Engineering: Double inversion of emulsions

 

Scientists have observed oil mixing in water and vice versa with nanoparticles and surfactants.

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Oil and water are not known to mix together but it is possible to combine both into an emulsion to act as a unit, such as in creams, body lotions, milk or in mayonnaise, for example.

In these substances, one of the two liquids is dispersed as tiny droplets in the other, which requires an emulsifier and vigorous shaking or stirring.

However, whether the oil droplets are suspended in water (oil-in-water emulsion, O/W) or the water droplets are suspended in oil (water-in-oil emulsion, W/O) depends on various factors.

Now, a British team from the University of Hull has reported a double inversion of a nanoparticle-containing emulsion: by the successive addition of a surfactant (a chemical compound that reduces the surface tension between two liquids), they were able to convert an O/W emulsion into a W/O emulsion and then back again.

The emulsifier’s job is to make droplet formation easier and to counteract separation.

In addition to surfactants, which are contained in detergents and the like, fine solid particles also have a stabilising effect - mustard powder has long been used to stabilise mayonnaise.

Both surfactants and particles aggregate at the phase boundary of the two liquids and keep the droplets from flowing together.

Many commercial formulations contain surfactants as well as solid particles.

If the conditions are changed, a phase inversion can occur, converting an O/W into a W/O emulsion, for example, if more and more surfactant is added.

First instance of double inversion

University of Hull researchers Bernard Binks and Johnny Rodrigues have now achieved a double inversion.

Their system initially contains silica nanoparticles and a small quantity of a surfactant with a water-loving (hydrophilic), positively charged head and two nonpolar, water-repellent (hydrophobic) tails.

The tiny silica spheres are negatively charged, hydrophilic, and easily wettable by water. In this state, they stabilise oil drops in water (O/W).

If more surfactant is added, a layer of surfactant molecules surrounds each sphere, all with their hydrophobic tails sticking out.

The spheres are now covered with a hydrophobic layer and are no longer wettable - they stop repelling each other and begin to aggregate.

This causes the emulsion to undergo its first inversion into W/O. If further surfactant is then added, these additional molecules lodge tail-to-tail with those already surrounding the spheres.

In turn, this forms a double layer around the spheres, with the positively charged heads of the second surfactant layer now sticking out.

The spheres thus once again have a charged, hydrophilic surface and again stabilise oil droplets in water. The emulsion undergoes its second inversion back into O/W.

The research was published in the journal Angewandte Chemie.

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