Bicontinuous Emulsions Research Articles

Stabilizing Bicontinuous Emulsions with Sub-Micrometer Domains Solely by Nanoparticles.

Nanoparticle-stabilized, bicontinuous interfacially jammed emulsion gels (bijels) find potential applications as battery, separation membrane, and chemical reactor materials. Decreasing the liquid domain sizes of bijels to sub-micrometer dimensions requires surfactants, complicating bijel synthesis and postprocessing into functional nanomaterials. This work introduces surfactant-free bijels with sub-micrometer domains, solely stabilized by nanoparticles. To this end, the covalent surface functionalization of silica nanoparticles is characterized by thermogravimetric analysis, mass spectrometry, Fourier-transform infrared spectroscopy, and contact angle measurements. Bijels are generated with the functionalized nanoparticles via solvent transfer induced phase separation (STrIPS), enabling the optimization of nanoparticle functionalization and surface ionization. Nanoparticles of intermediate functionalization and controlled negative surface charge stabilize bijels with sub-micrometer liquid domains. This remarkable control over bijel synthesis provides urgently needed progress to facilitate the widespread implementation of bijels as nanomaterials in research and applications.

Dual access to the fluid networks of colloid-stabilized bicontinuous emulsions through uninterrupted connections.

Large surface areas are important for enhancing mass and energy transfer in biological and technological processes. Bicontinuous interfacially jammed emulsion gels (bijels) increase the surface area between two fluids by intertwining them into particle stabilized networks. To facilitate efficient mass and energy exchange via the bijels' high surface area, the fluid networks need to be connected to their respective bulk phases. Here, we generate bijels between two bulk fluids and investigate the connections the bijel makes. We analyze these connections by investigating the colloidal stability, interfacial rheology and mass transfer dynamics during bijel formation. To this end, we employ confocal and electron microscopy, as well as dynamic light scattering, pendant drop analysis, electrophoretic mobility measurements and diffusion simulations. We find that the connections the bijel makes to the bulk fluid can be disrupted by severe colloidal aggregation and interruptions of the bicontinuous fluid network. However, the addition of alcohol to the bulk fluid moderates aggregation and allows undisturbed fluid network formation, facilitating open connections between bijel and bulk fluid. The unprecedented control of bijel pore connections from this research will be crucial for the application of bijels as separation membranes, electrochemical energy storage materials and chemical reactors.

Super-Macroporous Lightweight Materials Templated from Bicontinuous Intra-Phase Jammed Emulsion Gels Based on Nanochitin.

Non-equilibrium multiphase systems are formed by mixing two immiscible nanoparticle dispersions, leading to bicontinuous emulsions that template cryogels with interconnected, tortuous channels. Herein, a renewable, rod-like biocolloid (chitin nanocrystals, ChNC) is used to kinetically arrest bicontinuous morphologies. Specifically, it is found that ChNC stabilizes intra-phase jammed bicontinuous systems at an ultra-low particle concentration (as low as 0.6wt.%), leading to tailorable morphologies. The synergistic effects of ChNC high aspect ratio, intrinsic stiffness, and interparticle interactions produce hydrogelation and, upon drying, lead to open channels bearing dual characteristic sizes, suitably integrated into robust bicontinuous ultra-lightweight solids. Overall, it demonstrates the successful formation of ChNC-jammed bicontinuous emulsions and a facile emulsion templating route to synthesize chitin cryogels that form unique super-macroporous networks.

Fabrication of bijels with sub-micron domains via a single-channel flow device

Particle-stabilized bicontinuous emulsions (bijels) are made of two interwoven liquid channel systems. In contrast to emulsion droplets, the liquid-liquid interface of bijels curves both towards the oil and the water phases. Thus, particles with equal wettability for both oil and water are needed to stabilize the interface. In this research paper, we enhance the understanding of nanoparticle functionalization by a surfactant for controlling the particle wettability. To this end, we develop a novel, single-channel, continuous flow method, enabling the rapid synthesis and analysis of bijels formed via solvent transfer induced phase separation (STrIPS). Silica nanoparticles are functionalized with the positively charged surfactant (cetyltrimethylammonium+, CTA+). Zeta-potential and colloidal stability analysis are employed to characterize the CTA+ functionalization. Confocal and electron microscopy are used to visualize the resulting bijel structures. Bijels with nearly uniform, sub-micrometer channels are obtained when the particle functionalization by CTA+ is regulated. To this end, the initial negative zeta-potential of the particles needs to be low enough to prevent excessive CTA+ adsorption. The adsorption is further controlled by adjusting the concentrations of CTA+, salt and glycerol additive. This report shows that the nanoparticle surfactant modification depends on multiple parameters, providing guidance for future bijel synthesis approaches.

Development and characterization of food-grade bigel system for 3D printing applications: Role of oleogel/hydrogel ratios and emulsifiers

This study objected to widen the knowledge and applications of bigels by constructing food bigels for 3D printing. A serious of bigels were developed by incorporating a candelilla wax-based oleogel into a gelatin hydrogel at different oleogel/hydrogel ratios of 3:7, 5:5, and 7:3 under the presence of different emulsifiers (PGPR, monoglycerides and lecithin). And the influence of different oleogel/hydrogel ratios and emulsifiers on the physical properties and 3D printability of resulting bigels were studied. Bigels containing monoglycerides or lecithin could form homogenous, stable gel system with bicontinuous emulsion structure. Microstructure analysis found that PGPR could disturbed the crystalline process of candelilla wax resulting in weak mechanical strength of resulting oleogel and phase separation of bigels. These bigels exhibited shear-thinning behavior and improved mechanical properties (apparent viscosity, storage modulus, hardness, etc.) compared to pure oleogel and pure hydrogel. Bigels containing monoglyceride with oleogel/hydrogel ratio of 7:3 exhibited suitable mechanical properties for 3D printing. The results from this study indicated that 3D printable bigels could be obtained by combining candelilla wax (organogelators) with gelatin (hydrogelator), and the physical, printing properties of these bigels could be tailored by adjusting oleogel/hydrogel ratios and using different emulsifiers.

All-Aqueous Bicontinuous Structured Liquid Crystal Emulsion through Intraphase Trapping of Cellulose Nanoparticles.

Here, we describe the all-aqueous bicontinuous emulsions with cholesteric liquid crystal domains through hierarchical colloidal self-assembly of nanoparticles. This is achieved by homogenization of a rod-like cellulose nanocrystal (CNC) with two immiscible, phase separating polyethylene glycol (PEG) and dextran polymer solutions. The dispersed CNCs exhibit unequal affinity for the binary polymer mixtures that depends on the balance of osmotic and chemical potential between the two phases. Once at the critical concentration, CNC particles are constrained within one component of the polymer phases and self-assemble into a cholesteric organization. The obtained liquid crystal emulsion demonstrates a confined three-dimensional percolating bicontinuous network with cholesteric self-assembly of CNC within the PEG phase; meanwhile, the nanoparticles in the dextran phase remain isotropic instead. Our results provide an alternative way to arrest bicontinuous structures through intraphase trapping and assembling of nanoparticles, which is a viable and flexible route to extend for a wide range of colloidal systems.

Spontaneous Emulsions: Adjusting Spontaneity and Phase Behavior by Hydrophilic-Lipophilic Difference-Guided Surfactant, Salt, and Oil Selection.

Spontaneous emulsion behavior has been difficult to predict and could be influenced by many variables including salinity, temperature, and chemical composition of the oil and surfactant. In this work, the hydrophilic-lipophilic difference (HLD) framework was used to predict the formation of spontaneous emulsions using a mixture of Span-80 and SLES surfactants. The spontaneity and emulsion behavior of different systems were modeled by estimating the HLDmix. The influence of surfactant ratio, salinity, and oil type was investigated. Spontaneous emulsification could only be observed when the HLDmix was between -0.96 and 1.04. Within this range, a negative HLDmix resulted in a greater spontaneity to form o/w emulsion, and a w/o emulsion was more likely to form when the HLDmix was positive. When the HLDmix was close to 0 (between -0.22 and 0.56 in our systems), emulsions were formed in both the oil and aqueous phases with high spontaneity. A combined effect of ultralow interfacial tension, Span-80 micelle swelling, and interfacial turbulence due to Marangoni effects is likely the main mechanism of the spontaneous emulsification observed in this study. A synergistic reduction in interfacial tension was observed between Span-80 and SLES (<1 mN/m). When the HLD of the system was close to 0, a bicontinuous emulsion phase was formed at the oil-water interface. The bicontinuous emulsion broke-up over time due to the ultralow interfacial tension and interfacial turbulence, forming dispersed oil and water droplets. Results from this work provide a practical method to suggest what surfactant composition, salinity, and oil type could promote (or eliminate) the conditions favorable for spontaneous emulsification.

Large inorganic monolithic nanomaterials with a significant rigid hierarchical pore structure

Although there are many methods for preparing macroscopic nanomaterials, it is difficult to directly and completely maintain the structure from nanometer to macroscopic because of the insufficient structural strength. Here, large inorganic monolithic nanomaterials (LNMs) were directly synthesized by the complex nanostructure reinforced nanofilms self-regulating grown along with the bicontinuous emulsion (BE) interface in a three-dimensional bicontinuous film (3DBF) rigid structure. The Zn(OH)2/SiO2 nanofilms have complex nanostructures divided into the nanofoam structure woven by Zn(OH)2 nanoribbons in ionic bonds, and the silica aerogel cross-linked in covalent bonds, significantly strengthening the hard nanofilms. Therefore, the LNMs' structural strength is sufficient to directly overcome the surface tension and capillary pressure and completely maintain the structure from nanometer to macroscopic. Prepared with controllable shapes under mild condition, LNMs not only have the characteristics of traditional nanomaterials such as low density (0.0849 g mL−1) and high specific area (359.43 m2 g−1), but also have new advantages such as high structural strength (compressive strength, 62555 Pa), low-shrinkage (2.97%), hierarchical pores (1 nm ~ >100 μm), and high permeability. Moreover, the Zn(OH)2/SiO2 LNM columns (4 cm3) in a rigid hierarchical pore structure not only has the traditional catalytic and adsorption properties but also overcome fluid erosion and avoid the loss of nanoparticles, that the total Zn in the effluent without separation is 0.89 mg L−1. Therefore, this new method significantly facilitates the commercial synthesis and application of LNMs as convenient, safe, efficient, and stable components.

Nanostructure strengthened nanofilms self-regulating synthesize along with the oil-water interface to fabricate macroscopic nanomaterials

Few interfacial assembled or synthesized nanofilms are strong enough to self-support macrostructures. And almost no film prepared by the oil-water interface reaction system has a nanostructure that is hard enough to imprint the interface reaction process. Here, nanostructure strengthened nanofilms were self-regulating synthesized along with the oil-water interface. The oil-water interface is positively charged by cationic surfactants and accumulates OH− in the interfacial mixture layer. The nanoparticles are crystallized in the mixture, negatively changed by OH−, modified by cationic surfactants, grow and weave into monolithic Zn(OH)2 nanofoam and silica aerogel, significantly strengthening the nanofilm structural strength and imprinting the nanofilm formation process. With the nanofilm forming, the diffusions are blocked and then isolated, resulting in the mixture expansion gradually slows down and even shrinks to disappear, thereby self-limiting the nanofilm thickness and shaping the complex nanostructure. Macroscopic nanomaterials (2 cm3) were prepared by the nanofilm grown along with the bicontinuous emulsion interface. Moreover, the nanofilm is strong enough to maintain the complex structure from nano to macro, thereby the macroscopic nanomaterial demonstrating a low-shrinkage of 2.9 % after drying at 70 °C and atmosphere pressure. Together with the middle conditions, this provides a simple method to synthesize complex nanostructure strengthened hard nanofilm by the oil-water interface reaction system, which can prepare macroscopic nanomaterials or large nanodevices maintained the designed structure form nano to macro, an ideal bridge to connect nanomaterials and macromaterials.

Fabrication of bicontinuous double networks as thermal and pH stimuli responsive drug carriers for on-demand release.

The fabrication, via a two steps approach, of a novel bicontinuous Double Network (BCDN) material is reported. We first use a bicontinuous emulsion as template to obtain a poly(butyl acrylate) (PBA) and poly(acrylic acid) (PAA) bicontinuous amphiphilic material. The material is then swollen with the precursor of a second hydrophilic polymer (PNIPAM, poly(N-isopropylacrylamide)). After polymerization of these precursors, the two responsive polymers, PNIPAM and PAA, form a double-network within a bicontinuous templated material, i.e. a bicontinuous double network (BCDN) material. The advantages of using such unique and complex double network architecture are manifold. PBA increases the mechanical properties of the hydrogel all together with the hydrophilic double network that also decouples the pH and temperature responsiveness. Among different possible applications, we tested this responsive hydrogels for its biomedical application. It can be used as pH and temperature sensitive devices for on-demand drug delivery. In addition, the release of a drug confined in the amphiphilic bicontinuous structure follows different kinetics profiles, depending on pH and temperature. This last result indicates that it is possible to control and regulate the release of an encapsulated drug according to the fluctuations of physiological conditions.

Bicontinuous Intraphase Jammed Emulsion Gels: A New Soft Material Enabling Direct Isolation of Co-Continuous Hierarchial Porous Materials

Materials with hierarchical, bicontinuous porosity are desirable for numerous applications, providing a continuous pathway for reactants and products and a large interfacial surface area for reaction/storage. Herein, we demonstrate a low-energy and flexible route to dual scale co-continuous materials via a new class of soft material first reported here—bicontinuous intraphase-jammed emulsion gels, referred here as bipjels. Alumina-coated silica nanoparticles (AlO-SiO NPs) are shown to stabilize bicontinuous emulsion gels formed by the spinodal decomposition of water/2,6-lutidine (W/L) via a percolating network of attractive NPs within the water-rich phase. The prepared bipjels are tunable depending on pre-mixing conditions and NP concentration. Following bipjel formation, a free-standing co-continuous monolith of AlO-SiO NPs with macro- and mesoporosity is directly extractable owing to the high mechanical strength of the percolating network of stabilizing NPs. Overall, this new class of soft materials is shown to overcome the inherent difficulty in creating thermodynamically unstable bicontinuous morphologies via a scalable, template-, and organic binder-free method, producing materials that are near-ideal for catalytic, storage, or separation applications.

Bi-continuous emulsion using Janus particles.

Bi-continuous emulsion stabilized with amphiphilic Janus particles was achieved. Phase inversion of the as-formed emulsion was driven by increasing water content. The orientated Janus particle monolayer at the bi-continuous emulsion interface is interconnected by interfacial polymerization to form robust materials with amphiphilic channels.

Advanced emulsions via noncovalent interaction-mediated interfacial self-assembly.

We demonstrate that the traditional emulsification theory can be enriched by a self-assembly approach, in which hydrophilic copolymers with one block exhibiting noncovalent forces with the oil phase self-assemble at the oil-water interface, thereby reducing interfacial tension and forming emulsions. This approach was established using affinity diblock copolymers that can interact with oil molecules through electrostatic interactions or hydrogen-bonding. Nanoemulsions with excellent stability were successfully obtained simply via vortexing. The self-assembled emulsions showed unexpected catastrophic phase inversion, further extending the phase structures to bicontinuous and reverse emulsions. Complex emulsions could also be fabricated by this strategy. In addition, the thus prepared nanoemulsions can be used to engineer different nanomaterials.

Development of model systems for in vitro investigation of transdermal transport pathways

AbstractSkin is a complex structured system primarily involved in Transdermal Drug Delivery (TDD). The outer stratum corneum represents the main barrier to the entrance of external molecules. Nowadays, none of the recognized methods, used for the investigation of penetration processes, is able to give a complete overview of the transport mechanisms involved. Standard protocols are based on the use of human or animal skin samples, which are difficult and expensive to obtain. Here, we present a novel experimental setup to investigate TDD by using Confocal Laser Scanning Microscopy and image analysis. The methodology is based on diffusion experiments of fluorescent‐labelled fluids (water, oil, and oil‐in‐water emulsions), in a model matrix. Using an agarose gel as model for a proof of concept, different penetration efficiencies were observed, suggesting an important role of both chemical composition and fluid microstructure on the transport mechanism. An adequate model system should be used to better mimic the stratum corneum morphology and properties. To this aim, several bicontinuous emulsion gels, obtained from an emulsification process, were preliminary formulated and suggested as model systems to mimic the stratum corneum. In this work, we define the key directions for the development of an innovative approach to study the penetration of formulations through the skin.

Particle-size effects in the formation of bicontinuous Pickering emulsions.

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway, i.e., nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

Template-particle stabilized bicontinuous emulsion yielding controlled assembly of hierarchical high-flux filtration membranes.

A novel solvent-evaporation-based process that exploits template-particle stabilized bicontinuous emulsions for the formation of previously unreached membrane morphologies is reported in this article. Porous membranes have a wide range of applications spanning from water filtration, pharmaceutical purification, and battery separators to scaffolds for tissue engineering. Different situations require different membrane morphologies including various pore sizes and pore gradients. However, most of the previously reported membrane preparation procedures are restricted to specific morphologies and morphology alterations require an extensive optimization process. The tertiary system presented in this article, which consists of a poly(ether sulfone)/dimethylacetamide (PES/DMAc) solution, glycerol, and ZnO-nanoparticles, allows simple and exact tuning of pore diameters ranging from sub-20 nm, up to 100 nm. At the same time, the pore size gradient is controlled from 0 up to 840%/μm yielding extreme asymmetry. In addition to structural analysis, water flux rates of over 5600 L m(-2) h(-1) are measured for membranes retaining 45 nm silica beads.

Bicontinuous emulsions stabilized solely by colloidal particles

Recent large-scale computer simulations suggest that it may be possible to create a new class of soft solids, called 'bijels', by stabilizing and arresting the bicontinuous interface in a binary liquid demixing via spinodal decomposition using particles that are neutrally wetted by both liquids. The interfacial layer of particles is expected to be semi-permeable; hence, if realized, these new materials would have many potential applications, for example, as micro-reaction media. However, the creation of bijels in the laboratory faces serious obstacles. In general, fast quench rates are necessary to bypass nucleation, so that only samples with limited thickness can be produced, which destroys the three-dimensionality of the putative bicontinuous network. Moreover, even a small degree of unequal wettability of the particles by the two liquids can lead to ill-characterized, 'lumpy' interfacial layers and therefore irreproducible material properties. Here, we report a reproducible protocol for creating three-dimensional samples of bijel in which the interfaces are stabilized by essentially a single layer of particles. We demonstrate how to tune the mean interfacial separation in these bijels, and show that mechanically, they indeed behave as soft solids. These characteristics and their tunability will be of great value for microfluidic applications.

Emulsification of Partially Miscible Liquids Using Colloidal Particles: Nonspherical and Extended Domain Structures

We present microscopy studies of particle-stabilized emulsions with unconventional morphologies. The emulsions comprise pairs of partially miscible fluids and are stabilized by colloids. Alcohol-oil mixtures are employed; silica colloids are chemically modified so that they have partial wettability. We create our morphologies by two distinct routes: starting with a conventional colloid-stabilized emulsion or starting in the single-fluid phase with the colloids dispersed. In the first case temperature cycling leads to the creation of extended fluid domains built around some of the initial fluid droplets. In the second case quenching into the demixed region leads to the formation of domains which reflect the demixing kinetics. The structures are stable due to a jammed, semisolid, multilayer of colloids on the liquid-liquid interface. The differing morphologies reflect the roles in formation of the arrested state of heterogeneous and homogeneous nucleation and spinodal decomposition. The latter results in metastable, bicontinuous emulsions with frozen interfaces, at least for the thin-slab samples, investigated here.

Emulsion microstructure and energy input, roles in emulsion stability

The microstructure of a series of oil-in-water emulsions, formed in the toluene/triton X-100/water system, was investigated as a function of energy input during the formulation process. Emulsion microstructure could be manipulated for a single emulsion composition by changing only the amount of energy supplied during emulsification. Four different emulsion microstructures are realisable in this system: two distinct discrete oil droplet structures (low- and high-oil content) separated by a bicontinuous system and finally for the highest oil content a closed-cell foam structure. The two discrete oil droplet emulsions responded in an opposite sense on increasing formulation energy. For the low-oil content droplet emulsion the average droplet diameter decreased on increasing energy input whereas the average droplet diameter for the high-oil content emulsions increased. Indicating that the interfacial characteristics of the two droplet emulsions are different. Creaming results confirmed this differentiation. The bicontinuous emulsion, which bridges the two droplet emulsions in the phase diagram, could only be realised for very low energy inputs, whereas the closed-cell foam emulsion was almost completely independent of formulation energy used. Rheological investigations of a given emulsion composition, formed with varying input energy, showed a single response only, the result of one common microstructure being formed upon application of the external stress. These results show that the strength of the interfacial domain of these emulsions, controlled primarily by the triton X-100, is extremely weak, being readily manipulable during formulation but equally so upon processing.

Bicontinuous and Closed-Cell Foam Emulsions as Continuum Structures in an Oil in Water Emulsion System

Stable emulsions in the ternary toluene/Triton X-100/water system can be prepared for a surfactant to oil weight ratio of 1:5. Emulsions are formed on this continuum between 0.5 and 14 wt % Triton X-100. For all Triton X-100 wt %, an oil in water emulsion is formed. At high Triton X-100 concentrations, a closed-cell fluid foam is stabilized. In addition, a bicontinuous emulsion can be stabilized for several hours for a Triton X-100 composition of 6 wt %. This phase coexists with an oil in water emulsion. The bicontinuous phase is formed only when the input energy is minimized; for higher input energy no stable bicontinuous emulsion could be prepared. Moreover, the bicontinuous phase is extremely sensitive to external stimuli. Four techniques, pulsed gradient spin−echo NMR, rheology, conductivity, and freeze-fracture transmission electron microscopy, were used to probe the microstructures of the stabilized emulsions.