Doubly pH Responsive Emulsions by Exploiting Aggregation of Oppositely Charged Nanoparticles and Polyelectrolytes

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.

Electrostatic Heteroaggregation: Fundamentals and Applications in Interfacial Engineering

The aggregation of oppositely charged soft materials (particles, surfactants, polyelectrolytes, etc.) that differ in one or more physical or chemical attributes, broadly referred to as electrostatic heteroaggregation, has been an active area of research for several decades now. While electrostatic heteroaggregation (EHA) is relevant to diverse fields such as environmental engineering, food technology, and pharmaceutical formulations, more recently there has been a resurgence to explore various aspects of this phenomenon in the context of interface stabilization and the development of functional materials. In this Feature Article, we provide an overview of the recent contributions of our group to this exciting field with particular emphasis on fundamental studies of electrostatic heteroaggregation between oppositely charged systems in the bulk, at interfaces, and across the bulk/interface. The influence of the size and shape of particles and the surface charge of heteroaggregates on the formation of Pickering emulsions and their utilization in the development of porous ceramics is discussed.

Demulsification Performance and Mechanism of Tertiary Amine Polymer-Grafted Magnetic Nanoparticles in Surfactant-Free Oil-in-Water Emulsion

Numerous cationic magnetic nanoparticles (MNPs) have previously been developed for demulsifying oil-in-water (O/W) emulsion, and results showed that the cationic MNPs could effectively flocculate and remove the negatively charged oil droplets via charge attraction; however, to the best of our knowledge, there are no research reports regarding the synergetic influence of both the positive charge density and interfacial activity of MNPs on the demulsification performance. In this study, three tertiary amine polymer-grafted MNPs, namely, poly(2-dimethylaminoethyl acrylate)-grafted MNPs (M-PDMAEA), poly(2-dimethylamino)ethyl methacrylate)-grafted MNPs (M-PDMAEMA), and poly(2-diethylaminoethyl methacrylate)-grafted MNPs (M-PDEAEMA), were synthesized and evaluated for their demulsification performance. Results demonstrated that a high positive charge density and superior interfacial activity of MNPs could cause partial oil droplet re-dispersion when excessive MNPs were introduced, leading to a lower magnetic separation efficiency and narrower demulsification window. Herein, a demulsification window is defined as a range of nanoparticle dosages in which the MNPs can effectively demulsify the O/W emulsion under certain conditions. For highly positively charged MNPs, their good interfacial activity could aggravate the formation of a narrower demulsification window. When tertiary amine polymer-grafted MNPs carried a lower positive charge density or weak interfacial activity, that is, M-PDMAEA at pH 4.0, M-PDMAEMA at pH 5.0-9.0, and M-PDEAEMA at pH 9.0-10.0, wide demulsification windows were observed. Additionally, a recycling experiment suggested that MNPs could maintain high demulsification efficiency up to at least five cycles, indicating their satisfactory recyclability. The three tertiary amine polymer-grafted MNPs can be potentially used for efficient demulsification from surfactant-free O/W emulsion in various pH ranges.

Effect of the Interactions between Oppositely Charged Cellulose Nanocrystals (CNCs) and Chitin Nanocrystals (ChNCs) on the Enhanced Stability of Soybean Oil-in-Water Emulsions

Chitin nanocrystals (ChNCs) and cellulose nanocrystals (CNCs) have been recently used to stabilize emulsions; however, they generally require significant amounts of salt, limiting their applicability in food products. In this study, we developed nanoconjugates by mixing positively charged ChNCs and negatively charged CNCs at various ChNC:CNC mass ratios (2:1, 1:1, and 1:2), and utilized them in stabilizing soybean oil-water Pickering emulsions with minimal use of NaCl salt (20 mM) and nanoparticle (NP) concentrations below 1 wt%. The nanoconjugates stabilized the emulsions better than individual CNC or ChNC in terms of a reduced drop growth and less creaming. Oppositely charged CNC and ChNC neutralized each other when their mass ratio was 1:1, leading to significant flocculation in the absence of salt at pH 6. Raman spectroscopy provided evidence for electrostatic interactions between the ChNCs and CNCs, and generated maps suggesting an assembly of ChNC bundles of micron-scale lengths intercalated by similar-size areas predominantly composed of CNC. The previous measurements, in combination with contact angles on nanoparticle films, suggested that the conjugates preferentially exposed the hydrophobic crystalline planes of CNCs and ChNCs at a 1:1 mass ratio, which was also the best ratio at stabilizing soybean oil-water Pickering emulsions.

pH and Magnetism Dual-Responsive Pickering Emulsion Stabilized by Dynamic Covalent Fe3O4 Nanoparticles

Herein, we describe pH and magnetism dual-responsive liquid paraffin-in-water Pickering emulsion stabilized by dynamic covalent Fe₃O₄ (DC-Fe₃O₄) nanoparticles. On one hand, the Pickerinfigureg emulsions are sensitive to pH variations, and efficient demulsification can be achieved by regulating the pH between 10 and 2 within 30 min. The dynamic imine bond in DC-Fe₃O₄ can be reversibly formed and decomposed, resulting in a pH-controlled amphiphilicity. The Pickering emulsion can be reversibly switched between stable and unstable states by pH at least three times. On the other hand, the magnetic Fe₃O₄ core of DC-Fe₃O₄ allowed rapid separation of the oil droplets from Pickering emulsions under an external magnetic field within 40 s, which was a good extraction system for purifying the aqueous solution contaminated by rhodamine B. The dual responsiveness enables Pickering emulsions to have better control of their stability and to be applied more broadly.

Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces

We exploit the aggregation between oppositely charged particles to visualize and quantify the equilibrium position of charged colloidal particles at the fluid-water interface. A dispersion of commercially available charge-stabilized nanoparticles was used as the aqueous phase to create oil-water and air-water interfaces. The colloidal particles whose charge was opposite that of the nanoparticles in the aqueous phase were deposited at the chosen fluid-water interface. Heteroaggregation, i.e., aggregation between oppositely charged particles, leads to the deposition of nanoparticles onto the larger particle located at the interface; however, this only occurs on the surface of the particle in contact with the aqueous phase. This selective deposition of nanoparticles on the surfaces of the particles exposed to water enables the distinct visualization of the circular three-phase contact line around the particles positioned at the fluid-water interface. Since the electrostatic association between the nanoparticles and the colloids at interfaces is strong, the nanoparticle assembly on the larger particles is preserved even after being transferred to solid substrates via dip-coating. This facilitates the easy visualization of the contact line by electron microscopy and the determination of the equilibrium contact angle of colloidal particles (θ) at the fluid-water interface. The suitability of the method is demonstrated by the measurement of the three-phase contact angle of positively and negatively charged polystyrene particles located at fluid-water interfaces by considering particles with sizes varying from 220 nm to 8.71 μm. The study highlights the effect of the size ratio between the nanoparticles in the aqueous phase and the colloidal particles on the accuracy of the measurement of θ.

Macroporous Ceramic Monolith from Nanoparticle-Polyelectrolyte-Stabilized Pickering Emulsions

In this work, we present a simple and scalable approach for fabricating porous ceramic from emulsions stabilized by a binary mixture of oppositely charged nanoparticles and a polyelectrolyte. The electrostatic heteroaggregation is exploited to form weakly charged particle-polyelectrolyte complexes (PPCs) that readily stabilize oil-in-water emulsions. The concentration of surface-active PPCs is varied to obtain Pickering emulsion gels that can be processed and converted into the macroporous ceramic structure. The polyelectrolyte in the binary mixture not only enables the adsorption of particles to the oil-water interface and renders processability of the emulsions but also acts as a binder. Nearly one-to-one correspondence between the microstructure of the green ceramic obtained after the evaporation of solvents from the gel-like emulsions and the parent emulsions is observed. The green ceramic is further sintered under controlled conditions to obtain a porous ceramic monolith. We demonstrate that the microstructure and the pore size distribution in the final ceramic can be altered by tuning the composition of the individual species used in the emulsion formulation, i.e., by optimization of the particle-polyelectrolyte ratio used in the processing route.

Controlling the microstructure of emulsions by exploiting particle-polyelectrolyte association

HYPOTHESIS

Albeit solid stabilized emulsions are studied for several decades, the surface of the emulsion drops most often are coated with densely packed and jammed monolayer of particles. However, a control over the area that the particles occupy on the drop surface is necessary, especially in applications involving controlled release of active compounds from emulsions. We hypothesize that it is possible to achieve precise control over the concentration of particles on the surface of emulsions by tailoring the adsorption of different species in a multi-component dispersion used for emulsification.

EXPERIMENTS

To this end, we carry out emulsification of oil and aqueous dispersions consisting of a combination of oppositely charged colloidal particles and polyelectrolyte. The droplet size distribution and storage stability of the oil-in-water emulsions, the microstructure, the percentage area of the drop surface occupied by the particles and the adsorption behavior of particle-polyelectrolyte binary dispersions are investigated.

FINDINGS

Our results demonstrate that the association between oppositely charged colloidal particles and polyelectrolyte can be exploited to obtain surface active species that aid in the formation of emulsions. Moreover, we found that the concentration of particle-polyelectrolyte complexes and polyelectrolyte in the dispersions used in emulsification greatly influence the mean diameter of the emulsions and their microstructure. Our findings provide a strategy to achieve control over surface coverage of particles on the emulsion droplets across a wide range - from a theoretically possible maximum, ≈90%, to as low as ≈5%. Interestingly, the emulsions formulated are found to possess excellent storage stability irrespective of the particle coverage on the drop surface.

pH-Responsive Pickering emulsions stabilized solely by surface-inactive nanoparticles via an unconventional stabilization mechanism

Using solely highly hydrophilic particles to stabilize emulsions, especially high internal phase emulsions, has always been an important challenge. Here pH-responsive Pickering emulsions stabilized by a low concentration of bare highly hydrophilic Ludox CL nanoparticles without surface modification or addition of surfactants are developed at neutral pH. The dispersed nanoparticles can be transformed into an aggregate state with a network-like structure near the isoelectric point, which contributes to the stabilization of the emulsions. Moreover, the vdW attraction between particles and droplets also plays a key role in the formation of emulsions, which can make the aggregated nanoparticles adsorb tightly around the droplets rather than penetrate the oil-water interface. The formed protective armor and network-like aggregates separate droplets from each other to prevent coalescence. At a low nanoparticle concentration (0.5 wt%), a high internal phase emulsion can be formed and can last up to half a year. This system can emulsify not only the hydrocarbon oil but also the fluoroalkane oil phase. Finally, organic-inorganic composite particles are fabricated using the template action of the Pickering emulsions. The method of preparing composite particles is more convenient than the traditional Pickering emulsion polymerization which often requires the modification of the surface of the hydrophilic particles or the addition of auxiliary monomers. This study provides a simple green strategy for the preparation of a more stable Pickering emulsion stabilized by surface-inactive nanoparticles and will broaden the scope of applications.

Porous Ceramics via Processable Pickering Emulsion Stabilized by Oppositely Charged Colloids

We report a method to produce processable Pickering emulsions in which electrostatic heteroaggregation of oppositely charged fumed silica and alumina particles is exploited. The ability of weakly charged heteroaggregates to adsorb to the oil-water interface favors the formation of highly stable emulsions. A control over the microstructure and rheology of the Pickering emulsion is demonstrated by tuning parameters such as composition, pH, and total concentration of particles. The Pickering emulsions formed under optimal conditions are found to be gel-like and are excellent templates for the fabrication of porous ceramics. Furthermore, the porous mullite ceramic structure which has exceptional thermal and chemical resistance under harsh environments is obtained by drying and sintering of the emulsion template. The phase evolution, microstructure, and porosity of the resulting ceramic are characterized.

Interactions between polystyrene particles with diameters of several tens to hundreds of micrometers at the oil-water interface

HYPOTHESIS

The charged spherical colloidal particles at the fluid-fluid interface experience considerably strong and long-ranged electrostatic and capillary interactions. The contribution of capillary force becomes more significant as the particle size increases beyond a certain limit. The relative strengths of the two competing interactions between the spherical polystyrene particles at the oil-water interface are quantified depending on their size.

EXPERIMENTS

The studied particles, obtained using the microfluidic method, have diameters of tens to hundreds of micrometers. The scaling behaviors of the commercially available colloidal particles with diameters of ~3 μm are also compared. An optical laser tweezer apparatus is used to directly or indirectly measure the interparticle force. Subsequently, the capillary force that can be attributed to the gravity-induced interface deformation and contact line undulation is calculated and compared with the measured interaction force.

FINDINGS

Regardless of the particle diameter (~3-330 μm), the measured force is observed to decay as r^-4, where r denotes the center-to-center separation, demonstrating that the dipolar electrostatic interaction is important and that the gravity-induced capillary interaction is negligible. Furthermore, numerical calculations with respect to the undulated meniscus confirm that the magnitude of capillary interaction is significantly smaller than that of the measured electrostatic interaction.

Conformational Characterization of Polyelectrolyte Oligomers and Their Noncovalent Complexes Using Ion Mobility-Mass Spectrometry

Poly-l-lysine (PLL), polystyrenesulfonate (PSS), and a mixture of these polyelectrolytes were investigated by electrospray ionization ion mobility mass spectrometry. The IM step confirmed the formation of noncovalent (i.e., supramolecular) complexes between these polyelectrolytes, which were detected in various charge states and stoichiometries in the presence of their constituents. Experimental and theoretical collision cross sections (CCSs) were derived for both PLL and PSS oligomers as well as their noncovalent assemblies. PSS chains showed higher compactness with increasing size as compared to PLL chains, indicating that the intrinsic conformations of the polyelectrolytes depend on the nature of the functional groups on their side chains. The CCS data for the noncovalent complexes further revealed that assemblies with higher PLL content have higher CCS values than other compositions of similar mass and that PLL-PSS complex formation is accompanied by significant size contraction.

Temperature and CO2 Dual-Responsive Pickering Emulsions Using Jeffamine M2005-Modified Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) with excellent biodegradability are promising biomaterials for use as responsive Pickering emulsifiers. However, the high hydrophilicity of CNCs limits their emulsification ability. Some existing studies have utilized complicated covalent modification procedures to increase the hydrophobicity of CNCs. To simplify the modification process, we prepared hydrophobically modified CNCs (CNCs-M2005) via simple and controllable electrostatic interactions with thermosensitive M2005. The obtained CNCs-M2005 exhibited temperature and CO₂ dual-responsive properties. Subsequently, stable oil/water Pickering emulsions were prepared using the partially hydrophobic CNCs-M2005 at 20 °C. However, demulsification occurred when the temperature increased to 60 °C. This temperature-induced demulsification resulted from the dehydration of polyethylene oxide and polypropylene oxide, causing the aggregation of the CNCs-M2005, as shown by dynamic light scattering and transmission electron microscopy experiments. In addition, demulsification was also achieved after bubbling CO₂, which was attributed to the dissociation of the partially hydrophobic CNCs-M2005. The temperature and CO₂ dual-responsive biosafe Pickering emulsions open up opportunity for the design of intelligent food, cosmetic, and drug delivery systems.

Redox-Responsive Pickering Emulsions Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecules

In this paper, we report a novel redox-responsive water-in-oil Pickering emulsion stabilized by negatively charged silica nanoparticles in combination with a trace amount of redox switchable fluorescent molecule ferrocene azine (FcA), in which ferrocene serves as a redox-sensitive group and anthryl unit serves as a fluorescence emission center. By alternately adding oxidants and reducing agents at a moderate condition, the amphiphilicity of silica nanoparticles changes because of the adsorption of Fc⁺A and the desorption of FcA on the silica surface. On the one hand, the stability of emulsions can be transformed between stable and unstable at ambient temperature via redox trigger and the regulation process can be cycled at least three times. On the other hand, the fluorescent intensity of the FcA molecule can be regulated by redox stimuli; thus, the change in fluorescent behavior of the emulsion droplets is observed upon redox cycles, which makes it useful in the fluorescent label of stimuli-responsive Pickering emulsions. This work provides a deep understanding of the regulation mechanism of Pickering emulsions upon redox stimuli and opens the new way for in situ fluorescent label of stimulus-responsive Pickering emulsions without introducing additional fluorescent molecules.

Nonionic Glycolipids for Chromium Flotation- and Emulsion (W/O and O/W)-Based Bioactive Release

Biosourced surfactants are endeavored as a green alternative to biosurfactants and petrochemical surfactants having industrial utilization. Nine glycolipids with headgroup and chain length variation were derived from renewable resources like vegetable oils, carbohydrates, and amino acids. The concentration-dependent interfacial activity, foamability, wetting power, emulsification power, and solubilization capacities of glycolipids were investigated to provide a structure-activity relationship. Later, the metal flotation and emulsification experiments were performed. In general, for metal flotation, the surfactant should contain a hydrophobic tail, hydrophilic head, and chelating function. In the present investigation, it was observed that the headgroup of a glycolipid can serve as a hydrophilic head as well as perform a chelating function. Moreover, heat energy generated from the sunlight was utilized for metal flotation. Additionally, these glycolipids are capable to form stable sunflower oil-water (W/O and O/W) emulsions. The mechanical and thermal stabilities and hydrophobic chain length dependency of the prepared emulsions at different water volume fractions are explored. Furthermore, encapsulation and release of water-soluble (riboflavin and l-ascorbic acid) and oil-soluble (curcumin and α-tocopherol) bioactives in glycolipid emulsions were monitored. Thus, glycolipids under investigation had shown the possibility for pretreatment of chromium-containing wastewaters and bioactive-loaded emulsions toward the controlled release.