International. Researchers led by Lucio Isa, professor in the Department of Materials at ETH Zurich, have created a new type of silica particle capable of stabilizing emulsions in a new way. An emulsion is a finely dispersed mixture of two immiscible liquids, consisting of droplets of one liquid dispersed in the second.
An everyday example of this is an oil-vinegar salad dressing: its main components, vinegar (effectively water) and oil, do not mix on their own and must be vigorously beaten to create a uniform mixture. If this mixture is left to rest, the finely dispersed vinegar drops will fuse again and the liquids will be completely separated.
That is why it is necessary to stabilize emulsions: this can be achieved by using numerous different emulsifiers, such as surfactants, polymers or proteins. As early as the early 1900s, British chemists W. Ramsden and S. U. Pickering also demonstrated that emulsions could be stabilized using very fine solid particles, such as spherical silica particles (SiO2).
In this process, the particles spontaneously enter and bind to the interface between the two liquids. They form a kind of armor around the droplets and prevent their fusion, thus stabilizing the emulsion practically indefinitely. However, until now this required two types of particles: those with hydrophilic surfaces, that is, sitting mostly in water, which stabilized only oil emulsions in water and those with hydrophobic surfaces, that is, mostly sitting in oil, which stabilize only mixtures of water in oil.
Now the researchers say this will no longer be necessary: they have sprayed the surfaces of these tiny silica spheres, which measure from one to six micrometers in diameter, charging with silica nanoparticles of a much smaller diameter. As a result, these small balls take the form of raspberries. Michele Zanini, a PhD student in Isa's group, was able to alter the roughness of the surface in a controlled manner and create a complete collection of such particles.
In a recent study, researchers have shown that they can stabilize both types of emulsion using only one type of these raspberry-shaped particles. This depends solely on the liquid into which the particles are introduced before the emulsion is formed. If researchers add the particles to the oil phase, a water-in-oil emulsion is formed. Conversely, they are able to stabilize an oil-in-water emulsion (oil droplets finely dispersed in water), if they first dissolve their new particles in water. "These particles can therefore be used as a universal tool to create emulsions," says Isa.
This is because the rough surface reduces the mobility of the particles through the surface of the droplets, he explains. "Even though they push forward on the surface between liquids, they cannot move as far as silica particles comparable to a smooth surface – the rough particles get stuck before they can reach the most energetically favourable position at the interface," says Professor ETH.
There are many possible applications for these particles, i.e. when there is a need to stabilize emulsions; for example, in the chemical industry. Although this research focused on laboratory model systems, the same principles can be extended to the use of natural harsh particles as emulsion stabilizers, to find other potential uses in the food, cosmetic and pharmaceutical industries, although more research is needed in this direction.
Source: ETH Zurich.
