Robust porous organosilica monoliths via a surfactant-free high internal phase emulsion process for efficient oil-water separation

Superhydrophobic/ superoleophilic polystyrene-based porous material with superelasticity for highly efficient and continuous oil/water separation in harsh environments

Three-dimensional separation materials with robust physical/chemical stability have great demand for effective and continuous separation of immiscible oil/water mixtures and water-in-oil emulsions, resulting from chemical leakages and discharge of industrial oily wastewaters. Herein, a superelastic polystyrene-based porous material with superhydrophobicity/superoleophilicity was designed and prepared by high internal phase emulsion polymerization to meet the aforementioned requirements. A flexible and hydrophobic aminopropyl terminated polydimethylsiloxane (NH2-PDMS-NH2) segment was introduced into the rigid styrene-divinylbenzene copolymer through 1, 4-conjugate addition reaction with trimethylolpropane triacrylate. The addition of NH2-PDMS-NH2 simultaneously improved the mechanical and hydrophobic properties of the porous material (the water contact angle from 141.2° to 152.2°). The material exhibited outstanding reversible compressibility (80% strain, even in liquid N2 environments) and superhydrophobic stability, even after being repeatedly compressed 100 times, water contact angle still remained above 150°. Meanwhile, the as-prepared material had outstanding hydrophobic stability in corrosive solutions (strong acidic, alkaline, high-salty, and even strong polar solvent), presence of mechanical interference, strong UV radiations, and high/low temperature environments. More importantly, the material could continuously and efficiently separate immiscible oil/water mixture and water-in-oil emulsions under the above conditions, showing huge potential for the large-scale remediation of complex oily wastewaters.

Synthesis and Applications of Elastomeric Polymerized High Internal Phase Emulsions (PolyHIPEs)

Polymer foams (PFs) are among the most industrially produced polymeric materials, and they are found in applications including aerospace, packaging, textiles, and biomaterials. PFs are predominantly prepared using gas-blowing techniques, but PFs can also be prepared from templating techniques such as polymerized high internal phase emulsions (polyHIPEs). PolyHIPEs have many experimental design variables which control the physical, mechanical, and chemical properties of the resulting PFs. Both rigid and elastic polyHIPEs can be prepared, but while elastomeric polyHIPEs are less commonly reported than hard polyHIPEs, elastomeric polyHIPEs are instrumental in the realization of new materials in applications including flexible separation membranes, energy storage in soft robotics, and 3D-printed soft tissue engineering scaffolds. Furthermore, there are few limitations to the types of polymers and polymerization methods that have been used to prepare elastic polyHIPEs due to the wide range of polymerization conditions that are compatible with the polyHIPE method. In this review, an overview of the chemistry used to prepare elastic polyHIPEs from early reports to modern polymerization methods is provided, focusing on the applications that flexible polyHIPEs are used in. The review consists of four sections organized around polymer classes used in the preparation of polyHIPEs: (meth)acrylics and (meth)acrylamides, silicones, polyesters and polyurethanes, and naturally occurring polymers. Within each section, the common properties, current challenges, and an outlook is suggested on where elastomeric polyHIPEs can be expected to continue to make broad, positive impacts on materials and technology for the future.

Water-in-oil high internal phase Pickering emulsions formed by spontaneous interfacial hydrolysis of monomer oil

HYPOTHESIS

Alcohols can strongly reduce the interfacial tension between immiscible liquids, thus facilitating the formation of emulsions. By combining non-surface-active hydrophobic particles with medium-chain alcohols, stable water-in-oil (w/o) high internal phase Pickering emulsions (HIPPEs) can be easily prepared without high-energy emulsification methods.

EXPERIMENTS

The emulsions containing acrylate monomer as the oil phase were prepared at different pH values in the presence of hydrophobic silica particles. Further, by replacing monomer oil with organic solvents (e.g., toluene) and a certain concentration of alcohol, the promoted particle adsorption at the oil-water interface has been systematically investigated. The morphology and interfacial structure of HIPPEs were visualized by confocal laser scanning microscopy (CLSM).

FINDING

At high pH, stable water-in-acrylate monomer HIPPEs can be formed using commercial fumed silica nanoparticles alone with simple stirring or vortexing. The hydrolysis of the acrylate group at high pH can generate alcohols in situ which adsorb at the oil-water interface to reduce the interfacial tension and promote particle adsorption to hinder droplet coalescence. The novel strategy for forming stable and processable HIPPEs can be universally applied to different hydrophobic silica particles with the help of various alcohols as the co-stabilizer, which provides a flexible approach for the fabrication of lightweight, closed-cell solid foams for a range of applications.

All-natural oil-in-water high internal phase Pickering emulsions featuring interfacial bilayer stabilization

HYPOTHESIS

Synergistic stabilization of high internal phase Pickering emulsions (HIPPEs) by food-grade colloidal particles are necessary for food, pharmaceuticals or cosmetics owing to their biocompatibility and multi-functionality. By tuning the interfacial structure of adsorbed binary particles, the HIPPE may exhibit extraordinary characteristics compared to conventional all-natural HIPPEs solely stabilized by single-component particle or composite particle, which should have potential applications in varies fields.

EXPERIMENTS

HIPPEs were prepared by using zein protein nanoparticles (ZNPs) and starch nanocrystals (SNCs) as stabilizers. We systematically investigated the effect of particle concentration and internal phase fraction on HIPPEs morphology, stability and rheological behaviors. Further, the stabilization mechanism as well as potential applications were demonstrated.

FINDINGS

HIPPEs were prepared with excellent stability against centrifugation and high temperature (50 °C). Our result indicates the successful construction of unique bilayer interfacial structures consisting of inner ZNPs layer and outer SNCs layer. Since SNCs could gelatinize at 50 °C, dense shells can form around droplets afterwards. Such thermally responsive interfacial structures can be used to protect hydrophobic bioactive substances at higher temperatures while still allowing controlled release at certain conditions. Furthermore, with high internal phase fraction, HIPPEs can possibly replace mayonnaise and salad dressing on the market due to comparable appearance and properties. Following the removal of inner oil, porous materials can be further fabricated, which have potential applications in environmental protection or tissue engineering.

A comparison between superhydrophobic surfaces (SHS) and slippery liquid-infused porous surfaces (SLIPS) in application

Slippery liquid-infused porous surfaces inspired by the Nepenthes pitcher plant exhibit excellent performances and are known for their extremely low contact angle hysteresis (<5°) and smooth surface. In contrast, superhydrophobic surfaces (SHS) exhibit poor pressure stability, difficulty in self-healing, and difficulty in removing low surface tension liquids or organic solvents, which can affect the stable air layer. Thus, these issues can be avoided through the replacement of SHS with slippery liquid infused porous surfaces (SLIPS). In this review, the theoretical models of SHS and SLIPS are classified initially, and several design standards for the preparation of SLIPS are briefly described. Then, we focus on comparing the differences in the application of SHS and SLIPS, such as pressure stability, transparency, and droplet manipulation. However, there are still some problems that need to be improved during the preparation of SLIPS, such as the evaporation of the lubricant layer, the use of a lubricant layer of toxic perfluoropolyether and other substances, and easily lost nanostructured lubricant layer. Accordingly, several new improved methods are proposed in this review, and finally, the potential applications and development prospects of SLIPS are presented.

Flexible, Strong, Multifunctional Graphene Oxide/Silica-Based Composite Aerogels via a Double-Cross-Linked Network Approach

Multifunctional silica-based aerogels are an emerging material due to their unique properties and wide applications. However, their large-scale production and application are limited due to the high cost and cumbersome preparation process. Herein, we prepare graphene oxide (GO)/silica-based composite aerogels via a simple in situ sol-gel reaction. GO nanosheets (GOs) are functionalized with polyethylenimine (PEI) and 3-glycidyloxypropyltrimethoxysilane (GPTMS) successively. After a cohydrolysis and condensation of trimethoxymethylsilane (MTMS) and dimethoxydimethylsilane (DMDMS) in the presence of GOs and a convenient ambient-pressure drying process, the composite aerogels are obtained. In addition to the normal cross-linking of MTMS and DMDMS, the GOs also behave as cross-linking points to significantly enhance the mechanical properties and thermal stability of the network of the composite aerogels. The pore structure of the aerogels is tailored by varying the GO loads as well as its surface modification. The Young's modulus of a composite aerogel with a GO load of 0.5 wt % is about 5 times that for a neat polysiloxane aerogel, and the maximum degradation rate temperature is increased to over 90 °C. Compared with pure polysiloxane aerogel, the thermal insulation and flame resistance are also improved by a small addition of GOs. Moreover, GO/silica-based composite aerogels show stable piezo-resistive behavior. With the excellent mechanical properties, thermal stability, and multifunctionality, GO/silica-based composite aerogels show promising applications under some harsh and extreme conditions.