Mixing water and oil: no surfactants needed – Physics World
Oil and water famously don’t mix – at least, not without adding a surfactant such as soap to coax them into a stable combination. Now, however, researchers in France and US have turned this conventional wisdom on its head by showing that they can, in fact, mix without a surfactant. The finding could have wide-reaching implications for industries that make heavy use of such mixtures, including food, cosmetics, health, paints and packaging to name just a few.
A mixture of two immiscible liquids such as water and oil is known as an emulsion. When an emulsion is shaken vigorously, one of its component liquids may disperse into small droplets within the other. But if the emulsion is left to stand, its components invariably separate out again.
The main driver of this separation is that as droplets of each liquid move closer to each other, they coalesce into ever-larger droplets. To prevent this, a third component may be added that is amphiphilic, meaning that it has an affinity for the interface between the mixture’s two components. Today’s industrial emulsions rely on the use of such materials, which are termed surfactants. However, many surfactants are toxic for both humans and the environment. Reducing their use (or doing away with them altogether) would therefore be highly beneficial.
In the latest work, researchers from the Colloïdes et Matériaux Divisés Laboratory at the ESPCI in Paris, France; the French company Calyxia, which specializes in the design and manufacture of biodegradable microcapsules; and Harvard University in the US studied mixtures composed solely of water and various types of oil. Within these normally immiscible mixtures, they observed ultrathin but abnormally stable films of oil spontaneously appearing between the dispersed droplets of water.
