Phase Inversion of Ellipsoid-Stabilized Emulsions

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.

Molecular Insights into the Modified Silica Nanoparticle-Induced Emulsification of Crude Oil-in-Water Emulsions: Experimental and Simulation Study

Numerous experimental results have demonstrated that Pickering emulsions stabilized by modified silica nanoparticles exhibit excellent performance in enhanced oil recovery. This study investigates the microcosmic mechanism of emulsion stability formed by three typical silica nanoparticles (hydrophilic SiO2 (HSO), hydrophobic SiO2 (LSO), and Janus SiO2 (JSO)) by using experiments and molecular dynamics simulations. Based on the results of the interfacial tension and emulsification index (EI) measurements, JSO exhibits the greatest interfacial activity, whereas LSO possesses a similar ability to stabilize emulsions as JSO. Then, the number density distribution and solvent-accessible surface area (SASA) are calculated to explore in detail the interfacial distribution of nanoparticles affected by oil components in the aqueous phase. The mechanism of nanoparticle stabilization emulsion is further investigated via the radial distribution function (RDF), interaction energy, and independent gradient mode based on Hirshfeld partition (IGMH), which is verified via steered molecular dynamics (SMD) simulations. It is found that the more intensive hydrophobic effect among nanoparticles in comparison to the weaker interaction between asphaltenes and nanoparticles should be responsible for the special "nanoparticles channel" formed by LSO, which is beneficial to emulsion stability. The interfacial membrane barrier of JSO, caused by van der Waals interactions and weak hydrogen bonds with asphaltenes, significantly improves the stability of the emulsion. This work is of great significance to the in-depth understanding of the mechanism by which modified nanoparticles stabilize emulsions.

Pickering Emulsions: Role of Particle Wettability and Adhesive Force on Droplet Bridging

This study demonstrates the engineering of bridged Pickering emulsion (PE) gels by tuning the particle position at the interface and adhesive forces. This is achieved through controlled surface modification of hematite particles using oleic acid in a water-decane system. Microscopy observations revealed that the droplets are stabilized through a bridging mechanism, where oil droplets are connected by a shared monolayer of particles, with an intervening water layer between them. The experimental observations reveal that the concentration of oleic acid affects both the position of particles with respect to the interface (wettability) and adhesive forces, leading to the formation of emulsions with bridged droplets at specific oleic acid concentration ranges. To investigate this, the particle position at the interface and the strength of adhesive force are measured as a function of oleic acid concentration by direct visualization and droplet stretching technique, respectively. These studies confirm that at low oleic acid concentrations, the particle position favors the bridging, as particles are preferentially wettable by the continuous phase (water) but adhesive forces are not strong. Thus, this condition promotes the formation of oil-in-water emulsions without bridging. While, at higher oleic acid concentrations, the position of particles with respect to the interface hinders bridging, despite sufficient adhesive forces, because the particle surface becomes preferentially wettable by dispersed phase (oil), thereby supporting the inversion of emulsions. Therefore, a precise amount of oleic acid is necessary to achieve stable bridging with both factors contributing to the bridge formation. Further, the versatility of the process is illustrated by using different types of oil and particle surface modifiers. In all of the cases, stable emulsions are obtained by droplet bridging at a precise concentration of the modifier. The effect of the particle concentration and water-to-decane volume ratio on the stability of these emulsions is also studied. These emulsions show remarkable stability under undisturbed conditions due to gel-like nature despite the droplets being partially covered with particles. Moreover, after such emulsions are destabilized by external stimuli, emulsions with similar features can be readily and reversibly formed.

Pickering Emulsions Stabilized with Millimeter-Sized Polymer Plates

Hexagonal polymer plates of (sub)millimeter size that were uniform in shape and size were used as a stabilizer for emulsions, and the correlations of plate size, oil polarity, and plate dispersing media before emulsification with the formability, type, and droplet shape of emulsions were studied. The formability of the emulsions was improved by decreasing the plate size. The lower the oil polarity was, the more preferably O/W-type emulsions were formed, and as the oil polarity increased, the formability of W/O-type emulsions increased, whereas too high of an oil polarity resulted in no emulsion formation or macrophase separation of the oil dispersion of the plates and water. Furthermore, when the plate dispersing medium before emulsification was oil, the plates tended to be lipophilic compared with those dispersed in water before emulsification. In addition, we confirmed that there was a correlation between the droplet/stabilizer size ratio and droplet shape: when the droplet/plate size ratios are >2, droplets with near-spherical shapes are formed; when the size ratios are between 1 and 2, droplets with polyhedral shapes (e.g., hexahedral and tetrahedral shapes) are formed; and when the size ratios are <1, sandwich-shaped droplets are formed. Droplets with similar structures tended to form if the droplet/plate size ratios were close, even though the sizes of the plate and droplet were different.

Development of Hematite Nano Ellipsoids/Pectin Composite Films for Green Packaging Applications

Synthetic packaging materials are known to cause serious environmental and human health problems. Among the eco-friendly biopolymers from nonfood sources that are suitable for packaging applications, pectin is a promising candidate. However, native pectin films (NPF) exhibit poor mechanical strength, high hydrophilicity, and poor gas diffusion barrier properties. These shortcomings offset the advantages of pectin as a potential packaging material. To address these limitations, in this study, hematite nano ellipsoids (HNEs) were incorporated as fillers to reinforce native pectin films. This reinforcement resulted in substantial improvements in the mechanical properties, hydrophobicity, thermal stability, barrier properties, and optical attributes of pectin films. Compared to NPF, the pectin-hematite composite film exhibited a 35% increase in tensile strength, a 30° increase in contact angle, a 6-fold increase in the oxygen diffusion barrier properties, and a 20% increase in the water vapor barrier properties. This study presents a sustainable, biocompatible, and biodegradable packaging solution by capitalizing on eco-friendly biopolymer and nanoparticle engineering.

Elucidating shape-mediated drug carrier mechanics of hematite nanomaterials for breast cancer therapeutics

Metallic nanomaterials have gained significant attention in cancer therapy as potential nanocarriers due to their unique properties at the nanoscale. However, nanomaterials face several drawbacks, including biocompatibility, stability, and cellular uptake. Hematite (α-Fe2O3) nanoparticles are emerging as promising nano-carriers to reduce adverse outcomes of conventional chemotherapeutics. However, the shape-mediated drug carrier mechanics of hematite nanomaterials are not raveled. In this study, we tailored hematite nanoparticles in ellipsoidal (EHNP) and spherical (SHNP) shapes with excellent biocompatibility and efficient drug encapsulation and release. We elucidate that EHNP exhibits higher cellular uptake than SHNP. With effective cellular internalization, the cisplatin-loaded EHNP showed excellent cytotoxicity with an IC50 value of 200 nM compared to the cisplatin-loaded SHNP. The flow cytometry cell sorting (FACS) analysis showed a four-fold increase in cell death by arresting the cells at the G0/G1 and G1 phases for cis-EHNP compared to cis-SHNP. The results show that ellipsoidal-shaped hematite nanoparticles can act as attractive nanocarriers with improved therapeutic efficacy in cancer therapy.

Single-Step Formation of Pickering Double Emulsions by Exploiting Differential Wettability of Particles

We present a modular single-step strategy for the formation of single and Pickering double emulsions (DEs). To this end, we consider the role of surface modification of particles and their dispersibility in different phases in the context of the design of Pickering emulsions by varying the volume fraction of oil in the oil-water mixture (ϕoil) used for emulsification. In particular, the experiments are performed by considering (a) model spherical and nonspherical colloids of different wettabilities which are tailored by oleic acid treatment, (b) immiscible liquids with or without particles, and (c) varying ϕoil from 0.1 to 0.9. We show that it is possible to affect a transition from (i) oil-in-water (O/W) emulsion to water-in-oil (W/O) emulsion and (ii) oil-in-water (O/W) to oil-in-water-in-oil (O/W/O) to water-in-oil (W/O) as ϕoil is systematically varied. We elucidate that the range of ϕoil at which particle stabilized DEs of the O/W/O type form can be tuned by engineering surface modification of particles to different extents. Furthermore, the arrangement of particles on the surface of droplets in the Pickering DEs is discussed. Our results conclusively establish that the differential wettability of particles is the key for the design of Pickering DEs. The versatility of the proposed strategy is established by developing DEs using a number of model colloidal systems.

Oil-in-Water Emulsions Stabilized by Hydrophilic Homopolymers

Most of the polymeric emulsifiers have diblock and triblock copolymer architecture containing hydrophilic and hydrophobic domains. In this work, we show that hydrophilic homopolymers can be effective stabilizers of oil-in-water emulsions. Using polyethelyne oxide and poly(vinylpyrrolidone) as model hydrophilic homopolymers and n-decane and n-hexane as model nonpolar phases, we show that high-molecular weight polymers can stabilize emulsions over 24 h beyond a threshold concentration. We highlight the role of the molecular weight and concentration of the polymer in the stability of emulsions through kinetic measurements of emulsion volume, microscopic analysis, interfacial tension, and dilational rheology. We explain the mechanism of stabilization to stem from buoyancy-driven creaming of emulsion drops and film drainage and dilational elasticity of the interface in relation to the molecular weights and concentrations of polymers. This study demonstrates that water-soluble homopolymers can stabilize oil-in-water emulsions and open avenues for the use of eco-friendly biopolymers, which are inherently hydrophilic, as an alternative to synthetic emulsifiers.

Preparation and Emulsifying Properties of Carbon-Based Pickering Emulsifier

Water is increasingly being used as a solvent in place of organic solvent in order to meet the demand for green chemical synthesis. Nevertheless, many of the reaction substrates are organic matter, which have low water solubility, resulting in a low reaction interface and limiting the development of organic-water biphasic systems. A surfactant is typically added to the two-phase system to form an emulsion to increase the contact area between the organic phase and the water. Compared to ordinary emulsion stabilized with the surfactant, Pickering emulsion offers better adhesion resistance, biocompatibility, and environmental friendliness. It possesses unrivaled benefits as an emulsifier and catalyst in a two-phase interfacial catalysis system (PIC). In this study, the amine group (NNDB) was employed to alter the surface of graphene oxide (GO). A stable Pickering emulsion was created by adsorbing GO-NNDB on the toluene–water interface. It was determined that the emulsion system had good stability by analyzing digital photographs and microscope images of droplets at various temperatures, and fluorescence microscopy images of emulsion droplets created by both newly added and recovered emulsifiers. This work provided the groundwork for future applications of Pickering emulsion in interfacial catalysis.

Hydrophobically modified silica nanolaces-armored water-in-oil pickering emulsions with enhanced interfacial attachment energy

HYPOTHESIS

Anisotropic particles with a high aspect ratio led to favorable interfacial adhesion, thus enabling Pickering emulsion stabilization. Herein, we hypothesized that pearl necklace-shaped colloid particles would play a key role in stabilizing water-in-silicone oil (W/S) emulsions by taking advantage of their enhanced interfacial attachment energy.

EXPERIMENTS

We fabricated hydrophobically modified silica nanolaces (SiNLs) by depositing silica onto bacterial cellulose nanofibril templates and subsequently grafting alkyl chains with tuned amounts and chain lengths onto the nanograins comprising the SiNLs.

FINDINGS

The SiNLs, of which nanograin has the same dimension and surface chemistry as the silica nanospheres (SiNSs), showed more favorable wettability than SiNSs at the W/S interface, which was supported by the approximately 50 times higher attachment energy theoretically calculated using the hit-and-miss Monte Carlo method. The SiNLs with longer alkyl chains from C6 to C18 more effectively assembled at the W/S interface to produce a fibrillary interfacial membrane with a 10 times higher interfacial modulus, preventing water droplets from coalescing and improving the sedimentation stability and bulk viscoelasticity. These results demonstrate that the SiNLs acted as a promising colloidal surfactant for W/S Pickering emulsion stabilization, thereby allowing the exploration of diverse pharmaceutical and cosmetic formulations.

Destabilization of Pickering emulsions by interfacial transport of mutually soluble solute

HYPOTHESIS

Pickering emulsions (PEs) once formed are highly stable because of very high desorption energies (∼10⁷ k_BT) associated with particles adsorbed to the interfaces. The destabilization of PEs is required in many instances for recovery of valuable chemicals, products and active compounds. We propose to exploit interfacial instabilities develop by the addition of different types of solutes to PEs as a route to engineer their destabilization.

EXPERIMENTS

PEs stabilized by (i) spherical particles, (ii) non-spherical particles, (iii) oppositely charged particle-particle mixtures, and (iv) oppositely charged particle-polyelectrolyte mixtures are formulated. Different types of solutes are added to these highly stable PEs and the macroscopic as well as microscopic changes induced in the PEs is recorded by visual observation and bright field optical microscopy.

FINDINGS

Our results point to a simple yet robust method to induce destabilization of PEs by transiently perturbing the oil-water interface by transport of a mutually soluble solute across the interface. The generality of the method is demonstrated for different kind of solutes and stabilizers including particles of different sizes (nm to µm), shapes (sphere, spheroids, spherocylinders) and types (polystyrene, metal oxides). The method works for both oil-in-water (o/w) and water-in-oil (w/o) PEs with different kinds of non-polar solvents as oil-phase. However, the method fails when the solute is insoluble in one of the phases of PEs. The study opens up a new approach to destabilization of particle stabilized emulsions.

Tailoring Pickering Double Emulsions by in Situ Particle Surface Modification

Fundamental studies on the formation and stability of Pickering double emulsions are crucial for their industrial applications. Available methods of double emulsion preparation involve multiple tedious steps and can formulate a particular type of double emulsion, that is, water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). In this work, we proposed a simple single-step in situ surface modification method to stabilize different types of double emulsions using hematite and silica particle systems which involves the addition of oleic acid. In the emulsification studies, we use (i) a combination of hematite and oleic acid, which is termed the binary system, and (ii) a mixture of hematite and silica particles together with oleic acid, which is designated as the ternary system. The wettability of hematite particles is tuned by direct or sequential addition of oleic acid to the water-decane medium. The direct surface modification (which involves the addition of a known quantity of oleic acid to the oil-water mixtures at once) of hematite particles in both binary and ternary systems shows transitional phase inversion from oil-in-water (o/w) to water-in-oil (w/o) emulsions. However, sequential surface modification results in the transition of a single emulsion to double emulsions. In the case of the binary system, the sequential surface modification of the hematite-particle-stabilized o/w emulsion can be converted into double emulsions of o/w/o type. However, in the case of the ternary system, i.e., in the presence of silica particles, sequential surface modification of hematite particles stabilizes both single (o/w) and double (w/o/w and o/w/o) emulsions. The critical concentration of oleic acid required to form a double emulsion is observed to be dependent on the ratio of the surface area of the silica particle to the total surface area of particles (S) and mixing protocols. A study of the size distribution of oil and water droplets of double emulsions shows that droplet size can be controlled by oleic acid concentration and magnitude of S. The arrangements of the particles at interfaces are visualized by SEM imaging. In this way, we developed an easy and novel single-step method of double emulsion preparation and provide a strategy to tailor the formation of different types of emulsions with a single/binary particle system by sequential in situ surface modification of the particles.

Phase Inversion of Pickering Emulsions Induced by Interfacial Electrostatic Attraction

Phase inversion of Pickering emulsions from water-in-oil (W/O) to oil-in-water (O/W) is achieved by the formation of an interfacial particle bilayer using negatively charged and positively charged particles dispersed in water and oil, respectively, before emulsification. A mechanism based on electrostatic attraction across the toluene-water interface is proposed and verified by systematic investigation of the parameters that affect the surface charge of negatively charged particles such as pH and salt concentration. Cationic silica-FITC particles (600 nm) can be dispersed in toluene and stabilize W/O emulsions alone; phase inversion of this emulsion can be induced by the addition of anionic silica-RB particles in the aqueous phase at a concentration of 1.0 wt % or above. It is revealed that silica-RB particles of a smaller size (100 nm) can induce emulsion phase inversion at a much lower concentration (0.4 wt %) and an interfacial particle bilayer is clearly revealed by CLSM and SEM images. By tuning the surface charge density of silica-RB particles, the electrostatic attraction mechanism leading to the formation of the interfacial particle bilayer is confirmed and emulsion stability can be tuned as demonstrated by osmotic pressure enhancement results obtained from centrifugation.