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 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.     

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.     

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.     

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.     

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.     

A novel amphotropic polymer based on cellulose nanocrystals grafted with azo polymers

A novel amphotropic polymer which could exhibit liquid-crystalline behavior both in the solvent and in the heating process was synthesized through azo polymers grafting from cellulose nanocrystals (CNCs). The CNCs, prepared by acid hydrolysis of filter paper, were characterized by Atomic Force Microscopy (AFM). Poly{6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate}(PMMAZO), which was a liquid-crystalline polymers (LCP), was successfully to graft from CNC via Atom transfer radical polymerization (ATRP). The structure and thermal properties of the PMMAZO-grafted CNC were investigated using Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analyses (TGA). Its phase structures and transitions were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The experimental results showed that the PMMAZO-grafted CNC exhibited both types of liquid crystal formation, thermotropic and lyotropic.     

Pickering emulsions co-stabilised by cellulose nanofibres and nicotinamide mononucleotide

Cellulose - Emulsified solid particles adsorbed at the oil–water interface can stabilise Pickering emulsions by acting as a physical barrier to the coalescence of oil droplets. Cellulose...     

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.     

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.     

Simple model for grafted polymer brushes

The first theories of grafted polymer brushes assumed a step profile for the monomer density. Later, the real density profile was obtained from Monte Carlo or molecular dynamics simulations and calculated numerically using a self-consistent field theory. The analytical approximations of the solutions of the self-consistent field equations provided a parabolic dependence of the self-consistent field, which in turn led to a parabolic distribution for the monomer density in neutral brushes. As shown by numerical simulations, this model is not accurate for dense polymer brushes, with highly stretched polymers. In addition, the scaling laws obtained from the analytical approximations of the self-consistent field theory are identical to those derived from the earlier step-profile-approximation and predict a vanishing thickness of the brush at low graft densities, and a thickness exceeding the length of the polymer chains at high graft densities. Here a simple model is suggested to calculate the monomer density and the interaction between surfaces with grafted polymer brushes, based on an approximate calculation of the partition function of the polymer chains. The present model can be employed for both good and poor solvents, is compatible with a parabolic-like profile at moderate graft densities, and leads to an almost steplike density for highly stretched brushes. While the thickness of the brush depends strongly on solvent quality, it is a continuous function in the vicinity of the temperature. In good and moderately poor solvents, the interactions between surfaces with grafted polymer brushes are always repulsive, whereas in poor solvents the interactions are repulsive at small separations and become attractive at intermediate separation distances, in agreement with experiment. At large separations, a very weak repulsion is predicted.     

Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

Pickering–Ramsden emulsions stabilized with chemically and morphologically anisotropic particles

Recent developments in nanotechnology have facilitated the use of surface-active colloidal particles with tailor-made anisotropic properties. These surface-active agents have introduced unprecedented emulsion systems that exhibit qualitatively different self-assembled/organized structures and material properties from those of emulsions with conventional surfactants or isotropic colloidal particles. The author highlights the recent experimental works that elucidate the fundamental roles of anisotropy in the self-assembly/organization in emulsions, while focusing predominantly on amphiphilicity and morphological anisotropy in a particle. The author also introduces recent works that harness these fundamental properties of anisotropy for realizing the characteristic emulsion state and its functionality, together with a work with large particles beyond colloidal scale.     

Surface charge effect on Pickering encapsulation with ionic cellulose nanocrystals

Ionic cellulose nanocrystals (CNCs) are interesting surface-active particles for encapsulating a lipophilic liquid in water. A CNC is modified chemically to a negative charge (an S-CNC) by surface treatment with sulfuric acid. Despite the amphiphilic nature of S-CNCs, it is difficult to determine the degree of substitution for emulsification of lipophilic liquids, especially when the surface energy is low and polarity is high. Here, we control the substitution of S-CNCs by desulfation of S-CNCs (dS-CNCs) using a low-concentration hydrochloric acid solution. Decreased surface charge of S-CNCs was expected, and the lipophilic affinity of dS-CNCs increased compared with those of S-CNCs. Six oils with differing surface tensions were selected for determination of the effect of charged CNCs on emulsification. The stability of the emulsion was evaluated by emulsion fraction, emulsion particle size, and surface tension of emulsified solutions from dS-CNCs and oils.

Graphical abstract

     

Brownian dynamics simulation of grafted polymer brushes

We present results of computer simulations by the method of Brownian dynamics of polymeric brushes attached to impenetrable planes. For testing both model and method we have used one polymer brush attached to a repulsive plane and compare some results with Monte Carlo results of Lai and Binder on the bond fluctuation model. We have also studied two polymeric brushes attached to two parallel planes at different distances between planes, and investigate the interplay between the interpenetration of the brushes and the configurational properties of the grafted chains.     

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

     

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...