Nanoparticle‐Stabilized Double Emulsions and Compressed Droplets

Abstract

Emulsion droplets can be stabilized against coalescence by many types of interfacially active species, including proteins, small-molecule surfactants, polymers, nanoparticles and microparticles. When the fluid-fluid interface of an oil-inwater (o/w), or water-in-oil (w/o), emulsion is stabilized by adsorption of particles at the interface, a "Pickering emulsion" is obtained in which the particles occupy the fluid-fluid interface and prevent, or retard, droplet coalescence. Pioneering work of Ramsden [1] and Pickering [2] on paraffin/water emulsions containing solid particles, such as iron oxide, silicon dioxide, barium sulfate, and kaolin clays demonstrated the critically important role of particles in such interfacial stabilization. Theoretical treatment of nanoparticle-stabilized droplet structures, reported for example by Pieranski [3] and Binks, [4][5][6] describe the reduction of the overall surface energy of the system due to nanoparticle interfacial segregation as a function of particle size and the relative interfacial energies of the system (oil-water, oil-particle, and water-particle).The tendency of particles to localize to oil-water interfaces opens opportunities to fabricate new materials based on the individual and collective properties of the particles, including: 1) the optical properties derived from colloidal crystallization, [7,8] 2) self-assembled conducting structures, [9,10] and 3) encapsulation and release technologies. [11,12] For example, Weitz and co-workers reported the oil-water interfacial assembly of polystyrene microspheres into hexagonally packed capsule structures termed "colloidosomes." [13] For nanoparticles, the energy holding each particle at the fluid-fluid interface is smaller than that for microscale objects; nonetheless, there have been numerous reports of nanoparticle-stabilized emulsion droplets, in which the droplets are stable for hours, days, or longer. CdSe QDs, [14,15] Au NPs, [16][17][18] Fe 3 O 4 NPs, [19] "Janus" nanoparticles [20] and plantderived virus particles [21] all prove amenable to stabilizing fluid-fluid interfaces of different types. Moreover, embedding functional ligands on nanoparticle surfaces provides access to further droplet stabilization through chemical cross-linking of the ligands, thereby converting these dynamic self-assembled systems into robust structures. [22] Double-emulsion droplets, whether water-in-oil-in-water (w/o/w), or oil-in-water-in-oil (o/w/o), are attractive for providing a means to control release from the inner phase to the outer phase, while effectively shielding the interior phase from the continuous phase. Double emulsions can be prepared by a one-or two-step emulsification process, in the presence of a relatively hydrophilic surfactant that stabilizes o/w droplets, and a relatively hydrophobic surfactant that stabilizes the w/o interfaces.[23] Such emulsions have been generated during phase inversion processes, that is, when the continuous phase of the immiscible liquid-liquid dispersion becomes the dispersed phase. We n...

Nanoparticle Assembly at Liquid–Liquid Interfaces: From the Nanoscale to Mesoscale

“…In addition to micrometer‐sized colloidal particles, NPs, such as CdSe, Au, and SiO 2 , can self‐assemble at liquid–liquid interfaces to stabilize emulsions or double emulsions. However, considering the weak nature of the NP assemblies at the interface, methods to convert the assemblies into materials with mechanical integrity are necessary.…”

Microfluidic one-step fabrication of radiopaque alginate microgels with in situ synthesized barium sulfate nanoparticles

“…[20][21][22][23][24][25] For example, many research groups developed microfluidics devices to produce functionalized polymer microspheres 21,26 and inorganic nanoparticles, [27][28][29][30] with precise control over their shapes and sizes. Song et al reviewed various microfluidics synthesis techniques to form metallic and silica nanoparticles and quantum dots.…”

Magnetoresponsive Virus‐Mimetic Nanocapsules with Dual Heat‐Triggered Sequential‐Infected Multiple Drug‐Delivery Approach for Combinatorial Tumor Therapy

“…Only when oil phase contains hydrophobic IO, the stable double emulsion can be observed with a Lf–IO composite shell. The phenomenon can be understood by solid‐stabilized liquid–liquid interface by colloidal particles (known as the Pickering emulsion process) . The integrated application of Lf and IO as stabilizing core‐shell capsules cause to discover a new type of protein–IO‐based emulsion capsules.…”