image: (a) Schematic illustration of particle adsorption to the interface by depletion attraction. (b) Photographs of compared emulsions (0.18?vol% of 1?μm-silica particles) formed with the presence of PEG (10,000?g?mol?1, left) and the absence of PEG (right), taken after several hours from voltex emulsification (?3,000?r.p.m.). Here, φ is the oil fraction of the initial oil-water mixture before emulsifying. Scale bar, 1?cm. (c) Photograph of φ=0.6 emulsion (0.18?vol% silica particles) with oil-soluble fluorescence dyes and its confocal micrograph. Scale bar, 100?μm. (d) Photograph of φ=0.8 high internal phase Pickering emulsion (0.18?vol% silica particles) and its confocal micrographs. Scale bar, 100?μm. (e) Confocal microscope image of fluorescently labelled silica particles (1.08?vol% silica particles) well adsorbed on an oil droplet surface by depletion attraction. Scale bar, 30?μm. All emulsions and HIPPEs (b-e) are produced by the vortexing method. view more
Credit: KAIST
Professor Siyoung Choi's research team from the KAIST Department of Chemical & Biomolecular Engineering used physical force to successfully produce a stable emulsion.
Emulsions, commonly known as cosmetic products, refer to stably dispersed structures of oil droplets in water (or water droplets in oil). Pickering emulsions refer to emulsions stabilized using solid particles, instead of detergent. Traditionally, it is said that water and oil do not mix. Until recently, detergent was added to mix oil and water for dispersion. Emulsions have traditionally been produced using this technique and are currently used for products such as mayonnaise, sun block, and lotion.
On the other hand, Pickering emulsions have been used after stabilization of chemical treatments on solid particle surfaces to enhance adsorption power. However, there were limitations in its application, since the treatment process is complex and its applicable range remains limited. Instead of chemical treatment on Pickering emulsion surfaces, the research team mixed small macromolecules a few nanometer in size with larger solid particles (tens of nanometers to a few micrometers). This induced depletion force was used to successfully stabilize the emulsion.
Depletion force refers to the force a large number of small particles induces to aggregate the bigger particles, in order to secure free space for themselves. In short, the force induces an attraction between larger particles. Until now, depletion force could only be applied to solids and solid particles. However, the research team used macromolecules and large particles such as solid particles and oil droplets to show the applicability of depletion force between solids and liquids. By introducing macromolecules that act as smaller particles, hydrophilic solid particles enhanced the adsorption of solid particles to the oil droplet surface, while preventing dissociation from the particle surface, resulting in the maintenance of a stable state.
The research team confirmed the possibility of the simple production of various porous macromolecular materials using stable Pickering emulsions. Such porous macromolecules are expected to be applicable in separation film, systems engineering, drug delivery, and sensors, given their large surface area.
Professor KyuHan Kim, the first author said, "Until now, depletion force has only been used between solid colloid particles. This research has scientific significance since it is the first example of using depletion force between solid particles and liquid droplets."
Professor Choi said, "Beyond its academic significance, this technology could contribute to industries and national competitiveness." He continued, "Since this technology uses physical force, not chemical, to produce stable emulsion, it can be used regardless of the type of solid particle and macromolecule. Further, it could be used in customized porous material production for special purposes."
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The research was published in Nature Communications online on February 1. In particular, this research is significant since an undergraduate student, Subeen Kim, participated in the project as a second author through the KAIST Undergraduate Research Program (URP). This research was funded by the National Research Foundation of Korea.
Journal
Nature Communications