Silica nanoparticles stabilise emulsions in a new way

The stabilisation of emulsions can be achieved using numerous different emulsifiers, such as surfactants, polymers or proteins. As early as in the beginning of the 1900’s, British chemists also demonstrated that emulsions could be stabilised 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 sort of armour around the droplets and prevent their fusion, thus stabilising the emulsion practically indefinitely. However, until now, this required two types of particles: those with hydrophilic surfaces, i.e. mostly sitting in the water, that stabilise only oil-in-water emulsions and those with hydrophobic surfaces, i.e. mostly sitting in the oil, that stabilise only water-in-oil mixtures.

One emulsifier stabilises both emulsions

Now, this may no longer be necessary: Researchers at ETH Zurich’s Department of Materials have roughened the surfaces of these tiny silica spheres, which measure one to six micrometres in diameter, by loading them with silica nanoparticles of a much smaller diameter. As a result, these tiny balls take on the shape of raspberries. Michele Zanini, a doctoral student in Isa’s group, was able to alter the surface roughness in a controlled way and create a whole collection of such particles.

Universal tool

In a study, published in Nature Communications, the researchers have demonstrated that they can stabilise both types of emulsion using just 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 the researchers add the particles to the oil phase, a water-in-oil emulsion is formed. Conversely, they are able to stabilise an oil-in-water emulsion, if they dissolve their new particles in water first. “These particles can therefore be used as a universal tool for creating emulsions,” says Lucio Isa, Professor of Interfaces, Soft Matter and Assembly.

Coarse particles get stuck earlier

This is because the rough surface reduces the particles’ mobility through the droplet’s surface, he explains. “Although they push forward on the surface between the liquids, they cannot move as far across it as comparable silica particles with a smooth surface do – the rough particles get stuck before they can reach the energetically most favourable position at the interface,” says the ETH professor. The researchers have filed a patent for their new process of particle production as emulsion stabilisers.

New applications in sight

There are many possible applications for these particles, namely whenever there is the need to stabilise emulsions; e.g. in the chemical industry. Even though this research was focused on laboratory model systems, the same principles can be extended to the use of naturally occurring rough particles as emulsion stabilisers, to find other potential uses in the industry, even though further research is needed in this direction.