Functional membranes and capsules via nanoscale interfacial complexation (NICE)

Microcapsules that encapsulate and protect molecules and materials by forming isolated aqueous compartments inside hollow shells are widely used in a variety of applications in the food, pharmaceutical, cosmetics and agriculture industry. One promising method that has emerged in the recent years for the preparation of functional hollow microcapsules with aqueous cores involves layer-by-layer (LbL) assembly. Although the useful functionality of LbL microcapsules has been demonstrated, these successes have mostly been limited to laboratory/small-scale demonstration of such functionality. The widespread application of these LbL microcapsules in industry has not been fully realized. The major bottlenecks that have limited the widespread adoption of this promising technology are: 1) encapsulation efficiency is very low because this approach involves core dissolution and subsequent introduction of materials into the microcapsule lumen through the shell, and 2) although clever approaches to enhance encapsulation efficiency by using sacrificial porous particles have been developed, LbL assembly is nevertheless extremely tedious and time consuming.  The objective of this effort is to investigate the processing-structure-property relationship of a new class of polyelectrolyte microcapsules with high encapsulation efficiency that are fabricated by a recently developed method based on the nanoscale interfacial complexation in emulsions (NICE). The proposed work is motivated by our recent success in one-step generation of water-in-water polyelectrolyte microcapsules with high encapsulation efficiency.  The NICE method involves interfacial complexation of polymers at an oil-water interface in a multiphasic system such as double emulsions, followed by a spontaneous dewetting phenomenon.  This method can be readily extended to systems using conventional multiple emulsions or single emulsions suspended over a macroscopic water-oil interface. The NICE microcapsules are potentially transformative because 1) their fabrication method is highly scalable, 2) the NICE method results in high encapsulation efficiency, 3) NICE capsules can encapsulate both hydrophilic and hydrophobic species in the core and shell of the microcapsules, respectively, and 4) microcapsules with functionality that rival that of LbL capsules can be generated. 

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One-step formation of Polyelectrolyte Microcapsules via nanoscale interfacial complexation in emulsion (NICE)

Hierarchical and asymmetric bijel microparticles, fibers and membranes

Bijels (bicontinuous interfacially jammed emulsion gels) are a new class of multiphasic mixtures that are formed by arresting the spinodal decomposition of immiscible liquids with interfacial jammed colloidal particles. In contrast to conventional emulsions, bijels have stable bicontinuous liquid architectures, making them ideal for applications that require close contact between two immiscible liquid phases and potentially mass transfer through the interface. 

Although bijels clearly present important new opportunities, the conventional preparation method is limited to privileged pairs of liquids that undergo temperature-induced phase separation, and cannot be widely adopted for other types of liquid pairs and for broad composition ranges. Furthermore, bijels prepared by this technique lose their stability when the temperature is changed and also tend to have features that are in the micron-scale range, limiting their interfacial area. Finally, these temperature quench methods have been performed in batch process; a vessel filled with components is quenched to produce the bijel. To fully realize the promise of these unique bicontinuous liquid structures, it is highly desirable to develop methods that allow for continuous production of thermally stable bijels using a diverse pallet of liquids and particles. We recently presented the continuous production of thermally stable bijels using a diverse pallet of liquids and particles. We recently  presented the continuous generation of hierarchically and asymmetrically structured bijel fibers, microparticles and membranes based on solvent transfer-induced phase separation (STRIPS). In STRIPS, phase separation is induced by selective extraction of a solvent from an originally homogeneous mixture of three liquids. A homogeneous ternary liquid mixture comprising two immiscible liquids (typically, oil and water) and a solvent is introduced into a continuous phase that extracts the solvent from the ternary picture to induce phase separation. The nanoparticles to the oil-water interface. The bijel fibers that we form using STRIPS have bicontinuous internal structures and are able to support the contact of immiscible fluids throughout their structures within which species can selectively migrate, which we demonstrated by the diffusion of hydrophobic (green) and hydrophilic (blue) dyes in the oil and water domains within the fiber, respectively.

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Ternary phase diagram illustrating STRIPS. Diethylpthalate is a model oil phase, and ethanol is a co-solvent that makes all three fluids miscible. A silica nanoparticle along with a cationic surfactant is added to arrest spinodal decomposition via interfacial jamming. (b) Schematic illustration of STRIPS. (c-d) Confocal images of STRIPS fibers

Emulsion morphology control via polyelectrolyte-surfactant ion pairing

Based on the oil-water volume ratio, emulsifiers commonly stabilize either water continuous (oil in water) or oil continuous (water in oil) emulsions. A rare class of emulsifiers are those that able to stabilize both primary emulsion morphologies depending the physicochemical state of the system; even more efficient are those that can dynamically switch between the two morphologies in-situ upon the action of a physicochemical stimulus such as incident light. These emulsifiers often must be specially synthesized, and only function under specific environmental conditions. It would thus be beneficial to utilize readily available chemicals to stabilize multiple emulsion morphologies, as well as achieve dynamic in-situ emulsion morphology switches. In this research, we use commercially available polyelectrolytes and surfactants to achieve this aim. Chitosan from crustacean harvested sources is complexed with the anionic surfactant AOT to create a polyelectrolyte-surfactant complex that has an effectively adjustable HLB capable of stabilizing both common emulsion morphologies, as well as a multiple emulsion morphology, depending on the surfactant to polyelectrolyte monomer ratio. Dynamic phase inversion between the two morphologies is achieved via in-situ modulations of suspension pH or surfactant concentration. It is found that the surfactant to polyelectrolyte monomer ratio at which the morphology switches from water to oil continuous depends on the surfactant’s critical micelle concentration as well as its oil-water partition coefficient.

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Schematic illustration of emulsion morphology dependence on surfactant to polyelectrolyte monomer ratio

Continuous generation of nanocapsule via microfluidics

Nanocapsules are a promising material for use across multiple industries due to their enhanced colloidal stability, high surface area to volume ratio, and core-shell morphology. One method used for fabricating nanocapsules in a one step process is the co-precipitation of an oil and a polymer, such that the polymer is made to sit at the oil droplet to be subsequently crosslinked. The oil is precipitated via the “Ouzo effect” using acetone and water as the solvent and nonsolvent respectively, requiring precise oil and acetone mass fractions to be in the Ouzo domain on their ternary diagram. A remaining challenge in the translation towards widespread use is that this fabrication is often performed in small batches which lends to difficulties in scaling up and potential batch-to-batch variations.  It would thus be desirable to develop a continuous method of nanocapsule fabrication as a step towards larger scale production. In this research, coaxial glass capillary microfluidics is used to mix streams to achieve oil-polymer coprecipitation. The streams are mixed via vortices whose size and mixing capabilities are a function of the taper diameter of the inner capillary, the inner to outer capillary mass flowrate ratio, as well as the total mass flowrate. In an extension of the oil-polymer system, the formation of Pickering nanoemulsions is being investigated using hydrophobized silica nanoparticles via surfactant adsorption.