Hydrophilic polymer foams with well‐defined open‐cell structure prepared from pickering high internal phase emulsions

Open‐cell hydrophilic polymer foams are prepared through oil‐in‐water Pickering high internal phase emulsions (HIPEs). The Pickering HIPEs are stabilized by commercial titania (TiO₂) nanoparticles with adding small amounts of non‐ionic surfactant Tween85. The morphologies, such as average void diameter and interconnectivity, of the foams can be tailored easily by varying the TiO₂ nanoparticles and Tween85 concentrations. Further, investigation of the HIPE stability, emulsion structure and the location of TiO₂ nanoparticles in resulting foams shows that the surfactant tends to occupy the oil‐water interface at the contact point of adjacent droplets, where the interconnecting pores are hence likely to be formed after the consolidation of the continuous phase. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Effect of Emulsification Method on the Properties of Lecithin‐ and PGPR‐Stabilized Water‐in‐Oil‐Emulsions

Water‐in‐oil (w/o) emulsions were prepared with phosphatidylcholine‐depleted lecithin or polyglycerol polyricinoleate (PGPR) as emulsifying agents. The effect of different laboratory emulsification devices and the effect of sodium chloride on particle size distribution, coalescence stability, and water droplet sedimentation were investigated. The properties of lecithin‐stabilized w/o emulsions were found to depend more strongly on the emulsifying method than those prepared with PGPR. The rotor‐stator system was not suitable for preparing stable w/o emulsions with lecithin. Whereas the addition of salt was essential to achieve coalescence‐stable emulsions prepared with PGPR, the presence of NaCl favored the coalescence of water droplets and phase separation in emulsions containing lecithin.     

Polymerized pickering HIPEs: Effects of synthesis parameters on porous structure

A polyHIPE is a highly porous polymer synthesized from monomers within the external phase of a high internal phase emulsion (HIPE). The large amount of difficult to remove surfactant needed for HIPE stabilization can affect the properties of the resulting polymer. A Pickering emulsion is a surfactant‐free emulsion stabilized by solid particles that preferentially migrate to the interface. In this article, the synthesis of crosslinked polyacrylate polyHIPEs based on Pickering HIPEs stabilized using silane‐modified silica nanoparticles is described and the effects of the synthesis parameters on the porous structure are discussed. The silane chemistry, silane content, and nanoparticle content had significant effects on the size of the polyhedral, relatively closed‐cell polyHIPE voids that resulted from aqueous‐phase initiation. Increasing the mixing intensity reduced the wall thickness and produced a more open‐cell structure. The locus of initiation had a significant effect on polyHIPE morphology. Organic‐phase initiation yielded larger, more spherical voids from the more extensive coalescence before the structure could be “locked‐in” at the gel point. Most significantly, the nanoparticles were located within the polymer walls rather than at the interface, as might be expected. The void walls were shown to be an assembly of nanoparticle agglomerate shells that become embedded within the polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1516–1525, 2010     

Hydrophobic polyurethane polyHIPEs templated from mannitol within nonaqueous high internal phase emulsions for oil spill recovery

Although amphiphilicity is an integral component for the applications of polyHIPEs (PHs), it is challenging to produce hydrophobic PHs from hydrophilic monomers. Herein, hydrophobic polyurethane (PU) PHs have been fabricated from a water‐soluble mannitol within block copolymer surfactant‐stabilized, nonaqueous high internal phase emulsions (HIPEs). These highly porous, interconnected, macroporous PU PHs were hydrophobic with water contact angles between 102° and 140°, demonstrating that water‐soluble monomers could be used for fabrication of hydrophobic PHs. The block copolymer surfactant acted not only as the HIPE stabilizer, but also as a monomer, enhancing hydrophobicity and overcoming some drawbacks imposed by conventional inert stabilizers. The solvents used for PU PH synthesis and purification were easily recovered and reused, showing that nonaqueous HIPE templating for PU PH preparation is an efficient and facile route. The PU PHs were investigated for oil spill reclamation and they were demonstrated to be an ideal candidate for such an application. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1315–1321     

All‐Silica Submicrometer Colloidosomes for Cargo Protection and Tunable Release

Colloidosomes have received considerable attention for the controlled delivery of active ingredients in medicine, agrochemicals, and cosmetics. However, most reported colloidosomes are highly permeable and size is larger than 1 μm. All silica colloidosomes have now been prepared with adjustable size, compact shell and low permeability. Our approach is based on the formation of inverse water‐in‐oil (w/o) emulsions stabilized solely by hydrophobic silica nanoparticles and subsequent locking of the particle at the oil–water interface by a simple sol–gel reaction of silica precursor at room temperature. The colloidosomes obtained display a robust and closed shell, ensuring a long‐term retention of small hydrophilic molecules such as Methylene Blue. Remarkably, unlike all other reported silica colloidosomes, a timely and stepwise release of the encapsulated cargo can be triggered by adding ethanol or surfactant without destroying the capsule shell.     

Double inversion of emulsions induced by salt concentration

The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.     

Inversion of silica-stabilized emulsions induced by particle concentration

Emulsions of equal volumes of a cyclic silicone oil and water stabilized by fumed silica nanoparticles alone can be inverted from oil-in-water (o/w) to water-in-oil (w/o) by simply increasing the concentration of particles. The phenomenon is found to be crucially dependent both on the inherent hydrophobicity of the particles and on their initial location. Inversion only occurs in systems with particles of intermediate hydrophobicity when dispersed in oil; emulsions prepared from the same particles but initially dispersed in water remain o/w at all particle concentrations. The stability and drop size distributions in the different emulsions are compared. Various hypotheses are put forward and argued to explain this novel inversion route including adsorption of oil onto particle surfaces, hysteresis of contact angle affecting particle wettability in situ, and the structure of particle dispersions in oil or water prior to emulsification inferred from rheology and light scattering measurements. We propose that the tendency for particles to behave more hydrophobically at higher concentrations in oil is due to the reduction in the effective silanol content at their surfaces as a result of gel formation via silanol-silanol hydrogen bonds. In water, solvation of particle surfaces prevents this from occurring and particles behave as hydrophilic ones at all concentrations. A concentration-induced change in particle wettability is thus advanced.     

Porous polyurethanes synthesized within high internal phase emulsions

PolyHIPE are highly porous, emulsion‐templated polymers typically synthesized via free‐radical polymerization within a water‐in‐oil (W/O) high internal phase emulsion (HIPE) whose dispersed, aqueous phase occupies more than 74% of the volume. The synthesis of a polyHIPE containing biodegradable polymers is not straightforward because the presence of both an organic phase and an aqueous phase within the HIPE limits the type of polymerization reactions that can be used. This article describes the synthesis of polyHIPE containing biodegradable poly(ε‐caprolactone) (PCL) groups through the step‐growth reaction of a diisocyanate with a flexible PCL triol to form a crosslinked polyurethane. The reaction of the diisocyanate with the water in the HIPE produced urea groups and large bubbles from the generation of CO₂. The polymer walls between these bubbles consisted of a porous, emulsion‐templated structure. Polymerization with an excess of diisocyanate produced a significant enhancement in the amounts of urea and CO₂. The reduction in the flexible PCL content and the enhancement in the rigid urea content produced an increase in wall modulus that was over 20‐fold. The ability to synthesize polyHIPE through such step‐growth reactions is an important advance in the adaptation of polyHIPE for such applications as tissue engineering. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5806–5814, 2009     

Porosity control in high internal phase emulsion templated polyelectrolytes via ionic crosslinking

A series of novel macroporous materials based on poly(N‐isopropylacrylamide)‐b‐sodium polyacrylate is synthesized via aqueous reversible addition‐fragmentation chain transfer polymerization in an oil‐in‐water high internal phase emulsion (HIPE) utilizing both covalent and ionic crosslinkers (PEG diacrylate and calcium diacrylate, respectively). Porosity is directly related to the calcium diacrylate content of the polyHIPE. Depth profiling XPS of pressed samples reveal the segregation of less polar substituents (PNIPAM, PEGDA) to the interface, whereas ionic components are located deeper within the continuous aqueous phase, primarily driven by ionic strength. This segregation of components stabilizes the internal‐continuous phase interface and results in decreased void diameter. Calcium diacrylate also forms ionic crosslinks in the polyHIPE material, resulting in increased interconnecting pore diameter due to volume contraction upon polymerization. Evidence of volume contraction is provided by the stress induced on PEG at the o/w interface by internally located calcium polyacrylate crosslinks, resulting in a decrease in XRD peak intensity. It is therefore proposed that calcium diacrylate is capable of modifying polyHIPE morphology via two separate mechanisms. Published 2016 1 . Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2486–2492     

Preparation of emulsion‐templated fluorinated polymers and their application in oil/water separation

A series of emulsion‐templated fluorinated polymers (polyHIPEs) were first synthesized with introducing 2‐(perfluorohexyl)ethyl methacrylate (PEM) to the external phase of water‐in‐styrene high internal phase emulsion (HIPE) templates. The morphology (i.e., void size and its distribution) of these porous materials could be tuned simply by changing PEM and/or surfactant amount. The synergistic effect between the surface chemistry and surface architecture allowed the polyHIPEs to possess hydrophobicity with a water contact angle of 151°. The superhydrophobicity and oleophilicity of the polyHIPEs, together with their highly open porous structure, make the material a very competitive candidate as a filtration material for oil/water separation in practice with the efficiency of separating dichloromethane from the oil/water mixture of 95%. Such oil/water separating capacity was maintained after 10 cycles of filtration of oil/water, indicating the cyclic usage of the polyHIPE is feasible. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1508–1515     

A Visible Spectroscopic Study of Microstructure of O/W Microemulsions of Anionic Surfactants: Effect of Cosurfactant

The microstructure of o/w microemulsions, stabilized by sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) with different cosurfactants, has been studied by partitioning of a dye, phenol red, between the oil‐water interface and bulk water. The cosurfactants used are propan‐1‐ol, propan‐2‐ol, butan‐1‐ol, butan‐2‐ol, pentane‐1‐ol, pentane‐2‐ol, and pentan‐3‐ol. The effects of changing the oil volume fraction and surfactant‐cosurfactant w/w ratio on the oil‐water interface and droplet size have also been discussed. Larger droplet size was predicted for SDS than SDBS. The predicted droplet radius increased with increase in the oil fraction, decrease in the surfactant concentration, increase in the C‐number of the linear cosurfactant, and decrease in branching of the cosurfactant. Surfactant‐cosurfactant ratio and pH did not affect the droplet size significantly. The minimum concentrations of surfactants with which microemulsions were formed were found to be higher for larger oil fraction, smaller C‐number of the alcohol, more branching of the alcohol, and higher pH.     

A Simple and Fast Aqueous‐Phase Synthesis of Ultra‐Highly Concentrated Silver Nanoparticles and Their Catalytic Properties

A simple and fast synthetic route to ultra‐highly concentrated silver nanoparticles with long‐term stability by reducing AgNO₃ with ascorbic acid in the presence of polyethyleneimine (PEI) as a stabilizer in an aqueous phase is reported. The concentration of silver precursor was as high as 2000 mm (200 g of Ag nanoparticle per liter of water) and the reaction time was less than 10 min. The resulting silver nanoparticles show long‐term stability after two months of storage at room temperature without any signs of particle aggregation or precipitation in an aqueous phase. The successful ligand exchange of PEI‐stabilized silver nanoparticles to polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) without particle aggregation is also demonstrated. In addition, the catalytic activities of silver nanoparticles stabilized by various stabilizers prepared by the ligand exchange method was investigated. The PEI‐stabilized silver nanoparticles exhibited a higher stability than those of PEG‐ and PVP‐stabilized silver nanoparticles in the diffusion‐controlled catalytic reduction of 4‐nitrophenol to 4‐aminophenol by NaBH₄.     

High internal phase emulsions: catastrophic phase inversion, stability, and triggered destabilization

We have investigated the formation, drop sizes, and stability of emulsions prepared by hand shaking in a closed vessel in which the emulsion is in contact with a single type of surface during its formation. The emulsions undergo catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) as the oil volume fraction is increased. We find that the oil volume fraction required for catastrophic inversion exhibits a linear correlation with the oil-water-solid surface contact angle. W/o high internal phase emulsions (HIPEs) prepared in this way contain water drops of diameters in the range 10-100 μm; emulsion drop size depends on the surfactant concentration and method of preparation. W/o HIPEs with large water drops show water separation but w/o HIPEs with small water drops are stable with respect to water separation for more than 100 days. The destabilization of the w/o HIPEs can be triggered by either evaporation of the oil continuous phase or by contact the emulsion with a solid surface of the "wrong" wettability.     

Emulsion Inversion in an Oil‐Surfactant‐Water System Based on Model Naphthenic Acids under Alkaline Conditions

Emulsion inversion has been studied in a system based on oil (toluene/heptane), 5β‐cholanic acid, and an alkaline brine solution by varying the concentration of sodium hydroxide. At an intermediate pH w/o emulsions were formed, and in the high pH region o/w emulsions were formed. Emulsion inversion occurred in the pH range 8.5–10. The w/o emulsions were consistently more stable compared to the o/w emulsions. Increasing the amount of acid enhanced the stability of the emulsions. Maximum stability was observed close to pH 8, where the ratio between the undissociated and dissociated acid was approximately 1.5. From light microscopy, it can be seen that the emulsions are stabilized by a liquid gel phase. At equilibrium the system consists of an oil phase, a liquid gel phase, and an aqueous phase. Increasing the oil fraction eventually gave only w/o emulsions in the pH range between 7 and 14. For these emulsions, no obvious difference in stability was observed at pH 8, while the stability of the emulsions in the high pH region was significantly enhanced. An increase of the ratio between toluene and heptane gave no obvious difference in either stability or type of emulsion while varying the pH. Use of a less lipophilic acid, such as 4‐octylbenzoic acid, gave very unstable w/o emulsions in the intermediate pH region, while stable o/w emulsions were found in the high pH region.     

Assembly of a Metal–Organic Framework into 3 D Hierarchical Porous Monoliths Using a Pickering High Internal Phase Emulsion Template

3D Hierarchical porous metal–organic framework (MOF) monoliths are prepared by using Pickering high internal phase emulsion (HIPE) template. Pickering HIPEs were stabilized solely by UiO‐66 MOF particles with internal phase up to 90 % of the volume. The effects of internal phase type and volume, as well as MOF particle concentration on the stability of resulting Pickering HIPEs were investigated. Furthermore, by adding small amount of polyvinyl alcohol (PVA) as binder or polymerization in the continuous aqueous phase, followed by freeze‐drying, two types of MOF‐based 3D hierarchical porous monoliths with ultralow density (as low as 12 mg cm^?3) were successfully prepared. This Pickering HIPE template approach provides a facile and practical way for assembling of MOFs into complex structures.     

Porous multifunctional fluoropolymer composite foams prepared via humic acid modified Fe3O4 nanoparticles stabilized Pickering high internal phase emulsion using cationic fluorosurfactant as co-stabilizer

Fluoropolymers are very important owing to their excellent application performances, especially in extreme conditions. On the other hand, the preparation of porous fluoropolymers is a difficult task due to unavailability of suitable surfactants as well as tedious synthesis steps. Here we prepared multifunctional porous fluoropolymer composite foams with a simple process of “high internal phase emulsion (HIPE)” by using humic acid modified iron oxide nanoparticles (HA-Fe₃O₄ NPs) and cationic fluorosurfactant (CFS) (PDMAEMA-b-PHFBA) as co-stabilizer. The inclusion of HA-Fe₃O₄ NPs in the system made fluoro-HIPE more stable than the emulsion prepared using only CFS or other conventional stabilizers. Morphology of the prepared polyHIPE was easily controlled by altering the concentration of HA-Fe₃O₄ and/or CFS in the original formulation. Adjustment of the porous structure with open/close cells was performed and the average diameter of the pores tuned between 4.9 and 23 μm. With the increase in specific surface area by using nanoparticles (NPs) and CFS as co-surfactants, Pickering HIPE monoliths adsorbed double amount of oil compared to foams based solely on HIPE template. Multiple functional groups were bound onto Fe₃O₄ NPs through HA modification that made the fluoro-monolith capable of adsorbing dye, i.e. methylene blue, from water. A simple centrifugation enabled regeneration of the oil soaked foams and adsorption capacity was not decreased after 10 adsorption/regeneration cycles.     

Effect of alpha-lactalbumin on aerosol-OT phase structures in oil/water mixtures

The ability of water-soluble, globular proteins to tune surfactant/oil/water self-assemblies has potential for the formation of biocompatible microemulsions and also plays a role in protein function at biological interfaces. In this work, we examined the effect of the protein alpha-lactalbumin on Aerosol-OT (AOT) phase structures in equivolume mixtures of oil and 0.1 M brine. In this pseudo-ternary system, surfactants are free to move to either oil or water phase to adopt phase structures close to the spontaneous curvature of the surfactants. Using small-angle X-ray scattering, we observed that addition of this protein changed the spontaneous curvature of the surfactant monolayer substantially. In the absence of protein, AOT adopted a negative spontaneous curvature to form spherical w/o microemulsion droplets. When less than 1 wt % of alpha-lactalbumin was added into the system, the w/o droplets became nonspherical and larger in volume, corresponding to an increase in water uptake into the droplets. As the protein-to-surfactant ratio increased, protein, surfactant, and oil increasingly partitioned toward the aqueous phase. There the protein triggered the formation of o/w microemulsions with a positive spontaneous curvature. These protein-containing structures exhibited significant interparticle attraction. We also compared the influence of two oil types, isooctane and cyclohexane, on the protein/surfactant interactions. We propose that the more negative natural curvature of the AOT/cyclohexane monolayer in the absence of protein prevented protein incorporation within organic phase structures and consequently pushed the system self-assembly toward aqueous aggregate formation.     

Evaporation rates of water from concentrated oil-in-water emulsions

We have investigated the rate of water evaporation from concentrated oil-in-water (o/w) emulsions containing an involatile oil. Evaporation of the water continuous phase causes compression of the emulsion with progressive distortion of the oil drops and thinning of the water films separating them. Theoretically, the vapor pressure of water is sensitive to the interdroplet interactions, which are a function of the film thickness. Three main possible situations are considered. First, under conditions when the evaporation rate is controlled by mass transfer across the stagnant vapor phase, model calculations show that evaporation can, in principle, be slowed by repulsive interdroplet interactions. However, significant retardation requires very strong repulsive forces acting over large separations for typical emulsion drop sizes. Second, water evaporation may be limited by diffusion in the network of water films within the emulsion. In this situation, water loss by evaporation from the emulsion surface leads to a gradient in the water concentration (and in the water film thickness). Third, compression of the drops may lead to coalescence of the emulsion drops and the formation of a macroscopic oil film at the emulsion surface, which serves to prevent further water evaporation. Water mass-loss curves have been measured for silicone o/w emulsions stabilized by the anionic surfactant SDS as a function of the water content, the thickness of the stagnant vapor-phase layer, and the concentration of electrolyte in the aqueous phase, and the results are discussed in terms of the three possible scenarios just described. In systems with added salt, water evaporation virtually ceases before all the water present is lost, probably as a result of oil-drop coalescence resulting in the formation of a water-impermeable oil film at the emulsion surface.     

MULTIPLE EMULSIONS STABILIZED BY COLLOIDAL MICROGRYSTALLINE CELLULOSE

Multiple emulsions stabilized by colloidal microcrystalline cellulose (CMCC, Avicel RC591) at the w/o and o/w interfaces, and by the addition of Span 80 or Span 85 at the w/o interface, were studied by means of brightfield microscopy, freeze-etch electron microscopy, droplet size distribution analysis and rheologic measurements. Stable multiple emulsions were prepared by incorporation of sodium chloride in the innermost aqueous phase, thereby creating an osmotic gradient preventing loss of the inner aqueous phase to the external aqueous phase. Freeze-etch electron microscopy of the multiple emulsions indicated the presence of a network of microcrystalline cellulose at the outer o/w interface. It may be assumed that the surfactant directly stabilized the w/o interface by adsorption at the interface, as well as indirectly by facilitating wetting of the microcrystalline cellulose by the oil. From rheologic measurements, the existence of a three-dimensional network in the external aqueous phase was indicated by the considerable degrees of thlxotropy and significant static yield values of these multiple emulsions.     

Double emulsion droplets as microreactors for synthesis of magnetic macroporous polymer beads

An easy method is presented to fabricate monodisperse magnetic macroporous polymer beads（MMPBs）. Waterin-oil high internal phase emulsion（HIPE） is prepared by emulsifying aqueous iron ions solution in an oil phase containing monomers. The HIPE is introduced into a simple microfluidic device to fabricate monodisperse（water-in-oil）-in-water double emulsion droplets. The droplets serve as microreactors to synthesize Fe3O4 nanoparticles and are on-line polymerized to form MMPBs. The prepared MMPBs display uniform size, interconnected porous structure, superparamagnetic behavior and uniform distribution of Fe3O4 in polymer matrix. The MMPBs are characterized by scanning electron microscopy（SEM）, Fourier transform infrared spectroscopy（FTIR）, X-ray diffraction（XRD）, transmission electron microscopy（TEM）, vibrating sample magnetometry（VSM）. We believe that this method is a universal technique in preparing macroporous nanocomposite beads.