Oil and water may not mix but adding the right nanoparticles to the recipe can convert these two immiscible fluids into an exotic gel with uses ranging from batteries to water filters to tint-changing smart windows.
Such gels have been an area of research for decades. Part of their potentially broad utility is the complex set of interconnected microscopic channels that form within them, creating a spongelike structure. These channels not only offer passageways for other materials to travel through — making them useful for filtration — but also give the gel a high amount of internal surface area, a characteristic valuable for speeding up chemical reactions or as scaffolding on which living tissue can grow.
The gels are commonly formed of two liquid solvents mingled together. As with oil and water, these solvents do not mix well but to prevent them from completely separating, researchers add custom-designed nanoparticles that can stay at the interface between them. Carefully cooking these ingredients allows a cohesive gel to form; however, the process is demanding because custom-designing nanoparticles for each application has been difficult and forming the gels has required carefully controlled rapid temperature change. These constraints have made it hard to create this type of gel in any more than small quantities suitable for lab experiments rather than on an industrial scale.
Researchers have found ways to sidestep many of these problems. The novel approach they developed forms a SeedGel — an abbreviation for solvent segregation-driven gel. Instead of designing nanoparticles to remain at the interface between the two solvents, their chosen particles concentrate within one of them. While these particles tend to repel one another, the particles’ affinity toward one of the solvents is stronger and keeps them together in the channel.
The resulting gel could be far easier to create, as its two solvents are essentially oil and water and its nanoparticles are silicon dioxide — essentially tiny spheres of common quartz. It also could have a variety of industrial uses. The gel is thermoreversible, which refers to an optical property that the finished SeedGel possesses: It can switch from transparent to opaque and back again just by changing its temperature. This property could be harnessed in smart windows that sandwich a thin layer of the gel between two panes of glass or in sensors.
Because the team’s gel-creation approach could be used with other solvent-and-nanoparticle combinations, it could become useful in filters for water purification and possibly other filtration processes, depending on what type of nanoparticles are used. The creation approach allows for the size of the channels within the gel to be tuned by changing the rate at which the temperature changes during the formation process.