الخروج من عنق الزجاجة... كيف؟

Quote: Oil and water do mix after all Oil and water do not mix - the mantra is familiar to every schoolchild. You have to shake them to overcome the forces that hold the oil together. Now teachers may want to rewrite their lessons. If you first remove any gas that is dissolved in the water, it will mix spontaneously and even stay that way indefinitely, according to chemist Ric Pashley of the Australian National University in Canberra. "Many scientists are going to find this very hard to believe," says colloid scientist Len Fisher of the University of Bristol in England, "but Pashley has provided very strong proof that oil and water will mix." Pashley's observation is bound to cause controversy as the reason it happens is still unclear. Chemists are waiting to see whether the experiment can be repeated. If confirmed, the finding could provide clues to one of chemistry's most puzzling phenomena. This is the so-called long-range hydrophobic force, which causes oil surfaces to attract one another over what to chemists are remarkably long distances. French dressing The effect prevents oil's dispersion in water, and means that you can only make oil and water emulsions, such as French dressing for salads, by shaking them and adding stabilising agents. But although countless chemists have measured the force, no one has ever been able to explain how it works. Pashley was studying oil-like hydrophobic surfaces as they were being pulled apart, and spotted 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 Pashley suspected 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. To test his hunch, Pashley removed almost all the gas from a water-oil mixture by repeatedly freezing and thawing it while pumping off the gases as they evaporated out (Journal of Physical Chemistry B, vol 107, p 1714). What he saw then was completely unexpected. "The mix spontaneously formed a cloudy emulsion. I was as surprised as anybody," says Pashley. The result suggests that dissolved gas may be involved in how the force acts. Extremely close "He takes the air out and he doesn't get the long-range hydrophobic force. It doesn't nail the hydrophobic force down, but now we have something to work on," says James Quirk, a chemist at the University of Western Australia in Perth, who hopes that studying the spontaneous emulsions may lead to an explanation for the elusive force. Even more surprisingly, the mixture did not break up even when gas was put back into the water after the emulsion had formed. Pashley suggests that the gas might interfere with the hydrophobic force most effectively only when the oil droplets are extremely close together, such as when they are first separating as the emulsion starts to form. Once the emulsion has formed, hydroxyl groups from the water adsorb onto the surface of the oil droplets, making them similarly charged and thus preventing them from coming close together. If spontaneous emulsions can be made at will, they could have important applications in medicine and the chemical industry. Many injectable medicines are currently only soluble in oil. An alternative might be to disperse the medicine in degassed water, which is already produced on a large scale by the oil industry. Emulsion paints, which currently use chemical stabilisers to stop them separating, could also be made more cheaply if degassed water would do the tri [/QUOTE]