Surfactant-free switchable emulsions using CO2-responsive particles

A review of high internal phase Pickering emulsions: Stabilization, rheology, and 3D printing application

High internal phase Pickering emulsion (HIPPE) is renowned for its exceptionally high-volume fraction of internal phase, leading to flocculated yet deformed emulsion droplets and unique rheological behaviors such as shear-thinning property, viscoelasticity, and thixotropic recovery. Alongside the inherent features of regular emulsion systems, such as large interfacial area and well-mixture of two immiscible liquids, the HIPPEs have been emerging as building blocks to construct three-dimensional (3D) scaffolds with customized structures and programmable functions using an extrusion-based 3D printing technique, making 3D-printed HIPPE-based scaffolds attract widespread interest from various fields such as food science, biotechnology, environmental science, and energy transfer. Herein, the recent advances in preparing suitable HIPPEs as 3D printing inks for various applied fields are reviewed. This work begins with the stabilization mechanism of HIPPEs, followed by introducing the origin of their distinctive rheological behaviors and strategies to adjust the rheological behaviors to prepare more eligible HIPPEs as printing inks. Then, the compatibility between extrusion-based 3D printing and HIPPEs as building blocks was discussed, followed by a summary of the potential applications using 3D-printed HIPPE-based scaffolds. Finally, limitations and future perspectives on preparing HIPPE-based materials using extrusion-based 3D printing were presented.

CO2-Switchable colloids

The development and design of CO2-switchable colloidal particles is described. A presentation of the principles of CO2 switching, especially as they apply to colloids, is followed by recent progress in the preparation of several types of colloidal particles (polymer nanoparticles, metal-organic frameworks (MOFs), quantum dots, graphene, cellulose nanocrystals, carbon nanotubes) for various applications (Pickering stabilizers, catalysts, latexes), and our perspective on future opportunities.

CO2-Switchable Hierarchically Porous Zirconium-Based MOF-Stabilized Pickering Emulsions for Recyclable Efficient Interfacial Catalysis

Stimuli-responsive Pickering emulsions are recently being progressively utilized as advanced catalyzed systems for green and sustainable chemical conversion. Hierarchically porous metal-organic frameworks (H-MOFs) are regarded as promising candidates for the fabrication of Pickering emulsions because of the features of tunable porosity, high specific surface area and structure diversity. However, CO₂-switchable Pickering emulsions formed by hierarchically porous zirconium-based MOFs have never been seen. In this work, a novel kind of the amine-functionalized hierarchically porous UiO-66-(OH)₂ (H-UiO-66-(OH)₂) has been developed using a post-synthetic modification of H-UiO-66-(OH)₂ by (3-aminopropyl)trimethoxysilane (APTMS), 3-(2-aminoethylamino)propyltrimethoxysilane (AEAPTMS) and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (AEAEAPTMS), and employed as emulsifiers for the construction of Pickering emulsions. It was found that the functionalized H-UiO-66-(OH)₂ could stabilize a mixture of toluene and water to give an emulsion even at 0.25 wt % content. Interestingly, the formed Pickering emulsions could be reversibly transformed between demulsification and re-emulsification with alternate addition or removal of CO₂. Spectral investigation indicated that the mechanism of the switching is attributed to the reaction of CO₂ with amino silane on the MOF and the generation of hydrophilic salts, leading to a reduction in MOF wettability. Based on this strategy, a highly efficient and controlled Knoevenagel condensation reaction has been gained by using the emulsion as a mini-reactor and the emulsifier as a catalyst, and the coupling of catalysis reaction, product isolation and MOF recyclability has become accessible for a sustainable chemical process.

A CO2-Responsive Imidazole-Functionalized Fluorescent Material Mediates Cancer Chemotherapy

We present a breakthrough in the synthesis and development of functional gas-responsive materials as highly potent anticancer agents suitable for applications in cancer treatment. Herein, we successfully synthesised a stimuli-responsive multifunctional material (I-R6G) consisting of a carbon dioxide (CO₂)-sensitive imidazole moiety and spirolactam-containing conjugated rhodamine 6G (R6G) molecule. The resulting I-R6G is highly hydrophobic and non- or weakly fluorescent. Simple CO₂ bubbling treatment induces hydrophobic I-R6G to completely dissolve in water and subsequently form self-assembled nanoparticles, which exhibit unique optical absorption and fluorescence behaviours in water and extremely low haemolytic ability against sheep red blood cells. Reversibility testing indicated that I-R6G undergoes reversible CO₂/nitrogen (N₂)-dependent stimulation in water, as its structural and physical properties can be reversibly and stably switched by alternating cycles of CO₂ and N₂ bubbling. Importantly, in vitro cellular assays clearly demonstrated that the CO₂-protonated imidazole moiety promotes rapid internalisation of CO₂-treated I-R6G into cancer cells, which subsequently induces massive levels of necrotic cell death. In contrast, CO₂-treated I-R6G was not internalised and did not affect the viability of normal cells. Therefore, this newly created system may provide an innovative and efficient route to remarkably improve the selectivity, safety and efficacy of cancer treatment.

Dispersion of Hydrophilic "Hedgehog" Microparticles in Liquid CO2 Mixtures

Liquid and supercritical CO₂ are nontoxic and nonflammable reaction media with pressure-variable physical properties. These states of CO₂ also have high solubility limits for gas and liquid hydrocarbons, making them good candidates for "green" hydrophobic solvents in sustainable chemical technologies. However, the dispersion of hydrophilic colloidal nanoscale and microscale particles in CO₂ is challenging due to the tendency of polar particles to aggregate in nonpolar media, limiting their available surface area and catalytic efficiencies. Here we show that native hydrophilic semiconductor particles can be effectively dispersed in a liquid CO₂ mixture with acetonitrile (ACN) without additional chemical or mechanical dispersion techniques. Using surface corrugation as a method to prevent aggregation, we find that geometrically complex particles with a halo of stiff nanoscale spikes disperse and remain suspended longer in liquid CO₂ than those without or with less prominent nanoscale corrugation. For the particles of this size and liquid CO₂ mixtures, individual particle mass remains a prominent factor determining particle sedimentation rate even in the absence of aggregation. Particle dispersion and structural stability are confirmed using a combination of UV-vis spectroscopy, finite-difference time-domain modeling, and electron microscopy. The necessity of the cosolvent (ACN) indicates that particle behavior in liquid CO₂ is vastly different than in traditional liquid-phase solvents and highlights the need for future studies to understand the wetting behavior of hydrophilic particles in high-pressure nonpolar environments.

In-situ Spontaneous Fabrication of Tough and Stretchable Polyurethane-Polyethyleneimine Hydrogels Selectively Triggered by CO2

We develop CO₂ -triggered in-situ hydrogels from waterborne poly(ε-caprolactone)-based polyurethane (PU) dispersion and aqueous polyethyleneimine (PEI) solution without any other chemicals and apparatus (e.g., UV light). In our approach, non-toxic CO₂ in air is used as a selective trigger for the hydrogel formation. CO₂ adsorption onto PEI results in the formation of ammonium cations in PEI and the subsequent multiple ionic crosslinking between PU and PEI chains. Besides the amount of CO₂ in air, the rate of hydrogel formation can be controlled by NaHCO₃ in the PU-PEI mixture, which serves as a CO₂ supplier. The PU hydrogels exhibit tough and stretchable properties with high tensile strength (2.05 MPa) and elongation at break (438.24%), as well as biocompatibility and biodegradability. In addition, the PU hydrogels exhibit high adhesion strength on skin and injectability due to the in-situ formation. We believe that these PU hydrogels have the ideal features for various future applications, such as tissue adhesion barriers, wound dressing, artificial skin, and injectable fillers. This article is protected by copyright. All rights reserved.

Pickering foams and parameters influencing their characteristics

Pickering foams are available in many applications and have been continually gaining interest in the last two decades. Pickering foams are multifaceted, and their characteristics are highly dependent on many factors, such as particle size, charge, hydrophobicity and concentration as well as the charge and concentration of surfactants and salts available in the system. A literature review of these individual studies at first might seem confusing and somewhat contradictory, particularly in multi-component systems with particles and surfactants with different charges in the presence of salts. This paper provides a comprehensive overview of particle-stabilized foams, also known as Pickering foams and froths. Underlying mechanisms of foam stabilization by particles with different morphology, surface chemistry, size and type are reviewed and clarified. This paper also outlines the role of salts and different factors such as pH, temperature and gas type on Pickering foams. Further, we highlight recent developments in Pickering foams in different applications such as food, mining, oil and gas, and wastewater treatment industries, where Pickering foams are abundant. We conclude this overview by presenting important research avenues based on the gaps identified here. The focus of this review is limited to Pickering foams of surfactants with added salts and does not include studies on polymers, proteins, or other macromolecules.

Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications

As a growing hot research topic, manufacturing smart switchable surfaces has attracted much attention in the past a few years. The state-of-the-art study on reversibly switchable wettability of smart surfaces has been presented in this systematic review. External stimuli are brought about to render the alteration in chemical conformation and surface morphology to drive the wettability switch. Here, starting from the fundamental theories related to the surfaces wetting principles, highlights on different triggers for switchable wettability, such as pH, light, ions, temperature, electric field, gas, mechanical force, and multi-stimuli are discussed. Different applications that have various wettability requirement are targeted, including oil-water separation, droplets manipulation, patterning, liquid transport, and so on. This review aims to provide a deep insight into responsive interfacial science and offer guidance for smart surface engineering. It ends with a summary of current challenges, future opportunities, and potential solutions on smart switch of wettability on superwetting surfaces.

The effect of chitosan molecular weight on CO2-triggered switching between emulsification and demulsification

The role of molecular weight as a key physical property of macromolecules in determining the CO₂-triggered switching characteristics of responsive emulsions prepared using CO₂-switchable macromolecules has not been studied and is the focus of the current study. In this work, CO₂-switchable chitosan of four different molecular weights is used to investigate the effect of molecular weight on CO₂-triggered switching of CO₂-responsive emulsions. The molecular weight of chitosan is shown to have an opposite effect on emulsification and demulsification by the CO₂ trigger. Before bubbling of CO₂, chitosan of higher molecular weight forms a more stable three-dimensional network structure in the continuous phase of oil-in-water (O/W) emulsions, which leads to the formation of a more stable emulsion. After bubbling of CO₂, the chitosan of higher molecular weight makes the continuous phase more viscous, which leads to an incomplete demulsification as compared with the chitosan of lower molecular weight. Experimental evidence from the measurement of conductivity, interfacial tension and rheological properties is provided to support the proposed mechanism. This work is of great significance in guiding the selection of desirable CO₂-switchable polymers for switchable emulsions of desired switching characteristics.

Adhesion-Shielding based synthesis of interfacially active magnetic Janus nanoparticles

HYPOTHESIS

A unique adhesion-shielding (AS)-based method could be used to manufacture magnetic Janus nanoparticles (IM-JNPs) of promising interfacial activities, asymmetric surface wettability, and great performance on deoiling from oily wastewater under the external magnetic field.

EXPERIMENTS

The IM-JNPs were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). The interfacial properties of IM-JNPs were investigated by the measurements of interfacial pressure-area isotherms (π-A), oil-water interfacial tension, and the related crumpling ratio. The Langmuir-Blodgett (L-B) technique was used to determine the asymmetric surface wettability of the IM-JNPs. The performance and recyclability of IM-JNPs for treating oily wastewater were also investigated.

FINDINGS

Using the proposed AS-based method, 17.9 g IM-JNPs were synthesized at a time and exhibited excellent interfacial properties, as indicated by decreasing oil-water interfacial tension from 38 to 27 mN/m. The crumpling behavior of the oil droplet further demonstrated the irreversible deposition of IM-JNPs at the oil droplet surfaces. The L-B technique and water contact angle measurement confirmed the asymmetric surface wettability of the IM-JNPs. The IM-JNPs were applied to successful removal of > 90% emulsified oil droplets from the household-produced oily wastewater under the external magnetic field while realizing facile recyclability and regeneration.

Dynamic Covalent Nanoparticles for Acid-Responsive Nonaqueous Pickering Emulsions

Acid-responsive nonaqueous (glycerol in n-decane) Pickering emulsions were prepared using preferentially oil-wetted dynamic covalent silica (SiO₂-pDB) nanoparticles as the Pickering emulsifiers. The acid-responsive Pickering emulsifier SiO₂-pDB was prepared based on a Schiff base reaction between amino silica (SiO₂-NH₂) and p-decanoxybenzaldehyde (pDBA). The formation of SiO₂-pDB was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis. The preferentially oil-wetted character of SiO₂-pDB was indicated by contact angle measurement. Stable nonaqueous Pickering emulsions were prepared using preferentially oil-wetted SiO₂-pDB as the Pickering emulsifier. However, after adjusting the nonaqueous Pickering emulsions to an acidic environment, complete phase separation occurred. In the acidic environment, preferentially oil-wetted SiO₂-pDB decomposed into hydrophilic SiO₂-NH₂ and hydrophobic pDBA due to the decomposition of the dynamic imine bond in the SiO₂-pDB. Then, the hydrophilic SiO₂-NH₂ and hydrophobic pDBA desorbed from the two-phase interface, resulting in complete phase separation of the initially stable nonaqueous Pickering emulsions. The acid-responsive nonaqueous Pickering emulsions show great potential in application in water sensitive systems, such as oil-based drilling fluids.

Limonene-in-water Pickering emulsion and on-demand separation using thermo-responsive biodegradable nanoparticles

In the last few decades, Pickering emulsions have regained attention due to the possibility of forming stable oil-in-water emulsions with interesting interfacial properties. As an example, the more and more stringent regulations on the products for home and personal care are pushing the market towards the use of biodegradable materials in order to reduce their environmental impact. In this scenario, an appealing opportunity is offered by the use of biodegradable polymeric nanoparticles (NPs) for the stabilization of fragrance oils in water. In this work, modular biodegradable NPs have been synthesized through a combination of ring opening polymerization and reversible addition-fragmentation chain transfer emulsion polymerization and used to produce limonene-in-water Pickering emulsions. This strategy allowed controlling independently the NP size, polymer molecular weight, and hydrophobicity acting on the microstructure of the constituting copolymers. Stable limonene-in-water Pickering emulsions could be obtained, with the size of the oil phase and the wetting by limonene that can be strictly controlled by tuning the NP physico-chemical properties. Finally, the adoption of thermo-responsive polymer chains within the shell of the Pickering emulsifiers enabled the on-demand destabilization of the emulsions and hence the selective dispensing of limonene by simply increasing the temperature.

Ambient CO2/N2 Switchable Pickering Emulsion Emulsified by TETA-Functionalized Metal-Organic Frameworks

In recent years, metal-organic frameworks (MOFs) have been explored as emulsifiers for the fabrication of Pickering emulsions and then used for hybrid material synthesis and interface catalysis. Nevertheless, stimuli-responsive Pickering emulsions stabilized by MOFs have been rarely reported so far, although they are of great importance for fundamental research studies and practical applications. Herein, for the first time, triethylenetetramine (TETA)-functionalized MOFs (ZIF-90/TETA) have been designed, synthesized, and used for fabricating CO₂-/N₂-response Pickering emulsions. It is shown that even at the ZIF-90/TETA content of 0.25 wt %, the functional MOF can still efficiently emulsify n-hexane and water to form a high internal phase Pickering emulsion. Importantly, the Pickering emulsion can be easily and reversibly switched between emulsification and demulsification by bubbling of CO₂ and N₂ alternatively at atmospheric pressure. The possible mechanism of the CO₂/N₂ switchable emulsion is investigated by zeta potential, water contact angle, interfacial tension, ¹³C NMR spectroscopy, and an optical microscope. It is found that the acid-base reaction of CO₂ with TETA anchored on the surface of ZIF-90 leads to the production of hydrophilic ammonium bicarbonate and carbamate, which results in the emulsification of the Pickering emulsion. However, when N₂ is bubbled to remove CO₂, the reverse reaction takes place to cause the demulsification of the Pickering emulsion. Moreover, the CO₂/N₂ switchable Pickering emulsion has been successfully used as a microreactor for Knoevenagel reactions to demonstrate a highly efficient integration of chemical reaction, product separation, and ZIF-90/TETA recycling for a sustainable chemical process.

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.

Stimuli-Responsive Biomass Cellulose Particles Being Able to Reversibly Self-Assemble at Fluid Interface

Stimuli-responsive surface-active microcrystalline cellulose (MCC) particles are obtained by interaction with conventional cationic surfactants such as cetyltrimethylammonium bromide (CTAB) in aqueous media, where MCC are in situ hydrophobized by adsorption of the cationic surfactant in water via electrostatic interaction and with the in situ hydrophobization removed by adding an equimolar amount of an anionic surfactant such as sodium dodecyl sulfate (SDS). The trigger is that the electrostatic interaction between the oppositely charged ionic surfactants is stronger than that between the cationic surfactant and the negative charges on particle surfaces, or the anionic surfactant prefers to form ion pairs with the cationic surfactants and thus making them desorbed from surface of MCC. Reversible O/W Pickering emulsions can then be obtained by using the MCC in combination with trace amount of a cationic surfactant and an anionic surfactant, and the anionic surfactant with a longer alkyl chain is more efficient for demulsification. With excellent biocompatibility, biodegradability, and renewability, as well as low toxicity, the biomass cellulose particles that can be made stimuli-responsive and able to reversibly self-assemble at fluid interface become ideal biocompatible particulate materials with extensive applications involving emulsions and foams.

Dynamically Reconfigurable, Multifunctional Emulsions with Controllable Structure and Movement

Dynamically reconfigurable oil-in-water (o/w) Pickering emulsions are developed, wherein the assembly of particles (i.e., platinum-on-carbon and iron-on-carbon particles) can be actively controlled by adjusting interfacial tensions. A balanced adsorption of particles and surfactants at the o/w interface allows for the creation of inhomogeneity of the particle distribution on the emulsion surface. Complex Pickering emulsions with highly controllable and reconfigurable morphologies are produced in a single step by exploiting the temperature-sensitive miscibility of hydrocarbon and fluorocarbon liquids. Dynamic adsorption/desorption of (polymer) surfactants afford both shape and configuration transitions of multiple Pickering emulsions and encapsulated core/shell structured can be transformed into a Janus configuration. Finally, to demonstrate the intrinsic catalytic or magnetic properties of the particles provided by carbon bound Pt and Fe nanoparticles, two different systems are investigated. Specifically, the creation of a bimetallic microcapsule with controlled payload release and precise modulation of translational and rotational motions of magnetic emulsions are demonstrated, suggesting potential applications for sensing and smart payload delivery.

CO2-responsive surfactants with tunable switching pH

HYPOTHESIS

One of the major challenges in applying CO₂-responsive surfactants concerns their tunable switchability and robustness under operating conditions. We hypothesize that combining monoethanolamine (MEA) with long-chain fatty acids (LCFAs) of variable chain lengths through electrostatic attraction could develop a series of CO₂-responsive surfactants with tunable switching pH.

EXPERIMENTS

The tunability of switching pH for this group of surfactants was demonstrated by in situ probing of the CO₂-responsive characteristics at the oil/water interface using dynamic interfacial tension (IFT) measurements. Two protocols were applied to distinguish interfacial response and solution response. The key importance of interfacial response was demonstrated by two essential applications of CO₂-responsive surfactants: demulsification of stable emulsions, and alternation of the interfacial properties of ultra-heavy crude oil-water interfaces.

FINDINGS

The switching pH of the CO₂-responsive surfactants was controlled by the hydrocarbon chain length of LCFAs. More importantly, their switching behaviour was found to be different at the interface and in the bulk solution, which is attributed to the enhanced molecular interactions at the interface. Since most applications require surfactants to be switched at the interface, it is thereby most appropriate to determine the switching pH through their interfacial responses.

Simultaneous and Reversible Triggering of the Phase Transfer and Luminescence Change of Amidine-Modified Carbon Dots by CO2

The ability to reversibly manipulate the surface nature of luminescent nanoparticles upon external stimulation enables the development of advanced optical probes for biological sensing and data encoding. Herein, we report the synthesis of a new class of smart carbon dots (CDs) via surface modification of amine-enriched CDs with CO₂-responsive groups of amidine. We present that alternative CO₂ and N₂ bubbling can not only lead to a reversible phase transfer of the CDs between an organic phase and an aqueous phase but also give rise to a corresponding reversible luminescence change between blue and cyan-green. We attribute these observations to changes in both the surface chemistry and the emission states of the CDs triggered by the alternative CO₂/N₂ introduction. We also find a similar luminescence change of the CDs upon alternative exposure to a humid vapor of CO₂ and a mixture of NH₃ and N₂ at room temperature, allowing them to be used as a new class of optical materials for optical encoding.

Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

A novel CO₂/N₂ switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO₂/N₂ switchable surfactant [ N'-dodecyl- N, N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N₂ into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO₂ into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

Intelligent CO2- and photo-dual-responsive polymer vesicles with tunable wall thickness

CO₂- and photo-dual-responsive polymer vesicles with tunable wall thickness were explored and used as a potential “smart” platform for drug release.

Reversible flocculation of nanoparticles by a carbamate surfactant

Fatty alkyldiamine readily reacts with CO₂ in aqueous solution at pH 12 to reversibly form surface active carbamate species. The carbamate can be reverted to the amine by exposure to N₂ and heat. In this work, a carbamate-based surfactant (Y12-carbamate) has been used to disperse and stabilize hydrophobic nanoparticles. This state could be regarded as the "on" state of a series of cycle. The nanoparticles were then flocculated when the carbamate groups were cleaved by exposure to N₂ and heating, corresponding to the "off" state. In a subsequent cycle, the nanoparticles were re-dispersed by exposure to CO₂, while the pH remained at 12. This cycle of re-dispersion and flocculation could be repeated two times without impairing the particle size. However, further cycles increased the particle size, indicating that all particles could not be completely re-dispersed. In addition, we also investigated the effect of pH on the colloidal stability with sodium Y12-carbamate, by measuring particle size and electrophoretic mobility. The results showed that pH strongly influenced the stability of the nanoparticles. Sodium Y12-carbamate stabilized the particles with a negative electrophoretic mobility at pH well above pKa whereas at pH close to pKa of Y12-amine (pKa = 9.0), the particles quickly flocculated, as a result of an ion-pair formation between Y12-ammonium and Y12-carbamate.

CO2-Responsive Pickering Emulsions Stabilized by a Bio-based Rigid Surfactant with Nanosilica

A novel CO₂-responsive surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)propylamine imide (MPAGN), based on sustainable resource of rosin was synthesized and used to prepare a kind of CO₂-responsive Pickering emulsions with nanosilica. MPAGN can be reversibly responsive to CO₂ and N₂ between active cationic (MPAGNH⁺) and inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of nanosilica, then stabilize or break emulsion. CO₂-responsive behavior of MPAGN was verified by cycle change of pH and conductivity with bubbling CO₂ and N₂ alternately. The type of adsorption of MPAGNH⁺ at the particle-water interface was explained according to the adsorption isotherms. The mechanisms of stabilization, destabilization, and restabilization of Pickering emulsion were analyzed according to zeta potentials and droplet size. This Pickering emulsion can be reversible between stable and unstable by bubbling CO₂ and N₂ alternately. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has an environmentally friendly property.

An Oligoimide Particle as a Pickering Emulsion Stabilizer

A pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA)-based oligoimide (PMDA-ODA) was synthesized by a one-step procedure using water as a solvent. The PMDA-ODA particles showed excellent partial wetting properties and were stably dispersed in both water and oil phases. A stable dispersion was not obtained with comparison PMDA-ODA particles that were synthesized by a conventional two-step method using an organic solvent. Both oil-in-water and water-in-oil Pickering emulsions were prepared using the oligoimide particles synthesized in water, and the size of the emulsion droplet was controlled based on the oligoimide particle concentration. The oligoimide particles were tested to prepare Pickering emulsions using various kinds of oils. The oil-in-water Pickering emulsions were successfully applied to prepare microcapsules of the emulsion droplets. Our new Pickering emulsion stabilizer has the advantages of easy synthesis, no need for surface modification, and the capability of stabilizing both oil-in-water and water-in-oil emulsions.

Tailoring the Wettability of Colloidal Particles for Pickering Emulsions via Surface Modification and Roughness

Pickering emulsions are water or oil droplets that are stabilized by colloidal particles and have been intensely studied since the late 90s. The surfactant-free nature of these emulsions has little adverse effects such as irritancy and contamination of environment and typically exhibit enhanced stability compared to surfactant-stabilized emulsions. Therefore, they offer promising applications in cosmetics, food science, controlled release, and the manufacturing of microcapsules and porous materials. The wettability of the colloidal particles is the main parameter determining the formation and stability of Pickering emulsions. Tailoring the wettability by surface chemistry or surface roughness offers considerable scope for the design of a variety of hybrid nanoparticles that may serve as novel efficient Pickering emulsion stabilizers. In this review, we will discuss the recent advances in the development of surface modification of nanoparticles.

Dynamic Covalent Silica Nanoparticles for pH-Switchable Pickering Emulsions

Dynamic covalent surfactants have been recently reported for preparation of pH-switchable emulsions [ Sun , D. Langmuir , 2017 , 33 , 3040 ]. In this study, dynamic covalent silica (SiO₂-B) nanoparticles of switchable wettability were fabricated by a pH-responsive dynamic (covalent) imine bond between hydrophilic amino silica (SiO₂-NH₂) nanoparticles and hydrophobic benzaldehyde molecules. The properties of SiO₂-B were characterized by Fourier transform infrared spectroscopy, elemental analysis, contact angle measurement, and ζ potential measurement. The hydrophilicity and hydrophobicity of SiO₂-B were shown to be readily switchable by adjusting pH between 7.8 and 3.5. At pH 7.8, SiO₂-B was partially hydrophobic and adsorbed at oil-water interface to stabilize O/W Pickering emulsions, which were characterized by electrical conductivity, optical microscopy, and confocal laser scanning microscopy. Upon lowering the pH to 3.5, the dynamic covalent bond is dissociated to convert partially hydrophobic SiO₂-B into highly hydrophilic SiO₂-NH₂ and surface-inactive benzaldehyde. Both of them desorb from oil-water interface, resulting in a rapid oil-water separation of the Pickering emulsions. Alternating stabilization and phase separation of the Pickering emulsions over 3 cycles were demonstrated by adjusting the pH. The pH-switchable Pickering emulsions show great potential in application to effective oil-water separation of emulsions.

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.

CO2 and Redox Dual Responsive Pickering Emulsion

Herein, we described for the first time a CO₂ and redox dual responsive paraffin oil-in-water Pickering emulsion stabilized by the modified silica nanoparticles with Se-containing tertiary amine, SeTA, in which the tertiary amine serves as a CO₂-sensitive group, and the Se atom serves as a redox-sensitive center. The Pickering emulsion can be reversibly switched between stable and unstable states by bubbling CO₂ and N₂ in the reduced state, or with the addition of H₂O₂ and Na₂SO₃ in the absence of CO₂, because of the adsorption and desorption of SeTA on the silica surface. The former is mainly attributed to a CO₂-controllable electrostatic attraction, resulting from the transition of molecules between cationic and nonionic states; whereas, the latter is ascribed to a redox-tunable hydrogen bonding, originating from the transition of molecules between selenide and selenoxide. However, in the presence of CO₂, redox can only induce a change in the droplet size, not demulsification. These interesting and unique multiresponsive behaviors endow the Pickering emulsion with a capacity for intelligent control of emulsification and demulsification, as well as the droplet size, which may be an asset for a myriad of technological applications in biomedicine, microfluidics, drug delivery, and cosmetics.

pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant

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.

CO2/N2 triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

Switchable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) and equal moles of a CO₂/N₂ switchable surfactant.

CO2-Triggered Pickering Emulsion Based on Silica Nanoparticles and Tertiary Amine with Long Hydrophobic Tails

We describe a strategy of fabricating CO₂-triggered oil-in-water Pickering emulsion based on silica nanoparticles functionalized in situ by a trace amount of conventional CO₂-switchable surfactant, N-(3-(dimethylamino)propyl)alkyl amide (C_nPMA). By alternately bubbling CO₂ and N₂ at a moderate conditions (30 °C, 80 mL min^-1), silica nanoparticles reversibly switch between amphipathic and hydrophilic as a result of the adsorption of ammonium (CO₂) and the desorption of tertiary amine (N₂). The emulsion can then be smart switched "on (stable)" and "off (unstable)", along with homogenization, without needing cooling and heating. The switching of the current tertiary-based system is simple, moderate, and environmentally friendly, without contamination and the restriction of rigorous conditions. The surfactant concentration window of the Pickering emulsion is closely related to the length of hydrophobic tail, and the upper limit is no more than 0.20 cmc of that of the corresponding ammonium surfactant. Such a strategy is also suitable for commercial alkyl tertiary amines, without needing complicated organic synthesis.

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.

CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

CO2 is an ideal trigger for switchable or stimuli-responsive materials because it is benign, inexpensive, green, abundant, and does not accumulate in the system. Many different CO2-responsive materials including polymers, latexes, solvents, solutes, gels, surfactants, and catalysts have been prepared. This review focuses on the preparation, self-assembly, and functional applications of CO2-responsive polymers. Detailed discussion is provided on the synthesis of CO2-responsive polymers, in particular using reversible deactivation radical polymerization (RDRP), formerly known as controlled/living radical polymerization (CLRP), a powerful technique for the preparation of well-defined (co)polymers with precise control over molecular weight distribution, chain-end functional groups, and polymer architectural design. Self-assembly in aqueous dispersed media is highlighted as well as emerging potential applications.