To Ngai

High internal phase emulsions stabilized solely by microgel particles.

High internal phase emulsions (HIPEs) are often defined as very concentrated emulsions in which the volume fraction of the internal phase exceeds 0.74. Conventional HIPEs consisting of a continuous organic phase and an internal aqueous phase (water-in-oil, w/o emulsion) are commonly stabilized by large amounts of surfactant. If the continuous phase contains one or more monomeric species that are polymerizable, HIPEs can be used as templates for the fabrication of highly porous materials. Highly porous materials have proven to be useful in a variety of applications, including filtration membranes for molten metals and hot gases, bioreactors, catalyst carriers, and scaffolds for bone replacement and tissue engineering. In addition to surfactants, colloidal particles have also been used to stabilize HIPEs and produce macroporous materials after drying and sintering. 8] The concept of using particle-stabilized, or Pickering, emulsions as templates to manufacture porous materials provides a number of benefits that are not achievable using low-molecular-weight surfactants. Firstly, the particles used as stabilizers in Pickering emulsions are irreversibly adsorbed at the interface of emulsions because of their high energy of attachment, which makes the resultant Pickering emulsions extremely stable with shelf-life stabilities of months or even years. Secondly, the use of Pickering emulsion droplets as templates can functionalize the cell walls of the porous materials with a layer of solid particles, which could, for example, contain functional groups and lead to a variety of further applications. The stability and type of Pickering emulsions depend on the hydrophilicity of the particles. While considerable progress has been made on the fabrication of porous materials by Pickering HIPE templating, one important limitation has still remained. All previous reports on Pickering emulsions deal with emulsions for which the volume fractions of the dispersed phases are less than 0.70. On the basis of the developed thermodynamic model, Kralchevsky et al. have predicted that Pickering emulsions will phase-invert above an internal phase volume fraction of 0.5. In practice, phase inversion is usually observed at a volume fraction of 0.70 after kinetic factors are considered. Binks and co-workers 12] have experimentally demonstrated that Pickering emulsions phase-invert between volume fractions of 0.65 and 0.70, which means that the majority phase becomes the continuous phase. The formation of HIPEs stabilized solely by solid particles seems to be impossible. Menner et al. reported on the first successful preparation of an HIPE stabilized solely by carbon nanotubes with 60% internal phase volume. Akartuna et al. also reported on the preparation of macroporous materials from Pickering HIPE templates stabilized by inorganic particles, but about 35 vol% particles were needed to stabilize the emulsions with internal phase fractions of 72–78%. Recently, using titania or silica particles, stable Pickering HIPEs with an internal phase volume fraction greater than 90% could be prepared. However, the hydrophilicity of the particles should be tailored through prior chemical treatment. Furthermore, to avoid the collapse of the emulsion structure during the production of porous materials, consolidation of the HIPEs by gelation of the continuous phase requires additional strict control over the reaction chemistry, which may be challenging. Hence, the fabrication of porous materials using the Pickering HIPE templating approach that would not need chemical reactions and could be easily extended to a wide variety of chemical compositions is highly desirable. We report herein the stabilization of Pickering hexane-inwater (o/w) HIPEs with volume fractions as high as 0.9 by soft poly(N-isopropylamide)-co-(methacrylic acid) (PNIPAM-coMAA) microgel particles at concentrations as low as 0.05 wt %. Microgel particles are adsorbed at the oil–water interface to hinder extensive droplet coalescence. Excess microgel particles simultaneously form a gel in the continuous phase to trap oil droplets in the gel matrix, which in turn can inhibit creaming of the oil droplets and enhance the emulsion stability. Evaporation in air of the gelled continuous phase of the HIPEs directly leads to porous materials without any chemical reactions. Furthermore, the porosity of the final structure can be tailored by simply changing the microgel particle concentration. Fluorescent PNIPAM-co-MAA microgel particles were synthesized using surfactant-free precipitation polymerization as described before. The details are provided in the Supporting Information (Figures S1–S4). The resultant microgel particles have an average hydrodynamic diameter of 248 nm at room temperature with a solution pH value of 7.5, as determined by dynamic laser light scattering (see the [*] Z. F. Li, Prof. T. Ngai Department of Chemistry, The Chinese University of Hong Kong Shatin, N.T., Hong Kong (China) E-mail: tongai@cuhk.edu.hk Homepage: http://www.chem.cuhk.edu.hk/faculty_ngai_to.htm

Nitrogen-Rich and Fire-Resistant Carbon Aerogels for the Removal of Oil Contaminants from Water

Effective removal of crude oils, petroleum products, organic solvents, and dyes from water is of significance in oceanography, environmental protection, and industrial production. Various techniques including physical and chemical absorption have been developed, but they suffer from problems such as low separation selectivity, a complicated and lengthy process, as well as high costs for reagents and devices. We present here a new material, termed nitrogen-rich carbon aerogels (NRC aerogels,) with highly porous structure and nitrogen-rich surfaces, exhibiting highly efficient separation of specific substances such as oils and organic pollutants. More importantly, we demonstrate that the fabricated NRC aerogels can also collect micrometer-sized oil droplets from an oil-water mixture with high efficiency that is well beyond what can be achieved by most existing separation methods, but is extremely important in practical marine oil-spill recovery because a certain amount of oils often shears into many micrometer-sized oil droplets by the sea wave, resulting in enormous potential destruction to marine ecosystem if not properly collected. Furthermore, our fabricated material can be used like a recyclable container for oils and chemicals cleanup because the oil/chemical-absorbed NRC aerogels can be readily cleaned for reuse by direct combustion in air because of their excellent hydrophobicity and fire-resistant property. We demonstrate that they keep 61.2% absorption capacity even after 100 absorption/combustion cycles, which thus has the highest recyclability of the reported carbon aerogels. All these features make these fabricated NRC aerogels suitable for a wide range of applications in water purification and treatment.

Novel emulsions stabilized by pH and temperature sensitive microgels

Surfactant-free oil-in-water emulsions prepared with temperature and pH sensitive poly(N-isopropylacrylamide)(PNIPAM) microgel particles offer unprecedented control of emulsion stability.

Pure protein scaffolds from pickering high internal phase emulsion template.

The formation of hierarchical porous protein scaffolds from oil-in-water (o/w) high internal phase emulsions (HIPEs) stabilized by bovine serum albumin (BSA) protein nanoparticles (Pickering HIPE) is reported. The route consists of three principal steps. First, a stable o/w HIPE stabilized by BSA protein nanoparticles is formulated. Next, crosslinking the dispersed protein nanoparticles gives rise to a gel in the continuous water phase to freeze the emulsions microstructure. Finally, removal of the oil components and water directly leads to a three dimensional, bimodal meso-macroporous protein scaffold, which is suitable for a wide range of biomedical applications.

One-step formation of w/o/w multiple emulsions stabilized by single amphiphilic block copolymers.

Multiple emulsions are complex polydispersed systems in which both oil-in-water (O/W) and water-in-oil (W/O) emulsion exists simultaneously. They are often prepared accroding to a two-step process and commonly stabilized using a combination of hydrophilic and hydrophobic surfactants. Recently, some reports have shown that multiple emulsions can also be produced through one-step method with simultaneous occurrence of catastrophic and transitional phase inversions. However, these reported multiple emulsions need surfactant blends and are usually described as transitory or temporary systems. Herein, we report a one-step phase inversion process to produce water-in-oil-in-water (W/O/W) multiple emulsions stabilized solely by a synthetic diblock copolymer. Unlike the use of small molecule surfactant combinations, block copolymer stabilized multiple emulsions are remarkably stable and show the ability to separately encapsulate both polar and nonpolar cargos. The importance of the conformation of the copolymer surfactant at the interfaces with regards to the stability of the multiple emulsions using the one-step method is discussed.

Porous TiO2 Materials through Pickering High-Internal Phase Emulsion Templating

We report a facile method for preparing porous structured TiO2 materials by templating from Pickering high-internal phase emulsions (HIPEs). A Pickering HIPE with an internal phase of up to 80 vol %, stabilized by poly(N-isopropylacrylamide)-based microgels and TiO2 solid nanoparticles, was first formulated and employed as a template to prepare the porous TiO2 materials with an interconnected structure. The resultant materials were characterized by scanning electron microscopy, X-ray diffraction, and mercury intrusion. Our results showed that the parent emulsion droplets promoted the formation of macropores and interconnecting throats with sizes of ~50 and ~10 μm, respectively, while the interfacially adsorbed microgel stabilizers drove the formation of smaller pores (~100 nm) throughout the macroporous walls after drying and sintering. The interconnected structured network with the bimodal pores could be well preserved after calcinations at 800 °C. In addition, the photocatalytic activity of the fabricated TiO2 was evaluated by measuring the photodegradation of Rhodamine B in water. Our results revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules.

Poly(N-isopropylacrylamide) microgels at the oil–water interface: adsorption kinetics

Understanding the adsorption behaviors of soft poly(N-isopropylacrylamide) (PNIPAM) microgels to the oil–water interface has become increasingly important both in terms of fundamental science and applications of microgels as multi-stimuli responsive emulsion stabilizers. In the present work, we used pendant drop tensiometry to trace the evolution of oil–water interfacial tensions. We investigated two PNIPAM microgels with different cross-link density as well as poly(styrene-co-NIPAM) particles. We found that the adsorption of microgels from the aqueous phase to the oil–water interface is dominated by two steps. Microgels first diffuse to the oil–water interface and this diffusion process depends on microgel concentration in the bulk phase. The second process involves the deformation and spreading of microgels at the interface. The second process depends strongly on microgel deformability. The behavior of the different microgel systems is compared with conventional Pickering stabilizers and proteins. Our results demonstrate that the softness of the microgels dominates their properties at the oil–water interface. The change of microgel shape at the interface resembles the unfolding transitions observed with proteins. On the other hand, microgels are distinctly different from conventional, rigid Pickering stabilizers.

Macroporous polymer from core-shell particle-stabilized Pickering emulsions.

Poly(styrene-co-N-isopropylacrylamide) (PS-co-PNIPAM) core-shell particles were synthesized and used as particulate emulsifiers in the preparation of particle-stabilized (Pickering) emulsions. Highly concentrated oil-in-water emulsions with an internal phase up to 80 vol % can be produced using PS-co-PNIPAM core-shell particles along as the emulsifiers in emulsions. The core-shell particles are adsorbed at the liquid interface, acting as a barrier against oil droplet coalescence. In addition, it is likely that excess particles simultaneously form a gel in the continuous phase to trap oil droplets in the gel matrix, in turn inhibiting creaming and phase inversion. Evaporation in air of such a core-shell particle-stabilized emulsion directly leads to porous membranes in the absence of chemical reactions. The pore walls of the final structures are densely packed with layers of the core-shell particles. This provides great flexibility to prepare functionalized porous materials for opening up new applications.

Gelatin Particle-Stabilized High Internal Phase Emulsions as Nutraceutical Containers

In this paper, we report for the first time the use of a well-dispersed gelatin particle as a representative of natural and biocompatible materials to be an effective particle stabilizer for high internal phase emulsion (HIPE) formulation. Fairly monodispersed gelatin particles (∼200 nm) were synthesized through a two-step desolvation method and characterized by dynamic light scattering, ζ-potential measurements, scanning electron microscopy, and atomic force microscopy. Those protein latexes were then used as sole emulsifiers to fabricate stable oil-in-water Pickering HIPEs at different concentrations, pH conditions, and homogenization times. Most of the gelatin particles were irreversibly adsorbed at the oil-water interface to hinder droplet coalescence, such that Pickering HIPEs can be formed by a small amount of gelatin particles (as low as 0.5 wt % in the water phase) at pH far away from the isoelectric point of the gelatin particles. In addition, increasing homogenization time led to narrow size distribution of droplets, and high particle concentration resulted in more solidlike Pickering HIPEs. In vitro controlled-release experiments revealed that the release of the encapsulated β-carotene can be tuned by manipulating the concentration of gelatin particles in the formulation, suggesting that the stable and narrow-size-distributed gelatin-stabilized HIPEs had potential in functional food and pharmaceutical applications.

Plasmonic Gold−Superparamagnetic Hematite Heterostructures

Bifunctional heterostructures composed of gold nanorods and hematite nanoparticle aggregates are prepared facilely through the hydrothermal decomposition of ferric acetylacetonate on the surface of the nanorods. The gold nanorod in the heterostructure is partially coated with the hematite nanoparticle aggregates. The hematite coating is porous. These heterostructures exhibit both plasmonic and superparamagnetic properties. They are well-dispersed in aqueous solutions. Such heterostructures will be potentially useful for many biotechnological applications.