Interactions between interfaces dictate stimuli-responsive emulsion behaviour

Compression, expansion and relaxation of soft colloidal monolayers at the air/water interface

The phase behavior of soft and deformable microgels at fluid interfaces is typically studied with a Langmuir trough and using uniaxial compression. In situ investigations that shine light on the structural arrangements and changes during compression are scarce. Knowledge on the phase behavior is mostly generated from ex situ observations after transfer of monolayers from the fluid interface onto a substrate. Similarly, little is known about the impact of the compression geometry and potentially occurring relaxation processes after compression. Here, we use small-angle light scattering implemented in a Langmuir trough to follow the evolution of microgel monolayers in situ and in real time. We use core-shell microgels as soft model colloids where the rigid cores ensure large contrast for light diffraction. Not only the influence of compression but also the influence of expansion is studied as well as relaxation after previous compression/expansion. At sufficiently high surface pressures, densely packed monolayers of partially compressed microgels are obtained at air/water interfaces. In this regime, the monolayer response upon manipulation of the accessible interfacial area is reversible over many cycles. The uniaxial geometry of this manipulation leads to anisotropic deformation of the monolayer seen by the recorded structure factor. Upon stopping compression/expansion, anisotropy relaxes with two time constants but full isotropy is not recovered. This work underlines the potential and necessity of in situ ensemble techniques for investigating soft colloidal monolayers at fluid interfaces. With our results, we advance the understanding of how soft colloids react to uniaxial compression/expansion.

Dynamic Assembly of Microgels and Polymers at Non-Aqueous Liquid/Liquid Interfaces

Particle assembly at liquid-liquid interfaces presents a promising bottom-up strategy for creating supramolecular materials with advanced functionalities. However, the significantly lower interfacial tension observed in immiscible organic phases compared to traditional oil-water systems has hindered the effective adsorption and assembly of particles at oil-oil interfaces. In this work, a versatile and effective strategy is presented that utilizes the assembly and jamming of microgels and polymer ligands at non-aqueous liquid-liquid interfaces to create non-aqueous Pickering emulsions and reconfigurable droplet networks. The resulting microgel-polymer complexes form an asymmetric interfacial bilayer with high surface coverage, which effectively minimizes interfacial energy and improves interfacial elasticity. Through a combination of systematic interfacial measurements and molecular dynamics simulations, the underlying mechanisms governing interfacial self-assembly are elucidated. Notably, the stimuli-responsive nature of the microgel-polymer complexes allows for precise control over the interfacial assembly and disassembly by introducing competitive molecules. Furthermore, it is demonstrated that these non-aqueous Pickering emulsions serve as excellent templates for the fabrication of heterogeneous organogels and microgel-based colloidosomes through both covalent and non-covalent crosslinking strategies. This work underscores the potential of non-aqueous interfaces in advancing materials science and opens new avenues for developing multifunctional materials.

Self-assembly of defined core-shell ellipsoidal particles at liquid interfaces

HYPOTHESIS

Ellipsoidal particles confined at liquid interfaces exhibit complex self-assembly due to quadrupolar capillary interactions, favouring either tip-to-tip or side-to-side configurations. However, predicting and controlling which structure forms remains challenging. We hypothesize that introducing a polymer-based soft shell around the particles will modulate these capillary interactions, providing a means to tune the preferred self-assembly configuration based on particle geometry and shell properties.

EXPERIMENTS

We fabricate core-shell ellipsoidal particles with defined aspect ratios and shell thickness through thermo-mechanical stretching. Using interfacial self-assembly experiments, we systematically explore how aspect ratio and shell thickness affect the self-assembly configurations. Monte Carlo simulations and theoretical calculations complement the experiments by mapping the phase diagram of thermodynamically preferred structures as a function of core-shell properties.

FINDINGS

Pure ellipsoidal particles without a shell consistently form side-to-side "chain-like" assemblies, regardless of aspect ratio. In contrast, core-shell ellipsoidal particles exhibit a transition from tip-to-tip "flower-like" arrangements to side-to-side structures as aspect ratio increases. The critical aspect ratio for this transition shifts with increasing shell thickness. Our results highlight how we can engineer the self-assembly of anisotropic particles at liquid interfaces by tuning their physicochemical properties such as aspect ratio and shell thickness, allowing the deterministic realization of distinct structural configurations.

MnO2/porous spontaneously polarized ceramic with self-powered electric field and superior charge transfer to catalyze ozonation for efficient demulsification

Ozone (O3) demulsification shows great potential in emulsion wastewater treatment due to its strong oxidative properties. However, the low mass transfer efficiency and oxidation selectivity of O3 cannot be ignored. Herein, a MnO2/porous spontaneously polarized ceramic (MnO2/PSPC) composite with strong interfacial interactions and self-powered electric field was prepared for heterogeneous catalytic ozonation (HCO) to achieve efficient demulsification. Excellent remanent polarization (0.00858 μC/cm2) together with systematic electrochemical characterizations of MnO2/PSPC demonstrated its significant charge transfer capability, which is essential for the subsequent reduction of Mn4+ in the HCO demulsification process. O3- MnO2/PSPC exhibited excellent demulsification performance with 99 % demulsification rate of cetyltrimethylammonium bromide-stabilized emulsion within 30 min, outperforming O3 (130 min), O3-MnO2 (60 min), and O3-PSPC (90 min). O3-MnO2/PSPC showed effective demulsification of non-/anionic surfactant stabilized emulsions and excellent stability after 5 cycles. Density functional theory calculations together with characterizations illustrate that potential difference-induced rapid electron transfer and water flow-induced self-powered electric field were the fundamental motivation for the fast Mn3+/Mn4+ cycle and O3 adsorption/decomposition to generate reactive oxygen species (ROS). Notably, the oxidation of surfactants by ROS led to the coalescence of the oil droplets. This study provides an efficient, sustainable, and energy-efficient method to improve the O3 demulsification performance.

Elucidating the effect of chitosan microgel characteristics on the large amplitude oscillatory shear (LAOS) behavior of their stabilized high internal phase emulsions using the sequence of physical processes (SPP) approach and comparison with mayonnaise

Chitosan microgels (h-CSMs) were prepared by cross-linking hydrophobically modified chitosan with sodium phytate (SP). Emulsions stabilized by h-CSMs with different inter-phase fraction, microgel concentration and cross-linking density were studied of their microstructural and rheological properties. In particular, the large amplitude oscillatory shear (LAOS) of the high internal phase emulsions (HIPEs) stabilized by h-CSMs were systematically analyzed using the Fourier transform with Chebyshev polynomials (FTC) and sequence of physical processes (SPP) methods to explore their nonlinear rheological properties. It was found that the HIPEs showed Type III LOAS response with weak strain overshoot depending on the emulsion parameters and microgel characteristics. The FTC method enabled the extraction of nonlinearity measures at limiting conditions (γ → 0, γ → γmax) showing intracycle strain-hardening and intracycle shear thinning of the HIPEs under LOAS. By providing a detailed process of the emulsion microstructure transformation in each oscillation cycle, the SPP analysis showed that the HIPEs underwent a 3-step gradual sequence of physical processes, and magnified the influence of microgel characteristics on the rheology of the HIPEs. Comparing with commercially available traditional and low-fat mayonnaise samples, the h-CSM stabilized HIPEs exhibited higher flow compliance but stronger thixotropic recovery ability.

Sprayed Aqueous Microdroplets for Spontaneous Synthesis of Functional Microgels

The development of sustainable synthesis route to produce functional and bioactive polymer colloids has attracted much attention. Most strategies are based on the polymerization of monomers or crosslinking of prepolymers by enzyme- or cell-mediated reactions or specific catalysts in confined emulsions. Herein, a facile solution spray method was developed for spontaneous synthesis of microgels without use of confined emulsion, additional initiators/catalysts and deoxygenation, which addresses the challenges in traditional microgel synthesis. The polarization of air-water interface of the microdroplets can spontaneously split hydroxide ions in water to produce hydroxyl radicals, thereby initiating polymerization and crosslinking in air environment. This synthesis strategy is applicable to a variety of monomers and enables the fabrication of microgels with tunable chemical structures and variable sizes. Importantly, the synthesis route also allows for the preparation of enzyme- or drug-loaded microgels via the in situ encapsulation, which also display high enzymatic activity and stimuli-triggered drug release. Therefore, this work not only is of great significance to macromolecular science and microdroplet chemistry, but also may bring new insights into cellular biochemistry and even prebiotic chemistry due to the prevalence of microdroplets in the environment.

Assembly and jamming of polar additive-swollen microgels at liquid-liquid interfaces: From inverse Pickering emulsions to functional materials

HYPOTHESIS

Poly-N-isopropylacrylamide (PNIPAM)-based microgels have garnered significant interest as effective soft particulate stabilizers because of their deformability and functionality. However, the inherent hydrophilic nature of microgel restricts their potential use in stabilizing water-in-oil (W/O) Pickering emulsions. Employing diverse polar additives can improve the hydrophobicity of microgels, thus unlocking new possibilities in inverse Pickering emulsion formation and materials fabrication.

EXPERIMENTS

Different types of microgels were generated using free-radical precipitation polymerization with tailored physiochemical properties. The effect of various polar additives on the wettability, adsorption kinetics, and interfacial coverage of microgels was systematically investigated. Additive-swollen microgels were utilized to stabilize inverse W/O Pickering emulsions, which served as templates to develop functional materials with stimuli responsiveness and hierarchical structures.

FINDINGS

Additive-swollen PNIPAM-based microgels exhibited enhanced hydrophobicity and superior emulsifying capability, which spontaneously assembled and jammed at oil-water interfaces, resulting in a significant interfacial energy decrease. The additive-swollen microgels formed a tightly packed, elastic, and responsive microgel monolayer. The feasibility of the strategy was verified by preparing various inverse W/O Pickering emulsions and high internal phase Pickering emulsions (HIPPEs). More importantly, this straightforward formation strategy of microgel-stabilized inverse W/O Pickering emulsions offered a novel platform to create functional materials with customized inner structures from microscale (e.g., responsive core-shell hydrogel microspheres and colloidosomes) to macroscale (e.g., hierarchical porous materials) that can be used for potential applications, such as recyclable contaminant removal and droplet manipulation.

Fiber complex-stabilized high-internal-phase emulsion for allicin encapsulation: microstructure, stability, and thermal-responsive properties

BACKGROUND

Stimuli-responsive emulsions have garnered significant attention for their ability to enhance sensory qualities and control the release of encapsulated nutrient in emulsion-based products. However, the characteristics of synthetic materials of fabricating stimuli-responsive emulsions have been a crucial limitation in the food industry. Regulating the behavior of molecules at the interface could potentially achieve the desired stimuli-responsive behavior, but currently there is limited information available.

RESULTS

High-internal-phase emulsions (HIPEs) were fabricated for the encapsulation of allicin, stabilized by a complex of 20 g kg-1 whey protein amyloid fibrils (WPF) and 20 g kg-1 glycyrrhizin fibers (GA). The intermolecular interactions between WPF and GA in the fiber complexes were predominantly governed by hydrophobic and electrostatic forces. These complexes adsorbed and stacked around the oil droplets, forming a protective interfacial film that enhanced droplet stability. An increased proportion of WPF (WPF = 3:1 or 4:1) surrounding the oil droplets enhanced the accelerated storage stability of HIPEs, with instability indexes approaching 0.2. Additionally, HIPEs displayed a temperature-dependent modulus, with the emulsion stabilized by a WPF ratio of 3:1 showing the highest modulus at 85 °C. The encapsulation efficiency of allicin in HIPEs ranged from 88.69 ± 6.62% to 101 ± 1.37% at 25 °C, and from 31.95 ± 1.92% to 78.69 ± 4.63% after incubation at 85 °C for 8 h. The release profile of allicin from the HIPEs exhibited thermal responsiveness, depending on the interfacial content of GA.

CONCLUSION

These findings indicated that the thermal-responsive properties of HIPEs can be strategically engineered by manipulating their interfacial characteristics. © 2024 Society of Chemical Industry.

Elastomer Particle Monolayers Formed by the Compression of Poly(methyl acrylate) Microparticles at an Air/Water Interface

In the previous study (Green Chem., 2023, 25, 3418), highly stretchable and mechanically tough poly(methyl acrylate) (pMA) microparticle-based elastomers can be formed by drying a microparticle-containing aqueous dispersion. This discovery has the potential to overcome the mechanical weakness of industrially produced aqueous latex films. However, in 3D-arranged particle films, structural complexity, such as the existence of defects, makes it difficult to clearly understand the relationship between the particle film structure and its mechanical properties. In this study, 2D-ordered pMA particle monolayers at the air/water interface of a Langmuir trough are prepared. Under high compression at the air/water interface, the microparticles contact their neighboring particles, and the resulting monolayers can be successfully transferred onto a solid substrate. The compression of the monolayer films is linked to an increase in the elastic modulus of the monolayer film on the solid substrate as evident from the local Young's modulus mapping using atomic force microscopy. Thus, pMA particle films with different mechanical properties can be created using a Langmuir trough.

Bridged emulsion gels from polymer-nanoparticle enabling large-amount biomedical encapsulation and functionalization

Large-amount encapsulation and subsequent expressing are common characteristics for many biomedical applications, such as cosmetic creams and medical ointments. Emulsion gels can accomplish that, but often undergo exclusive, complex, multiple synthesis steps, showing extremely laborious and non-universal. The method here is simple via precisely interfacial engineering in homogenizing a nanoparticle aqueous dispersion and a polymer oil solution, gaining interfacial 45° three-phase-contact-angle for the nanoparticle that can bridge across oil emulsions' interfaces and ultimately form interconnected macroscopic networks. Their bridged skeletons and rheology are tunable over a vast range and deterministic on the basis of components' inputs. Furthermore, emulsion gels with high encapsulation and storage ability encapsulating active sunscreen ingredients, as a proof-of-concept, outperform commercial products. The ease (only seconds by strongly mixing two solutions) and the versatile chemical selection of our synthetic emulsion gels suggest an exciting general, scalable strategy for the next-generation cosmetic, ointment or otherwise food gel systems.

Nanogels with tailored hydrophobicity and their behavior at air/water interfaces

The interfacial behavior of micro-/nanogels is governed to a large extent by the hydrophobicity of their polymeric network. Prevailing studies to examine this influence mostly rely on external stimuli like temperature or pH to modulate the particle hydrophobicity. Here, a sudden transition between hydrophilic and hydrophobic state prevents systematic and gradual modulation of hydrophobicity. This limits detailed correlations between interfacial behavior and network hydrophobicity. To address this challenge, we introduce a nanogel platform that allows accurate tuning of hydrophobicity on a molecular level. For this, via post-functionalization of active ester-based particles, we prepare poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) nanogels as a hydrophilic benchmark and introduce gradually varied amounts of hydrophobic propyl or dodecyl moieties to increase the nanogel hydrophobicity. We study the deformation and arrangement of these particles at an air/water interface and correlate the results with quantitative measures for nanogel hydrophobicity. We observe that increasing hydrophobicity of nanogels, either by increasing the hydrophobic moiety ratio or the alkyl chain length, leads to decreased particle deformability and aggregation of an interfacially-adsorbed monolayer. Contrary to what may be intuitively assumed, these changes are not gradual, but rather occur suddenly above a threshold in hydrophobicity. Our study further shows that the effect of hydrophobicity affects the nanogel properties differently in bulk and when adsorbed at liquid interfaces. Thus, this study establishes the transition of interfacial behavior between soft gel-like particles to a solid spherical morphology triggered by the increase in hydrophobicity.

Control of emulsion crystal growth in low-temperature environments

Currently, various types of emulsions can be applied to a wide range of systems. Emulsions are thermodynamically unstable systems, and their crystallization can be affected by a variety of factors. The nucleation and growth processes of emulsion crystal networks are determined on the basis of reported theoretical and experimental methods. The issues addressed include changes in the apparent crystal morphology of samples, changes in thermal properties with respect to temperature, changes in boundary conditions, and changes in the various applications of emulsions as feedstocks or in processing and storage methods. Changes in a variety of common emulsions during constant-temperature storage and unavoidable temperature fluctuations (e.g., multiple freeze-thaw cycles) are considered. Different methods for controlling the crystalline stability of these colloidal systems are also discussed. This review outlines the crystallization mechanism of emulsions during their food processing and storage.

Myosin supramolecular self-assembly: The crucial precursor that manipulates the covalent aggregation, emulsification and rheological properties of myosin

The transformation of molecular conformation and self-assembly properties of myosin during the heating process at different ionic strengths (0.2 M, 0.4 M and 0.6 M NaCl) and its effect on rheological behavior and emulsification properties were investigated. Under incubation temperatures between 40 °C and 50 °C, myosin underwent a supramolecular self-assembly stage dominated by noncovalent forces (hydrogen bonding, ionic bonding and hydrophobic interactions). Higher ionic strength facilitated molecular rearrangement through enhanced swelling of myosin heads and head-to-head assemblies, which contributed to enhanced ordering and homogeneity of myosin covalent aggregates (above 60 °C) and manifested itself macroscopically as enhanced gel viscoelasticity and emulsion stability. In contrast, at lower ionic strength, the tail-to-tail assemblies of myosin led to the preferential formation of covalent cross-links in the tails, which resulted in the inability of molecular rearrangement and the formation of disordered aggregates and finally led to the deterioration of the gel and the destabilization of the emulsion. In conclusion, the supramolecular self-assembly behavior of myosin, as an intermediate process in myosin's sol-gel transition, is crucial for the orderliness of myosin assemblies, gel network strengthening, and emulsion stability. The obtained insight provides a reference for the precise implementation of quality improvement strategies for meat products.

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.

Demulsification of Pickering emulsions: advances in understanding mechanisms to applications

Pickering emulsions are ultra-stable dispersions of two immiscible fluids stabilized by solid or microgel particles rather than molecular surfactants. Although their ultra-stability is a signature performance indicator, often such high stability hinders their demulsification, i.e., prevents the droplet coalescence that is needed for phase separation on demand, or release of the active ingredients encapsulated within droplets and/or to recover the particles themselves, which may be catalysts, for example. This review aims to provide theoretical and experimental insights on demulsification of Pickering emulsions, in particular identifying the mechanisms of particle dislodgment from the interface in biological and non-biological applications. Even though the adhesion of particles to the interface can appear irreversible, it is possible to detach particles via (1) alteration of particle wettability, and/or (2) particle dissolution, affecting the particle radius by introducing a range of physical conditions: pH, temperature, heat, shear, or magnetic fields; or via treatment with chemical/biochemical additives, including surfactants, enzymes, salts, or bacteria. Many of these changes ultimately influence the interfacial rheology of the particle-laden interface, which is sometimes underestimated. There is increasing momentum to create responsive Pickering particles such that they offer switchable wettability (demulsification and re-emulsification) when these conditions are changed. Demulsification via wettability alteration seems like the modus operandi whilst particle dissolution remains only partially explored, largely dominated by food digestion-related studies where Pickering particles are digested using gastrointestinal enzymes. Overall, this review aims to stimulate new thinking about the control of demulsification of Pickering emulsions for release of active ingredients associated with these ultra-stable emulsions.

Amphiphilic Nanogels as Versatile Stabilizers for Pickering Emulsions

Pickering emulsions (PEs) are stabilized by particles at the water/oil interface and exhibit superior long-term stability compared to emulsions with molecular surfactants. Among colloidal stabilizers, nano/microgels facilitate emulsification and can introduce stimuli responsiveness. While increasing their hydrophobicity is connected to phase inversion from oil-in-water (O/W) to water-in-oil (W/O) emulsions, a predictive model to relate this phase inversion to the molecular structure of the nano/microgel network remains missing. Addressing this challenge, we developed a library of amphiphilic nanogels (ANGs) that enable adjusting their hydrophobicity while maintaining similar colloidal structures. This enabled us to systematically investigate the influence of network hydrophobicity on emulsion stabilization. We found that W/O emulsions are preferred with increasing ANG hydrophobicity, oil polarity, and oil/water ratio. For nonpolar oils, increasing emulsification temperature enabled the formation of W/O PEs that are metastable at room temperature. We connected this behavior to interfacial ANG adsorption kinetics and quantified ANG deformation and swelling in both phases via atomic force microscopy. Importantly, we developed a quantitative method to predict phase inversion by the difference in Flory-Huggins parameters between ANGs with water and oil (χwater - χoil). Overall, this study provides crucial structure-property relations to assist the design of nano/microgels for advanced PEs.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Stimuli-Responsive Polymers for Engineered Emulsions

Emulsions are complex. Dispersing two immiscible phases, thus expanding an interface, requires effort to achieve and the resultant dispersion is thermodynamically unstable, driving the system toward coalescence. Furthermore, physical instabilities, including creaming, arise due to presence of dispersed droplets of different densities to a continuous phase. Emulsions allow the formulation of oils, can act as vehicles to solubilize both hydrophilic and lipophilic molecules, and can be tailored to desirable rheological profiles, including "gel-like" behavior and shear thinning. The usefulness of emulsions can be further expanded by imparting stimuli-responsive or "smart" behaviors by inclusion of a stimuli-responsive emulsifier, polymer or surfactant. This enables manipulation like gelation, breaking, or aggregation, by external triggers such as pH, temperature, or salt concentration changes. This platform generates functional materials for pharmaceuticals, cosmetics, oil recovery, and colloid engineering, combining both smart behaviors and intrinsic benefit of emulsions. However, with increased functionality comes greater complexity. This review focuses on the use of stimuli-responsive polymers for the generation of smart emulsions, motivated by the great adaptability of polymers for this application and their efficacy as steric stabilizers. Stimuli-responsive emulsions are described according to the trigger used to provide the reader with an overview of progress in this field.

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.

Body temperature responsive capsules templated from Pickering emulsion for thermally triggered release of β-carotene

In recent years, functional foods with lipophilic nutraceutical ingredients are gaining more and more attention because of its potential healthy and commercial value, and developing of various bioderived food-grade particles for use in fabrication of Pickering emulsion has attracted great attentions. Herein, the bio-originated sodium caseinate-lysozyme (Cas-Lyz) complex particles were firstly designed to be used as a novel interfacial emulsifier for Pickering emulsions. Pickering emulsions of various food oils were all successfully stabilized by the Cas-Lyz particles without addition of any synthetic surfactants, while the fluorescence microscopy and SEM characterizations clearly evidenced Cas-Lyz particles were attached on the surface of emulsion droplets. Additionally, the Cas-Lyz particles stabilized emulsion can also be used to encapsulate the β-carotene-loaded soybean oil, suggestion a potential method to carry lipophilic bioactive ingredients in an aqueous formulation for food, cosmetic and medical industry. At last, we present a Pickering emulsion strategy that utilizes biocompatible, edible and body temperature-responsive lard oil as the core material in microcapsules, which can achieve hermetic sealing and physiological temperature-triggered release of model nutraceutical ingredient (β-carotene).

Charge Regulation at the Nanoscale as Evidenced from Light-Responsive Nanoemulsions

Emulsions are indispensable in everyday life, and the demand for emulsions' diversity and control of properties is therefore substantial. As emulsions possess a high internal surface area, an understanding of the oil/water (o/w) interfaces at the molecular level is fundamental but often impaired by experimental limitations to probe emulsion interfaces in situ. Here, we have used light-responsive surfactants (butyl-AAP) that can photoisomerize between E and Z isomers by visible and UV light irradiation to tune the emulsion interfaces. This causes massive changes in the interface tension at the extended o/w interfaces in macroemulsions and a drastic shift in the surfactants' critical micelle concentration, which we show can be used to control both the stability and phase separation. Strikingly different from macroemulsions are nanoemulsions (RH ∼90 nm) as these are not susceptible to E/Z photoisomerization of the surfactants in terms of changes in their droplet size or ζ-potential. However, in situ second-harmonic scattering and pulsed-field gradient nuclear magnetic resonance (NMR) experiments show dramatic and reversible changes in the surface excess of surfactants at the nanoscopic interfaces. The apparent differences in ζ-potentials and surface excess provide evidence for a fixed charge to particle size ratio and the need for counterion condensation to renormalize the particle charge to a critical charge, which is markedly different compared to the behavior of very large particles in macroemulsions. Thus, our findings may have broader implications as the electrostatic stabilization of nanoparticles requires much lower surfactant concentrations, allowing for a more sustainable use of surfactants.