Oil and water mix and un-mix on demand

Oil and water can for the first time be mixed and separated on demand thanks to a new, reversible surfactant.
The liquid molecule could prove invaluable in mitigating the environmental damage caused by oil spills, such as the one currently spreading along the coasts of Lebanon and Syria.
Such a chemical could also simplify commercial oil extraction from currently inaccessible deposits, its designers say. And it would prove equally valuable in the food and cosmetics industries, simplifying processes and products which rely on the mixing or separation of oily and watery components.
The genius of the new surfactant lies in its reversibility. Unlike existing ones, it can be switched "on" or "off" repeatedly. The switches are very simple too: carbon dioxide and air.
Bubbling away
“Bubble carbon dioxide through a solution and the surfactant switches on, leading the oil-and-water mix to form an emulsion,” explains Philip Jessop at Queen’s University in Kingston, Ontario, Canada, who led the research team that developed the new surfactant.
“To switch it off again, you bubble air through it, and the oil and water separate again,” he says.
Surfactants must have two ends – a water-repelling hydrophobic bit that binds to the oily substance; and a water-attracting hydrophilic bit, which has an ionic charge that binds to water.
An example is a soap or detergent, says Jessop, which enables oily grime to mix with water when you wash. But a mixture held together by soap can only be separated by adding other chemicals such as acids, which are “messy, expensive and hazardous”, he says.
Temporary separation
The same is true of most surfactants in current industrial use, Jessop adds. So, while it is easy to use a surfactant to bind oil and water into an emulsion – so they can be transported more easily through a pipeline, for example – it is difficult to re-separate the oil and water once they reach their destination.
To get around this, the team fashioned molecules called long-chain alkyl amidines which consist of a long, stringy alkyl chain of hydrocarbons that bind to oils. At the other end of the alkyl chain is an amidine group which also binds to oils.
But on exposure to carbon dioxide, the amidine end changes into a bicarbonate salt, which is hydrophilic and binds to water. This makes the long molecule into a functioning surfactant that binds oil and water.
Simply bubbling air through the liquid reforms the amidine, reversing the process – the oil-water mixture separates again.
Oil mush
Jessop and colleagues have demonstrated the surfactant’s effectiveness by using it to mix and separate mixtures of water and the oil, hexadecane. It also worked with a mixture of crude oil and water, and enabled polystyrene to be formed and separated in water.
The surfactant could be invaluable for extracting abundant but relatively inaccessible oil deposits, such as those in “oilsands” found in Venezuela and Alberta, in Canada, the team says.
These mushes of oil, water and rock are very difficult to handle. But the surfactant could be used first with carbon dioxide to emulsify the oil and water, making the mixture easier to extract. Then, on exposure to air, the oil and water could be separated, enabling the oil to be extracted and the water re-cycled, Jessop explains.
Another important advantage is that the surfactant is not likely to be environmentally hazardous or long-lived, because air will convert it to the inactive form which then breaks down, the researchers say.
Journal reference: Science (vol 313, p 958)