A pH-switched Pickering emulsion catalytic system: high reaction efficiency and facile catalyst recycling

Recent Advances of Stimuli-Responsive Liquid-Liquid Interfaces Stabilized by Nanoparticles

Liquid-liquid interfaces offer highly controlled, flexible, and adaptable platforms for precise molecular assemblies, enabling the construction of sophisticated functional materials. Interfacial assemblies of specific nanoparticles (NPs) and ligands can alter their physicochemical states under external stimuli, leading to macroscopic dynamic transformations at the interface. This Review summarizes and analyzes the recent advances of the assembly and disassembly behaviors of various stimuli-responsive nanoparticle surfactants (NPSs) at liquid-liquid interfaces, focusing on their responsive behaviors when exposed to external stimuli and the interaction forces between interfacial molecules. Additionally, we outline recent advancements in applications such as reconfigurable all-liquid devices, all-liquid 3D printing, and chemical reaction platforms. Finally, we discuss current challenges and future prospects for the development of applications in this rapidly evolving field.

Complex Droplet Microreactor for Highly Efficient and Controllable Esterification and Cascade Reactions

A highly efficient complex emulsion microreactor has been successfully developed for multiphasic water-labile reactions, providing a powerful platform for atom economy and spatiotemporal control of reaction kinetics. Complex emulsions, composing a hydrocarbon phase (H) and a fluorocarbon phase (F) dispersed in an aqueous phase (W), are fabricated in batch scale with precisely controlled droplet morphologies. A biphasic esterification reaction between 2-bromo-1,2-diphenylethane-1-ol (BPO) and perfluoro-heptanoic acid (PFHA) is chosen as a reversible and water-labile reaction model. The conversion reaches up to 100 % under mild temperature without agitation, even with nearly equivalent amounts of reactants. This efficiency surpasses all reported single emulsion microreactors, i. e., 84~95 %, stabilized by various emulsifiers with different catalysts, which typically necessitate continuous stirring, a high excess of one reactant, and/or extended reaction time. Furthermore, over 3 times regulation threshold in conversion rate is attained by manipulating the droplet morphologies, including size and topology, e. g., transition from completely engulfed F/H/W double to partially engulfed (F+H)/W Janus. Addition-esterification, serving as a model for triple phasic cascade reaction, is also successfully implemented under agitating-free and mild temperature with controlled reaction kinetics, demonstrating the versatility and effectiveness of the complex emulsion microreactor.

Switchable CO2-Responsive Janus Nanoparticle for Lipase Catalysis in Pickering Emulsion

The use of convertible immobilized enzyme carriers is crucial for biphasic catalytic reactions conducted in Pickering emulsions. However, the intense mechanical forces during the conversion process lead to enzyme leakage, affecting the stability of the immobilized enzymes. In this study, a CO2-responsive switchable Janus (CrSJ) nanoparticle (NP) was developed using silica NP, with one side featuring aldehyde groups and the other side adsorbing N,N-dimethyldodecylamine. A switchable Pickering emulsion catalytic system for biphasic interface reactions was prepared by covalently immobilizing lipase onto the CrSJ NPs. The CO2-responsive nature of the CrSJ NPs allowed for rapid conversion of the Pickering emulsion, and covalent immobilization substantially reduced lipase leakage while enhancing the stability of the immobilization during the conversion process. Impressively, after repeated transformations, the Pickering emulsion still maintains its original structure. Following 10 consecutive cycles of esterification and hydrolysis reactions, the immobilized enzyme's activity remains at 77.7 and 79.5% of its initial activity, respectively. The Km of the CrSJ catalytic system showed no significant change compared to the free enzyme, while its Vmax values were 1.2 and 1.6 times that of the free enzyme in esterification and hydrolysis reactions, respectively.

Redox-switchable Pickering emulsion stabilized by hexaniobate-based ionic liquid for oxidation catalysis

Pickering emulsions provide an efficient platform for interfacial catalysis, but product separation and catalyst recycling rely on time- and energy-consuming centrifugation or filtration. Herein, three hexaniobate-based ionic liquids, [C_nMIM]Nb₆ (n = 12, 14 and 16), have been successfully synthesized by self-assembly of hexaniobate (Nb₆) with long alkyl chain-modified imidazole cations (C_nMIM). Interestingly, the surface wettability of [C₁₆MIM]Nb₆ can be regulated by redox reactions, and the rapid switch between emulsification and demulsification can be achieved by alternately adding oxidant (H₂O₂) and reductant (Na₂SO₃) agents. Furthermore, studies suggest that the redox-responsive behavior is related to the reversible transformation between [C₁₆MIM]Nb₆ and peroxohexaniobate [C₁₆MIM]Nb₆-O₂, which leads to the rearrangement of hydrophobic long chains on imidazole cations around hydrophilic Nb₆. Moreover, [C₁₆MIM]Nb₆ can effectively catalyze oxidative desulfurization (conversion > 99%), and the separation of clean model oil and the recycling of the interfacial catalyst were realized in a facile route.

Destabilization of Pickering emulsions by interfacial transport of mutually soluble solute

HYPOTHESIS

Pickering emulsions (PEs) once formed are highly stable because of very high desorption energies (∼10⁷ k_BT) associated with particles adsorbed to the interfaces. The destabilization of PEs is required in many instances for recovery of valuable chemicals, products and active compounds. We propose to exploit interfacial instabilities develop by the addition of different types of solutes to PEs as a route to engineer their destabilization.

EXPERIMENTS

PEs stabilized by (i) spherical particles, (ii) non-spherical particles, (iii) oppositely charged particle-particle mixtures, and (iv) oppositely charged particle-polyelectrolyte mixtures are formulated. Different types of solutes are added to these highly stable PEs and the macroscopic as well as microscopic changes induced in the PEs is recorded by visual observation and bright field optical microscopy.

FINDINGS

Our results point to a simple yet robust method to induce destabilization of PEs by transiently perturbing the oil-water interface by transport of a mutually soluble solute across the interface. The generality of the method is demonstrated for different kind of solutes and stabilizers including particles of different sizes (nm to µm), shapes (sphere, spheroids, spherocylinders) and types (polystyrene, metal oxides). The method works for both oil-in-water (o/w) and water-in-oil (w/o) PEs with different kinds of non-polar solvents as oil-phase. However, the method fails when the solute is insoluble in one of the phases of PEs. The study opens up a new approach to destabilization of particle stabilized emulsions.

Ultrasound-assisted Pickering Interfacial Catalysis for Transesterification: Optimization of Biodiesel Yield by Response Surface Methodology

Recently, Pickering interfacial catalyst (PIC) was widely applied for liquid-liquid reactions, in view of not only intensifying the mass transfer through significant reducing both the drop sizes and the diffusion distance, but also supplying a flexible platform for the immobilization of valuable active sites. However, the restriction of the mobility of catalyst somehow decreases the activity of a catalyst. To obtain a promise reaction efficiency, we firstly report a synergistic method to enhance the biphasic reaction by Pickering emulsion and ultrasound concepts, targeted at efficient production of biodiesel. Response surface methodology based on Box-Behnken design was applied to optimize the reaction conditions, such as composition of catalyst, reaction temperature, ultrasound power, methanol to oil molar ratio and catalyst amount. An over 98% yield of biodiesel could be achieved within 2.5 hours by ultrasound assisted Pickering interfacial catalysis, which is over two times higher than that of ultrasound assisted homogeneous transesterification system. Besides, the ultrasound assisted Pickering emulsion shortened the reaction time by 3.6 fold when compared to mechanical stirring assisted Pickering emulsion system.

Recyclable Nonionic-Anionic Bola Surfactant as a Stabilizer of Size-Controllable and pH-Responsive Pickering Emulsions

A novel nonionic-anionic Bola surfactant (abbreviated as CH₃O(EO)₇-R₁₁-COOH) was designed and synthesized by condensation of methyl polyoxyethylene (7) ether with 12-bromododecanoic acid. In neutral aqueous solution, the surfactant behaves as a nonionic one and can stabilize oil-in-water (O/W) conventional emulsions alone and costabilize O/W Pickering emulsions with positively charged alumina nanoparticles with n-decane as the oil. In alkaline solution, the carboxylic acid group is deprotonated, becoming anionic and the surfactant is converted to Bola form, which is an inferior emulsifier and does not adsorb on particle surfaces, resulting in demulsification of both kinds of emulsions. With strong hydrophilicity, both the Bola surfactant and the bare particles return to the aqueous phase after demulsification, which is therefore recyclable and reusable in accordance with sustainable chemistry and engineering. In acidic media between pH 3 and 6, the ethyleneoxy groups tend to desorb from particle surfaces, slightly reducing the hydrophobicity of the particles. However, Pickering emulsions are still stable but their droplet size increases on lowering the pH. The Pickering emulsions are therefore pH-responsive and size-controllable. This newly designed Bola surfactant is effective in preparing smart emulsions, which are extensively applied in heterogeneous catalysis, oil product transportation, emulsion polymerization, and new material preparation.

Synthesis and stability of switchable CO2-responsive foaming coupled with nanoparticles

CO₂-responsive foaming has been drawing huge attention due to its unique switching characteristics in academic research and industrial practices, whereas its stability remains questionable for further applications. In this paper, a new CO₂-switchable foam was synthesized by adding the preferably selected hydrophilic nanoparticle N20 into the foaming agent C₁₂A, through a series of analytical experiments. Overall, the synergy between cationic surfactants and nanoparticles with a contact angle of 37.83° is the best. More specifically, after adding 1.5 wt% N20, the half-life of foam is 14 times longer than that of pure C₁₂A foam. What’s more, the C₁₂A-N20 solution is validated to own distinctive CO₂-N₂ switching features because very slight foaming degradations are observed in terms of the foaming volume and half-life time even after three cycles of CO₂-N₂ injections. This study is of paramount importance pertaining to future CO₂ foam research and applications in energy and environmental practices.

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Cationic surfactants have the best synergy with NPs with a contact angle of 37.83°

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The foam stability increased with the increase of NPs concentration

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CO₂/N₂ can control the foaming properties of C₁₂A-N20 solution and are reversible

Multiphase Microreactors Based on Liquid-Liquid and Gas-Liquid Dispersions Stabilized by Colloidal Catalytic Particles

Pickering emulsions, foams, bubbles, and marbles are dispersions of two immiscible liquids or of a liquid and a gas stabilized by surface-active colloidal particles. These systems can be used for engineering liquid-liquid-solid and gas-liquid-solid microreactors for multiphase reactions. They constitute original platforms for reengineering multiphase reactors towards a higher degree of sustainability. This Review provides a systematic overview on the recent progress of liquid-liquid and gas-liquid dispersions stabilized by solid particles as microreactors for engineering eco-efficient reactions, with emphasis on biobased reagents. Physicochemical driving parameters, challenges, and strategies to (de)stabilize dispersions for product recovery/catalyst recycling are discussed. Advanced concepts such as cascade and continuous flow reactions, compartmentalization of incompatible reagents, and multiscale computational methods for accelerating particle discovery are also addressed.

Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions

Pickering emulsion systems have emerged as platforms for the synthesis of organic molecules in biphasic biocatalysis. Herein, the catalytic performance was evaluated for biotransformation using whole cells exemplified for the dehydration of n-octanaloxime to n-octanenitrile catalysed by an aldoxime dehydratase (OxdB) overexpressed in E. coli. This study was carried out in Pickering emulsions stabilised solely with silica particles of different hydrophobicity. We correlate, for the first time, the properties of the emulsions with the conversion of the reaction, thus gaining an insight into the impact of the particle wettability and particle concentration. When comparing two emulsions of different type with similar stability and droplet diameter, the oil-in-water (o/w) system displayed a higher conversion than the water-in-oil (w/o) system, despite the conversion in both cases being higher than that in a "classic" two-phase system. Furthermore, an increase in particle concentration prior to emulsification resulted in an increase of the interfacial area and hence a higher conversion.

Catalysis in Pickering emulsions

Particle-stabilised or Pickering emulsions are versatile systems. In the past 10 years a new application has emerged in the field of catalysis to use them as vehicles to carry out catalytic reactions, allowing a more environmentally friendly process with high conversions and selectivities and important advantages for catalyst recovery. As the area has advanced rapidly, the intention of this review is to summarize the latest innovations being reported. An overview is given regarding the kinds of liquid phases comprising the emulsion system, the different types of solid particle stabilizers (whether they contain catalyst or not) and the catalytic reactions studied. A section describing methods for recovering the catalyst is also included, in which various stimuli are discussed. Finally, the importance of using Pickering emulsions to carry out reactions in flow and in multi-step cascade systems is highlighted with various examples to support the benefits of transferring this technology to industrial processes.

pH-Responsive Pickering emulsion stabilized by polymer-coated silica nanoaggregates and applied to recyclable interfacial catalysis

We first synthesized a diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-(trimethoxysilyl)propyl methacrylate] (PtBMA-b-PTMSPMA) through reversible addition–fragmentation chain transfer (RAFT) living radical polymerization and grafted it onto fumed silica by converting the PTMSPMA segment to silanol and the PtBMA segment to polymethylacrylic acid (PMAA) in the presence of trifluoroacetic acid in order to obtain PMAA brush-coated silica nanoaggregates P-Si. TEM, DLS, FTIR, and TGA results confirmed the successful modification of the starting materials. The nanoaggregates flocculated and stabilized a toluene-in-water Pickering emulsion at low pH, while the nanoaggregates were well dispersed in water and broke the emulsion under both neutral and basic conditions. Alternatively, the addition of acid/base induced emulsification/demulsification cycles that were sustained for several cycles. Moreover, when the P-Si was mixed with Rh-loaded silica, Rh-Si, the mixture had the same pH-responsive Pickering emulsion behavior as the single P-Si. This Pickering emulsion system can be used in the biphasic interfacial catalytic hydrogenation of olefins and had excellent yields under a hydrogen atmosphere. The yield of Pickering emulsion catalysis rapidly reached more than 99% in 3 h, while that of the demulsified mixture failed to reach 20% in 4 h, which verified the promotion of catalysis by the Pickering emulsion. Base-induced demulsification can be used to separate the products and recycle the catalyst. This pH-responsive Pickering emulsion catalytic system was capable of several cycles of reuse, and there was no significant decrease in catalytic efficiency even after eight cycles.

We synthesized a diblock copolymer and grafted it onto fumed silica in the presence of trifluoroacetic acid to obtain a pH-responsive Pickering emulsion system stabilized by polymer-coated nanoaggregates, P-Si.

Spontaneous particle desorption and "Gorgon" drop formation from particle-armored oil drops upon cooling

We study how the phenomenon of drop "self-shaping" (Denkov et al., Nature, 528, 2015, 392), in which oily emulsion drops undergo a spontaneous series of shape transformations upon emulsion cooling, is affected by the presence of adsorbed solid particles, like those used in Pickering emulsion stabilization. Experiments with several types of latex particles, and with added surfactant of low concentration to enable drop self-shaping, revealed several new unexpected phenomena: (1) adsorbed latex particles rearranged into regular hexagonal lattices upon freezing of the surfactant adsorption layer. (2) Spontaneous particle desorption from the drop surface was observed at a certain temperature - a remarkable phenomenon, as the solid particles are known to irreversibly adsorb on fluid interfaces. (3) Very strongly adhered particles to drop surfaces acted as a template to enable the formation of tens to hundreds of semi-liquid fibres, growing outwards from the drop surface, thus creating a shape resembling the Gorgon head from Greek mythology. Mechanistic explanations of all observed phenomena are provided using our understanding of the rotator phase formation on the surface of the cooled drops. The surface rotator phase creates positive line tension at the contact line formed between the particle surface and the fluid interface, which causes the particle ejection from the drop surface.

Inverse Pickering Emulsion Stabilized by Binary Particles with Contrasting Characteristics and Functionality for Interfacial Biocatalysis

Water-in-oil (w/o) Pickering emulsions have received considerable attention in biphasic enzymatic catalysis for their advantages of good stability, large interfacial area, and ease of product separation. However, enzymes are commonly encapsulated in the interior of aqueous droplets, which inevitably increases the diffusional resistance to catalysis. Alternatively, enzymes are immobilized or trapped into Pickering stabilizers. Often, however, these approaches suffer from leaching and a decrease of enzyme activity during the chemical treatments. We report here a new Pickering interfacial biocatalysis platform with efficient enzyme encapsulation, binary particle composition, and high catalytic performance. Our approach is based on w/o Pickering emulsions stabilized by binary particles consisting of hard silica and soft, pH-responsive microgel particles. We demonstrate that pH-responsive microgels can simultaneously stabilize a w/o Pickering emulsion, encapsulate enzymes, and catalyze reactions at the water/oil interface. In addition, we show that the coordination with rigid silica nanoparticles as additional stabilizers markedly improves the emulsion structure and will provide a new avenue for the preparation of w/o Pickering emulsion and concept of biphasic catalysis.

A Pickering emulsion of a bifunctional interface prepared from Pd nanoparticles supported on silicane-modified graphene oxide: an efficient catalyst for water-mediated catalytic hydrogenation

A Pickering emulsion of bifunctional interface that prepared by Pd nanoparticles supported on silicane-modified graphene oxide exhibited high catalytic performance for hydrogenation of CAL.

Novel amphiphilic cellulose nanocrystals for pH-responsive Pickering emulsions

Development of a green, recyclable emulsifier for pH-responsive Pickering emulsion would be of great importance to many industries. To this end, a novel emulsifier, benzyl-polyethyleneimine modified cellulose nanocrystals (Ben-PEI-CNCs), was developed via the periodate oxidation of cellulose nanocrystals and reductive amination. Ben-PEI-CNCs possess pH-responsive amphiphilicity due to the existence of hydrophilic amino and hydrophobic benzyl groups. The Pickering emulsions stabilized by Ben-PEI-CNC2 and Ben-PEI-CNC18 are very responsive to pH changes, and adjusting the pH from 3 to 7 effectively triggers oil-water separation and emulsification. Additionally, cyclic testing establishes the robustness of this process. Overall, this study demonstrates that Ben-PEI-CNCs can promote the transition from a stable emulsion to an unstable emulsion by adjusting the pH, allowing the recovery of oil and the recycling of the emulsifier.

CO2-switchable Pickering emulsions: efficient and tunable interfacial catalysis for alcohol oxidation in biphasic systems

CO₂-responsive Pickering emulsions were fabricated on the basis of polymeric nanoaggregates with adjustable surface wettability. The static Pickering emulsion system provides an efficient and sustainable platform for in situ separation and reuse of catalysts in biphasic reactions.

Building Reconfigurable Devices Using Complex Liquid-Fluid Interfaces

Liquid-fluid interfaces provide a platform both for structuring liquids into complex shapes and assembling dimensionally confined, functional nanomaterials. Historically, attention in this area has focused on simple emulsions and foams, in which surface-active materials such as surfactants or colloids stabilize structures against coalescence and alter the mechanical properties of the interface. In recent decades, however, a growing body of work has begun to demonstrate the full potential of the assembly of nanomaterials at liquid-fluid interfaces to generate functionally advanced, biomimetic systems. Here, a broad overview is given, from fundamentals to applications, of the use of liquid-fluid interfaces to generate complex, all-liquid devices with a myriad of potential applications.

Enzyme-Loaded Mesoporous Silica Particles with Tuning Wettability as a Pickering Catalyst for Enhancing Biocatalysis

Pickering emulsion systems have created new opportunities for two-phase biocatalysis, however their catalytic performance is often hindered by biphasic mass transfer process relying on the interfacial area. In this study, lipase-immobilized mesoporous silica particles (LMSPs) are employed as both Pickering stabilizers and biocatalysts. A series of alkyl silanes with the different carbon length are used to modify LMSPs to obtain suitable wettability and enlarge the interfacial area of Pickering emulsion. The results show the water/paraffin oil Pickering emulsions stabilized by 8 carbon atoms silane grafted LMSPs (LMSPs_C8) with a three-phase contact angles of 95° get the relatively large interfacial area. Moreover, the conversion of enzymatic reaction catalyzed by LMSPs_C8 Pickering emulsion system is 3.4 times higher than that unmodified LMSPs with the reaction time of 10 min. Additionally, the effective recycling of LMSPs is achieved by simple low-speed centrifugation. As evidenced by a 6-cycles reaction of remaining 75% of relative enzymatic activity, the protection of 350–450 nm mesoporous silica particles can alleviate the inactivation of enzyme from the shear stress and make a benefit to form stabile Pickering emulsion. Therefore, the biphasic reactions in the Pickering emulsion system can be effectively enhanced through changing interfacial area only by the means of adjusting the wettability of biocatalysts.

Control of Particle Adsorption for Stability of Pickering Emulsions in Microfluidics

Studying the stability of Pickering emulsion is of great interest for applications including catalysis, oil recovery, and cosmetics. Conventional methods emphasize the overall behavior of bulk emulsions and neglect the influence of particle adsorbing dynamics, leading to discrepancies in predicting the shelf-life of Pickering emulsion-based products. By employing a microfluidic method, the particle adsorption is controlled and the stability of the Pickering emulsions is consequently examined. This approach enables us to elucidate the relationship between the particle adsorption dynamics and the stability of Pickering emulsions on droplet-level quantitatively. Using oil/water emulsions stabilized by polystyrene nanoparticles as an example, the diffusion-limited particle adsorption is demonstrated and investigated the stability criteria with respect to particle size, particle concentration, surface chemistry, and ionic strength. This approach offers important insights for application involving Pickering emulsions and provides guidelines to formulate and quantify the Pickering emulsion-based products.

Magnetic-responsive switchable emulsions based on Fe3O4@SiO2–NH2 nanoparticles

Reversible magnetic control of emulsification and demulsification behavior based on engineered Fe₃O₄@SiO₂–NH₂ nanoparticles.

Catalyst surfaces with tunable hydrophilicity and hydrophobicity: metal–organic frameworks toward controllable catalytic selectivity

MOFs with controllable wettability are obtained by tuning the reduction degree of graphene oxide which display additional catalytic selectivity.

Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant

We put forward a simple protocol to prepare thermoresponsive Pickering emulsions. Using hydrophilic silica nanoparticles in combination with a low concentration of alkyl polyoxyethylene monododecyl ether (C₁₂E_n) nonionic surfactant as emulsifier, oil-in-water (o/w) emulsions can be obtained, which are stable at room temperature but demulsified at elevated temperature. The stabilization can be restored once the separated mixture is cooled and rehomogenized, and this stabilization-destabilization behavior can be cycled many times. It is found that the adsorption of nonionic surfactant at the silica nanoparticle-water interface via hydrogen bonding between the oxygen atoms in the polyoxyethylene headgroup and the SiOH groups on particle surfaces at low temperature is responsible for the in situ hydrophobization of the particles rendering them surface-active. Dehydrophobization can be achieved at elevated temperature due to weakening or loss of this hydrogen bonding. The time required for demulsification decreases with increasing temperature, and the temperature interval between stabilization and destabilization of the emulsions is affected by the surfactant headgroup length. Experimental evidence including microscopy, adsorption isotherms, and three-phase contact angles is provided to support the mechanism.

An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications

Pickering emulsion, a kind of emulsion stabilized only by solid particles locating at oil-water interface, has been discovered a century ago, while being extensively studied in recent decades. Substituting solid particles for traditional surfactants, Pickering emulsions are more stable against coalescence and can obtain many useful properties. Besides, they are more biocompatible when solid particles employed are relatively safe in vivo. Pickering emulsions can be applied in a wide range of fields, such as biomedicine, food, fine chemical synthesis, cosmetics, and so on, by properly tuning types and properties of solid emulsifiers. In this article, we give an overview of Pickering emulsions, focusing on some kinds of solid particles commonly serving as emulsifiers, three main types of products from Pickering emulsions, morphology of solid particles and as-prepared materials, as well as applications in different fields.

Flow Pickering Emulsion Interfaces Enhance Catalysis Efficiency and Selectivity for Cyclization of Citronellal

Cyclization of citronellal is a necessary intermediate step to produce the important flavor chemical (-)-menthol. Here, a continuous-flow Pickering emulsion (FPE) strategy for selective cyclization of citronellal to (-)-isopulegol by using water droplets hosting a heteropolyacid (HPA) catalyst to fill a column reactor is demonstrated. Owing to the large liquid-liquid interface and the excellent confinement ability of droplets toward HPA, the FPE system exhibited a much higher catalysis efficiency than its batch counterpart (2-5-fold) and an excellent durability (two months). Moreover, a remarkably enhanced selectivity was observed from 34.8 % for batch reactions to 64 % for the FPE reactions. It was found that the water droplet size and the flow rate significantly impact the catalysis selectivity and efficiency. This study not only represents an unprecedented and sustainable process for the selective cyclization of citronellal but also demonstrates a new flow-interface catalysis effect that can be useful for designing innovative catalysis systems in the future.

Submicron Colloidosomes of Tunable Size and Wall Thickness

We present a simple method for the fabrication of colloidal capsules of different sizes ranging from below 100 nm up to one micron. The capsules are produced by self-assembling 5 nm gold nanoparticles at the interface of oil-in-water emulsion droplets. The size of the capsules is regulated by tuning the wetting properties of the nanoparticles by changing either the composition of their ligand shell or the composition of the oil phase. The modified wettability affects not only the size but also the thickness of capsule walls. The wall can be thin if it is made of a single layer of the nanoparticles or thick when composed of multilayers. The durability of such capsules is quite high, although it can be improved by chemical cross-linking with UV light. Such capsules have low permeability, so they can store a molecular cargo and then release it on demand.

Tuning Amphiphilicity of Particles for Controllable Pickering Emulsion

Pickering emulsions with the use of particles as emulsifiers have been extensively used in scientific research and industrial production due to their edge in biocompatibility and stability compared with traditional emulsions. The control over Pickering emulsion stability and type plays a significant role in these applications. Among the present methods to build controllable Pickering emulsions, tuning the amphiphilicity of particles is comparatively effective and has attracted enormous attention. In this review, we highlight some recent advances in tuning the amphiphilicity of particles for controlling the stability and type of Pickering emulsions. The amphiphilicity of three types of particles including rigid particles, soft particles, and Janus particles are tailored by means of different mechanisms and discussed here in detail. The stabilization-destabilization interconversion and phase inversion of Pickering emulsions have been successfully achieved by changing the surface properties of these particles. This article provides a comprehensive review of controllable Pickering emulsions, which is expected to stimulate inspiration for designing and preparing novel Pickering emulsions, and ultimately directing the preparation of functional materials.

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

The aim of this paper is to review the key findings about how particle-stabilised (or Pickering) emulsions respond to stress and break down. Over the last ten years, new insights have been gained into how particles attached to droplet (and bubble) surfaces alter the destabilisation mechanisms in emulsions. The conditions under which chemical demulsifiers displace, or detach, particles from the interface were established. Mass transfer between drops and the continuous phase was shown to disrupt the layers of particles attached to drop surfaces. The criteria for causing coalescence by applying physical stress (shear or compression) to Pickering emulsions were characterised. These findings are being used to design the structures of materials formed by breaking Pickering emulsions.

Fabrication of mesoporous solid acid catalysts with tunable surface wettability for efficient catalysis

Mesoporous solid acid catalysts with tunable surface wettability were fabricated successfully and hydrophobic surfaces are beneficial for catalytic activity.

Enzyme-conjugated ZIF-8 particles as efficient and stable Pickering interfacial biocatalysts for biphasic biocatalysis

Enzyme-based biphasic catalytic reactions were successfully accomplished by utilizing CRL-conjugated ZIF-8 particles as robust Pickering interfacial biocatalysts.

pH-Responsive Gas-Water-Solid Interface for Multiphase Catalysis

Despite their wide utility in laboratory synthesis and industrial fabrication, gas-water-solid multiphase catalysis reactions often suffer from low reaction efficiency because of the low solubility of gases in water. Using a surface-modification protocol, interface-active silica nanoparticles were synthesized. Such nanoparticles can assemble at the gas-water interface, stabilizing micrometer-sized gas bubbles in water, and disassemble by tuning of the aqueous phase pH. The ability to stabilize gas microbubbles can be finely tuned through variation of the surface-modification protocol. As proof of this concept, Pd and Au were deposited on these silica nanoparticles, leading to interface-active catalysts for aqueous hydrogenation and oxidation, respectively. With such catalysts, conventional gas-water-solid multiphase reactions can be transformed to H2 or O2 microbubble reaction systems. The resultant microbubble reaction systems exhibit significant catalysis efficiency enhancement effects compared with conventional multiphase reactions. The significant improvement is attributed to the pronounced increase in reaction interface area that allows for the direct contact of gas, water, and solid phases. At the end of reaction, the microbubbles can be removed from the reaction systems through changing the pH, allowing product separation and catalyst recycling. Interestingly, the alcohol oxidation activation energy for the microbubble systems is much lower than that for the conventional multiphase reaction, also indicating that the developed microbubble system may be a valuable platform to design innovative multiphase catalysis reactions.