Elena Tervoort

Stabilization of Oil-in-Water Emulsions by Colloidal Particles Modified with Short Amphiphiles

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.

Materials from foams and emulsions stabilized by colloidal particles

Foams and emulsions stabilized by colloidal particles can lead to new materials with unique structures and properties. In this Highlight article, we describe the underlying mechanisms of this new enabling technology, highlighting some of the processing routes to obtain capsules and porous structures for a variety of applications.

General Route for the Assembly of Functional Inorganic Capsules

Semipermeable, hollow capsules are attractive materials for the encapsulation and delivery of active agents in food processing, pharmaceutical and agricultural industries, and biomedicine. These capsules can be produced by forming a solid shell of close packed colloidal particles, typically polymeric particles, at the surface of emulsion droplets. However, current methods to prepare such capsules may involve multistep chemical procedures to tailor the surface chemistry of particles or are limited to particles that exhibit inherently the right hydrophobic-hydrophilic balance to adsorb around emulsion droplets. In this work, we describe a general and simple method to fabricate semipermeable, inorganic capsules from emulsion droplets stabilized by a wide variety of colloidal metal oxide particles. The assembly of particles at the oil-water interface is induced by the in situ hydrophobization of the particle surface through the adsorption of short amphiphilic molecules. The adsorption of particles at the interface leads to stable capsules comprising a single layer of particles in the outer shell. Such capsules can be used in the wet state or can be further processed into dry capsules. The permeability of the capsules can be modified by filling the interstices between the shell particles with polymeric or inorganic species. Functional capsules with biocompatible, bioresorbable, heat-resistant, chemical-resistant, and magnetic properties were prepared using alumina, silica, iron oxide, or tricalcium phosphate as particles in the shell.

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.

Controlling Phase Distributions in Macroporous Composite Materials through Particle-Stabilized Foams

Aqueous foams stabilized by ceramic and thermoplastic polymeric particles provide a general method for producing novel porous materials because their extraordinary stability against disproportionation and drainage allows them to be dried and sintered into solid materials. Here, we report the different microstructures that can be obtained from liquid foams stabilized by binary mixtures of particles when the interfacial energies between the particles and the air-liquid interfaces are manipulated to promote either preferential or competitive self-assembly of the particles at the foam interface. Modification of the interfacial energies was accomplished through surface modification of the particles or by decreasing the surface tension of the aqueous phase. Materials derived from liquid foams stabilized by poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles are investigated. However, as is shown, the method can be extended to other polymeric and ceramic particles and provides the possibility to manufacture a wide range of porous composite materials.

Designing macroporous polymers from particle-stabilized foams

Particle-stabilized liquid foams provide a general route for producing low-density macroporous materials from melt-processable and intractable thermoplastic polymers. In this paper, we demonstrate how these liquid foams can be used to design macroporous polymers with tailored microstructures and properties by adjusting the various processing parameters. By varying the size, concentration, and wettability of the particles in the colloidal suspensions and controlling the frothing, drying, and sintering conditions, macroporous materials with porosities between 33 and 95% and median pore sizes (D50) between 13 and 634 μm were obtained. This foaming process is applicable to a wide range of hydrophobic materials and is demonstrated here on commercially available polymeric powders of poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF), poly(ether imide) (PEI), and poly(ether ether ketone) (PEEK).

Unifying Model for the Electrokinetic and Phase Behavior of Aqueous Suspensions Containing Short and Long Amphiphiles

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.

Liquid-phase deposition of ferroelectrically switchable nanoparticle-based BaTiO3 films of macroscopically controlled thickness

BaTiO3 films are extensively used in many electrical devices, because they offer remarkable dielectric and ferroelectric properties. Here, we demonstrate a powerful, nanoparticle-based deposition route towards BaTiO3 films with systematic thickness control over a wide range up to several microns. The unusual control over the film thickness with the maintenance of crack free nanostructures, phase and ferroelectric properties of the BaTiO3 films allows us to fabricate various future devices of different thicknesses by a single deposition method. For this, films are deposited from stable dispersions of BaTiO3 nanocrystals, synthesized via an efficient microwave-assisted non-aqueous sol–gel approach. Crack-free films of controlled thickness are obtained by a carefully elaborated, alternating process of spin-coating and intermediate drying. According to X-ray diffraction and confocal Raman microscopy, the final, sintered films consist of BaTiO3 nanocrystals of about 20 nm in a hexagonal–tetragonal phase mixture. The nanoparticulate films display outstanding optical characteristics exceeding 90% transparency above 500 nm and a band gap of 3.5 eV. The latter, band gap, is larger than the classic bulk materials band gap of 3.2 eV, indicating a more electrically insulating nature of the films. Piezoresponse force microscopy gives evidence for potent ferroelectric switching. This newly accessible film processing route with wide film thickness tuning allows for desired ferroelectric response with the advantage of a wide film thickness to implicate building blocks for various applications e.g. ferroelectric random access memory devices, microelectromechanical system devices or Bragg reflectors.

Nano-Sized Structurally Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors

A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered Ni xFe yO z@rGO delivers a capacity of 388 mAh g-1 at 5 A g-1, which is 6 times that of the crystalline sample. Disordered Ni xFe yO z@rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory calculations reveal a smaller volume expansion during Li+ insertion for the structurally disordered Ni xFe yO z nanoparticles, and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

Pickering emulsions stabilized by in situ grown biologically active alkyl gallate microneedles

We report on the stabilization of water-in-oil Pickering emulsions through the in situ growth of surface and biologically active alkyl gallate microneedles. Octyl gallate needles are shown to form a rigid interlocked network upon adsorption at the oil–water interface, generating emulsions that are stable for several months. Emulsions containing food-grade oils with potentially high antioxidant and antimicrobial activities were successfully prepared using solely octyl gallate microneedles as the surface active species. The safety and efficacy of alkyl gallates as antioxidant and antimicrobial agents combined with their ability to form strong interwoven needle-like structures at the oil–water interface makes this a promising approach for the preparation of functional, ultrastable Pickering emulsions for food, cosmetic and pharmaceutical applications.