High Internal Phase Emulsions Stabilized by a Zeolite-Surfactant Combination in a Composition-Dependent Manner

pH-Induced reversible conversion between non-Pickering and Pickering high internal phase emulsion

Solid-stabilized high internal phase emulsions have received extensive attention. Many previous studies have confirmed that solid emulsifiers in high internal phase Pickering emulsions (HIPPEs) provide a great interface mechanical barrier. With the development of research, novel solid-stabilized emulsions have emerged. These emulsions are stabilized by the electrostatic repulsion between the surfactants and hydrophilic solid particles. They are distinct from Pickering emulsions in that the solid particles do not exist at the oil-water interface, but are dispersed in the continuous phase, so it is called a non-Pickering emulsion. However, high internal phase non-Pickering emulsions (HIPNPEs) are rarely reported. Herein, HIPNPEs that are synergistically stabilized by anionic surfactants with dynamic covalent bonds and negatively charged nano-SiO₂ particles were prepared. In the presence of dodecylamine, the acidity causes the dynamic covalent bonds to break and the surfactant to be inactivated. Additionally, the long-chain amine is protonated and adsorbed on nano-SiO₂ particles to form a new surfactant for stabilizing HIPPEs. However, alkalinity causes the HIPNPEs to form again. In addition, rheological tests confirmed that the HIPNPEs and HIPPEs had similar rheological behaviors, which were typical gel-like fluids. The emulsion can quickly respond to realize the conversion between the different types of high internal phase emulsion by simple stimulation, which provides a new direction for stimulus-responsive high internal phase emulsions.

Water-in-oil high internal phase Pickering emulsions formed by spontaneous interfacial hydrolysis of monomer oil

HYPOTHESIS

Alcohols can strongly reduce the interfacial tension between immiscible liquids, thus facilitating the formation of emulsions. By combining non-surface-active hydrophobic particles with medium-chain alcohols, stable water-in-oil (w/o) high internal phase Pickering emulsions (HIPPEs) can be easily prepared without high-energy emulsification methods.

EXPERIMENTS

The emulsions containing acrylate monomer as the oil phase were prepared at different pH values in the presence of hydrophobic silica particles. Further, by replacing monomer oil with organic solvents (e.g., toluene) and a certain concentration of alcohol, the promoted particle adsorption at the oil-water interface has been systematically investigated. The morphology and interfacial structure of HIPPEs were visualized by confocal laser scanning microscopy (CLSM).

FINDING

At high pH, stable water-in-acrylate monomer HIPPEs can be formed using commercial fumed silica nanoparticles alone with simple stirring or vortexing. The hydrolysis of the acrylate group at high pH can generate alcohols in situ which adsorb at the oil-water interface to reduce the interfacial tension and promote particle adsorption to hinder droplet coalescence. The novel strategy for forming stable and processable HIPPEs can be universally applied to different hydrophobic silica particles with the help of various alcohols as the co-stabilizer, which provides a flexible approach for the fabrication of lightweight, closed-cell solid foams for a range of applications.

Basic Amino Acid-Modified Lignin-Based Biomass Adjuvants: Synthesis, Emulsifying Activity, Ultraviolet Protection, and Controlled Release of Avermectin

Avermectin (AVM) is a highly effective and safe biopesticide but is very sensitive to ultraviolet (UV) light and exhibits poor water solubility. Developing green and multifunctional adjuvants is important for the protection and controlled release of AVM. In this work, a number of water-soluble enzymatic hydrolysis lignins (W-EHLs) were prepared via grafting basic amino acids and used as emulsifiers with co-surfactants to prepare high-internal phase emulsions (HIPEs). The results showed that W-EHLs with co-surfactants could be prepared with HIPEs that contained 90 vol % green oil phases such as turpentine, and the stability of the HIPEs first increased and then decreased when the rate of grafting of basic amino acids on lignin increased from 0.26 to 1.46 mmol/g. The more polar oil droplets were less deformable due to their higher viscosity, thereby affording a stability advantage to HIPEs. Subsequently, the relations between the stability and interfacial viscoelasticity of the emulsion were effectively correlated by interfacial rheology, droplet size, and physical stability tests. The results showed that HIPEs with smaller droplets had poor fluidity and strong interfacial viscoelasticity due to their higher droplet packing density, which resulted in good macroscopic stability. Like the AVM carrier, the retention rate of AVM in HIPEs was 80.1% after UV radiation for 72 h, which represented the highest UV protection efficiency in AVM delivery systems. The release curves showed that the rate of release of AVM from HIPEs was adjusted by controlling the pH value of the medium. In addition, the release of HIPEs is completely in accord with both diffusion and the matrix erosion mechanism. The strategy could be extended to other sensitive pesticides and used to promote the development of sustainable agriculture.

All-natural oil-in-water high internal phase Pickering emulsions featuring interfacial bilayer stabilization

HYPOTHESIS

Synergistic stabilization of high internal phase Pickering emulsions (HIPPEs) by food-grade colloidal particles are necessary for food, pharmaceuticals or cosmetics owing to their biocompatibility and multi-functionality. By tuning the interfacial structure of adsorbed binary particles, the HIPPE may exhibit extraordinary characteristics compared to conventional all-natural HIPPEs solely stabilized by single-component particle or composite particle, which should have potential applications in varies fields.

EXPERIMENTS

HIPPEs were prepared by using zein protein nanoparticles (ZNPs) and starch nanocrystals (SNCs) as stabilizers. We systematically investigated the effect of particle concentration and internal phase fraction on HIPPEs morphology, stability and rheological behaviors. Further, the stabilization mechanism as well as potential applications were demonstrated.

FINDINGS

HIPPEs were prepared with excellent stability against centrifugation and high temperature (50 °C). Our result indicates the successful construction of unique bilayer interfacial structures consisting of inner ZNPs layer and outer SNCs layer. Since SNCs could gelatinize at 50 °C, dense shells can form around droplets afterwards. Such thermally responsive interfacial structures can be used to protect hydrophobic bioactive substances at higher temperatures while still allowing controlled release at certain conditions. Furthermore, with high internal phase fraction, HIPPEs can possibly replace mayonnaise and salad dressing on the market due to comparable appearance and properties. Following the removal of inner oil, porous materials can be further fabricated, which have potential applications in environmental protection or tissue engineering.

Transition between a Pickering Emulsion and an Oil-in-Dispersion Emulsion Costabilized by Alumina Nanoparticles and a Cationic Surfactant

The transition between a novel oil-in-dispersion emulsion and an oil-in-water (O/W) Pickering emulsion triggered by pH was achieved using alumina nanoparticles in combination with a cationic surfactant. In acidic and neutral aqueous media, positively charged particles and the surfactant both at very low concentrations costabilize an oil-in-dispersion emulsion with the surfactant adsorbed at droplet interfaces and particles dispersed in the aqueous phase between the droplets. In alkaline media, however, particles become negatively charged and are hydrophobized in situ by adsorption of the surfactant to become surface-active and stabilize an O/W Pickering emulsion. The transition between the two is also possible by lowering the pH. The transformation can be achieved several times in a mixture of 0.1 wt % nanoparticles and 0.01 mM surfactant. This transition is significant, since particles can be made to either adsorb at the oil-water interface, which is beneficial for applications like biphasic catalysis, or remain dispersed in the aqueous phase, which is favorable for their recovery and reuse.