New Pickering emulsions stabilized by bacterial cellulose nanocrystals

We studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge density, forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diameter stable for several months. We characterized the emulsion and visualized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.     

Pickering emulsions stabilized by cellulose nanocrystals grafted with thermo-responsive polymer brushes

Cellulose nanocrystals (CNCs) from ramie fibers are studied as stabilizers of oil-in-water emulsions. The phase behavior of heptane and water systems is studied, and emulsions stabilized by CNCs are analyzed by using drop sizing (light scattering) and optical, scanning, and freeze-fracture electron microscopies. Water-continuous Pickering emulsions are produced with cellulose nanocrystals (0.05-0.5 wt%) grafted with thermo-responsive poly(NIPAM) brushes (poly(NIPAM)-g-CNCs). They are observed to be stable during the time of observation of 4 months. In contrast, unmodified CNCs are unable to stabilize heptane-in-water emulsions. After emulsification, poly(NIPAM)-g-CNCs are observed to form aligned, layered structures at the oil-water interface. The emulsions stabilized by poly(NIPAM)-g-CNCs break after heating at a temperature above the LCST of poly(NIPAM), which is taken as indication of the temperature responsiveness of the brushes installed on the particles and thus the responsiveness of the Pickering emulsions. This phenomenon is further elucidated via rheological measurements, in which viscosities of the Pickering emulsions increase on approach of the low critical solution temperature of poly(NIPAM). The effect of temperature can be counterbalanced with the addition of salt which is explained by the reduction of electrostatic and steric interactions of poly(NIPAM)-g-CNCs at the oil-water interface.     

Nanocomposite hydrogels enhanced by cellulose nanocrystal-stabilized Pickering emulsions with self-healing performance in subzero environment

Cellulose - Nowadays, hydrogels as flexible materials have attracted considerable attention in frontier fields such as wearable electronic devices, soft actuators and robotics. However, water-based...     

Pickering emulsions stabilized by ultrashort nanotubes

We describe a simple method to prepare high-efficiency ultrashort nanotube Pickering emulsifiers. The polydivinylbenzene (PDVB) nanotubes with a slight degree of sulfonation, then interrupted to several microns in length, can stabilize hundred times their own mass of oil or water phase and form different Pickering emulsion types. The emulsion is very stable and can be stored for more than half a year without demulsification. A layer of magnetic Fe₃O₄ nanoparticles can be grown on the surface of the ultrashort sulfonated PDVB nanotubes. After being emulsified, oil-phase and magnetic nanotubes can be collected using a magnet, which have huge potential application for separation and recovery of organic solvents in environmental protection.     

Pickering emulsions stabilized by nanoparticle surfactants

Amphiphilic gold nanoparticles are demonstrated to effectively stabilize emulsions of hexadecane in water. Nanoparticle surfactants are synthesized using a simple and scalable one-pot method that involves the sequential functionalization of particle surfaces with thiol-terminated polyethylene glycol (PEG) chains and short alkane-thiol molecules. The resulting nanoparticles are shown to be highly effective emulsifying agents due to their strong adsorption at oil-water and air-water interfaces. The original nonfunctionalized gold nanoparticles are unable to effectively stabilize oil-water emulsions due to their small size and low adsorption energy. Small-angle X-ray scattering and electron microscopy are used to demonstrate the formation of nanoparticle-stabilized colloidosomes that are stable against coalescence and show significant shifts in plasmon resonance enhancing the near-infrared optical absorption.     

Pickering emulsions stabilized by anatase nanoparticles

Abstract  

Oil-in-water emulsions can be stabilized by solid particles. These so-called Pickering emulsions are regularly used in many technological applications. Here we describe the efficiency of sol–gel-synthesized anatase nanoparticles with a diameter of 6 nm in stabilizing emulsions. Key parameters were the surface charge of the particles—depending on pH and salt concentration—and their contact angle—depending on the surface groups and the polarity of the oil phase. The effect of these properties on the stability of the emulsions was investigated. The sol–gel nanoparticles were most efficient in stabilizing emulsions at pH 3 (depending on the salt and particle concentration). Highly apolar oil phases (cyclohexane, n-hexane) were required to obtain stable emulsions with the investigated system and addition of salt or hydrophobic coupling molecules in the oil phase, such as long alkyl chain containing phosphonates, increased the stability of the emulsions.     

Protein nanocage-stabilized Pickering emulsions

The utilization of surface-active engineered protein nanocages as stabilizers for emulsions provides avenues for the design of new tailor-made functional materials in various fields including food, pharmaceutical, and biotechnology. They can be used to codeliver bioactive molecules of different polarities in a tailored manner to the body, act as a platform for screening cells or enzymes, or function as targeted drug delivery systems. Knowledge on the mechanisms that underlie the protein nanocage-driven stabilization of emulsions and their colloidal structure can have direct implications for the rational design of the new advanced functional colloids.This contribution summarizes the recent progress in protein nanocage-stabilized emulsions. It discusses the advances in the precision bioengineering of protein nanocages for emulsion design, highlights challenges in the characterization of structure and dynamics in these materials, and demonstrates selected applications in the field of functional food materials.     

Aggregation in Pickering emulsions

For the first time, the particle distribution and aggregation in Pickering emulsions were made visible by transmission X-ray microscopy. Oil/water emulsions were stabilized by heterocoagulates of a clay mineral and magnesium aluminum hydroxide. Stability is optimum when the particles surround the oil droplets and also assemble to form a network extending through the coherent phase. Received: 18 September 1998 Accepted: 28 September 1998     

An approach to enhanced redispersibility of cellulose nanocrystals via freeze-drying their Pickering emulsions

To enhance the redispersibility of dried nanocellulose, cellulose nanocrystal (CNC) cryogels were produced by freeze-drying CNC-stabilized cyclohexane-in-water Pickering emulsions. The CNC cryogels were easily redispersed in water and organic solvents; thus, the approach proposed made it possible to significantly improve CNC redispersibility in aqueous and nonaqueous media.     

Sustainable food-grade Pickering emulsions stabilized by plant-based particles

This review summarizes the major advances that have occurred over the last 5 years in the use of plant-based colloidal particles for the stabilization of oil-in-water and water-in-oil emulsions. We consider the characteristics of polysaccharide-based particles, protein-based particles and organic crystals (flavonoids) with respect to their particle size, degree of aggregation, anisotropy, hydrophobicity and electrical charge. Specific effects of processing on particle functionality are identified. Special emphasis is directed towards the issue of correctly defining the stabilization mechanism to distinguish those cases where the particles are acting as genuine Pickering stabilizers, through direct monolayer adsorption at the liquid–liquid interface, from those cases where the particles are predominantly behaving as ‘structuring agents’ between droplets without necessarily adsorbing at the interface, for example, in many so-called high internal phase Pickering emulsions. Finally, we consider the outlook for future research activity in the field of Pickering emulsions for food applications.     

High internal phase Pickering emulsions

Macroemulsions rendered stable by adsorbed colloidal particles are termed Pickering emulsions. If the volume fraction of dispersed phase exceeds around 0.75, the emulsions are named high internal phase Pickering emulsions abbreviated to HIPPEs, which present new properties and potential applications. We review here the recent progress in preparing and studying HIPPEs of both oil-in-water and water-in-oil types. This includes discussion of the range of solid particle emulsifiers, the choice of the two immiscible liquids and methods for their preparation. As a result of their high interfacial area and long-term stability, HIPPEs are being put to use in many potential applications including drug delivery, catalysis, and in the production of novel porous materials.     

Pickering emulsions with controllable stability

We prepare solid-stabilized emulsions using paramagnetic particles at an oil/water interface that can undergo macroscopic phase separation upon application of an external magnetic field. A critical field strength is found for which emulsion droplets begin to translate into the continuous-phase fluid. At higher fields, the emulsions destabilize, leading to a fully phase-separated system. This effect is reversible, and long-term stability can be recovered by remixing the components with mechanical agitation.     

Optimizing organoclay stabilized Pickering emulsions

Oil-in-water emulsions were prepared using montmorillonite clay platelets, pre-treated with quaternary amine surfactants. In previous work, cetyl trimethylammonium bromide (CTAB) has been used. In this study, two more hydrophilic quaternary amine surfactants, Berol R648 and Ethoquad C/12, were used and formed Pickering emulsions, which were more stable than the emulsions prepared using CTAB coated clay. The droplets were also more mono-disperse. The most hydrophilic surfactant Berol R648 stabilizes the emulsions best. Salt also plays an important role in forming a stable emulsion. The droplet size decreases with surfactant concentration and relatively mono-disperse droplets can be obtained at moderate surfactant concentrations. The time evolution of the droplet size indicates a good stability to coalescence in the presence of Berol R648. Using polarizing microscopy, the clay platelets were found to be lying flat at the water oil interface. However, a significant fraction (about 90%) of clay stayed in the water phase and the clay particles at the water-oil interface formed stacks, each consisting of four clay platelets on average.     

Shear-induced coalescence of oil-in-water Pickering emulsions

Thermodynamic treatment of thin liquid films in Part III of this series was applied to foam films stabilized by sodium dodecyl sulfate. Miscibility of sodium chloride and sodium dodecyl sulfate in the adsorbed films at the film surfaces and transition between the black films were studied by measuring film thickness and contact angle. A discontinuous change in the thickness and a break on the contact angle vs. concentration curve appeared at the transition. Judging from the phase diagram of adsorption, sodium chloride and sodium dodecyl sulfate are a little miscible in the adsorbed films. The miscibility was ascribed to specific interaction between sodium ion and dodecyl sulfate ion in the adsorbed films. The miscibility in an adsorbed film was compared between the film surface and meniscus and between the common black and Newton black films.     

Novel green strategy to improve the hydrophobicity of cellulose nanocrystals and the interfacial elasticity of Pickering emulsions

The development of Pickering emulsions as ecologically correct stabilized with bio-based material by substituting synthetic petroleum-derived tensoactives assumed a very attractive level, representing the current guideline of the global market for homecare industry, food and beverage applications. In this wor, cellulose nanocrystals (CNCs), a hierarchically advanced biomaterial, were produced to stabilize innovative emulsions formulated with western soapberry Sapindus saponaria L. oil (SO). Besides, green surfactants (triterpene saponins extracted from S. saponaria L. pericarp; SAP) were also investigated to stabilize the oil/water interface. The synergistic combination between cellulose nanowhiskers and the bioactive glycosides has never been reported in the literature. Dynamic interfacial tensions of SAP and SO were firstly investigated, and their capacity to form a plastic membrane at oil/water interface was revealed. Response surface methodology (RSM) was employed to study the influence of the binary systems (CNC:SAP) on the stability of emulsified systems, such as size and zeta potential. In addition, a new calculation was proposed to determine the coverage of the oil droplets formed by the mixture of cellulose crystallites and natural surfactants. The optimal nanoemulsion composition was determined to be 60 w/w (%) of water, 23.905 w/w % of SO, 5 w/w % of CNC and 8.095 w/w% of SAP to produce of smallest droplet (165.1 nm) combined with higher zeta potential module (?46.7 mV). Results highlight the potential of Sapindus saponins and cellulose nanowhiskers for efficient producing label-friendly nanoemulsions applicable for drug, cosmeceutical or edible delivery systems.

Graphical abstract

     

Magnetic Pickering emulsions stabilized by Fe3O4 nanoparticles

Superparamagnetic Fe(3)O(4) nanoparticles prepared by a classical coprecipitation method were used as the stabilizer to prepare magnetic Pickering emulsions, and the effects of particle concentration, oil/water volume ratio, and oil polarity on the type, stability, composition, and morphology of these functional emulsions were investigated. The three-phase contact angle (θ(ow)) of the Fe(3)O(4) nanoparticles at the oil-water interface was evaluated using the Washburn method, and the results showed that for nonpolar and weakly polar oils of dodecane and silicone, θ(ow) is close to 90°, whereas for strongly polar oils of butyl butyrate and 1-decanol, θ(ow) is far below 90°. Inherently hydrophilic Fe(3)O(4) nanoparticles can be used to prepare stable dodecane-water and silicone-water emulsions, but they cannot stabilize butyl butyrate-water and decanol-water mixtures with macroscopic phase separation occurring, which is in good agreement with the contact angle data. Emulsions are of the oil-in-water type for both dodecane and silicone oil, and the average droplet size increases with an increase in the oil volume fraction. For stable emulsions, not all of the particles are adsorbed to drop interfaces; the fraction adsorbed decreases with an increase in the initial oil volume fraction. Changes in the particle concentration have no obvious influence on the stability of these emulsions, even though the droplet size decreases with concentration.     

Surface compaction versus stretching in Pickering emulsions stabilised by microgels

Colloidal gel particles called microgels have shown their ability to adsorb at an oil–water interface and stabilise emulsion named Pickering emulsions. Such particles are soft, deformable, and porous, and they can swell or contract under the action of an external stimulus. These specificities make them emulsifiers of special interest as they offer a large versatility to emulsions and materials elaborated thereof. This modularity is in counterpart at the origin of an abundant and often contradictory literature. The aim of this paper is to review recent advances in the emulsion stabilisation mechanism, particularly focusing on the microgel conformation at the interface in relation with the mechanical interface behaviour and the emulsion macroscopic stability. A sum up of the unambiguous knowledge is also proposed as well as few central questions that remain to be answered to in the domain.     

Pickering emulsions stabilized by surface-modified Fe3O4 nanoparticles

Unmodified Fe(3)O(4) nanoparticles do not stabilize Pickering emulsions of a polar oil like butyl butyrate. In order to obtain stable emulsions, the Fe(3)O(4) nanoparticles were modified by either carboxylic acid (RCOOH) or silane coupling agents (RSi(OC(2)H(5))(3)) to increase their hydrophobicity. The influence of such surface modification on the stability of the resultant Pickering emulsions was investigated in detail for both a non-polar oil (dodecane) and butyl butyrate in mixtures with water. The stability of dodecane-in-water emulsions in the presence of carboxylic acid-coated particles decreases as the length of the alkyl group (R) and the coating extent increase. However, such particles are incapable of stabilizing butyl butyrate-water emulsions even when the carboxylic acid length is decreased to two. However, the silane-coated Fe(3)O(4) nanoparticles can stabilize butyl butyrate-in-water emulsions, and they also increase the stability of dodecane-in-water emulsions. Thermal gravimetric analysis indicates that the molar quantity of silane reagent is much higher than that of carboxylic acid on nanoparticle surfaces after modification, raising their hydrophobicity and enabling enhanced stability of the resultant polar oil-water emulsions.