Thermodynamically stable pickering emulsions

Designer colloids in structured food for the future

Recent advances in the understanding of colloids has enabled the design of food products that are healthier and tastier, in line with consumer expectations. Specifically, emulsion design and hydrocolloid structuring can be used to address the issue of fat reduction in foods by allowing the production of reduced fat products that provide similar sensory attributes. Additionally, various techniques for encapsulating molecules, such as flavour, nutraceuticals or drugs, are now being developed. The application of such techniques in food products can improve micronutrient bioavailability by means of targeted and controlled delivery, increasing the nutritional value. Colloidal structures can also be designed to enhance consumer experience, mimic fat or control satiety. Such novel improvements, as well as their potential translation into commercial food products, are highlighted in this paper, which focuses primarily on the areas of emulsion technologies and hydrocolloids.

Preparation of Composite Materials by Using Graphene Oxide as a Surfactant in Ab Initio Emulsion Polymerization Systems

In this letter, we report a simple and unexpected method of producing polymer-graphene oxide (GO) composite materials via ab initio emulsion polymerization in water. On the basis of the recent reports concerning the surfactant-like behavior of GO for stabilizing oil-in-water emulsions, we prepared exfoliated GO sheets with lateral dimension approximately 200 nm for use as surfactant in the emulsion polymerization of styrene. We observed an expected "classic" surfactant behavior to produce stable nanoparticles at extremely low GO loadings (<0.1% w/w); however, at higher concentrations a highly aggregated, fibrous morphology was formed. This morphology is predominantly due to the electrolyte concentration (ionic strength) of the aqueous phase resulting in heterocoagulation of growing oligomers with dispersed GO sheets, which offers a convenient route toward preparing hybrid materials.

Double hydrophilic Janus cylinders at an air-water interface

Colloidal particles spontaneously attach to the interface between two immiscible fluids to minimize the interfacial area between the two phases. The shape and wettability of particles have a strong influence on their configuration and interactions at fluid-fluid interfaces. In this study, we investigate the behavior of asymmetrically hydrophilic Janus cylinders (or double hydrophilic Janus cylinders with two different hydrophilic regions) trapped at an air-water interface. We find that these double hydrophilic Janus cylinders with aspect ratios of 0.9, 1.2, and 2.4 adopt both end-on and tilted configurations with respect to the interface. Our numerical calculations show that the coexistence of these configurations is a result of multiple energy minima present in the attachment energy profile that can be represented as a complex energy landscape. Double hydrophilic Janus cylinders with tilted orientations induce hexapolar interface deformation, which accounts for the pair interactions between the particles as well as the nondeterministic assembly behaviors of these particles at the interface.

Simulation of charged systems in heterogeneous dielectric media via a true energy functional

For charged systems in heterogeneous dielectric media, a key obstacle for molecular dynamics (MD) simulations is the need to solve the Poisson equation in the media. This obstacle can be bypassed using MD methods that treat the local polarization charge density as a dynamic variable, but such approaches require access to a true free energy functional, one that evaluates to the equilibrium electrostatic energy at its minimum. In this Letter, we derive the needed functional. As an application, we develop a Car-Parrinello MD method for the simulation of free charges present near a spherical emulsion droplet separating two immiscible liquids with different dielectric constants. Our results show the presence of nonmonotonic ionic profiles in the dielectric with a lower dielectric constant.

Fabrication of anisotropic porous silica monoliths by means of magnetically controlled phase separation in sol-gel processes

Sol-gel accompanied by phase separation is an established method for the preparation of porous silica monoliths with well-defined macroporosity, which find numerous applications. In this work, we demonstrate how the addition of (superpara)magnetic nanocolloids as templates to a system undergoing a sol-gel transition with phase separation leads to the creation of monoliths with a strongly anisotropic structure. It is known that magnetic nanocolloids respond to the application of an external magnetic field by self-assembling into columnar structures. The application of a magnetic field during the chemically driven spinodal decomposition induced by the sol-gel transition allows one to break the symmetry of the system and promote the growth of elongated needle-like silica domains incorporating the magnetic nanocolloids, aligned in the direction of the field. It is found that this microstructure imparts a strong mechanical anisotropy to the materials, with a ratio between the Young's modulus values measured in a direction parallel and perpendicular to the one of the field as high as 150, and an overall smaller average macropores size as compared to isotropic monoliths. The microstructure and properties of the porous monoliths can be controlled by changing both the system composition and the strength of the applied magnetic field. Our monoliths represent the first example of materials prepared by magnetically controlling a phase transition occurring via spinodal decomposition.

Stabilization of miniemulsion droplets by cerium oxide nanoparticles: a step toward the elaboration of armored composite latexes

Stable methyl methacrylate (MMA) miniemulsions were successfully prepared using for the first time cerium oxide (CeO(2)) nanoparticles as solid stabilizers in the absence of any molecular surfactant. The interaction between MMA droplets and CeO(2) nanoparticles was induced by the use of methacrylic acid (MAA) as a comonomer. Both MAA and CeO(2) contents played a key role on the diameter and the stability of the droplets formed during the emulsification step. Cryo-transmission electron microscopy (TEM) images of the suspensions formed with 35 wt % of CeO(2) showed the presence of polydisperse 50-150 nm spherical droplets. More surprisingly, some nonspherical (likely discoidal) objects that could be the result of the sonication step were also observed. The subsequent polymerization of these Pickering miniemulsion droplets led to the formation of composite PMMA latex particles armored with CeO(2). In all cases, the conversion was limited to ca. 85%, concomitant with a loss of stability of the latex for CeO(2) contents lower than 35 wt %. This stability issues were likely related to the screening of the cationic charges present on CeO(2) nanoparticles upon polymerization. TEM images showed mostly spherical particles with a diameter ranging from 100 to 400 nm and homogeneously covered with CeO(2). Besides, for particles typically larger than 200 nm, a buckled morphology was observed supporting the presence of residual monomer at the end of the polymerization and consistent with the limited conversion. The versatility of these systems was further demonstrated using 35 wt % of CeO(2) and replacing MMA by n-butyl acrylate (BA) either alone or in combination with MMA. Stable monomer emulsions were always obtained, with the droplet size increasing with the hydrophobicity of the oil phase, pointing out the key influence of the wettability of the solid stabilizer. The polymerization of Pickering miniemulsion stabilized by CeO(2) nanoparticles proved to be an efficient strategy to form armored composite latex particles which may find applications in coating technology.

Clustering and gelation of hard spheres induced by the Pickering effect

A mixture of hard-sphere particles and model emulsion droplets is studied with a Brownian dynamics simulation. We find that the addition of nonwetting emulsion droplets to a suspension of pure hard spheres can lead to both gas-liquid and fluid-solid phase separations. Furthermore, we find a stable fluid of hard-sphere clusters. The stability is due to the saturation of the attraction that occurs when the surface of the droplets is completely covered with colloidal particles. At larger emulsion droplet densities a percolation transition is observed. The resulting networks of colloidal particles show dynamical and mechanical properties typical of a colloidal gel. The results of the model are in good qualitative agreement with recent experimental findings [E. Koos and N. Willenbacher, Science 331, 897 (2011)] in a mixture of colloidal particles and two immiscible fluids.

Microcapsules composed of cross-linked organoclay

Polyelectrolyte-modified montmorillonite particles were used to stabilize oil-in-water Pickering emulsions, which were then bound together by an oil-soluble cross-linker to obtain microcapsules. It was determined how the morphology and rigidity of the microcapsules changed as polyelectrolyte and cross-linker concentrations were varied. Well-defined microcapsules could be formed by using a moderate concentration of polyelectrolyte, and the higher the cross-linker concentration, the more rigid the microcapsules. Dried microcapsules were observed using SEM, and it was shown that the clay platelets lie flat next to each other on the microcapsule surface, forming an armor-like structure.

Creating opal-templated continuous conducting polymer films with ultralow percolation thresholds using thermally stable nanoparticles

We propose a novel and robust strategy for creating continuous conducting polymer films with ultralow percolation thresholds using polymer-coated gold nanoparticles (Au NPs) as surfactant. Continuous poly(triphenylamine) (PTPA) films of high internal phase polymeric emulsions were fabricated using an assembly of cross-linked polystyrene (PS) colloidal particles as template. Polymer-coated Au NPs were designed to be thermally stable even above 200 °C and neutral to both the PS and PTPA phases. Therefore, the Au NPs localize at the PS/PTPA interface and function as surfactant to efficiently produce a continuous conducting PTPA polymer film with very low percolation thresholds. The volume fraction threshold for percolation of the PTPA phase with insulating PS colloids (as measured by electron microscopy and conductivity measurements) was found to be 0.20. In contrast, with the addition of an extremely low volume fraction (φ(p) = 0.35 vol %) of surfactant Au NPs, the volume fraction threshold for percolation of the PTPA phase was dramatically reduced to 0.05. The SEM and TEM measurements clearly demonstrated the formation of a continuous PTPA phase within the polyhedral phase of PS colloids. To elucidate the influence of the nanoparticle surfactant on the blend films, the morphology and conductivity of the blends at different PS colloid/PTPA volume ratios were carefully characterized as a function of the Au NP concentration. Our approach provides a methodology for a variety of applications that require a continuous phase for the transport of molecular species, ions, or electrons at low concentrations and a second phase for mechanical support or the conduction of a separate species.

Measuring single-nanoparticle wetting properties by freeze-fracture shadow-casting cryo-scanning electron microscopy

Nanoparticles at fluid interfaces are central to a rapidly increasing range of cutting-edge applications, including drug delivery, uptake through biological membranes, emulsion stabilization and the fabrication of nanocomposites. Understanding nanoscale wetting is a challenging issue, still unresolved for individual nanoparticles, and is essential in designing nanoparticle-building blocks with controlled surface properties. The core information about the structural and thermodynamic properties of particles at fluid interfaces is enclosed in the three-phase contact angle θ. Here we present a novel in situ method, on the basis of freeze-fracture shadow-casting cryo-scanning electron microscopy, that allows the measurement of contact angles of individual nanoparticles with 10 nm diameter, and thus greatly surpasses the current state of the art. We study hydrophilic and hydrophobic, organic and inorganic nanoparticles, demonstrating general applicability to systems of fundamental and applied nanotechnological interest. Significant heterogeneity in the wetting of nanoparticles is observed.

Highly compressed assembly of deformable nanogels into nanoscale suprastructures and their application in nanomedicine

Assembly of nanoparticles as interfacial stabilizers at oil-in-water (O/W) interfaces into microscopic suprastructures for stabilizing Pickering emulsions is an intriguing focus in the fields of chemical industry and material sciences. However, it is still a major challenge to assemble nanoscale suprastructures using nanoparticles as building blocks at O/W interfaces for fabricating nanoscale emulsion droplets with applicable potential in nanomedicine. Here, we show that it is possible to fabricate the nanodroplets by assembling highly deformable nanogels into the nanoscale suprastructures at spatially confined O/W interfaces. The compressed assembly of the nanogels induced the formation of the nanoscale suprastructures upon energy input at the nanoscale O/W interface. The hydrogen bonding interaction between the nanogels at the O/W interface are possibly responsible for the stabilization of the nanoscale suprastructures. The nanoscale suprastructures are further employed to stabilize the paclitaxel-loaded nanodroplets, which are found to provide sustained release of the drug, enhanced in vitro cytotoxicity, and prolonged in vivo blood circulation. Furthermore, the tissue distribution and antitumor efficacy studies show that the nanodroplets could induce a higher drug accumulation at the tumor site and enhance tumor growth inhibition when compared with the commercial product. This approach provides a novel universal strategy to fabricate nanoscale suprastructures for stabilizing nanodroplets with built-in payloads using deformable nanoparticles and displays a promising potential in nanomedicine.

The effect of interfacial microstructure on the lipid oxidation stability of oil-in-water emulsions

A novel approach to reduce lipid oxidation in oil-in-water emulsions has been taken and involves the manipulation of the emulsions' interfacial microstructure. Oil-in-water emulsions stabilised by sodium caseinate (CAS), Tween 20 and silica particles were prepared and their lipid oxidation stability was assessed over a week. Lipid oxidation was monitored by measuring the concentration of primary lipid oxidation product, using the peroxide value method and secondary lipid oxidation products formation were evaluated with the p-anisidine technique. Oil-phase volume fraction and emulsifier type both play key roles in influencing the rate of lipid oxidation. Decreasing the oil fraction from 30% to 5% was found to promote lipid oxidation as a result of an increase in the amount of pro-oxidant iron per gram of oil. It was further shown that, CAS in the continuous phase reduces lipid oxidation at pH 7 due to its metal chelating ability. In addition, the results show that, emulsions stabilised with silica particles (at pH 2) inhibit lipid oxidation to a greater extent than emulsions stabilised with surfactants alone. The present study demonstrates that emulsions' physical properties such as oil-phase volume fraction, droplet size and droplet interfacial microstructure are all formulation parameters that can be used to significantly reduce the rate of lipid oxidation.

Kinetic stability of water-dispersed oil droplets encapsulated in a polyelectrolyte multilayer shell

The original theoretical model of polyelectrolyte adsorption onto water-dispersed colloid particles is extended to the system of polydisperse droplets of sunflower oil. Polycation (poly(allylamine hydrochloride)) and polyanion (poly(sodium 4-styrenesulfonate)) are taken in the theoretically projected concentrations to perform Layer-by-Layer assembly of a multilayer shell on the surface of oil droplets preliminary stabilized with a protein emulsifier (bovine serum albumin). The velocity of gravitational separation in suspension of encapsulated oil droplets is theoretically predicted and experimentally measured depending on the coating shell's thickness, aiming to clarify the mechanism to control over the separation process. Combining the theory and experimental data, the mass density of a polyelectrolyte multilayer shell assembled in a Layer-by-Layer fashion is obtained. Polyelectrolyte multilayer coated oil droplets are characterized by means of ζ-potential, and particle size measurements, and visualized by scanning electron microscopy.

Interfacial properties of emulsions stabilized with surfactant and nonsurfactant coated boehmite nanoparticles

The properties of emulsions stabilized with surface-modified boehmite particles of 26 and 8 nm in diameter have been investigated. The surface-modified particles were prepared by mixing aqueous dispersions of cationic boehmite particles with aqueous solutions of the surfactant p-dodecylbenzenesulfonic acid (DBSA) or the nonsurfactant p-toluenesulfonic acid (TSA). For the 26 nm particles, interfacial tension measurements indicate that p-dodecylbenzenesulfonic acid partitions between the particle surface and the oil-water interface, while p-toluenesulfonic acid remains on the particle surface. The partitioning of p-dodecylbenzenesulfonic acid supports the formation of emulsions, although in the absence of the particles the same surfactant concentration is not sufficient for emulsion stabilization. Due to the fast exchange kinetics, p-dodecylbenzenesulfonic acid is gradually replaced by particles. At equilibrium, the interfacial tension in the presence of the surface-modified particles is between the values for the pure particles and the pure surfactant solutions. However, the interfacial tension is independent of the surfactant concentration used in the preparation of the particles. Reducing the particle size to 8 nm leads to increased emulsion stability, and thus, the minimum particle concentration required to prepare stable emulsions was reduced to 0.1 g/L. However, above approximately 3.5 mmol/L of the sulfonic acids, the small particles dissolve slowly, and the emulsion stability is lost. This mechanism can be used to trigger the collapse of the emulsions.

Superparamagnetic nanoclusters coated with oleic acid bilayers for stabilization of emulsions of water and oil at low concentration

Emulsions of water and dodecane with drop sizes down to 1 microm were stabilized with 30-100 nm interfacially active nanoclusters of sub-15 nm iron oxide primary particles at an extremely low loading of 0.14 wt.%. The nanoclusters, coated with a bilayer of oleic acid, formed stable dispersions in water at pH 7-10. The phase behavior and droplet morphologies of the emulsions of water and dodecane were tuned with pH. The oil/water emulsions at pH 9-10 were converted to middle phase emulsions at pH 6-7 and water/oil emulsions as the pH was further lowered. The magnetization per gram of Fe is similar for the nanoclusters and the primary particles, indicating the spacing between the particles is sufficient to avoid magnetic coupling. The larger volume of nanoclusters relative to the individual primary particles is beneficial for magnetomotive sensing applications including imaging of oil reservoirs, as it increases the force on the particles for a given magnetic field.

In vitro digestibility and emulsification properties of phytoglycogen octenyl succinate

This paper reports our recent studies on the in vitro digestibility and emulsification properties of an amphiphilic carbohydrate nanoparticle, phytoglycogen octenyl succinate (PG-OS). Phytoglycogen (PG), a glycogen-like alpha-d-glucan isolated from sugary-1 sweet corn endosperms, was subjected to octenyl succinate substitution to prepare PG-OS. Waxy corn starch octenyl succinate (WCS-OS) was also prepared as the reference. The degree of substitution (DS), molecular weight, particle size, dispersed molecular density, and zeta-potential of PG-OS and WCS-OS were determined. Transmission electron microscope (TEM) was used to image PG and its derivatives. In vitro digestibility and emulsification properties of PG-OS and WCS-OS were compared. The results showed that the dispersed molecular density of PG and PG-OS was much greater than that of WCS and WCS-OS. Zeta-potential of PG-OS decreased as the pH of dispersion increased. In general, the digestibility of PG and PG-OS was lower than that of WCS and WCS-OS at equivalent DS, suggesting the effect of glucan structure on glucan digestibility. At equivalent DS, PG-OS showed similar or even greater capability than WCS-OS to physically stabilize fish oil emulsions. This work revealed the potential of amphiphilic carbohydrate nanoparticles in the applications of emulsions.

Mixed-monolayer-protected gold nanoparticles for emulsion stabilization

Nanometer-sized gold nanoparticles have been prepared and surface-modified in order to stabilize alkane-in-water emulsions. A mixed hexane-undecanol ligand layer at the surface of the nanoparticles allowed us to tune their wettability and thus the adsorption at the oil-water interface. Oil droplets of the stable emulsions have been evidenced by confocal fluorescence microscopy, freeze-fracture transmission electron microscopy, and dynamic light scattering. Prepared emulsions were stable during performed cooling-heating cycles, in which the temperature stability of the emulsions has been studied by means of dynamic light scattering. The interfacial structure of the oil droplets was investigated by small-angle X-ray scattering. The obtained area per nanoparticle at the oil droplet interface was 30 nm(2). The investigation of the nanoparticle adsorption at the curved interface of the emulsion droplets is in agreement with our previous study at a planar oil-water interface, in which the nanoparticles started to interact with each other at about the same area per particle.

Triangular tessellation scheme for the adsorption free energy at the liquid-liquid interface: Towards nonconvex patterned colloids

We present a numerical technique, namely, triangular tessellation, to calculate the free energy associated with the adsorption of a colloidal particle at a flat interface. The theory and numerical scheme presented here are sufficiently general to handle nonconvex patchy colloids with arbitrary surface patterns characterized by a wetting angle, e.g., amphiphilicity. We ignore interfacial deformation due to capillary, electrostatic, or gravitational forces, but the method can be extended to take such effects into account. It is verified that the numerical method presented is accurate and sufficiently stable to be applied to more general situations than presented in this paper. The merits of the tessellation method prove to outweigh those of traditionally used semianalytic approaches, especially when it comes to generality and applicability.

Rheology of emulsions

The review is devoted to the historical and modern understanding of rheological properties of emulsions in a broad range of concentration. In the limiting case of dilute emulsions, the discussion is based on the analogy and differences in properties of suspensions and emulsions. For concentrated emulsions, the main peculiarities of their rheological behaviour are considered. Different approaches to understand the concentration dependencies of viscosity are presented and compared. The effects of non-Newtonian flow curves and the apparent transition to yielding with increasing concentration of the dispersed phase are discussed. The problem of droplet deformation in shear fields is touched. The highly concentrated emulsions (beyond the limit of closest packing of spherical particles) are treated as visco-plastic media, and the principle features of their rheology (elasticity, yielding, concentration and droplet size dependencies) are considered. A special attention is paid to the problem of shear stability of drops of an internal phase starting from the theory of the single drop behaviour, including approaches for the estimation of drops' stability in concentrated emulsions. Polymer blends are also treated as emulsions, though taking into account their peculiarities due to the coexistence of two interpenetrated phases. Different theoretical models of deformation of polymer drops were discussed bearing in mind the central goal of predictions of the visco-elastic properties of emulsions as functions of the properties of individual components and the interfacial layer. The role of surfactants is discussed from the point of view of stability of emulsions in time and their special influence on the rheology of emulsions.

Scenario for equilibrium solid-stabilized emulsions

We show theoretically that under certain conditions colloidal particles can give rise to spontaneous emulsification of oil/water systems. The capillary penalty to create a large interface is compensated by entropic contributions connected to ionic dissociation on the colloid surfaces. The colloids themselves are absorbed on the oil/water interface. The conditions for spontaneous emulsification are: (1) oil-water interfacial tension is low (a few mN/m or lower); (2) interfacial tension between colloids and oil is smaller than between colloids and water (in the absence of charge effects); (3) density of chargeable groups on the colloids is large (order 1 nm-2); (4) Debye length is comparable to colloid size.

Magnetic capsules and pickering emulsions stabilized by core-shell particles

Polystyrene-covered magnetic iron oxide particles (referred to as FeO(x)@PS) with different shell thickness are used as a novel type of nanostructured stabilizers in the preparation of magnetoresponsive Pickering emulsions. Microscopical methods (optical microscope and flow particle image analyzer) and magnetic characterization (vibrating sample magnetometry) were applied to investigate how the properties of the produced magnetic capsules are influenced by the solid concentration and the molar mass of the polymeric shell. We demonstrate the potential of these emulsions for possible applications, such as transporting and release systems.

Curvature dependence of the electrolytic liquid-liquid interfacial tension

The interfacial tension of a liquid droplet surrounded by another liquid in the presence of microscopic ions is studied as a function of the droplet radius. An analytical expression for the interfacial tension is obtained within a linear Poisson-Boltzmann theory and compared with numerical results from nonlinear Poisson-Boltzmann theory. The excess liquid-liquid interfacial tension with respect to the pure salt-free liquid-liquid interfacial tension is found to decompose into a curvature-independent part due to short-ranged interfacial effects and a curvature-dependent electrostatic contribution. Several curvature-dependent regimes of different scalings of the electrostatic excess interfacial tension are identified. Symmetry relations of the interfacial tension upon swapping droplet and bulk liquid are found to hold in the low-curvature limit, which, e.g., lead to a sign change of the excess Tolman length. For some systems a low-curvature expansion up to the second order turns out to be applicable if and only if the droplet size exceeds the Debye screening length in the droplet, independent of the Debye length in the bulk.

Internally structured pickering emulsions stabilized by clay mineral particles

The present study aims to describe emulsion particles containing a dispersed phase composed of nanostructured lipid mesophases and stabilized by montmorillonite and/or Laponite clay platelets. The size distributions of these emulsion particles were found independent of the clay mineral content and of the initial internal composition that determines the internal structure. The stabilization of the droplets by a shell of smectite layers was found possible even by montmorillonite which has a length of the same order or more than the droplets to stabilize. The clay platelets give a local flatness to the droplets that may influence the internal structure. In this paper, we describe the conditions to obtain such soft particles of about 220 nm, and we show by direct visualization the internal mesophase complexity and the shape of the particles. In particular, TEM analysis showed elongated particles with bent-back channels at their center but a different morphology at the periphery due to flat border conditions imposed by the presence of the clay minerals.