Switchable surfactants give on-demand emulsions
17 August 2006
Oil and water can now be mixed or separated simply by bubbling carbon dioxide or air through the blend, thanks to a molecule developed by Canadian chemists.
Dispersions of oil droplets in water are called emulsions, and are used in numerous processes in the chemical industry. In order to prevent the oil and water separating into two layers, the droplets are stabilised by surfactants. These molecules, which usually have a charged end attracted to water and a hydrocarbon chain attracted to oil, self-assemble at the oil-water interface and prevent droplets combining into a distinct phase.
But these emulsions are very stable, so separating the two components again can be a real problem, explains Philip Jessop of Queen's University, Kingston, Canada.
Jessop and his co-workers now think they may have the answer. They have developed a molecule whose surfactant properties can be switched on and off by a simple, reversible reaction, they report in the journal Science.
The scientists found that bubbling carbon dioxide through a solution of a neutral amidine molecule made it react to form a positively charged group at one end. Bubbling air through the solution returned the molecule to its original form. Jessop's team successfully used this chemistry to form and break emulsions of the hydrocarbon hexadecane and water.
The switchable surfactant may be useful in the oil industry, said Jessop. High-pressure water and carbon dioxide are often used to pump oil out of oilfields. The presence of natural surfactants in the crude mix means 'there's a good chance you get an emulsion out the other end', Jessop said. If the amidine was added with the carbon dioxide and water, it would be much easier to break up the emulsion to separate oil from the mixture, he predicts.
'We haven't done the tests with real oil in real ground, but we have proved the chemistry does everything we wanted and more' he said. Tests on crude oil mixtures in the lab have shown that the neutral amidine can indeed break up emulsions formed by the natural surfactants.
Surfactant chemist Bernie Binks, of the University of Hull, UK, says that industrial chemists are likely to take an interest in the work. 'It would be nice to see what other chemical structures could possess this behaviour,' he added. Jessop now intends to develop the technique further and plans to make the molecule biodegradable by modifying the chemical structure of the hydrocarbon tail.
Y Liu et al, Science, 2006, 313, 958
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