Is there a difference between surfactant-stabilised and Pickering emulsions?

Kraft lignin-hydrotrope emulsifier for shelf-stable oil-in-water emulsions

Oil-in-water (O/W) shelf-stable emulsions were successfully produced using Lignoboost Kraft lignin (LKL) and its tailored fractions, derived from aqueous hydrotropic fractionation. The process accommodates various oils (soybean, olive, almond, and mineral) and LKL concentrations (0.25-1 wt%). A 50/50 oil/water ratio with 0.5 wt% LKL yielded the narrowest droplet size distribution, exhibiting D90, D50, and D10 values of 1.75 μm, 1.19 μm, and 0.49 μm, respectively. Emulsions formulated with low-molecular-weight LKL demonstrated superior emulsification indices and stability. Dynamic light scattering (DLS) and interfacial tension analyses suggest an emulsion formation mechanism based on the synergistic interaction of kraft lignin, sodium xylene sulfonate (SXS), and water. Specifically, the reduction in hydrotrope concentration promotes the formation of lignin-rich particles, which adsorb at the oil-water interface, functioning as emulsifiers.

What makes oil-in-water emulsions with pea protein stable? The role of excess protein in network formation and yield stress development

Emulsions stabilized with pea protein exhibit enhanced stability only if excess protein is present in the continuous aqueous phase. We hypothesize that the additional protein, beyond the interfacial layer surrounding the oil droplets, is important for the emergence of a yield stress as well as for the overall stability and properties. Stable emulsions with oil concentrations of 40-60% v/v were prepared and compared to layers from various separated emulsions. Confocal microscopy visualized both the oil droplets and the protein distribution. Rheological measurements were used to assess mechanical properties and network formation. Small angle X-ray scattering provided quantitative structural information. Results identified that stable emulsions have a protein layer encapsulating the oil droplets and that excess protein forms irregular aggregates in the aqueous phase. Rheological analysis indicated that the protein aggregates contribute to network formation and give rise to a yield stress which enhances stability. Only for sufficiently high protein concentrations were the emulsions stable. Other samples separated and the upper phases were always similar in emulsion composition regardless of the initial component fractions. This study highlights the dual role of pea protein in emulsions as a dispersed protein network and as interfacial material. Determination of the most favourable emulsion composition provides insight into design of stable emulsions for applications.

Pickering stabilization of double emulsions: Basic concepts, rationale, preparation, potential applications, challenges, and future perspectives

Double emulsions (DEs) offer unique compartmentalized structures but are inherently unstable, prompting significant scientific and industrial efforts to enhance their stability. One promising strategy is the use of solid particles-known as Pickering stabilization-resulting in Pickering double emulsions (PDEs), which overcome many limitations of conventional low-molecular-weight (LMW) surfactants. However, the term "Pickering" is often misused in the literature to describe any formulation containing particles, regardless of whether the interface is fully stabilized by them. This review aims to clarify the concept of Pickering stabilization, outline the rationale for its application to DEs, and examine preparation mechanisms, interfacial approaches, potential applications, and current challenges. Particles with dual wettability and high desorption energy irreversibly adsorb at interfaces, forming robust mechanical barriers that inhibit coalescence and reduce diffusion or escape of internal droplets. PDEs can be prepared via two-step emulsification, one-step processes, or advanced microfluidic methods. A variety of Pickering approaches have been developed to engineer particles capable of dual interfacial stabilization, enabling sophisticated functions such as (co-)encapsulation, controlled release, and the formation of hierarchical structures like microspheres, colloidosomes, and antibubbles. To unlock the full potential of PDEs for industrial applications, future research should prioritize eliminating surfactant use, developing safe and sustainable particles, and advancing scalable production methods without compromising emulsion stability or performance.

Lignin from aldehyde-assisted fractionation can provide light-colored Pickering emulsions through colloidal particles formed using alkaline antisolvent

Colloidal lignin particles (CLPs) are gaining attention as eco-friendly stabilizers for Pickering emulsions. Still, conventional lignin sources, like kraft lignin, are often limited by their dark color and strong odor. This study explores, for the first time, the use of a light-colored lignin derived from an aldehyde-assisted fractionation with glyoxylic acid (GA-lignin) for producing CLPs and derived Pickering emulsions. CLPs were produced by antisolvent precipitation with water (CLPs-W, pH 6) and alkaline buffer (CLPs-B, pH 8) as the antisolvents. The results revealed that the selected antisolvent significantly influenced the CLPs' properties. CLPs-W were larger, uniform in size, and hydrophobic, whereas CLPs-B were smaller, agglomerated into clusters, and exhibited greater hydrophilicity. Despite both CLPs' effectiveness in stabilizing oil-in-water emulsions, the stabilization mechanisms differed markedly; CLPs-W formed a robust membrane barrier at the oil-water interface, while CLPs-B facilitated oil droplet bridging. Overall, this work demonstrates that GA-lignin's light color nature offers advantages for Pickering emulsions design, surpassing a lignin typical limitation. This advancement highlights the versatility of GA-lignin-derived CLPs and supports the development of sustainable lignin-based products with significant commercial prospects.

Preparation of Janus Polymer Nanosheets and Corresponding Oil Displacement Properties at Ultralow Concentration

Conventional methods for preparing Janus nanosheets, including graphene oxide-based nanosheets, molybdenum disulfide-based nanosheets, and silicon dioxide-based nanosheets, as well as polymer-based nanosheets, involve complicated procedures, poor repeatability, and difficulty in imparting Janus properties, which hinder further application. Here, the present authors develop a facile modified suspension polymerization method for preparing Janus polymer nanosheets, in which deep eutectic solvents completely replace water as the continuous phase. Janus polymer nanosheets can be fabricated using common hydrophobic and hydrophilic monomers, such as styrene (St), butyl acrylate (BA), acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acryloyloxyethyl trimethylammonium chloride (DAC), and maleic anhydride (MAH). Additionally, the thickness of the Janus polymer nanosheets can be precisely controlled in a range from 40 to 100 nm by adjusting the volume ratio of higher alkanes to the hydrophobic monomer. Subsequently, the emulsification properties of polystyrene-based nanosheets were evaluated, showing better performance at concentrations ranging from 1 to 50 mg/L compared with higher concentrations. This observation aligns with the corresponding reduction in interfacial tension and changes in the moduli of the interfacial film. Moreover, the adsorption of the nanosheets onto the core alters its wettability, changing it from a water-wettable state to an oil-wettable state. Consequently, a series of core flooding tests reveal that the poly(St-co-AM), poly(St-co-MAH), and poly(St-co-AMPS) nanosheets enhance oil recovery and reduce injection pressure at ultralow concentrations (50 mg/L).

Study about the effect of cellulose nanocrystals on a polyacrylate miniemulsion

Cellulose nanocrystals (CNC) are widely used due to their biodegradability, high strength, large surface area, and functional versatility. This study investigates the interaction between CNC and acrylate emulsions, which mainly focuses on their impact on emulsion characteristics, polymerization behaviour, and storage stability. CNC was incorporated into an acrylate miniemulsion system at varying concentrations, followed by the systematic study of its effects on particle size, interfacial tension, zeta potential, yield, and viscosity. The morphology of CNC-acrylate systems was analysed using infrared spectroscopy and scanning electron microscopy (SEM). The results demonstrated that CNC effectively co-stabilized acrylate miniemulsions and enhanced their stability before polymerization. Although CNC did not directly participate in polymerization or affect yield or reaction rates, it slowed the diffusion of free radicals. However, CNC concentrations higher than 1 wt% negatively impacted post-polymerization storage stability and caused aggregation of droplets. These findings reveal the dual role of CNC as both a stabilizing and aggregating agent, offering new insights into its potential for the design of advanced polymer systems.

Size-controlled assembly of phase separated protein condensates with interfacial protein cages

Phase separation of specific proteins into liquid-like condensates is a key mechanism for forming membrane-less organelles, which organize diverse cellular processes in space and time. These protein condensates hold immense potential as biomaterials capable of containing specific sets of biomolecules with high densities and dynamic liquid properties. Despite their appeal, methods to manipulate protein condensate materials remain largely unexplored. Here, we present a one-pot assembly method to assemble coalescence-resistant protein condensates, ranging from a few μm to 100 nm in sizes, with surface-stabilizing protein cages. We discover that large protein cages (~30 nm), finely tuned to interact with condensates, efficiently localize on condensate surfaces and prevent the merging (coalescence) of condensates during phase separation. We precisely control condensate diameters by modulating condensate/cage ratios. In addition, the 3D structures of intact protein condensates with interfacial cages are visualized with cryo-electron tomography (ET). This work offers a versatile platform for designing size-controlled, surface-engineered protein condensate materials.

Mechanistic study of synergetic stabilization of Pickering emulsions by corn glutelin and starch complexes

The hydrophobicity of glutelin, zein, and carotenoids has limited the development of corn-based functional food products. This paper aims to construct emulsions stabilized by multiple corn-derived components using a simple and organic solvent-free method. The emulsions comprised oil droplets dispersed in the water, where glutelin and starch were stabilizers. Optimal stability, smaller droplet sizes, and moderate viscosity were achieved with a glutelin/starch ratio of 1:4. The results of the dynamic rheological measurements of bulk emulsions as well as interfacial properties and microstructure revealed that the stability mechanism of glutelin-starch complex was the interplay of the increased continuous phase viscosity and stronger interfacial viscoelastic films. Thus, these combined factors effectively inhibited the creaming and coalescence of oil droplets. Interfacial films also protected the carotenoids. The results of this study elucidate the stabilization mechanism among different corn-derived components and therefore guide the design of corn-based personalized nutritional systems.

From theoretical aspects to practical food Pickering emulsions: Formation, stabilization, and complexities linked to the use of colloidal food particles

We noticed that in literature, the term Pickering emulsion (PE) is used as soon as ingredients contain particles, and in this review, we ask ourselves if that is done rightfully so. The basic behavior taking place in particle-stabilized emulsions leads to the conclusion that the desorption energy of particles is generally high making particles highly suited to physically stabilize emulsions. Exceptions are particles with extreme contact angles or systems with very low interfacial tension. Particles used in food and biobased applications are soft, can deform when adsorbed, and most probably have molecules extending into both phases thus increasing desorption energy. Besides, surface-active components will be present either in the ingredients or generated by the emulsification process used, which will reduce the energy of desorption, either by reduced interfacial tension, or changes in the contact angle. In this paper, we describe the relative relevance of these aspects, and how to distinguish them in practice. Practical food emulsions may derive part of their stability from the presence of particles, but most likely have mixed interfaces, and are thus not PEs. Especially when small particles are used to stabilize (sub)micrometer droplets, emulsions may become unstable upon receiving a heat treatment. Stability can be enhanced by connecting the particles or creating network that spans the product, albeit this goes beyond classical Pickering stabilization. Through the architecture of PEs, special functionalities can be created, such as reduction of lipid oxidation, and controlled release features.

Drying of Soft Colloidal Films

Thin films made of deformable micro- and nano-units, such as biological membranes, polymer interfaces, and particle-laden liquid surfaces, exhibit a complex behavior during drying, with consequences for various applications like wound healing, coating technologies, and additive manufacturing. Studying the drying dynamics and structural changes of soft colloidal films thus holds the potential to yield valuable insights to achieve improvements for applications. In this study, interfacial monolayers of core-shell (CS) microgels with varying degrees of softness are employed as model systems and to investigate their drying behavior on differently modified solid substrates (hydrophobic vs hydrophilic). By leveraging video microscopy, particle tracking, and thin film interference, this study shed light on the interplay between microgel adhesion to solid surfaces and the immersion capillary forces that arise in the thin liquid film. It is discovered that a dried replica of the interfacial microstructure can be more accurately achieved on a hydrophobic substrate relative to a hydrophilic one, particularly when employing softer colloids as opposed to harder counterparts. These observations are qualitatively supported by experiments with a thin film pressure balance which allows mimicking and controlling the drying process and by computer simulations with coarse-grained models.

Distinct Contributions of Particle Adsorption and Interfacial Compression to the Surface Pressure of a Fluid Interface

Particle-laden interfaces stabilize emulsions and foams and can serve as a platform for multiscale materials. Favorable wetting of a particle to a fluid interface reduces the apparent interfacial tension through area replacement with a linear relationship between the apparent surface pressure and the particle area fraction. The area replacement model is widely employed, often up to particle area fraction reaching the maximum hexagonal packing. However, data directly supporting the area replacement model are limited, and the description ignores contributions from particle-particle interactions and does not describe the surface pressure during the compression of a particle-laden interface. This work reports on the direct validation of the area replacement model through the direct measurement of the adsorption energy, surface pressure, and area fraction of adsorbed particles. Experiments combining tensiometry and confocal imaging during the adsorption of colloidal particles to the oil-water interface confirm the area replacement model within the observed range of area fraction, but only when the drop area is kept constant. Results highlight the importance of keeping the droplet area constant during particle adsorption to extract the adsorbed amount from tensiometry experiments. As particles adsorb to the interface, the droplet area tends to change and compresses or expands the interface. This change in area is associated with an increase in area fraction at nearly constant surface pressure, which deviates from the area replacement model. In contrast to particle adsorption, slow compression of the fluid interface leads to a negligible change in surface pressure up to an area fraction of η ∼ 0.26 for the materials systems investigated. Increase in surface pressure during compression is due to particle-particle interactions, while compression at higher strain rates introduces additional contributions from interfacial rheology.

Hybrid particle-phase field model and renormalized surface tension in dilute suspensions of nanoparticles

We present a two-phase field model and a hybrid particle-phase field model to simulate dilute colloidal sedimentation and flotation near a liquid-gas interface (or fluid-fluid interface in general). Both models are coupled to the incompressible Stokes equation, which is solved numerically using a combination of sine and regular Fourier transforms to account for the no-slip boundary conditions at the boundaries. The continuum two-phase field model allows us to analytically solve the equilibrium interfacial profile using a perturbative approach, demonstrating excellent agreement with numerical simulations. Notably, we show that strong coupling to particle dynamics can significantly alter the liquid-gas interface, thereby modifying the liquid-gas interfacial tension. In particular, we show that the renormalized surface tension is monotonically decreasing with increasing colloidal particle concentration and decreasing buoyant mass.

Pickering Emulsions Biocatalysis: Recent Developments and Emerging Trends

Biocatalysis within biphasic systems is gaining significant attention in the field of synthetic chemistry, primarily for its ability to solve the problem of incompatible solubilities between biocatalysts and organic compounds. By forming an emulsion from these two-phase systems, a larger surface area is created, which greatly improves the mass transfer of substrates to the biocatalysts. Among the various types of emulsions, Pickering emulsions stand out due to their excellent stability, compatibility with biological substances, and the ease with which they can be formed and separated. This makes them ideal for reusing both the emulsifiers and the biocatalysts. This review explores the latest developments in biocatalysis using Pickering emulsions. It covers the structural features, methods of creation, innovations in flow biocatalysis, and the role of interfaces in these processes. Additionally, the challenges and future directions are discussed in combining chemical and biological catalysts within Pickering emulsion frameworks to advance synthetic methodologies.

Spearheading a new era in complex colloid synthesis with TPM and other silanes

Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful tool to create novel anisotropic colloids with increasing complexity. In this Review, we highlight the advances in complex colloid synthesis facilitated by the use of TPM and show how this has driven remarkable new applications. The focus is on TPM as the current state-of-the-art in colloid science, but we also discuss other silanes and their potential to make an impact. We outline the remarkable properties of TPM colloids and their synthesis strategies, and discuss areas of soft matter science that have benefited from TPM and other SCAs.

Softness matters: effects of compression on the behavior of adsorbed microgels at interfaces

Deformable colloids and macromolecules adsorb at interfaces as they decrease the interfacial energy between the two media. The deformability, or softness, of these particles plays a pivotal role in the properties of the interface. In this study, we employ a comprehensive in situ approach, combining neutron reflectometry with molecular dynamics simulations, to thoroughly examine the profound influence of softness on the structure of microgel Langmuir monolayers under compression. Lateral compression of both hard and soft microgel particle monolayers induces substantial structural alterations, leading to an amplified protrusion of the microgels into the aqueous phase. However, a critical distinction emerges: hard microgels are pushed away from the interface, in stark contrast to the soft ones, which remain firmly anchored to it. Concurrently, on the air-exposed side of the monolayer, lateral compression induces a flattening of the surface of the hard monolayer. This phenomenon is not observed for the soft particles as the monolayer is already extremely flat even in the absence of compression. These findings significantly advance our understanding of the key role of softness on both the equilibrium phase behavior of the monolayer and its effect when soft colloids are used as stabilizers of responsive interfaces and emulsions.

Adding nanoparticles to improve emulsion efficiency and enhance microbial degradation in Pickering emulsions

Interfacial microbial degradation of alkane in Pickering emulsions stabilized by hydrophobic bacterial cells is a new mechanism for microbial degradation of water-insoluble chemicals, where both water-insoluble chemicals in the oil phase and water-soluble nutrients (such as nitrogen and phosphorus) in the water phase are bio-accessible to living microorganisms anchoring onto the oil-water interfaces. In the present work, super-hydrophobic Mycobacterium sp. (contact angle 168.6°) degradation of tetradecane was set up as a model. Addition of fumed SiO2 particles (Aerosil® R974) as a new strategy was developed to enhance tetradecane degradation where the biodegradation rate (based on the accumulated biomass) increased by approximately 80%. The enhanced effect of SiO2 particles on the tetradecane degradation attributed to the synergistic effect of SiO2 particles on the emulsion efficiency of Pickering emulsions stabilized by bacterial cells and then on the enhancement of interfacial microbial degradation in Pickering emulsions. KEY POINTS: • Interfacial microbial degradation in bacterial cells stabilized Pickering emulsions. • Adding fumed SiO2 particles to enhance microbial degradation of tetradecane. • Correlation relationship between emulsion efficiency and interfacial microbial degradation.