的组装形貌.这一方法的建立,为新型表面功能化材料的制备提供了一个新的思路.     

Structure and catalytic properties of sulfated zirconia foams

Porous sulfated zirconia foams were manufactured by a simple methodology based on the sol–gel process combined with a liquid foam template that used a surfactant mixture. A block copolymer (Pluronic F-127) and an anionic surfactant [sodium dodecylsulfate (SDS)] were mixed in different proportions in order to optimize the porous and surface properties of the ceramic material. By adjusting the SDS/Pluronic ratio, it was possible to obtain sulfated zirconia with a combination of macropores and mesopores that provided high porosity (≈90 %) and surface area (≈80 m² g^?1). The sulfate groups linked to the zirconia surface stabilized the tetragonal phase, to the detriment of the thermodynamically stable monoclinic phase. The sulfate groups and the tetragonal phase decreased as a function of the amount of SDS in the liquid foam template. The combined porous and structural characteristics, together with surface acidity, provided enhanced catalytic activity when the sulfated zirconia foams were employed in the isopropanol dehydration reaction. A further benefit was the selectivity towards propene and negligible formation of acetone.     

Interfacial criteria for stabilization of liquid foams by solid particles

The stability criteria of liquid foams, stabilized by solid particles have been derived, based on the interfacial separating pressure, acting between two neighboring bubbles (foam cells). Different structures of solid particles in the cell walls have been considered, all being able to stabilize liquid foams with an increasing probability, according to the following row: structure LP1 (loosely packed single layer of particles) → structure CP1 (closely packed single layer of particles) → structure LP2C (loosely packed double layer of clustered particles) → structure LP2+C (loosely packed ‘double+’ layer of clustered particles) → structure CP2 (closely packed double layer of particles) → structure CP2+ (closely packed ‘double+’ layer of particles). It has been shown that the contact angle should be higher than a certain value Θ_o, in order to ensure stability of bubble–particles agglomerates. On the other hand, different structures of particles can stabilize the foam, if the contact angle is below the certain value (90° for the CP1 and LP1 structures, 129° for the CP2, LP2C and LP2+C structures and 180° for the CP2+ structure). The optimum value of the contact angle, being able to stabilize the foam is a difficult function of different parameters, but has been found in the interval between 50 and 90°. It has been shown that the possibility to stabilize liquid foams is connected with the value of the dimensionless quantity PR_s/σ (P: the pressure, destabilizing the foam; R_s: the radius of the stabilizing particles; σ: the surface tension of the liquid). When PR_s/σ>40, foam stabilization is absolutely impossible. When PR_s/σ<40, foam stabilization becomes possible, but it has high probability only at PR_s/σ<4. From this condition the maximum size of the particles, being able to stabilize liquid foams can be found. Trial calculations showed that particles smaller than 3 and 30 μm in diameter are requested for stabilizing water based, and liquid aluminum based foams, respectively.     

表面能与晶体生长/溶解动力学研究的新动向

界面现象使物质在结晶过程中出现了临界现象.但最近的研究指出在物质溶解过程中,在表面能量的控制下也存在着临界现象以及尺寸效应.实验发现,当晶体自身小到一 定的程度时(通常在纳米尺度上并和临界蚀坑的大小相近),在溶解过程中其速度会自发降 低,反应被抑制乃至停止.尽管在热力学上表面能的因素可以赋予小颗粒晶体较大的溶解度 ,但表面能却也能通过对临界条件的控制而使这些微粒在动力学上不被溶解.这个发现不仅 解决了纳米颗粒在水溶液中稳定性的问题,而且还从动力学的角度解释了生物矿物选择纳米 尺度作为其基本构成单元的原因.由于表面能和晶体生长/溶解的动力学有着密切的关系, 我们可以通过对表面能的调节来修改它们的动力学速度和晶体的形貌.反过来,也可以用动力学的方法来测定表面能及表面吸附/脱附常数等.相对于常规的界面研究手段,通过生 长和溶解动力学途径所得的数据有着很好的可靠性及重复性.我们认为,晶体生长和溶解的 动力学和表面能的研究相结合,不仅为界面研究提供了新的思路和方法,而且也会推动晶体生长和材料科学的发展.     

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.     

Modeling Pendular Liquid Bridges with a Reducing Solid-Liquid Interface

Liquid bridges formed between particles of dissimilar surface properties are important in many processes involving the handling of powders in mixtures. For instance, growth kinetic models for wet granulation frequently incorporate the evolution and resistance to breakage of individual liquid bridges between particles in a statistical form. These models generally propose a confusing definition of liquid-to-solid contact angles. Taken as a single thermodynamic value, they typically neglect the influence of wetting hysteresis on the liquid bridge. In this paper, a simple model based on the interfacial energies is proposed for the evolution of liquid bridges when one solid-liquid interface reduces. This receding process is well described by a balance between the adhesion energy of the bridge liquid on the particle surface and the capillary energy stored by the liquid free surface. The extent of solid-liquid interfacial area reduction can hence be predicted from the initial liquid bridge configuration. The liquid bridge shape is approximated by a parabolic curve, which is validated from the good agreement between measured and calculated contact angles or liquid-vapor interfacial area. Copyright 2000 Academic Press.     

Foam superstabilization by polymer microrods

Few foam systems stabilized by solid particles have been reported, and usually the particles have been used in combination with surfactants. We report how foams can be stabilized solely with a new class of anisotropic particles, hydrophobic polymer microrods of diameter less than 1 mum and length of a few tens of micrometers. The obtained foams were extraordinary stable, retaining a constant volume over many days and even surviving drying of most of the free liquid. The bubbles in these foams were sterically stabilized by dense thick "hairy" layers. The rigid intertwined protective shells around the bubbles did not allow the formation of thin films between them. The lifetime of these foams was orders of magnitude longer than the ones stabilized with typical foaming surfactants such as sodium dodecyl sulfate. The addition of sodium dodecyl sulfate led to hydrophilization of the microrods and suppressed the superstabilization effect. Thus, common foaming agents effectively act as defoamers for the ultrastable foams stabilized by microrods.     

Foams from non-aqueous systems

The multi-phase approach to clarify the fundamentals of the stability of foams from non-aqueous liquids has been further developed. The two-phase liquid/liquid crystal line of attack for the liquid oil variety has been extended to foams stabilized by solid particles, in turn leading to progress in the fundamental science of solid metal foams and plastic foams.     

Interfacial water on hydrophobic surfaces recognized by ions and molecules

Recent spectrophotometric and molecular dynamics simulation studies have shown that the physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ from those of the bulk water. However, the interfacial water acting as a separation medium on hydrophobic surfaces has never been detected and quantified experimentally. In this study, we show that small inorganic ions and organic molecules differentiate the interfacial water formed on the surfaces of octadecyl-bonded (C(18)) silica particles from the bulk water and the chemical separation of these solutes in aqueous media with hydrophobic materials can be interpreted with a consistent mechanism, partition between the bulk water phase and the interfacial water formed on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in the nanopores of the C(18) silica materials and the solubility parameter of the water calculated from the distribution coefficients of organic compounds have indicated that the interfacial water may have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit of distance from the hydrophobic surface at which molecules and ions can sense the surface was estimated to be 1.25 ± 0.13 nm from the volume of the interfacial water obtained by a liquid chromatographic method and the surface area, suggesting that the hydrophobic effect may extend beyond the first solvation shell of water molecules directly surrounding the surfaces.     

Glass foam from window panes and bottle glass wastes

Glass foams are building materials that now compete with classic insulating polymeric and fiber materials for thermal enveloping. The low flammability, high chemical durability and thermal stability are distinct advantages over polymeric materials. The present paper proposes the possibility of producing glass foam using two types of recycled glass wastes (window panes and bottle glass) together with plaster wastes from used ceramic casting molds as foaming agent. Optical microscopy, measurements of apparent porosity and density, hydrolytic and chemical stability, as well as thermal conductivity were used in order to characterize the obtained glass foams as insulator materials for the building industry. The apparent porosity of glass foams ranges between 20.19–54.54% when using window glass wastes, and 18.77–51.75% with bottle glass wastes. Thermal conductivity was less than 0.25 W mK-1 for all the studied glasses. The obtained results confirm that there exists an alternative method for producing glass foams, for example, from glass wastes and used ceramic plaster molds, which are utilized as foaming agents with good chemical stability and insulating properties.     

pH-responsive aqueous foams stabilized by hairy latex particles

Polystyrene (PS) latex particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair (PDEA-PS particles) were synthesized by dispersion polymerization and characterized in terms of diameter, diameter distribution, morphology, chemical composition, surface chemistry, and pH-response using scanning electron microscopy (SEM), elemental microanalysis, (1)H nuclear magnetic resonance spectroscopy, the laser diffraction method, and zeta potential measurements. The hairy particles can act as pH-responsive stabilizers of aqueous foams by adsorption at the air-water surface. Above pH 8.0, where particles have nonprotonated PDEA hair, which is relatively hydrophobic, particle-stabilized foams are stable for at least 1 month. Optical microscopy and SEM confirmed that flocculated PDEA-PS latex particles were adsorbed at the air-water interface and stabilized the aqueous foams. At pH 6.1 and 7.1, relatively stable foams can be prepared that remain stable for at least 24 h. SEM studies indicated that the PDEA-PS particles were adsorbed at the air-water interface as a monolayer at pH 6.1. At pH 5.1 and 3.1, where the particles have cationic water-soluble PDEA hairs with hydrophilic character, no foam was formed. Rapid defoamation can be induced by lowering the solution pH; the addition of acid caused the in situ protonation of 2-(diethylamino)ethyl methacrylate residues, which impart water-soluble hydrophilic character to the PDEA hair, and the PDEA-PS particles desorbed from the air-water interface. The foaming and defoaming cycles could be repeated at least five times.     

Development of carbon coatings for cordierite foams: an alternative to cordierite honeycombs

Cordierite foams were prepared by replication of polyurethane foams and were coated with three types of carbon xerogels. The dip coating and synthesis conditions were optimized, and the coated foams were characterized exhaustively. The composition of the starting solution, coat loading, and carbonization temperature are the most important parameters determining both textural and mechanical properties. Carbon xerogel coatings obtained from aqueous solutions of resorcinol (R) and formaldehyde (F) are macro-, meso-, and microporous but present the greatest shrinkage, which causes a loss of adhesion between ceramic foams and carbon coatings. The coatings from polyfurfuryl alcohol (PFA) and RF-poly(vinyl butyral) (Butvar) resin are highly microporous and present very good adhesion even after carbonization. In all cases, coatings induce the improvement of the mechanical properties, which is related to the fact that the coating fills the defects present in the cordierite foams, thereby affecting both the rigidity and the way cracks propagate through the coated samples. These materials, due to the synergetic role of the highly porous coatings and the tortuous channels of the ceramic foams, are suitable materials for adsorption or catalytic treatments of fluids.     

Foaming in aqueous solutions of hexadecyltrimethylammonium bromide and silica nanoparticles: Measurement and analysis of rheological and interfacial properties

The properties of aqueous foams stabilized by a mixture of negatively charged silica nanoparticles and hexadecyltrimethylammonium bromide were studied in this work. Rheological properties of the foams were studied. The interaction between nanoparticles and surfactant molecules in the bulk phase was studied by zeta potential and size measurements of the particles. The interaction at the interface was studied by means of interfacial shear rheology, surface pressure measurement, and atomic force microscopy. It was found that foams were more stable at low surfactant concentrations, though the foamability was low. This was due to the formation of a strong viscoelastic film of surfactant-laden particles at the air–water interface. A suitable mechanism has been proposed to explain the stability of foams in the presence of nanoparticles at different surfactant concentrations.     

锂离子电池凝胶聚合物隔膜的研究进展

作为锂离子电池重要组分,隔膜由多孔聚烯烃高分子材料组成;电解质体系由有机碳酸酯和六氟磷酸锂混合组成,虽具有高离子电导率,但因液态碳酸酯的易燃特性给锂离子电池带来了安全隐患。利用能够将液态电解质体系凝胶化的聚合物制备得到的凝胶聚合物隔膜,结合了液态电解质体系高电导率和固态电解质高安全性的优点。凝胶聚合物隔膜的研究从简单微孔凝胶聚合物隔膜开始,经历了引入少量纳米无机颗粒的掺杂凝胶聚合物隔膜,到引入大量纳米颗粒的凝胶陶瓷隔膜的发展历程。本文详细介绍这三种类型凝胶聚合物隔膜的物理化学特性,最后展望凝胶聚合物隔膜的发展趋势。     

pH-responsive aqueous foams stabilized by ionizable latex particles

We have designed a type of colloidal particle whose surface characteristics are sensitive to the pH of the aqueous phase in which they are dispersed. Particles of polystyrene latex stabilized by poly(acrylic acid) can act as stabilizers of aqueous foams by adsorbing at the air-water surface. Foams can be prepared and stabilized only at pH values below the isoelectric point where particles are either uncharged and flocculated or acquire a positive charge. At high pH where particles are anionic, no foam forms. This influence of pH on foamability and stability applies to both pH-dependent and pH-responsive systems.     

Composite ceramic monoliths with poly(ester acrylate) and poly(ester acrylate)-poly(<Emphasis Type="Italic">N</Emphasis>-vinylcaprolactam) coatings

Polymerization of a series of oligo(ester acrylates) on the surface of ceramic monoliths was studied with the aim to prepare composite monolithic sorbents, highly porous permeable ceramic materials with polymeric coatings. A chelating organoceramic sorbent was prepared by copolymerization of an oligoester with N-vinylcaprolactam on a ceramic support.     

Stabilization of foams with inorganic colloidal particles

Wet foams are used in many important technologies either as end or intermediate products. However, the thermodynamic instability of wet foams leads to undesired bubble coarsening over time. Foam stability can be drastically improved by using particles instead of surfactants as foam stabilizers, since particles tend to adsorb irreversibly at the air-water interface. Recently, we presented a novel method for the preparation of high-volume particle-stabilized foams which show neither bubble growth nor drainage over more than 4 days. The method is based on the in-situ hydrophobization of initially hydrophilic particles to enable their adsorption on the surface of air bubbles. In-situ hydrophobization is accomplished through the adsorption of short-chain amphiphiles on the particle surface. In this work, we illustrate how this novel method can be applied to particles with various surface chemistries. For that purpose, the functional group of the amphiphilic molecule was tailored according to the surface chemistry of the particles to be used as foam stabilizers. Short-chain carboxylic acids, alkyl gallates, and alkylamines were shown to be appropriate amphiphiles to in-situ hydrophobize the surface of different inorganic particles. Ultrastable wet foams of various chemical compositions were prepared using these amphiphiles. The simplicity and versatility of this approach is expected to aid the formulation of stable wet foams for a variety of applications in materials manufacturing, food, cosmetics, and oil recovery, among others.     

Surface adsorption effects in the inkjet printing of an aqueous polymer solution on a porous oxide ceramic substrate

The inkjet printing of a polymeric solution into a porous substrate was studied, with the focus on phenomena occurring within the pore space during infiltration. Lines of aqueous polyacrylic acid (PAA) solution were printed onto the surface of porous, high-green-density ceramic powder beds. The PAA is a binder for the ceramic particles, allowing removal of the printed line structure ("primitive") and characterization of the extent of polymer penetration. Large differences in cross section of the retrieved printed structure were observed between ceramic systems and for different specific surface area powders. A mechanism for "filtration" of the polymer by adsorption onto the ceramic particle surfaces during infiltration was proposed. The adsorption of PAA onto Al2O3, SiO2, and TiO2 was characterized via adsorption isotherms, and the trend of primitive cross section with PAA adsorption was consistent with the filtration hypothesis, as was the variation with powder-specific surface area. These results can be generalized to other systems where a solution is inkjet printed onto a porous substrate (e.g., inks on plain paper, porous coated papers, etc.) Utilization of the adsorption effects may allow confinement of the solute molecules (e.g., colorant) to a small region near the substrate surface.     

Stabilization of Liquid Foams through the Synergistic Action of Particles and an Immiscible Liquid

Liquid foams are familiar from beer, frothed milk, or bubble baths; foams in general also play important roles in oil recovery, lightweight packaging, and insulation. Here a new class of foams is reported, obtained by frothing a suspension of colloidal particles in the presence of a small amount of an immiscible secondary liquid. A unique aspect of these foams, termed capillary foams, is the particle‐mediated spreading of the minority liquid around the gas bubbles. The resulting mixed particle/liquid coating can stabilize bubbles against coalescence even when the particles alone cannot. The coated bubbles are further immobilized by entrapment in a network of excess particles connected by bridges of the minority liquid. Capillary foams were prepared with a diverse set of particle/liquid combinations to demonstrate the generality of the phenomenon. The observed foam stability correlates with the particle affinity for the liquid interface formed by spreading the minority liquid at the bubble surface.