As every primary school pupil and salad dressing maker will attest, oil and water do not mix. Until today, that is.
One of science's incontrovertible truths, taught in standard chemistry lessons for generations, has been turned on its head by an Australian scientist.
Instead of the usual rule, Ric Pashley's research may lead to young children reciting the rule that "oil and water do not mix unless in certain circumstances when they become a spontaneous emulsion".
And his discovery could offer clues to one of the biggest mysteries of chemistry, the so-called long-range hydrophobic force – which causes oil surfaces to attract one another over long distances.
According to Mr Pashley, of the Australian National University in Canberra, the secret is to remove all of the gas dissolved in the water.
He was studying oil-like hydrophobic surfaces as they were being pulled apart, and noticed microscopic cavities appearing on their surfaces. Water that has been exposed to air contains the equivalent of several teaspoonfuls of dissolved gas per litre, and Mr Pashley noticed that the cavities contained bubbles of gas that had been drawn out of the water, maybe as a consequence of the long-range hydrophobic force.
"I was as surprised as anyone," he told New Scientist magazine. To test his theory, he removed virtually all the gas from a mixture of water and oil by repeated freezing and thawing. The result was a spontaneous merging of the mixture, which was completely unexpected. The liquids combined readily to form a cloudy mixture, which does not separate even when it is left to settle.
But scientists are unclear as to why it happened and chemists are waiting to see if it can be repeated. Len Fisher, of the University of Bristol, said: "Many scientists are going to find this very hard to believe, but Mr Pashley has provided very strong proof that oil and water will mix."
Potentially, Mr Pashley's find could have important applications in medicine and the chemical industry.
Many injected drugs are currently only soluble in oil. An alternative might be to disperse the medicine in water that has had the gas taken out, New Scientist reported. Emulsion paints, which use chemical stabilisers to stop them separating, could also be made more cheaply.
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