Designing bijels formed by solvent transfer induced phase separation with functional nanoparticles

Engineering Interconnected Open-Porous Particles via Microfluidics Using Bijel Droplets as Structural Templates

Designing porous structures is key in materials science, particularly for separation, catalysis, and cell culture systems. Bicontinuous interfacially jammed emulsion gels represent a unique class of soft matter formed by kinetically arresting the separation of the spinodal decomposition phase, which is stabilized by colloidal particles with neutral wetting. This study introduces a microfluidic technique to create highly interconnected open-porous particles using bijel droplets stabilized with hexadecyltrimethylammonium bromide (CTAB)-modified silica particles. Monodisperse droplets comprising a hydrophobic monomer, water, ethanol, silica particles, and CTAB were initially formed in the microfluidic device. The diffusion of ethanol from these droplets into the continuous cyclohexane phase triggered spinodal decomposition within the droplets. The phase-separated structure within the droplets was stabilized by the CTAB-modified silica particles, and subsequent photopolymerization yielded microparticles with highly interconnected, open pores. Moreover, the influence of the ratio of the CTAB and silica particles, fluid composition, and microchannel direction on the final structure of the microparticles was explored. Our findings indicated that the phase-separated structure of the particles transitioned from oil-in-water to water-in-oil as the CTAB/silica ratio was increased. At intermediate CTAB/silica ratios, microparticles with bicontinuous structures were formed. Regardless of the fluid composition, the pore size of the particles increased with time after phase separation. However, this coarsening was arrested 15 s after droplet formation in the CTAB-modified silica particles, accompanied by a change in the particle shape from spherical to ellipsoidal. In situ observations of the bijel droplet formation revealed that the particle shape deformation is caused by the rolling of elastic bijel droplets at the bottom of the microchannel. As such, the channel setup was altered from horizontal to vertical to prevent the deformation of bijel droplets, resulting in spherical particles with open pores.

Bicontinuous Interfacially Jammed Emulsion Gels (Bijels): Preparation, Control Strategies, and Derived Porous Materials

Bicontinuous interfacially jammed emulsion gels, also known as Bijels, are a new type of soft condensed matter. Over the last decade, Bijels have attracted considerable attention because of their unique morphology, property, and broad application prospects. In the present review, we summarize the preparation methods and main control strategies of Bijels, focusing on the research progress and application of Bijels as templates for porous materials preparation in recent years. The potential future directions and applications of Bijels are also envisaged.

Nanocomposite colloids prepared by the Ouzo effect

HYPOTHESIS

The organization of nanoparticles within nanocomposite colloids can imbue added functionality to these suprastructures. We hypothesize that the arrangement of nanoparticles in nanocomposite colloids can be systematically controlled by inducing co-precipitation of oil and a hydrophilic polymer in the presence of nanoparticles with a range of wetting properties. This process will produce oil core/polymer shell nanocapsules with nanoparticles strategically positioned within the suprastructures.

EXPERIMENTS

Coprecipitation of oil and polymer in the presence of nanoparticles is performed in glass capillary microfluidics. Silica nanoparticles of varying surface properties and morphology are used to investigate the relationship between nanoparticle wetting properties and nanocolloid morphology. The features of the nanocomposites formed are investigated using electron microscopy, sessile drop, and zeta potential measurements.

FINDINGS

When spherical nanoparticles with wetting properties ranging from hydrophilic to hydrophobic are used, the nanocomposite morphologies formed range from nanoparticles partially engulfed in the polymer shell to nanoparticles embedded in the oil core of the nanocapsule. The number of nanoparticles introduced in the nanocomposite is adjusted by changing their concentration in the precursor solution. The structure of nanocolloids formed with non-spherical or hollow silica nanoparticles depends on their wetting properties.

Polymerizable bijels prepared by a direct-mixing method

A simple direct-mixing method was proposed to use a union of silica particles and amino-capped silicone oil (diNH2-PDMS) as stabilizers to form bicontinuous emulsion gels (bijels) in wide mixing proportions of silica particle/diNH2-PDMS and oil/water with a tunable channel size of the spatial continuity, which was verified by the three-dimensional reconstruction viewer and curvature analysis. By direct polymerization of the oil phase as a template, solid materials were obtained with interconnected structures.

Particle-Polymer Union with Changeable Wettability for Constructing Bijels Using a Simple Mixing Method

Bicontinuous emulsion gels (bijels) are nonequilibrium dispersed systems with particle-stabilized continuous fluid domains, and the internal connectivity of channels brings the possibility of efficient mass transport, endowing bijels great potential in diverse applications. Different from the common method to produce bijels, the spinodal decomposition, which needs precise temperature control and is restricted by the selection of liquid pairs, in this work, a direct mixing method was performed to construct bijels, simplifying the fabrication process. The hydrophilic rod-shaped cellulose nanocrystalline (CNC) particles were in situ combined with the hydrophobic polymer, aminopropyl-terminated polydimethylsiloxane (PDMS-NH2), to acquire a controllable interfacial wettability of CNC. The CNC@mPDMS-NH2 complexes were adsorbed at the water-toluene interface and achieved a change of Pickering emulsion types, oil-in-water, bijel, and water-in-oil, through tuning the interfacial performance of CNC@mPDMS-NH2 complexes. A three-dimensional scanning image and curvature calculation were applied to verify the obtained bijel, further demonstrating the successful preparation of the bicontinuous structure. This work enriched the members of particles for stabilizing bijels and was considered to be scalable in manufacturing for applications on a large scale.

Fabrication and Evaluation of Multiwalled Carbon Nanotube-Containing Bijels and Bijels-Derived Porous Nanocomposites

Structures having continuous porous networks are of great interest for applications in areas such as separation, energy storage, and tissue engineering. Bicontinuous interfacially jammed emulsion gels ("bijels") have been actively investigated as templates for fabricating useful structures for such applications. However, the fabrication of bijels-templated porous nanocomposites incorporated with reinforcing or functional nanoparticles (or nanofibers) to provide specific, targeted functions is still a challenge, stemming from the difficulties of fabricating functional nanoparticle-containing bijels. In this study, bijels-derived porous nanocomposites incorporated with multiwalled carbon nanotubes (MWCNTs), which possessed interconnected channels inside the structures, were made via a facile phase inversion technique for bijels fabrication. For the composite manufacture, in the first step of bijels fabrication, MWCNT adsorption into the oil phase of bijels was observed. It was revealed that MWCNTs were physically absorbed into the oil-rich phase without disrupting the bicontinuous structure of bijels. The successful fabrication of non-crosslinked and crosslinked porous structures containing MWCNTs was evidenced through imaging by confocal laser scanning microscopy and scanning electron microscopy, respectively. For potential controlled release applications, an anticancer drug, doxorubicin hydrochloride (DOX), was incorporated into bijels-derived structures and nanocomposites. The in vitro DOX release profiles from drug delivery systems based on bijels-derived MWCNT-containing nanocomposites suggested that the photothermal effect of MWCNTs initiated by near-infrared irradiation could modulate the drug release behavior. Overall, this study has developed a facile approach to fabricate bijels-templated bicontinuous porous structures incorporated with functional nanoparticles (or nanofibers) and opened an avenue for making MWCNT-containing porous nanocomposites for controlled drug release applications.

Synthesis and Polyelectrolyte Functionalization of Hollow Fiber Membranes Formed by Solvent Transfer Induced Phase Separation

Ultrafiltration membranes are important porous materials to produce freshwater in an increasingly water-scarce world. A recent approach to generate porous membranes is solvent transfer induced phase separation (STrIPS). During STrIPS, the interplay of liquid-liquid phase separation and nanoparticle self-assembly results in hollow fibers with small surface pores, ideal structures for applications as filtration membranes. However, the underlying mechanisms of the membrane formation are still poorly understood, limiting the control over structure and properties. To address this knowledge gap, we study the nonequilibrium dynamics of hollow fiber structure evolution. Confocal microscopy reveals the distribution of nanoparticles and monomers during STrIPS. Diffusion simulations are combined with measurements of the interfacial elasticity to investigate the effect of the solvent concentration on nanoparticle stabilization. Furthermore, we demonstrate the separation performance of the membrane during ultrafiltration. To this end, polyelectrolyte multilayers are deposited on the membrane, leading to tunable pores that enable the removal of dextran molecules of different molecular weights (>360 kDa, >60 kDa, >18 kDa) from a feed water stream. The resulting understanding of STrIPS and the simplicity of the synthesis process open avenues to design novel membranes for advanced separation applications.

Nanostructured, Fluid-Bicontinuous Gels for Continuous-Flow Liquid-Liquid Extraction

Fluid-bicontinuous gels are unique materials that allow two distinct fluids to interact through a percolating, rigid scaffold. Current restrictions for their use are the large fluid-channel sizes (>5 µm), limiting the fluid-fluid interaction surface-area, and the inability to flow liquids through the channels. In this work a scalable synthesis route of nanoparticle stabilized fluid-bicontinuous gels with channels sizes below 500 nm and specific surface areas of 2 m² cm^-3 is introduced. Moreover, it is demonstrated that liquids can be pumped through the fluid-bicontinuous gels via electroosmosis. The fast liquid flow in the fluid-bicontinuous gel facilitates their use for molecular separations in continuous-flow liquid-liquid extraction. Together with the high surface areas, liquid flow through fluid-bicontinuous gels enhances their potential as highly permeable porous materials with possible uses as microreaction media, fuel-cell components, and separation membranes.

Centrifugal Assembly of Helical Bijel Fibers for pH Responsive Composite Hydrogels

In microfluidics, centrifugal forces are important for centrifugal microfluidic chips and curved microchannels. Here, an unrecognized use of the centrifugal effect in microfluidics is introduced. The assembly of helical soft matter fibers in a rotating microcapillary is investigated. During assembly, the fibers undergo phase separation, generating particle stabilized bicontinuous interfacially jammed emulsions gels. This process is accompanied by a transition of the fiber density over time. As a result, the direction of the centrifugal force in the rotating microcapillary changes. The authors analyze this effect systematically with high-speed video microscopy and complementary computer simulations. The resulting understanding enables the control of the helical fiber assembly into microropes. These microropes can be converted into pH responsive hydrogels that swell and shrink with potential applications in tissue engineering, soft robotics, controlled release, and sensing. More generally, the knowledge gained from this work shows that centrifugal forces potentially enable directed self-assembly or separation of colloids, biological cells, and emulsions in microfluidics.

Phase Inversion-Based Technique for Fabricating Bijels and Bijels-Derived Structures with Tunable Microstructures

Bicontinuous interfacially jammed emulsion gels ("bijels") are a new class of soft matter containing two interpenetrating continuous phases. They have great potential for applications in many areas. However, difficulties in fabricating bijels and controlling structural features of interest have posed severe barriers to their wide applications. In this study, a phase inversion-based technique was developed for fabricating bijels and bijels-derived structures. The effects of varying the composition of casting solutions for the fabrication of bijels on the porosity, oil-to-water percentage, and domain size of bijels were investigated. Composite bijels prepared from two organic monomers were also made, demonstrating the flexibility of the phase inversion-based technique for the fabrication of bijels. Interestingly, the incorporation of a second monomer into the casting solution also affected the porosity and domain size of bijels formed, which may provide a new strategy for the controlled fabrication of bijels. Doxorubicin hydrochloride (DOX, as a model drug)-loaded bijels-derived hybrid hydrogels comprising two continuous phases were successfully made, with one phase being cross-linked alginate that carried the drug. Controlled release of DOX from the bijels-derived structures could be achieved. In vitro degradation study indicated that cross-linking of alginate in bijels-derived hybrid hydrogels controlled alginate degradation, thereby affecting the DOX release behavior. Our current work has provided a facile and reproducible protocol for the controlled fabrication of bijels and bijels-derived structures, which facilitates expanding their applications in the biomedical field.

Controlling Surfactant Adsorption on Highly Charged Nanoparticles to Stabilize Bijels

Bicontinuous particle-stabilized emulsions (bijels) are networks of interpenetrating oil/water channels with applications in catalysis, tissue engineering, and energy storage. Bijels can be generated by arresting solvent transfer induced phase separation (STrIPS) via interfacial jamming of nanoparticles. However, until now, STrIPS bijels have only been formed with silica nanoparticles of low surface charge densities, limiting their potential applications in catalysis and fluid transport. Here, we show how strongly charged silica nanoparticles can stabilize bijels. To this end, we carry out a systematic study employing dynamic light scattering, zeta potential, acid/base titrations, turbidimetry, surface tension, and confocal microscopy. We find that moderating the adsorption of oppositely charged surfactants on the particles is crucial to facilitate particle dispersibility in the bijel casting mixture and bijel stabilization. Our results potentially introduce a general understanding for bijel fabrication with different inorganic nanoparticle materials of variable charge density.

Mechanical Properties of Solidifying Assemblies of Nanoparticle Surfactants at the Oil-Water Interface

The effect of polymer surfactant structure and concentration on the self-assembly, mechanical properties, and solidification of nanoparticle surfactants (NPSs) at the oil-water interface was studied. The surface tension of the oil-water interface was found to depend strongly on the choice of the polymer surfactant used to assemble the NPSs, with polymer surfactants bearing multiple polar groups being the most effective at reducing interfacial tension and driving the NPS assembly. By contrast, only small variations in the shear modulus of the system were observed, suggesting that it is determined largely by particle density. In the presence of polymer surfactants bearing multiple functional groups, NPS assemblies on pendant drop surfaces were observed to spontaneously solidify above a critical polymer surfactant concentration. Interfacial solidification accelerated rapidly as polymer surfactant concentration was increased. On long timescales after solidification, pendant drop interfaces were observed to spontaneously wrinkle at sufficiently low surface tensions (approximately 5 mN m^-1). Interfacial shear rheology of the NPS assemblies was elastic-dominated, with the shear modulus ranging from 0.1 to 1 N m^-1, comparable to values obtained for nanoparticle monolayers elsewhere. Our work paves the way for the development of designer, multicomponent oil-water interfaces with well-defined mechanical, structural, and functional properties.

Excellent porous environmental nanocatalyst: tactically integrating size-confined highly active MnOx in nanospaces of mesopores enables the promotive catalytic degradation efficiency of organic contaminants

A nanoporous molecular sieve catalyst containing size-confined MnO_x species, which affords excellent environmental catalytic efficiency, was synthesized using a micelle-assisted in situ embedding strategy.