Particle-stabilized Emulsions Research Articles

Molecularly imprinted polymers fabricated using Janus particle-stabilized Pickering emulsions and charged monomer polymerization

Janus particle-stabilized Pickering emulsions and their applications in protein interfacial imprinting.

Preparation of Ag/TiO 2 composite foams via Pickering emulsion for bactericide and photocatalysis

Abstract This research aims to prepare macroporous Ag/TiO2 composite foams via particle-stabilized Pickering emulsion for bactericidal and photocatalytic activities. The Ag/TiO2 particles used for preparing the composite foams consisted of Ag core surrounded by mesoporous TiO2 shell. By selective adsorption of nonionic 1-hexadecylamine molecules on the Ag/TiO2 particles, an initially hydrophilic surface was tuned to partly hydrophobic with contact angle of ∼80°. Stable foams were then prepared by a simple mechanical frothing of aqueous suspensions consisting of the hydrophobic Ag/TiO2 particles as the filler phase, followed then by calcination to obtain the macroporous Ag/TiO2 foams. The composite foams impart photo-degradation capability against methylene-blue dye in water under ultraviolet irradiation. The composite foams also render a synergistic antimicrobial activity against Escherichia coli, in which the bactericidal efficiency reached 99% under dark conditions in 3 h.

Colloidal Particles at a Range of Fluid-Fluid Interfaces.

The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil-water, air-water, oil-oil, air-oil, and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.

Effect of solids concentration on TiO2/Ni composite foams prepared by aqueous gelcasting of particle-stabilized emulsions

Porous TiO2/Ni composite foams were prepared from sintering of gelcasted particulate emulsions; in which, surface of the initially hydrophilic TiO2 nanoparticles was hydrophobized through selective adsorption of benzethonium chloride. The adsorption led to a preferential deposition of the TiO2 particles at the air-water interface to stabilize the emulsions upon mechanical frothing. The remaining TiO2 and hydrophilic Ni particles would reside between the emulsion bubbles to form scaffolding struts, together with addition of agar as shape-setting agent by freeze gelcasting. Sintered foams with a porosity of 66–79% and a multi-modal pore-size distribution were obtained without phase segregation. Three-point rupture strength increased from 175 to 760kPa as the solids concentration was increased. Electrical resistance yet decreased by more than 62% as the solids concentration exceeded 7vol%, revealing that the increased cell-wall thickness would facilitate formation of percolative Ni network for electrical conductance when the solids concentration was increased.

Highly permeable macroporous cordierite ceramics with controlled microstructure produced by particle-stabilized emulsions with a reactive thermal treatment

Macroporous cordierite ceramics, comprising hierarchical microstructures, are produced by a method of particle-stabilized emulsions, combined with a followed reactive thermal treatment. The microstructure is tailored by altering sintering temperature and solid content in the emulsion templates. Pore throats generate in-situ by introducing magnesite in templates, in contrast to conventional methods, which use either surfactants or depending on thin film contact. Moreover, microstructural evolution of samples is studied by DTA/TG, XRD, and SEM analyses. The results of analyses show that the formation of much more pore throats is closely related to the release of gas from the raw materials and volume shrinkage. The optimal process conditions are a temperature of 1300°C and a solid content of 30vol.%. The as-prepared sample displays a nitrogen permeability of ∼1.8×10−11m2. The method shows great promise for producing many other highly permeable ceramics using pore former agents in the emulsion templates.

Gelatin Particle-Stabilized High-Internal Phase Emulsions for Use in Oral Delivery Systems: Protection Effect and in Vitro Digestion Study.

The potential application of Pickering high-internal phase emulsions (HIPEs) in the food and pharmaceutical industries has yet to be fully developed. Herein, we synthesized fairly monodisperse, nontoxic, autofluorescent gelatin particles for use as sole stabilizers for fabricating oil-in-water (O/W) HIPEs in an effort to improve the protection and bioaccessibility of entrapped β-carotene. Our results showed that the concentration of gelatin particles determined the formation, microstructure, droplet size distribution, and digestion profile of the HIPEs. For storage stability, the retention of β-carotene in HIPEs was significantly higher than in dispersion in bulk oil, even after storage for 27 days. In addition, in vitro digestion experiments indicated that the bioaccessibility of β-carotene was improved 5-fold in HIPEs. This study will help establish a correlation between the physicochemical properties of gelatin particle-stabilized HIPEs with their applications in the oral delivery of bioactive nutraceuticals.

High temperature stable W/O emulsions prepared with in-situ hydrophobically modified rodlike sepiolite

HypothesisHydrophobic modification can influence interparticle interaction, their interfacial adsorption, and stability of particle-stabilized emulsions. Emulsions stabilized by rodlike particles are more stable than those prepared with spherical particles even at low concentrations. Moreover, interfacial adsorption of particles will be tuned by controlling the modification. Thus, it is possible to prepare stable W/O emulsions with in-situ modified rodlike particles. ExperimentsRodlike sepiolite particles were in-situ modified in oil using dimethyldioctadecylammonium chloride (DODMAC). High salinity solution (water) in paraffin oil (W/O) emulsion was prepared with the modified particles. Stability of emulsions at room temperature and after aging at 160°C for 24h was studied. Mechanism of emulsion stability was explored by rheological measurements and confocal fluorescent microscopy. FindingsRemarkable stability against coalescence was found at high temperature. The enhanced stability is due to the high viscosity of continuous phase. Moreover, modification of sepiolite particles at high DOMDAC concentrations enhances particle adsorption at water-oil interfaces and network in continuous phase, which improve the stability against sedimentation and coalescence of the W/O emulsions.

Synthesis of emulsion-templated macroporous materials via Diels-Alder polymerization

Emulsion-templated macroporous materials, known in the literature for many years as polyHIPEs, have found an increasing number of applications due to their well-defined structures. However, most of the emulsion-templated materials are synthesized using conventional radical polymerization, and most of the radical polymerization are thermally initiated. Expanding the polymerization mechanisms available for emulsion-templated material synthesis is still highly desired. In this work, macroporous polymers were firstly synthesized by Diels-Alder reaction of furan derivatives and bismaleimide within the continuous phase of particle-stabilized emulsions. These polymers have a well-defined controllable porous structure and good self-healing performance.

Hydrophilic and hydrophobic modifications of colloidal silica particles for Pickering emulsions

Colloidal silica particles, functionalized with hydrophilic and hydrophobic groups, have been studied for utilization in particle-stabilized emulsions, so called Pickering emulsions. The amounts of attached groups have been characterized using NMR spectroscopy and elemental analysis. A range of particles were prepared, with sizes from around 13 to 70nm in diameter. Hydrophilic functionalization of the silica sols was achieved by attaching methyl poly(ethylene glycol) (mPEG) silane to the silica particle surface. This provides a reduction of surface charge density, a pH dependent and controllable flocculation behavior and surface activity. The hydrophobic functionalization of the silica sols was accomplished by attaching organosilanes containing mainly propyl and methyl groups. The emulsification abilities were evaluated by preparing Pickering emulsions using particles, with varying degrees and combinations of surface functionalization, as stabilizers and comparing the obtained emulsion droplet size distributions. It was found that colloidal silica functionalized with hydrophobic groups produced emulsions with smaller droplets compared to using unmodified silica. The emulsification performance was further improved by the combination of both hydrophilic and hydrophobic groups. All particles having this heterogeneous modification were found to generate emulsions with high stability towards coalescence (from five weeks to 1.5 years).

3D Printing of Emulsions and Foams into Hierarchical Porous Ceramics

Bulk hierarchical porous ceramics with unprecedented strength-to-weight ratio and tunable pore sizes across three different length scales are printed by direct ink writing. Such an extrusion-based process relies on the formulation of inks in the form of particle-stabilized emulsions and foams that are sufficiently stable to resist coalescence during printing.

Evaporation of Particle-Stabilized Emulsion Sunscreen Films

We recently showed (Binks et al., ACS Appl. Mater. Interfaces, 2016, DOI: 10.1021/acsami.6b02696) how evaporation of sunscreen films consisting of solutions of molecular UV filters leads to loss of UV light absorption and derived sun protection factor (SPF). In the present work, we investigate evaporation-induced effects for sunscreen films consisting of particle-stabilized emulsions containing a dissolved UV filter. The emulsions contained either droplets of propylene glycol (PG) in squalane (SQ), droplets of SQ in PG or droplets of decane in PG. In these different emulsion types, the SQ is involatile and shows no evaporation, the PG is volatile and evaporates relatively slowly, whereas the decane is relatively very volatile and evaporates quickly. We have measured the film mass and area, optical micrographs of the film structure, and the UV absorbance spectra during evaporation. For emulsion films containing the involatile SQ, evaporation of the PG causes collapse of the emulsion structure with some loss of specular UV absorbance due to light scattering. However, for these emulsions with droplets much larger than the wavelength of light, the light is scattered only at small forward angles so does not contribute to the diffuse absorbance and the film SPF. The UV filter remains soluble throughout the evaporation and thus the UV absorption by the filter and the SPF remain approximately constant. Both PG-in-SQ and SQ-in-PG films behave similarly and do not show area shrinkage by dewetting. In contrast, the decane-in-PG film shows rapid evaporative loss of the decane, followed by slower loss of the PG resulting in precipitation of the UV filter and film area shrinkage by dewetting which cause the UV absorbance and derived SPF to decrease. Measured UV spectra during evaporation are in reasonable agreement with spectra calculated using models discussed here.

Macrostructure-controlled titania ceramics derived from particle-stabilized emulsions: Preparation and photocatalysis performance

To enhance the photocatalytic efficiency and resolve the titania recovery problem, we studied the processing of particle-stabilized emulsions for preparing porous TiO2 ceramics in this study. The samples were synthesized with different solid contents from 15 vol% to 30 vol%. The solid content markedly affected the pore structure and morphology. When the solid content in the precursor was controlled at 30 vol%, the sample had the three-dimensional interpenetrating structure with high porosity and uniform pore structure and the average pore size was about 3–18 μm. With excellent pore structures, the samples show the high adsorption capacity and excellent photocatalysis performance for Rhodamine B. Furthermore, when porous TiO2 ceramic was covered by a 2 nm carbon film layer (C/TiO2), the adsorption efficiency and photocatalytic degradation rates were efficiently improved.

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion.

The aim of this paper is to review the key findings about how particle-stabilised (or Pickering) emulsions respond to stress and break down. Over the last ten years, new insights have been gained into how particles attached to droplet (and bubble) surfaces alter the destabilisation mechanisms in emulsions. The conditions under which chemical demulsifiers displace, or detach, particles from the interface were established. Mass transfer between drops and the continuous phase was shown to disrupt the layers of particles attached to drop surfaces. The criteria for causing coalescence by applying physical stress (shear or compression) to Pickering emulsions were characterised. These findings are being used to design the structures of materials formed by breaking Pickering emulsions.

Emulsification Characteristics Using a Dynamic Woven Metal Microscreen Membrane.

An oscillatory emulsification system for the production of oil in water emulsions using a commercially available low-cost woven metal microscreen (WMMS) is investigated. The system allows for independent control of both the oscillation frequencies and amplitudes such that it provides two degrees of freedom for controlling the emulsion properties. The investigations included the production of both surfactant and particle-stabilized emulsions. The average droplet size was found to decrease when both the oscillation frequency and amplitude was increased. For surfactant-stabilized emulsions, using bi-surfactants in both the continuous and dispersed phases resulted in a smaller droplet size due to lower interfacial tension. For particle-stabilized emulsions, both the hydrodynamics of the system and the hydrophobic and hydrophilic nature of the stabilizing particles influenced the interfacial properties at the oil–water interface, which in turn affected the final droplet size and distribution with potential droplet breakage. In absence of the latter, a simple torque balance model can be used to reasonably predict the average emulsion droplet size.

Macroporous Polymers with Aligned Microporous Walls from Pickering High Internal Phase Emulsions.

A novel class of macroporous polymers, open macroporous polymers with aligned microporous void walls, were prepared by combining particle-stabilized high internal phase emulsion (Pickering HIPE) and unidirectional freezing technique. These Pickering HIPEs were prepared by utilizing poly(urethane urea)/(vinyl ester resin) nanoparticles as the sole stabilizer, and this nanoparticles also acted as building blocks for the resulting macroporous polymers. Moreover, the morphology and compression modulus of the resulting porous materials could be tuned easily. This means now Pickering-HIPE templated open-cell foams can be prepared, and this route was normally achieved with surfactant and/or chemical reaction involved.

Preparation of paraffin/porous TiO2 foams with enhanced thermal conductivity as PCM, by covering the TiO2 surface with a carbon layer

In the present study, porous TiO2 foams (PTF) were prepared by particle-stabilized emulsion method. The samples having solid contents ranging from 20wt.% to 35wt.% were prepared and successfully impregnated with paraffin without using a surfactant. SEM images showed that the prepared PTFs had three-dimensional interpenetrating structures with good porosities. The pore structure and morphology of PTF was markedly influenced by the solid content. With increase in solid content, porosity decreased, along with a decrease in the homologous encapsulation ratio of paraffin. Differential scanning calorimetry (DSC) evaluated the thermal properties of the paraffin/PTF (PTFP) composites. DSC results showed that with a solid content of 20wt.%, the paraffin adsorption reached 62vol.% and latent heat of the composite PCM was 94.05J/g after 200 melting/freezing cycles. TGA results showed that the form-stable composite PCMs had good thermal stabilities. In order to further enhance the thermal conductivity and improve the adsorption capacity of PTF, sucrose was added to the emulsion and carbonized in situ to form carbon/PTF (PTFC). This could also be successfully impregnated with paraffin to obtain the form-stable composite paraffin/PTFC (PTFCP). The TEM images and results of XPS analysis confirmed that the surfaces of TiO2 particles were covered with a 2nm thick carbon film (C/TiO2). The thermal conductivity of PTFCP increased from 0.302W/mK to 1.059W/mK compared to PTFP, along with a further improvement in adsorption capacity. Furthermore, the results of FT-IR analysis and thermal cyclic tests showed that the form-stable composite PCMs had good chemical stability and thermal reliability after 200melting/freezing cycles. Therefore, the prepared form-stable composite PCMs having excellent pore structures, thermal properties, thermal reliabilities, and chemical stabilities are promising PCM candidates for heat energy storage applications.

Polydopamine Particle as a Particulate Emulsifier.

“Pickering-type” emulsions were prepared using polydopamine (PDA) particles as a particulate emulsifier and n-dodecane, methyl myristate, toluene or dichloromethane as an oil phase. All the emulsions prepared were oil-in-water type and an increase of PDA particle concentration decreased oil droplet diameter. The PDA particles adsorbed to oil–water interface can be crosslinked using poly(ethylene imine) as a crosslinker, and the PDA particle-based colloidosomes were successfully fabricated. Scanning electron microscopy studies of the colloidosomes after removal of inner oil phase revealed a capsule morphology, which is strong evidence for the attachment of PDA particles at the oil–water interface thereby stabilizing the emulsion. The colloidosomes after removal of inner oil phase could retain their capsule morphology, even after sonication. On the other hand, the residues obtained after oil phase removal from the PDA particle-stabilized emulsion prepared in the absence of any crosslinker were broken into small fragments of PDA particle flocs after sonication.

Nano ellipsoids at the fluid-fluid interface: effect of surface charge on adsorption, buckling and emulsification.

In this contribution, we discuss the role of surface charge on the adsorption of shape anisotropic particles to fluid-fluid interfaces in the context of their application in particle-stabilized emulsions. Starting with a pendent aqueous drop containing nano-ellipsoids of known surface charge density suspended in an oil medium, we study the kinetics of adsorption of the ellipsoids to the water-decane interface using pendant drop tensiometry. The interfacial tension of the drop is recorded as a function of time by analyzing the shape of the drop. We show that the particles that are weakly charged readily adsorb to the water-decane interface and the adsorption behavior is influenced by the particle surface charge density. Furthermore, as the area available for the particles deposited at the interface is reduced, the interface populated with self-assembled ellipsoids shows wrinkles indicating buckling of the particle-laden interface under compression. However, the buckling is not observed if nano-ellipsoids are highly charged confirming that the particles do not adsorb to the interface when they are highly charged. This suggests that in several examples where the particles at interfaces concept is exploited, the repulsive energy barrier due to the particle surface charge plays a key role in the adsorption of particles to the interfaces. However, once the particles are adsorbed, the interfacial properties of the monolayer depend on the particle-particle interactions. Thus a combination of these interactions determines the concentration of particles at the interface, their microstructure and interfacial properties. The effect of these interactions on the quantity and size of the emulsion drops stabilized by ellipsoidal particles is also explored.

Preparation and attachment of liquid-infused porous supra-particles to liquid interfaces

We prepared model porous composite supra-particles and investigated the effect of the initial infused fluid phase on their attachment at the liquid-fluid interface. We used a simple method for fabrication of millimetre-sized spherical porous supra-particles from much smaller monodisperse latex microparticles as building blocks by evaporation of a polystyrene sulphate latex suspension on a hot super-hydrophobic surface. We annealed the dried supra-particles at the polymer's glass transition temperature to fuse partially their latex particle building blocks. Spherical porous supra-particles were produced above 40 wt% initial concentration of the latex particles in the suspension, which had a rough surface, with a porous and amorphous structure. We controlled the supra-particle size by varying the initial volume of the latex suspension drop, the latex particle concentration and the drop evaporation temperature. This preparation technique allowed limited control over the porosity of the supra-particles by varying the initial concentration of the latex particle suspension, the rate of evaporation and the annealing temperature. We characterised the surface morphology and the inner structure of supra-particles by SEM imaging. We report for the first time results of an MRI study of supra-particles attached to an air-water or an oil-water interface, which indicated that only the surface layer of the building block particles attaches to the liquid interface while the pore fluid was not displaced by the outer fluid. We observed that supra-particles infused with water had different wettability and attachment positions at the oil-water interface compared with the same particles infused with oil. Similarly, the infusion of the porous supra-particles with water led to a different attachment at the air-water interface compared to the attachment of the same supra-particle when dry. The fundamental importance of this result is that the porous particles (or colloid particle agglomerates) may give an oil-in-water or water-in-oil Pickering emulsion depending on whether they are initially impregnated with oil or water. The results of this study are relevant for particle-stabilised emulsions and foams in a range of pharmaceutical, food and cosmetic formulations as well as ore flotation.

Three-dimensional fully interconnected highly porous hydroxyapatite scaffolds derived from particle-stabilized emulsions

Three-dimensional (3D) macroporous hydroxyapatite (HAp) ceramics were fabricated via a facile emulsion templating technique for bone tissue engineering applications. The prepared scaffolds have an open, uniform, and 3D fully interconnected pore structure with high porosities, mechanical strength, and a large surface/volume ratio. The porous structure, which is composed of an array of quasi-spherical large cells interconnected through windows, has a pore diameter in the range of 5–30μm. The samples sintered at the optimal temperature (1200°C) have an open porosity of ~76.5% and a high compressive strength of 4.26MPa. The technique discussed here is a significant advancement over other previously used methods to produce HAp ceramics and is expected to enable much improvement in the topology of the porous structure of scaffolds.