Oil, water don't mix, but they do communicate

by Tom Parisi

Everyone knows oil and water don't mix, but the story doesn't end there. They do, in a sense, talk.

Scientists have known that two immiscible (unmixable) solvents must communicate to each other that they should not mix. But researchers have never known for sure what goes on at the molecular boundary between two immiscible liquids.

New research, published in the Jan. 13 issue of the prestigious journal Science, for the first time shows a seldom predicted layering phenomenon.

“We provide the first ever experimental evidence showing that the boundary is comprised of a layered structure of ions—charged particles at the smallest possible dimension,” said Petr Vanýsek, an NIU electrochemist and a key member of the team of researchers publishing in Science.

“There actually is some degree of mixing of the two solvents right at the interface,” he added. “In addition, the ions near the interface are stacked in a couple of layers rather than just distributing smoothly throughout.”

A team of nine scientists used the Advanced Photon Source at Argonne National Laboratory, the most powerful X-ray facility in the Western Hemisphere , to closely examine the boundary between water and nitrobenzene, an oil-like substance. NIU's Vanýsek and physicist Mark Schlossman of the University of Illinois at Chicago were principal investigators on the experiment, supported by the National Science Foundation.

“Down the road, this new understanding could have numerous applications, from improving oil-spill cleanups to providing superior delivery of drugs through human cell membranes, which are actually oil-like materials,” Vanýsek said.

In the past, scientists were divided over what occurs at the interface of immiscible liquids. Most ascribed to the Gouy-Chapman theory, which predicted a diffuse, gradual change from oil to water.

“We were able show that ions, charged particles of salts dissolved in these liquids, are distributed near the interface in a fashion quite different from what was predicted by the theory generally held to be true,” Vanýsek said. “The Gouy-Chapman theory predicted the distribution of ions would be gradual and continuous, but the theory doesn't hold true at the molecular level. We found it was gradual, but layered.

“This is important because the electric field, responsible for the communication with adjacent space, is then layered as well,” he added. “This new information should lead to a better understanding of how immiscible liquids communicate with each other.”

1-23-06