High internal phase emulsion templates solely stabilised by functionalised titania nanoparticles

Mechanical and Thermal Properties of Porous Nanocellulose/Polymer Composites: Influence of the Polymer Chemical Structure and Porosity

The excellent emulsifying capacity of nanocellulose allows for the preparation of porous nanocellulose/polymer composites through the emulsion templating process. However, the effects of the polymer chemical structure and porosity on the material properties have not been extensively explored. Here, we discuss the effects of these two factors on the thermal and mechanical properties of the composites. Two types of porous nanocellulose/polymer composites were fabricated with styrene-divinylbenzene (poly(St-co-DVB)) or styrene-poly(ethylene glycol) dimethacrylate (poly(St-co-EGDMA)) copolymers as the polymer phases. The porosity of the composite was changed up to ∼50% v/v by varying the aqueous phase volume fraction in the original nanocellulose-stabilized w/o emulsions. As the porosity increased, the thermal conductivity of the composite decreased. The mechanical properties were strongly influenced by the polymer type; the nanocellulose/poly(St-co-DVB) composite showed stiff but brittle behavior, whereas the nanocellulose/poly(St-co-EGDMA) composite showed higher strength and toughness. In both types of composites, the nanocelluloses served as reinforcing agents, contributing to the improvement of the mechanical properties.

Covalent organic frameworks based hierarchical porous hybrid monolithic capillary: Synthesis, characterization, and applications in trace metals analysis

The preparation of hierarchical porous monolithic column with high covalent organic frameworks (COF) loading and micropores accessibility is challenging due to the easy aggregability and sedimentation of COFs. Herein, a novel strategy based on high internal phase emulsion (HIPE) polymerization was proposed for preparing COF hybrid capillary monolithic column with hierarchical porosity. COFs with different frameworks including imine COFs (COF-OMe, COF-F and COF-SH), triazine COF (CTF-1) and boron-based COF (COF-5) were selected to investigate the universality of the preparation strategy. The presence of COF in the monolithic capillary was confirmed by scanning electron microscope, X-ray diffraction and fourier transform infrared spectroscopy. Nitrogen adsorption/desorption experiments and thermogravimetric analysis showed that the prepared COF hybrid monolithic capillary exhibited high COF loading (e.g., 28.7% for COF-SH) and accessibility (e.g., 98.5% for COF-SH), mainly due to the thin walls of void-window structures originated from polymerization of HIPE. The successful preparation of water-stable COF-F, COF-OMe, COF-SH and CTF-1 hybrid monolithic columns demonstrated the proposed synthesis strategy is universal to water-stable COF without tedious optimization of dispersion system, effectively avoiding the sedimentation of COF in pre-polymerization solution. Then, the sulfhydryl-modified COF hybrid polymer (poly(COF-SH-HIPE)) monolithic column was evaluated for the extraction of heavy metal ions, and a method based on poly(COF-SH-HIPE) monolithic capillary microextraction on-line coupled with inductively coupled plasma mass spectrometry detection was developed for analysis of trace Cd, Hg and Pb in human fluid samples.

Transparent Oil-Water Separating Hydrophobic Sponge Prepared from a Pickering High Internal Phase Emulsion Stabilized by Octadecyltrichlorosilane Grafting Carbon Nanotubes

Increasingly, oil spills and industrial discharges are wreaking havoc on the water environment; in order to efficiently separate oil and water from sewage containing oil or organic solvents, a novel porous polymer (P(EHA-co-BA)) was prepared by Pickering high internal phase emulsion (HIPE) template method. To obtain polyHIPE with better oil/water separation capacities, octadecyltrichlorosilane (OTS)-modified carbon nanotubes (CNTs) and surfactants were used as costabilizers for HIPE, which improved the stability of HIPE as well as the mechanical properties and the separation efficiency of polyHIPE. In the presence of 1 wt % OTS-CNT adding in the oil phase, 1%OTS-CNT polyHIPE has high porosity (92.21%), favorable hydrophobicity (a water contact angle of 128°), and excellent mechanical properties. As a result, 1%OTS-CNT polyHIPE has high absorption of oils and oily solvents, e.g., dichloromethane up to 36 g/g, and maintains an absorption efficiency of >97% after 20 reapplications. In the formulation of polyHIPE, cinnamaldehyde (CA) has been added to provide superior antibacterial properties against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). It appears that the novel polyHIPE proposed in this work is a reusable antibacterial porous polymer with promising applications for oil-water separation.

Cellulose-based, flexible polyurethane polyHIPEs with quasi-closed-cell structures and high stability for thermal insulation

Cellulose-based, closed-cell porous materials templated from emulsions are promising for thermal insulation, but their low stability imposed by physical interaction hinders the materials from real applications. Herein, we report the fabrication of cellulose-based, flexible polyurethane polyHIPEs with quasi-closed-cell structures, high stability and flexibility for thermal insulation. The polyHIPEs were prepared from cellulose-stabilized Pickering high internal phase emulsions through interfacial crosslinking using isocyanate. The resulting polyurethane polyHIPEs showed controllable external shapes, quasi-closed-cell structures, high flexibility, low density, and robust compression (without fracture even after compression to 30 % original height). The crosslinking enabled the polyHIPEs to show hydrophobicity, good stability (without breakage and dissolution observed after immersing in NaOH solution at pH 12, HCl solution at pH 1 and hot water at 100 °C, for 24 h) and decreased moisture uptake (below 1 %). The low density and quasi-closed-cell structures endowed the polyHIPEs with high thermal insulation, with thermal conductivity as low as 33.1 mW/(m K). These features make the cellulose-based, closed-cell polyHIPEs as an excellent candidate for thermal insulting.

Preparation of Interconnected Pickering Polymerized High Internal Phase Emulsions by Arrested Coalescence

Emulsion templating is a method that enables the production of highly porous and interconnected polymer foams called polymerized high internal phase emulsions (PolyHIPEs). Since emulsions are inherently unstable systems, they can be stabilized either by surfactants or by particles (Pickering HIPEs). Surfactant-stabilized HIPEs form materials with an interconnected porous structure, while Pickering HIPEs typically form closed pore materials. In this study, we describe a system that uses submicrometer polymer particles to stabilize the emulsions. Polymers fabricated from these Pickering emulsions exhibit, unlike traditional Pickering emulsions, highly interconnected large pore structures, and we related these structures to arrested coalescence. We describe in detail the morphological properties of this system and their dependence on different production parameters. This production method might provide an interesting alternative to poly-surfactant-stabilized-HIPEs, in particular where the application necessitates large pore structures.

Water-in-oil high internal phase Pickering emulsions formed by spontaneous interfacial hydrolysis of monomer oil

HYPOTHESIS

Alcohols can strongly reduce the interfacial tension between immiscible liquids, thus facilitating the formation of emulsions. By combining non-surface-active hydrophobic particles with medium-chain alcohols, stable water-in-oil (w/o) high internal phase Pickering emulsions (HIPPEs) can be easily prepared without high-energy emulsification methods.

EXPERIMENTS

The emulsions containing acrylate monomer as the oil phase were prepared at different pH values in the presence of hydrophobic silica particles. Further, by replacing monomer oil with organic solvents (e.g., toluene) and a certain concentration of alcohol, the promoted particle adsorption at the oil-water interface has been systematically investigated. The morphology and interfacial structure of HIPPEs were visualized by confocal laser scanning microscopy (CLSM).

FINDING

At high pH, stable water-in-acrylate monomer HIPPEs can be formed using commercial fumed silica nanoparticles alone with simple stirring or vortexing. The hydrolysis of the acrylate group at high pH can generate alcohols in situ which adsorb at the oil-water interface to reduce the interfacial tension and promote particle adsorption to hinder droplet coalescence. The novel strategy for forming stable and processable HIPPEs can be universally applied to different hydrophobic silica particles with the help of various alcohols as the co-stabilizer, which provides a flexible approach for the fabrication of lightweight, closed-cell solid foams for a range of applications.

High Internal Phase Emulsions Stabilized with Polyphenol-Amyloid Fibril Supramolecules for Encapsulation and Protection of Lutein

High internal phase emulsions (HIPEs), also called highly concentrated emulsions with a minimal internal phase volume fraction of 74%, have been paid increasing attention in the development of functional foods due to their high potential in loading with large amounts of hydrophobic nutriceuticals. In the present study, HIPEs stabilized by polyphenol-amyloid supramolecular filaments were prepared for encapsulation of olive oil and loading with lutein. Binding and stacking of the green tea polyphenol epigallocatechin gallate (EGCG) on the surface of amyloid fibrils fabricated from hen egg lysozyme resulted in the hybrid supramolecules, which assembled to form hydrogels. The amyloid fibril clusters shrouded by EGCG were observed in the microstructure of the hydrogels characterized by atomic force microscopy (AFM). HIPEs stabilized by the EGCG-amyloid fibril supramolecules showed the typical microstructure of highly packed polyhedral geometric oil droplets. The gel strength of the HIPEs stabilized by the hybrid supramolecules was greater than that of HIPEs stabilized by pure amyloid fibrils. The droplet size of the HIPEs first decreased and then increased with the increase of EGCG contents in the hybrid supramolecules, which was consistent with the corresponding emulsion morphologies obtained from the images of confocal laser scanning microscopy (CLSM). Aggregation of the protein-based nanofibrils appeared in the continuous phase at higher EGCG contents. The droplet size of the HIPEs decreased with the increase of the amyloid fibril concentration, accompanied by more packed and homogenously dispersed lipid droplets, as shown in the CLSM images. A high loading content of lutein of up to 10 mg/mL in the prepared HIPEs was realized, and the stability of lutein against ultraviolet irradiation, heat, iron, and hydrogen peroxide was promoted significantly. In addition, encapsulation with the HIPEs prevented the oxidization of olive oil, and this effect was enhanced with the increase of the EGCG content in the hybrid supramolecules ranging from 0 to 0.25 wt %. The protection function of the HIPEs might be ascribed to the membrane of interfacial amyloid fibrils and the crowded oil droplet environment, both of which could shield the pro-oxidation factors.

Microphase-separated, magnetic macroporous polymers with amphiphilic swelling from emulsion templating

PolyHIPEs are promising for various applications associated with liquid uptakes. PolyHIPEs from a reactive, monomeric block copolymer can exhibit amphiphilic swelling, but such swelling usually tends to disappear upon copolymerization...

The role of excess attractive particles in the elasticity of high internal phase Pickering emulsions

A high internal phase emulsion (HIPE), which has a volume fraction of dispersed phase of over 74%, shows a solid-like property because of concentrated polyhedral droplets. Although many studies have proposed theoretical and empirical models to explain the rheological properties of HIPEs, most of them are only limited to the emulsions stabilized by surfactants. In the case of high internal phase Pickering emulsions (HIPPEs), much greater values of elastic modulus have been reported, compared to those of surfactant-stabilized HIPEs, but so far, there have been no clear explanations for this. In this study, we investigate how colloidal particles attribute to the significantly high elasticity of HIPPEs, specifically considering two different contributions, namely, interfacial rheological properties and bulk rheological properties. Our results reveal that the flocculated structures of colloidal particles that possess a significant elasticity can be interconnected between dispersed droplets. Furthermore, this elastic structure is a crucial factor in the high elasticity of HIPPEs, which is also supported by a simple theoretical model.

High internal phase Pickering emulsions stabilized by co-assembled rice proteins and carboxymethyl cellulose for food-grade 3D printing

In this study, high internal phase Pickering emulsions (HIPPEs) stabilized by protein-polysaccharide complexes were used as inks for food-grade three-dimensional printing (3DP). The complexes (RCs) structured by synergistic interactions between rice proteins (RPs) and carboxymethyl cellulose (CMC) displayed outstanding biphasic wettability with excellent ability to reduce the oil/water interfacial tension. The interfacial structures formed by RCs provided a steric barrier and sufficient electrostatic repulsion, preventing droplet coalescence against heating treatment as well as long-term storage. Moreover, the rheological behaviors of the HIPPEs can be tuned by the substitution degree (DS) of CMC for tailorable hydrophobic/hydrophilic properties of RCs, allowing their controllable injectability and printability during 3DP. The HIPPEs stabilized by RCs with a DS 1.2 showed the most favorable printing resolution, hardness, adhesiveness, and chewiness. Associating the hydrophobic RPs with hydrophilic CMC, our study enabled on-demand amphiphilicity of RCs for effective stabilization of HIPPEs that can be manipulated for 3DP.

Metal-Organic Framework-Stabilized High Internal Phase Pickering Emulsions Based on Computer Simulation for Curcumin Encapsulation: Comprehensive Characterization and Stability Mechanism

High internal phase Pickering emulsions (HIPPEs) have taken a center stage in the arena of delivery systems in the food industry because of their high loading capacity and stability. In addition, metal-organic frameworks (MOFs), a type of cutting-edge designable porous scaffolding material, have attracted attention in reticular chemistry, which satisfies fundamental demands for delivery research in the past years. Here, we demonstrate a novel metal-organic framework (MOF)-stabilized HIPPE delivery system for hydrophobic phytochemicals. First, a novel high-biocompatibility and stable MOF particle, UiO-66-NH₂, was selected from atomic simulation screening, which showed proper electronegativity and amphiphilic properties to develop the HIPPE system. Monodispersed UiO-66-NH₂ nanoparticles with the particle size of 161.36 nm were then prepared via solvothermal synthesization. Pickering emulsions with inner phase ratios from 50 to 80% with varied contents of polyethylene glycol (PEG) were prepared by in situ high-pressure homogenization, and their physicochemical properties including crystallography, morphology, and rheology were systematically characterized. Subsequently, curcumin, a model antioxidant, was loaded in the HIPPE system and named cur@UiO-66-NH₂/HIPPE. It exhibited high loading capacity, up to 6.93 ± 0.41%, and encapsulation efficiency (19.76 ± 3.84%). This novel MOF nanoparticle-stabilized HIPPE delivery system could be practically utilized for other bioactive components and antimicrobial agents, which would find applications in food safety and biomedical areas in the future.

All-natural oil-in-water high internal phase Pickering emulsions featuring interfacial bilayer stabilization

HYPOTHESIS

Synergistic stabilization of high internal phase Pickering emulsions (HIPPEs) by food-grade colloidal particles are necessary for food, pharmaceuticals or cosmetics owing to their biocompatibility and multi-functionality. By tuning the interfacial structure of adsorbed binary particles, the HIPPE may exhibit extraordinary characteristics compared to conventional all-natural HIPPEs solely stabilized by single-component particle or composite particle, which should have potential applications in varies fields.

EXPERIMENTS

HIPPEs were prepared by using zein protein nanoparticles (ZNPs) and starch nanocrystals (SNCs) as stabilizers. We systematically investigated the effect of particle concentration and internal phase fraction on HIPPEs morphology, stability and rheological behaviors. Further, the stabilization mechanism as well as potential applications were demonstrated.

FINDINGS

HIPPEs were prepared with excellent stability against centrifugation and high temperature (50 °C). Our result indicates the successful construction of unique bilayer interfacial structures consisting of inner ZNPs layer and outer SNCs layer. Since SNCs could gelatinize at 50 °C, dense shells can form around droplets afterwards. Such thermally responsive interfacial structures can be used to protect hydrophobic bioactive substances at higher temperatures while still allowing controlled release at certain conditions. Furthermore, with high internal phase fraction, HIPPEs can possibly replace mayonnaise and salad dressing on the market due to comparable appearance and properties. Following the removal of inner oil, porous materials can be further fabricated, which have potential applications in environmental protection or tissue engineering.

High Internal Phase Emulsions Synergistically Stabilized by Sodium Carboxymethyl Cellulose and Palm Kernel Oil Ethoxylates as an Essential Oil Delivery System

High internal phase emulsions (HIPEs) with an internal phase fraction of 84 vol % were prepared using carboxymethyl cellulose (CMC) and palm kernel oil ethoxylates (SOE-N-60) as a dual emulsifier. Effects of the oil-phase volume fraction, CMC concentration, and SOE-N-60 concentration on oil-in-water HIPEs stability were systematically studied by a Mastersizer 2000 instrument, Lx POL polarizing microscope, rheometer, etc. The bioavailability of tea tree oil can be effectively protected using HIPEs as a delivery system. The experimental results showed that, with the increase of the concentrations of CMC and SOE-N-60, the droplet size of HIPEs gradually decreases and the HIPEs showed good static stability. In addition, it was observed by scanning electron microscopy that the polyHIPEs materials using HIPEs stabilized by different SOE-N-60 and CMC concentrations as templates had different structures. Moreover, the synergism between CMC and SOE-N-60 surfactants plays a significant role in the preparation and stability of HIPEs.

Novel Pickering High Internal Phase Emulsion Stabilized by Food Waste-Hen Egg Chalaza

A massive amount of chalaza with nearly 400 metric tons is produced annually as waste in the liquid-egg industry. The present study aimed to look for ways to utilize chalaza as a natural emulsifier for high internal phase emulsions (HIPEs) at the optimal production conditions to expand the utilization of such abundant material. To the author’s knowledge, for the first time, we report the usage of hen egg chalaza particles as particulate emulsifiers for Pickering (HIPEs) development. The chalaza particles with partial wettability were fabricated at different pH or ionic strengths by freeze-drying. The surface electricity of the chalaza particles was neutralized when the pH was adjusted to 4, where the chalaza contained a particle size around 1500 nm and held the best capability to stabilize the emulsions. Similarly, the chalaza reaches proper electrical charging (−6 mv) and size (700 nm) after the ionic strength was modified to 0.6 M. Following the characterization of chalaza particles, we successfully generated stable Pickering HIPEs with up to 86% internal phase at proper particle concentrations (0.5–2%). The emulsion contained significant stability against coalescence and flocculation during long term storage due to the electrical hindrance raised by the chalaza particles which absorbed on the oil–water interfaces. Different rheological models were tested on the formed HIPEs, indicating the outstanding stability of such emulsions. Concomitantly, a percolating 3D-network was formed in the Pickering HIPES stabilized by chalaza which provided the emulsions with viscoelastic and self-standing features. Moreover, the current study provides an attractive strategy to convert liquid oils to viscoelastic soft solids without artificial trans fats.

pH-Sensitive W/O Pickering High Internal Phase Emulsions and W/O/W High Internal Water-Phase Double Emulsions with Tailored Microstructures Costabilized by Lecithin and Silica Inorganic Particles

Synergistic stabilization of Pickering emulsions by a mixture of surfactants and colloidal particles has received increasing interest in recent years but only a few of them can produce high internal phase double emulsions (HIPDEs) with good stability. In this study, we present a feasible and common method of preparing Pickering high internal phase emulsions (HIPEs) with tunable inner morphology costabilized by a biosurfactant lecithin and silica nanoparticles. We investigate the influence of the pH value on the interfacial behavior of lecithin and elucidate the synergistic mechanism between lecithin and silica nanoparticles in different conditions in the stability of as-prepared emulsions. Specifically, water-in-oil (W/O) Pickering HIPEs can be successfully stabilized by lecithin and hydrophobic silica nanoparticles in a wide pH range (pH 1-10), while catastrophic phase inversion occurred at high pH values (pH ≥ 11). Interestingly, stable water-in-oil-in-water (W/O/W) high internal phase double emulsions (HIPDEs) can also be prepared via a two-step method by the cooperation of lecithin and silica nanoparticles. Moreover, functional interconnected porous monoliths and microspheres are facilely fabricated by emulsion templates and their potential applications are explored.

Magnetic hydrophobic solids prepared from Pickering emulsions for the extraction of polycyclic aromatic hydrocarbons from chamomile tea

Two types of magnetic hydrophobic solids were prepared by Pickering emulsion photopolymerization using polystyrene-modified magnetic nanoparticles (PS-MNPs) as emulsion stabilizers. Additionally, PS-MNPs provided magnetic character to the final solids. W/O Pickering emulsions were produced with high amounts of oily phase (above 50 wt%), while O/W Pickering emulsions were formed with higher amounts of aqueous phase (above 60 wt%). These two types of emulsions led to two kind of solids with very different structures despite being formed by the same components. In this way, W/O Pickering emulsions produced monolithic solids, while O/W Pickering emulsions formed magnetic microparticles. Multi-walled carbon nanotubes (MWCNTs) were also added to the emulsions to provide higher hydrophobic character to the final solids. The structure and morphology of both magnetic solids containing the MWCNTs was characterized by scanning electron microscopy (SEM). Finally, their extraction efficiency was evaluated using polycyclic aromatic hydrocarbons (PAHs) as target analytes, both qualitatively (visually by the fluorescence emitted before and after the extraction) and quantitatively (using gas chromatography coupled to mass spectrometry). Therefore, the LODs ranged from 1 to 4 μg L^-1 and the LOQs were between 3 and 12 μg L^-1. The reproducibility of the extraction procedure with different batches of emulsions was acceptable with RSD values <13%. Finally, a recovery study was carried out in complex matrices such as chamomile tea, obtaining excellent recovery values which ranged from 99 to 108%.

Porous polycaprolactone and polycarbonate poly(urethane urea)s via emulsion templating: structures, properties, cell growth

Macroporous, emulsion-templated, linear poly(urethane urea) elastomers were synthesized from polyols (poly(ε-caprolactone)s or polycarbonates) and a diisocyanate. Growing cells adhered to the walls, spread, and penetrated into the porous structures.

Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n-alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.

Hofmeister Effect-Assistant Fabrication of All-Natural Protein-based Porous Materials Templated from Pickering Emulsions

Porous materials derived from natural and biodegradable polymers have received growing interest. We demonstrate here an attractive method for the preparation of protein-based porous materials using emulsions stabilized by gliadin-chitosan hybrid particles (GCHPs) as the template, with the addition of gelatin and kosmotropic ions to improve the mechanical strength. The microstructure, mechanical properties, cytotoxicity, and fluid absorption behavior of porous materials were systematically investigated. This strategy facilitated the formation of porous materials with highly open and interconnected pore structure, which can be manipulated by altering the mass ratio of hexane or gelatin in the matrix. The Hofmeister effect resulted from kosmotropic ions greatly enhanced the Young's modulus and the compressive stress at 40% strain of porous materials from 0.56 to 6.84 MPa and 0.26 to 1.11 MPa, respectively. The developed all-natural porous materials were nontoxic to HaCaT cells; they also had excellent liquid (i.e., simulated body fluid and rabbit blood) absorption performance and advantages in resisting stress and maintaining geometry shape. The effects of different concentration amounts and type of salts in the Hofmeister series on the formation and performance of porous materials were also explored. Mechanical strength of porous materials was gradually enhanced when the (NH₄)₂SO₄ concentration increased from 0 to 35 wt %, and the other four kosmotropic salts, including Na₂S₂O₃, Na₂CO₃, NaH₂PO₄, and Na₂SO₄, also showed positive effects. This work opens a simple and feasible way to produce nontoxic and biodegradable porous materials with favorable mechanical strength and controllable pore structure. These materials have broad potential application in many fields involving biomedical and material science, such as cell culture, (bio)catalysis, and wound or bone defect healing.

Recent advances in separation applications of polymerized high internal phase emulsions

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

Advances and Opportunities of Oil-in-Oil Emulsions

Emulsions are mixtures of two immiscible liquids in which droplets of one are dispersed in a continuous phase of the other. The most common emulsions are oil-water systems, which have found widespread use across a number of industries, for example, in the cosmetic and food industries, and are also of advanced scientific interest. In addition, the past decade has seen a significant increase in both the design and application of nonaqueous emulsions. This has been primarily driven by developments in understanding the mechanism of effective stabilization of oil-in-oil (o/o) systems, either using block copolymers (BCPs) or solid (Pickering) particles with appropriate surface functionality. These systems, as highlighted in this review, have enabled emergent applications in areas such as pharmaceutical delivery, energy storage, and materials design (e.g., polymerization, monolith, and porous polymer synthesis). These o/o emulsions complement traditional emulsions that utilize an aqueous phase and allow the use of materials incompatible with water. We assess recent advances in the preparation and stabilization of o/o emulsions, focusing on the identity of the stabilizer (BCP or particle), the interplay between stabilizer and oils, and highlighting applications and opportunities associated with o/o emulsions.

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.

•

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.

•

We discuss recent 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.

•

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.

Bio-Compatible Ca-BDC/Polymer Monolithic Composites Templated from Bio-Active Ca-BDC Co-Stabilized CO2-in-Water High Internal Phase Emulsions

Because of the nontoxic solvents contained in CO2-in-water emulsions, porous polymer composites templated from these emulsions are conducive for bio-applications. Herein, bio-active rod-like calcium-organic framworks (Ca-BDC MOFs, BDC= 1,4-benzenedicarboxylate anion) particles co-stabilized CO2-in-water high internal phase emulsion (C/W HIPE) in the presence of polyvinyl alcohol (PVA) is first presented. After curing of the continuous phase, followed by releasing CO2, integral 3D macro-porous Ca-BDC monolith and Ca-BDC/Poly(2-hydroxyethyl methacrylate-co-acrylamide) HIPEs monolithic composites [Ca-BDC/P(AM-co-HEMA)HIPEs] with open-cell macro-porous structures were successfully prepared. The pore structure of these porous composite can be tuned by means of tailoring the Ca-BDC dosage, carbon dioxide pressure, and continuous phase volume fractions in corresponding C/W HIPEs. Results of bio-compatibility tests show that these Ca-BDC/P(AM-co-HEMA)HIPEs monoliths have non-cytotoxicity on HepG2 cells; also, the E. coli can grow either on the surfaces or inside these monoliths. Furthermore, immobilization of β-amylase on these porous composite presents that β-amylase can be well-anchored into the porous polymer composites, its catalytic activity can be maintained even after 10 cycles. This work combined bio-active MOFs Ca-BDC, bio-compatible open-cell macroporous polymer PAM-co-HEMA and green C/W HIPEs to present a novel and facile way to prepare interconnected macro-porous MOFs/polymer composites. Compared with the existing other well-known materials such as hydrogels, these porous composites possess well-defined tunable pore structures and superior bio-activity, thereby have promising applications in bio-tissue engineering, food, and pharmaceutical.

High Internal Phase Oil-in-Water Pickering Emulsions Stabilized by Chitin Nanofibrils: 3D Structuring and Solid Foam

Chitin nanofibrils (NCh, ∼10 nm lateral size) were produced under conditions that were less severe compared to those for other biomass-derived nanomaterials and used to formulate high internal phase Pickering emulsions (HIPPEs). Pre-emulsification followed by continuous oil feeding facilitated a "scaffold" with high elasticity, which arrested droplet mobility and coarsening, achieving edible oil-in-water emulsions with internal phase volume fraction as high as 88%. The high stabilization ability of rodlike NCh originated from the restricted coarsening, droplet breakage and coalescence upon emulsion formation. This was the result of (a) irreversible adsorption at the interface (wettability measurements by the captive bubble method) and (b) structuring in highly interconnected fibrillar networks in the continuous phase (rheology, cryo-SEM, and fluorescent microscopies). Because the surface energy of NCh can be tailored by pH (protonation of surface amino groups), emulsion formation was found to be pH-dependent. Emulsions produced at pH from 3 to 5 were most stable (at least for 3 weeks). Although at a higher pH NCh was dispersible and the three-phase contact angle indicated better interfacial wettability to the oil phase, the lower interdroplet repulsion caused coarsening at high oil loading. We further show the existence of a trade-off between NCh axial aspect and minimum NCh concentration to stabilize 88% oil-in-water HIPPEs: only 0.038 wt % (based on emulsion mass) NCh of high axial aspect was required compared to 0.064 wt % for the shorter one. The as-produced HIPPEs were easily textured by taking advantage of their elastic behavior and resilience to compositional changes. Hence, chitin-based HIPPEs were demonstrated as emulgel inks suitable for 3D printing (millimeter definition) via direct ink writing, e.g., for edible functional foods and ultralight solid foams displaying highly interconnected pores and for potential cell culturing applications.

PDMS polymerized high internal phase emulsions (polyHIPEs) with closed-cell, aqueous-filled microcavities

Emulsion templates can produce a wide range of unique microstructures via solidification of the continuous phase. Some of these structures result in unique, fluid-filled composites reminiscent of biological tissue when the templating droplets develop into closed-cell structures. However, the state-of-the-art falls short in replicating the mechanical and functional response of biological structures due to stiff, fragile, and bio-incompatible materials while lacking systematic processing parameters. This article describes the synthesis of high internal phase, closed-cell, polydimethylsiloxane (PDMS) elastomeric foams which simultaneously achieve biocompatibility, mechanical robustness, flexibility, and selective permeability. Water-in-oil high internal phase emulsions (HIPEs) stabilized by silica nano-particles (SNPs) provide the microstructural template, resulting in a >74% by volume aqueous phase (up to 82%). To overcome the prohibitive barrier to HIPE formation when using a mechanically-superior, but highly viscous commercial PDMS kit, we produce HIPE templates via centrifugation of low internal phase emulsions (LIPEs, <30% by volume dispersed phase). This oil phase crosslinks into an aqueous-filled (water + glycerol + NaCl) elastomeric composite. The composite's microstructural dependence on viscosity ratio, mixing speed, emulsifier concentration, and centrifugal force are systematically characterized. The resulting microstructured, fluid-filled elastomer composites exhibit mechanically robust and highly flexible behavior due to the excellent properties of the PDMS continuous phase.