Latex-particle-stabilized emulsions of anti-Bancroft type

Hydroxyapatite nanoparticle-modified porous bone grafts with improved cell attachment

Emulsion-templated foams have displayed promise as injectable bone grafts; however, the use of a surfactant as an emulsifier resulted in relatively small pores and impedes cell attachment. Hydroxyapatite nanoparticles were explored as an alternative stabilizer to address these limitations. To this end, hydroxyapatite nanoparticles were first modified with myristic acid to generate the appropriate balance of hydrophobicity to stabilize a water-in-oil emulsion of neopentyl glycol diacrylate and 1,4-butanedithiol. In situ surface modification of the resulting foam with hydroxyapatite was confirmed with elemental mapping and transmission electron microscopy. Nanoparticle-stabilized foams displayed improved human mesenchymal stem cell viability (91 ± 5%) over surfactant-stabilized foams (23 ± 11%). Although the pore size was appropriate for bone grafting applications (115 ± 71 μm), the foams lacked the interconnected architecture necessary for cell infiltration. We hypothesized that a co-stabilization approach with both surfactant and nanoparticles could be used to achieve interconnected pores while maintaining improved cell attachment and larger pore sizes. A range of hydroxyapatite nanoparticle and surfactant concentrations were investigated to determine the effects on microarchitecture and cell behavior. By balancing these interactions, a co-stabilized foam was identified that possessed large, interconnected pores (108 ± 67 μm) and improved cell viability and attachment. The co-stabilized foam was then evaluated as an injectable bone graft including network formation, microscale integration with bone, push out strength, and compressive properties. Overall, this work demonstrated that in situ surface modification with nHA improved cell attachment while retaining desirable bone grafting features and injectability.

Capsules templated from water-in-oil Pickering emulsions for enzyme encapsulation

HYPOTHESIS

Encapsulation of sensitive water-soluble bioactive materials such as fragrances, polyphenols and enzymes poses an immense challenge with capsules templated from water-in-oil (w/o) emulsions. Generation of radicals, heating, and extreme pH that are necessary for shell formation through interfacial reactions may have undesired influences on the active ingredients and thus lower their activity.

EXPERIMENTS

To overcome these limitations, we present a method to encapsulate sensitive ingredients, whereby capsules are templated from a w/o Pickering emulsion stabilized by binary particles of different hydrophilicity levels; the particles assembled at the water/oil interface are then crosslinked by polydiisocyanate (PHDI) at room temperature and neutral pH. Zein and casein nanoparticles were used as hydrophilic stabilizers and lipase was chosen as a model sensitive biomolecule that was encapsulated in the water core.

FINDINGS

Our results indicated that the enzymes encapsulated in colloid capsules had higher activity than those encapsulated in traditional w/o Pickering emulsion without shell crosslinking. The capsule structure effectively protected enzymes from the outer environment. This method is well suited for the encapsulation and protection of sensitive ingredients and shows great application in food, drug, and cosmetic industries.

Biosourced Polymeric Emulsifiers for Miniemulsion Copolymerization of Myrcene and Styrene: Toward Biobased Waterborne Latex as Pickering Emulsion Stabilizer

Biobased waterborne latexes were synthesized by miniemulsion radical copolymerization of a biosourced β-myrcene (My) terpenic monomer and styrene (S). Biobased amphiphilic copolymers were designed to act as stabilizers of the initial monomer droplets and the polymer colloids dispersed in the water phase. Two types of hydrophilic polymer backbones were hydrophobically modified by terpene molecules to synthesize two series of amphiphilic copolymers with various degrees of substitution. The first series consists of poly(acrylic acid) modified with tetrahydrogeraniol moieties (PAA-g-THG) and the second series is based on the polysaccharide carboxymethylpullulan amino-functionalized with dihydromyrcenol moieties (CMP-g-(NH-DHM)). The produced waterborne latexes with diameters between 160 and 300 nm and were composed of polymers with varying glass transition temperatures (T_g, PMy = -60 °C, T_{g, P(My-co-S)} = -14 °C, T_g, PS = 105 °C) depending on the molar fraction of biobased β-myrcene (f_My,0 = 0, 0.43, or 1). The latexes successfully stabilized dodecane-in-water and water-in-dodecane emulsions for months at all compositions. The waterborne latexes composed of low T_g poly(β-myrcene) caused interesting different behavior during drying of the emulsions compared to polystyrene latexes.

Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels

HYPOTHESIS

Stabilization of water-in-water (W/W) emulsions resulting from the separation of polymeric phases such as dextran (DEX) and poly(ethyleneoxide) (PEO) is highly challenging, because of the very low interfacial tensions between the two phases and because of the interface thickness extending over several nanometers. In the present work, we present a new type of stabilizers, based on bis-hydrophilic, thermoresponsive microgels, incorporating in the same structure poly(N-isopropylacrylamide) (pNIPAM) chains having an affinity for the PEO phase and dextran moieties. We hypothesize that these particles allow better control of the stability of the W/W emulsions.

EXPERIMENTS

The microgels were synthesized by copolymerizing the NIPAM monomer with a multifunctional methacrylated dextran. They were characterized by dynamic light scattering, zeta potential measurements and nuclear magnetic resonance as a function of temperature. Microgels with different compositions were tested as stabilizers of droplets of the PEO phase dispersed in the DEX phase (P/D) or vice-versa (D/P), at different concentrations and temperatures.

FINDINGS

Only microgels with the highest DEX content revealed excellent stabilizing properties for the emulsions by adsorbing at the droplet surface, thus demonstrating the fundamental role of bis-hydrophilicity. At room temperature, both pNIPAM and DEX chains were swollen by water and stabilized better D/P emulsions. However, above the volume phase transition temperature (VPTT ≈ 32 °C) of pNIPAM the microgels shrunk and stabilized better P/D emulsions. At all temperatures, excess microgels partitioned more to the PEO phase. The change in structure and interparticle interaction induced by heating can be exploited to control the W/W emulsion stability.

Pickering Emulsions Stabilized by Polystyrene Particles Possessing Different Surface Groups

Colloidal polystyrene (PS) latex particles in water can undergo interesting charge reversal in the presence of particular electrolytes. It is worth exploring the effect of charge reversal on the properties of Pickering emulsions they stabilize. Herein, emulsions stabilized by PS latex particles possessing different surface groups (sulfate, amidine, or carboxyl) were prepared in the presence of tetrapentylammonium bromide (TPeAB) or sodium thiocyanate (NaSCN) electrolytes. The effect of salt concentration on the charge of the particles and their colloid stability was measured. Emulsions were prepared from aqueous dispersions, and their type and stability were determined. The three-phase contact angle of particles at the planar oil-water interface was also measured using a gel trapping technique. It was found that the type of emulsion stabilized by latex particles is dominated by the hydrophobic PS portion on particle surfaces, although their surface charge is strongly affected by electrolyte addition. Preferred emulsions were always water-in-oil with dodecane, and charge reversal had little influence on the emulsion type and stability. However, transitional phase inversion of emulsions stabilized by carboxyl latex particles occurred on adding salt when the oil was a low-viscosity polydimethylsiloxane.

Structurally nanoengineered antimicrobial peptide polymers: design, synthesis and biomedical applications

Antimicrobial resistance not only increases the contagiousness of infectious diseases but also a threat for the future as it is one of the health care concern around the globe. Conventional antibiotics are unsuccessful in combating chronic infections caused by multidrug-resistant (MDR) bacteria, therefore it is important to design and develop novel strategies to tackle this problems. Among various novel strategies, Structurally Nanoengineered Antimicrobial Peptide Polymers (SNAPPs) have been introduced in recent years to overcome this global health care issue and they are found to be more efficient in their performance. Many facile methods are adapted to synthesize complex SNAPPs with required dimensions and unique functionalities. Their unique characteristics and remarkable properties have been exploited for their immense applications in various fields including biomedicine, targeting therapies, gene delivery, bioimaging, and many more. This review article deals with its background, design, synthesis, mechanism of action, and wider applications in various fields of SNAPPs.

Tuning Surface Chemistry and Ionic Strength to Control Nanoparticle Adsorption and Elastic Dilational Modulus at Air-Brine Interface

The relationship between the interfacial rheology of nanoparticle (NP) laden air-brine interfaces and NP adsorption and interparticle interactions is not well understood, particularly as a function of the surface chemistry and salinity. Herein, a nonionic ether diol on the surface of silica NPs provides steric stabilization in bulk brine and at the air-brine interface, whereas a second smaller underlying hydrophobic ligand raises the hydrophobicity to promote NP adsorption. The level of NPs adsorption at steady state is sufficient to produce an interface with a relatively strong elastic dilational modulus E' = dγ/d ln A. However, the interface is ductile with a relatively slow change in E' as the interfacial area is varied over a wide range during compression and expansion. In contrast, for silica NPs stabilized with only a single hydrophobic ligand, the interfaces are often more fragile and may fracture with small changes in area. The presence of concentrated divalent cations improves E' and ductility by screening electrostatic dipolar repulsion and strengthening the attractive forces between nanoparticles. The ability to tune the interfacial rheology with NP surface chemistry is of great interest for designing more stable gas/brine foams.

A new method to prepare microparticles based on an Aqueous Two-Phase system (ATPS), without organic solvents

HYPOTHESIS

Aqueous Two-Phase Systems (ATPS) are aqueous droplets dispersed in an aqueous phase. This specific behavior arises from interactions between at least two water-soluble entities, such as thermodynamically incompatible polymers. A simple, fast, and "green" process to produce ATPS with an aqueous core would be of high interest to the pharmaceutical field for drug delivery. However, to date, rapid destabilization of ATPS represents the main hurdle for their use. Herein we present a novel process to achieve a stabilized microparticle-ATPS, without the use of organic solvents.

EXPERIMENTS

ATPS composed of dextran and polyethylene oxide were prepared. A Pickering-like emulsion technique was used to stabilize the ATPS by adsorbing semi-solid particles (chitosan-grafted lipid nanocapsules) at the interface between the two aqueous phases. Finally, microparticles were formed by a polyelectrolyte complexation and gelation. The structure and stability of ATPS were characterized using microscopy and Turbiscan analysis.

FINDINGS

Adding chitosan-grafted lipid nanocapsules induced ATPS stabilization. Adding a polyelectrolyte such as sodium alginate allowed the formation of microparticles with a gelled shell that strengthened the formulation against shear stress and improved long-term stability, thus demonstrating that is possible to use ATPS to form delivery systems to encapsulate hydrophilic molecules.

Spontaneous particle desorption and "Gorgon" drop formation from particle-armored oil drops upon cooling

We study how the phenomenon of drop "self-shaping" (Denkov et al., Nature, 528, 2015, 392), in which oily emulsion drops undergo a spontaneous series of shape transformations upon emulsion cooling, is affected by the presence of adsorbed solid particles, like those used in Pickering emulsion stabilization. Experiments with several types of latex particles, and with added surfactant of low concentration to enable drop self-shaping, revealed several new unexpected phenomena: (1) adsorbed latex particles rearranged into regular hexagonal lattices upon freezing of the surfactant adsorption layer. (2) Spontaneous particle desorption from the drop surface was observed at a certain temperature - a remarkable phenomenon, as the solid particles are known to irreversibly adsorb on fluid interfaces. (3) Very strongly adhered particles to drop surfaces acted as a template to enable the formation of tens to hundreds of semi-liquid fibres, growing outwards from the drop surface, thus creating a shape resembling the Gorgon head from Greek mythology. Mechanistic explanations of all observed phenomena are provided using our understanding of the rotator phase formation on the surface of the cooled drops. The surface rotator phase creates positive line tension at the contact line formed between the particle surface and the fluid interface, which causes the particle ejection from the drop surface.

Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion

Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the Pickering emulsion were outlined along with the expected results of these methods on the emulsions. Besides, the application of modelling techniques has also been elaborated for the effective characterization of Pickering emulsions. Additionally, approaches to assess the stability of Pickering emulsions against physical deformation such as coalescence and gravitational separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the standard procedures of Pickering emulsion assessment and the advanced methods available to date to study these emulsions, down to the minute details.

How Do Charged End-Groups on the Steric Stabilizer Block Influence the Formation and Long-Term Stability of Pickering Nanoemulsions Prepared Using Sterically Stabilized Diblock Copolymer Nanoparticles?

Reversible addition-fragmentation chain transfer (RAFT) solution polymerization is used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing carboxylic acid, tertiary amine, or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate to form spherical nanoparticles as confirmed by transmission electron microscopy (TEM) analysis. Dynamic light scattering studies indicated an intensity-average diameter of approximately 25 nm. Aqueous electrophoresis measurements confirmed that the amine-functional nanoparticles became cationic at low pH owing to end-group protonation. In contrast, carboxylic acid-functional nanoparticles became appreciably anionic at pH 10 owing to end-group ionization. Finally, nanoparticles bearing neutral end-groups exhibited zeta potentials close to zero over a range of solution pH. High-shear homogenization of n-dodecane in the presence of such sterically stabilized nanoparticles led to the formation of oil-in-water Pickering macroemulsions with volume-average diameters of 20-30 μm. High-pressure microfluidization was then used to prepare the three corresponding Pickering nanoemulsions. Each Pickering nanoemulsion was characterized by analytical centrifugation and TEM studies of the dried nanoemulsion droplets confirmed their original nanoparticle superstructure. The nanoparticle adsorption efficiency at the oil-water interface was assessed by gel permeation chromatography (using a UV detector) for each nanoparticle type at both pH 3 and 7. Nanoparticles with charged end-groups exhibited relatively low adsorption efficiency, whereas up to 90% of the neutral nanoparticles were adsorbed onto the oil droplets. This observation was supported by small-angle X-ray scattering experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Analytical centrifugation was used to evaluate the colloidal stability of the aged Pickering nanoemulsions. Pickering nanoemulsions stabilized with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups.

Adsorption of charged anisotropic nanoparticles at oil-water interfaces

The adsorption of nanoparticles at fluid interfaces is of profound importance in the field of nanotechnology. Recent developments aim at pushing the boundaries beyond spherical model particles towards more complex shapes and surface chemistries, with particular interest in particles of biological origin. Here, we report on the adsorption of charged, shape-anisotropic cellulose nanocrystals (CNCs) for a wide range of oils with varying chemical structure and polarity. CNC adsorption was found to be independent of the chain length of aliphatic n-alkanes, but strongly dependent on oil polarity. Surface pressures decreased for more polar oils due to lower particle adsorption energies. Nanoparticles were increasingly wetted by polar oils, and interparticle Coulomb interactions across the oil phase thus increase in importance. No surface pressure was measurable and the O/W emulsification capacity ceased for the most polar octanol, suggesting limited CNC adsorption. Further, salt-induced charge screening enhanced CNC adsorption and surface coverage due to lower interparticle and particle-interface electrostatic repulsion. An empiric power law is presented which predicts the induced surface pressure of charged nanoparticles based on the specific oil-water interface tension.

Mechanical phase inversion of Pickering emulsions via metastable wetting of rough colloids

The possibility to invert emulsions from oil-in-water to water-in-oil (or vice versa) in a closed system, i.e. without any formulation change, remains an open fundamental challenge with many opportunities for industrial applications. Here, we propose a mechanism that exploits particle surface roughness to induce metastable wetting and obtain mechanically-responsive Pickering emulsions. We postulate that the phase inversion is driven by an in situ switch of the particle wettability from metastable positions at the interface following the input of controlled mechanical energy. Oil-in-water emulsions can be prepared at low energy using mildly hydrophobic rough colloids, which are dispersed in water and weakly pinned at the interface, and switched to water-in-oil emulsions by a second emulsification at higher energy, which triggers the relaxation of the particle contact angle. The same principle is demonstrated for the complementary emulsions using mildly hydrophilic colloids initially dispersed in oil. Our experiments and simulations support that the delicate interplay between particle surface design during synthesis and the energy of the emulsification process can encode a kinetic pathway for the phase inversion. Both organic and inorganic nanoparticles can be used, allowing for the future implementation of our strategy in a broad range of smart industrial formulations.

Preparation of Macroporous Polymers from Microcapsule-Stabilized Pickering High Internal Phase Emulsions

The ability to make stable water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs) is demonstrated using microcapsules as a stabilizer. The microcapsules are prepared with glycidyl methacrylate (GMA) and poly(melamine-formaldehyde) (PMF) as core and wall materials, respectively. Using these GMA-loaded PMF-walled microcapsules as a sole stabilizer of water-in-styrene/divinylbenzene HIPE, the polymerization of this HIPE causes a closed-cell porous polymer. While with the addition of a certain amount of the nonionic surfactant Span80 to the microcapsule-stabilized HIPEs, a series of open-cell porous materials are obtained. The morphologies of the porous materials are tunable with changing the microcapsules content and/or surfactant amount in the HIPE templates. When Raft polymerization is introduced to cure these HIPEs, owing to both the self-healing agent GMA within the microcapsules and the residue of the chain-transfer agent from Raft polymerization of HIPEs, the resulting porous polymers are proved to be self-repairable. This work suggests a new type of Pickering HIPE and provides a novel idea for preparing self-healing porous materials.

Biosurfactant-modified palygorskite clay as solid-stabilizers for effective oil spill dispersion

An effective and conventional remediation technique in marine oil spills is to apply chemical dispersants to emulsify oil slicks into small oil droplets. Still, the potential hazards of chemical dispersants onto the marine ecosystem have motivated the research for environmentally friendly alternative while keeping exceptional dispersion ability. Here, we showed that the mixture of palygorskite (PAL) and rhamnolipid (Rha) formed a biocompatible alternative to synthetic surfactants used for oil spill dispersion. The oil droplets dispersed by R-PAL presented a small average size and long-term stability, which illustrated the synergistic interactions between Rha and PAL acting as an efficient dispersant in artificial sea water (ASW). Due to the strong flocculation caused by high salinity, PAL alone was not effective emulsifiers in ASW. A small amount of Rha could played a major role in modifying the surface characteristics of PAL and decreasing oil-water interfacial tension. Therefore, PAL particles irreversibly adsorbed onto the oil-ASW interface and formed a rigid interfacial film around oil droplets in the presence of Rha, which offered an efficient barrier to droplet coalescence. The synergistic interactions between PAL and Rha could enable the dispersion of tetradecane in ASW. Such a functionality was further tested in dispersing crude oil in ASW. The study presents a new strategy of using a mixture of PAL and Rha for oil dispersion, thus providing an ecofriendly alternative to conventional dispersants.

Pickering emulsion-templated polymers: insights into the relationship between surfactant and interconnecting pores

Pickering high internal phase emulsions (HIPEs) using micron-size polymeric particles as stabilizer were developed. By adding a small amount of surfactant to the Pickering HIPEs, macroporous polymers with a well-define open-cell structure were synthesized with these HIPEs as templates. Owing to the micron-size of the particles, the particle locations could be observed directly by laser scanning confocal microscopy. It was found that the excess and attached particles aggregated and formed thick particle layers around the droplets when the HIPE was stabilized solely by particles. These thick particle layers were extremely stable, and did not easily rupture during or after polymerization, which caused the resulting polymers to have a closed-cell structure. When a small amount of surfactant was added, it was found that the surfactant disaggregated the particles, leaving them well-dispersed in the continuous phase. Moreover, the surfactant tended to occupy the oil–water interface at the contact point of adjacent droplets, where the interconnecting pores were hence likely to be formed after consolidation of the continuous phase. This observation confirmed experimentally the mechanism of interconnecting pore formation in Pickering-HIPE-templated porous polymers proposed theoretically in previous works.

Pickering high internal phase emulsions (HIPEs) stabilized by micron-size polymer particles were developed, and the relationship between surfactant and interconnecting pores of these HIPE-templated porous polymers was experimentally clarified.

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.

Pickering Emulsions Electrostatically Stabilized by Cellulose Nanocrystals

Cellulose Nanocrystals (CNC) are explored to stabilize oil/water emulsions for their ability to adsorb at the oil/water interface. In this work, the role of electrostatic forces in the CNC ability to stabilize oil/water emulsions is explored using canola oil/water and hexadecane/water as model systems. Canola oil/water and Hexadecane/ water (20/80, v/v) emulsions were stabilized with the addition of CNCs using ultrasonication. Emulsion droplet sizes range from 1 to 4 μm as measured by optical microscopy. It is found that CNC can stabilize oil/water emulsions regardless of their charge density. However, reducing the surface charge density, by adding salts and varying pH, can reduce the amount of CNC's required to form a stable emulsion. Just by adding 3 mM Na⁺ or 1 mM or less Ca⁺² to a CNC suspension, the amount of CNC reduced by 30% to stabilized 2 mL of Canola oil. On the other hand, adding salt increases the emulsion volume. The addition of 100 mM Na⁺ or the reduction of pH below 2 leads to the aggregation of CNC; emulsions formed under these conditions showed gel-like behavior. This work shows the potential of nanocellulose crystal in stabilizing food and industrial emulsions. This is of interest for applications where biodegradability, biocompatibility, and food grade requirements are needed.

Hardening of particle/oil/water suspensions due to capillary bridges: Experimental yield stress and theoretical interpretation

Suspensions of colloid particles possess the remarkable property to solidify upon the addition of minimal amount of a second liquid that preferentially wets the particles. The hardening is due to the formation of capillary bridges (pendular rings), which connect the particles. Here, we review works on the mechanical properties of such suspensions and related works on the capillary-bridge force, and present new rheological data for the weakly studied concentration range 30-55 vol% particles. The mechanical strength of the solidified capillary suspensions, characterized by the yield stress Y, is measured at the elastic limit for various volume fractions of the particles and the preferentially wetting liquid. A quantitative theoretical model is developed, which relates Y with the maximum of the capillary-bridge force, projected on the shear plane. A semi-empirical expression for the mean number of capillary bridges per particle is proposed. The model agrees very well with the experimental data and gives a quantitative description of the yield stress, which increases with the rise of interfacial tension and with the volume fractions of particles and capillary bridges, but decreases with the rise of particle radius and contact angle. The quantitative description of capillary force is based on the exact theory and numerical calculation of the capillary bridge profile at various bridge volumes and contact angles. An analytical formula for Y is also derived. The comparison of the theoretical and experimental strain at the elastic limit reveals that the fluidization of the capillary suspension takes place only in a deformation zone of thickness up to several hundred particle diameters, which is adjacent to the rheometer's mobile plate. The reported experimental results refer to water-continuous suspension with hydrophobic particles and oily capillary bridges. The comparison of data for bridges from soybean oil and hexadecane surprisingly indicate that the yield strength is greater for the suspension with soybean oil despite its lower interfacial tension against water. The result can be explained with the different contact angles of the two oils in agreement with the theoretical predictions. The results could contribute for a better understanding, quantitative prediction and control of the mechanical properties of three-phase capillary suspensions solid/liquid/liquid.

Ethyl Cellulose Nanoparticles at the Alkane-Water Interface and the Making of Pickering Emulsions

Pickering emulsions stabilized by nanoparticles have recently received great attention for their remarkable stability, in part a consequence of irreversible adsorption. In this study, we generate Pickering oil-in-water emulsions stabilized by ethyl cellulose (EC) nanoparticles without the addition of surfactants. Over a range of ionic strength and EC nanoparticle concentrations, a series of dynamic interfacial tension (IFT) measurements complemented by extended DLVO theoretical computations are conducted to quantitatively describe the behavior of EC nanoparticles at the interface of water with different alkanes. Regardless of ionic strength, there is no barrier against the adsorption of EC nanoparticles at the alkane-water interfaces studied and the particles tightly cover these interfaces with near maximal coverage (i.e., 91%). Remarkably, the rate of approach to maximum coverage of the alkane-water interface by EC nanoparticles during the later stages of adsorption is accelerated in the presence of salt at concentrations below the critical coagulation concentration (CCC), unlike the air-water interface. Above the CCC, alkane-water interfaces behave similar to air-water interfaces, showing decay in the adsorption flux which is attributed to an increase in surface blocking originating from the attachment of nanoparticles to nanoparticles already adsorbed at the interface. These findings shed light on particle-particle and particle-interface colloidal interactions at and near fluid-fluid interfaces, thereby improving our ability to use hydrophobic EC nanoparticles as emulsion stabilizers.

Interfacial tension of reactive, liquid interfaces and its consequences

Dispersions of immiscible liquids, such as emulsions and polymer blends, are at the core of many industrial applications which makes the understanding of their properties (morphology, stability, etc.) of great interest. A wide range of these properties depend on interfacial phenomena, whose understanding is therefore of particular importance. The behaviour of interfacial tension in emulsions and polymer blends is well-understood - both theoretically and experimentally - in the case of non-reactive stabilization processes using pre-made surfactants. However, this description of the interfacial tension behaviour in reactive systems, where the stabilizing agents are created in-situ (and which is more efficient as a stabilization route for many systems), does not yet find a consensus among the community. In this review, we compare the different theories which have been developed for non-reactive and for reactive systems, and we discuss their ability to capture the behaviour found experimentally. Finally, we address the consequences of the reactive stabilization process both on the global emulsions or polymer blend morphologies and at the interfacial scale.

Enhanced interfacial activity of multi-arm poly(ethylene oxide) star polymers relative to linear poly(ethylene oxide) at fluid interfaces

Interfacial tension reduction, dynamic dilatational elasticity and extent of adsorption were investigated for linear poly(ethylene oxide) (PEO) chains of varying molecular weight and for PEO star polymers with an average of 64 arms per star at air/water, xylene/water, and cyclohexane/water interfaces.

Steady-state droplet size in montmorillonite stabilised emulsions

The formation of hexadecane-in-water emulsions stabilised by montmorillonite platelets was studied. In this system the platelets form a monolayer around the droplets and the droplet size decreases with increasing platelet volume fraction. However, the number of platelets present exceeds that required for monolayer coverage. The kinetics of emulsification were investigated and coalescence of droplets during turbulent mixing was found to continue even after the droplets had reached their ultimate size. Non-spherical droplets, resulting from arrested coalescence, were not observed suggesting that particles may be desorbing from the interface during the turbulent flow. A kinetic model based on a competition between droplet break-up and coalescence, mediated by particle adsorption and desorption, reproduces experimental trends in droplet diameter. The model can be used to predict the most efficient formulation to minimise droplet diameters for given materials and mixing conditions and sheds light on the processes occurring during emulsification in this system.

Soft electrostatic repulsion in particle monolayers at liquid interfaces: surface pressure and effect of aggregation

Non-densely packed interfacial monolayers from charged micrometre-sized colloid particles find applications for producing micropatterned surfaces. The soft electrostatic repulsion between the particles in a monolayer on an air/water (or oil/water) interface is mediated by the non-polar fluid, where Debye screening is absent and the distances between the particles are considerably greater than their diameters. Surface pressure versus area isotherms were measured at the air/water interface. The experiments show that asymptotically the surface pressure is inversely proportional to the third power of the interparticle distance. A theoretical model is developed that predicts not only the aforementioned asymptotic law but also the whole surface pressure versus area dependence. An increase in the surface pressure upon aggregation of charged particles in the interfacial monolayers is experimentally established. This effect is explained by the developed theoretical model, which predicts that the surface pressure should linearly increase with the square root of the particle mean aggregation number. The effect of added electrolyte on the aggregation is also investigated. The data lead to the conclusion that 'limited aggregation' exists in the monolayers of charged particles. In brief, the stronger electrostatic repulsion between the bigger aggregates leads to a higher barrier to their coalescence that, in turn, prevents any further aggregation, i.e. negative feedback is present.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Star Polymers

Recent advances in controlled/living polymerization techniques and highly efficient coupling chemistries have enabled the facile synthesis of complex polymer architectures with controlled dimensions and functionality. As an example, star polymers consist of many linear polymers fused at a central point with a large number of chain end functionalities. Owing to this exclusive structure, star polymers exhibit some remarkable characteristics and properties unattainable by simple linear polymers. Hence, they constitute a unique class of technologically important nanomaterials that have been utilized or are currently under audition for many applications in life sciences and nanotechnologies. This article first provides a comprehensive summary of synthetic strategies towards star polymers, then reviews the latest developments in the synthesis and characterization methods of star macromolecules, and lastly outlines emerging applications and current commercial use of star-shaped polymers. The aim of this work is to promote star polymer research, generate new avenues of scientific investigation, and provide contemporary perspectives on chemical innovation that may expedite the commercialization of new star nanomaterials. We envision in the not-too-distant future star polymers will play an increasingly important role in materials science and nanotechnology in both academic and industrial settings.

Enzymatic cross-linking of α-lactalbumin to produce nanoparticles with increased foam stability

Hard colloidal nanoparticles (e.g. partly hydrophobised silica), are known to make foams with very high foam-stability. Nanoparticles can also be produced from proteins by enzymatic cross-linking. Such protein based particles are more suitable for food applications, but it is not known if they provide Pickering foam stabilisation to the same extent as hard colloidal particles. α-Lactalbumin (α-LA) was cross-linked with either microbial transglutaminase (mTG) or horseradish peroxidase (HRP) to produce α-LA/mTG and α-LA/HRP nanoparticles. With both enzymes a range of nanoparticles were produced with hydrodynamic radii ranging from 20-100 nm. The adsorption of nanoparticles to the air-water interface was probed by increase in surface pressure (Π) with time. In the beginning of the Π versus time curves, there was a lag time of 10-200 s, for nanoparticles with Rh of 30-100 nm, respectively. A faster increase of Π with time was observed by increasing the ionic strength (I = 0-125 mM). The foam-ability of the nanoparticles was also found to increase with increasing ionic strength. At a fixed I, the foam-ability of the nanoparticles decreased with increasing size while their foam-stability increased. Foams produced by low-shear whipping were found to be 2 to 6 times more stable for nanoparticles than for monomeric α-LA (Rh≈ 2 nm). At an ionic strength of 125 mM ionic strength and protein concentration ≥ 10 g L(-1), the foam-stability of α-LA/mTG nanoparticles (Rh = 100 nm, ρapp = 21.6 kg m(-3)) was 2-4 times higher than α-LA/HRP nanoparticles (Rh = 90 nm, ρapp = 10.6 kg m(-3)). This indicated that foam-stablity of nanoparticles is determined not only by size but also by differences in mesoscale structure. So, indeed enzymatic cross-linking of proteins to make nanoparticles is moving a step towards particle like behavior e.g. slower adsorption and higher foam stability. However, the cross-link density should be further increased to obtain hard particle-like rigidity and foam-stability.

Monolayers of charged particles in a Langmuir trough: Could particle aggregation increase the surface pressure?

The effect of aggregation on the surface pressure, Π, of monolayers from charged micrometer-sized colloidal particles on the air/water interface is investigated. Π is completely due to the long-range electrostatic repulsion between the particles mediated by their electrostatic field in the air. The most probable origin of particle aggregation is the attraction between capillary quadrupoles due to undulated contact lines on particle surfaces. Aggregates have higher charge and repel each other stronger than single particles. The data analysis by means of a theoretical model implies that Π linearly increases with n(1/2); n is the mean aggregation number, which can be determined from the experimental Π vs. area curves. The presence of electrolyte promotes aggregation, which tends to increase Π, but simultaneously reduces the surface charge that leads to lower Π. For our system, the first effect prevails and apparently paradoxical behavior is observed: the addition of salt in water enhances the electrostatic surface pressure. The data indicate limited aggregation: the rise of the electrostatic barrier prevents the further coalescence of aggregates if they have become sufficiently large. The results contribute for a better understanding of the factors that control the interactions in monolayers of charged particles at liquid interfaces.

Stabilization of Pickering Emulsions with Oppositely Charged Latex Particles: Influence of Various Parameters and Particle Arrangement around Droplets

In this study we explore the fundamental aspects of Pickering emulsions stabilized by oppositely charged particles. Using oppositely charged latex particles as a model system, Pickering emulsions with good long-term stability can be obtained without the need for any electrolyte. The effects of parameters like oil to water ratio, mixed particle composition, and pH on emulsion type and stability are explored and linked to the behavior of the aqueous particle dispersion prior to emulsification. The particle composition is found to affect the formation of emulsions, viz., stable emulsions were obtained close to a particle number ratio of 1:1, and no emulsion was formed with either positively or negatively charged particles alone. The emulsions in particle mixtures exhibited phase inversion from oil-in-water to water-in-oil beyond an oil volume fraction of 0.8. Morphological features of emulsion droplets in terms of particle arrangement on the droplets are discussed.

An ionic liquid-in-water microemulsion as a potential carrier for topical delivery of poorly water soluble drug: Development, ex-vivo and in-vivo evaluation

In this paper, we report an ionic liquid-in-water (IL/w) microemulsion (ME) formulation which is able to solubilize etodolac (ETO), a poorly water soluble drug for topical delivery using BMIMPF6 (1-butyl-3-methylimidazolium hexafluorophosphate) as IL, Tween 80 as surfactant and ethanol as co-surfactant. The prepared ME was characterized for physicochemical parameters, subjected to ex-vivo permeation studies as well as in-vivo pharmacodynamic evaluation. The ex-vivo drug permeation studies through rat skin was performed using Franz-diffusion cell and the IL/w based ME showed maximum mean cumulative percent permeation of 99.030±0.921% in comparison to oil-in-water (o/w) ME (61.548±1.875%) and oily solution (48.830±2.488%) of ETO. In-vivo anti-arthritic and anti-inflammatory activities of the prepared formulations were evaluated using different rodent models and the results revealed that ETO loaded IL/w based ME was found to be more effective in controlling inflammation than oily solution, o/w ME and marketed formulation of ETO. Histopathological studies also demonstrated that IL/w based ME caused no anatomical and pathological changes in the skin.

Hard and soft colloids at fluid interfaces: Adsorption, interactions, assembly & rheology

Soft microgel particles inherently possess qualities of both polymers as well as particles. We review the similarities and differences between soft microgel particles and stiff colloids at fluid-fluid interfaces. We compare two fundamental aspects of particle-laden interfaces namely the adsorption kinetics and the interactions between adsorbed particles. Although it is well established that the transport of both hard particles and microgels to the interface is driven by diffusion, the analysis of the adsorption kinetics needs reconsideration and a proper equation of state relating the surface pressure to the adsorbed mass should be used. We review the theoretical and experimental investigations into the interactions of particles at the interface. The rheology of the interfacial layers is intimately related to the interactions, and the differences between hard particles and microgels become pronounced. The assembly of particles into the layer is another distinguishing factor that separates hard particles from soft microgel particles. Microgels deform substantially upon adsorption and the stability of a microgel-stabilized emulsion depends on the conformational changes triggered by external stimuli.