A paper detailing the National Science Foundation-funded work is published in the current issue of Nature and is also highlighted in a news and views article in the same issue. In addition to Ben-Amotz, co-authors include Purdue graduate student Joel Davis and postdoctoral fellows Kamil Gierszal and Ping Wang.
The team's observations add to a more than 70-year debate over the interaction of oil and water, with some studies suggesting that water forms little icebergs around the oil molecules, while others point to a more disordered, vaporlike water structure.
"This question was really up for grabs until we introduced an experimental method that could see these subtle changes in water structure," Ben-Amotz said. "Surprisingly, we found that both sides are right, and it depends on the size of the oil."
The challenge of the experiment was that the team needed to see the very small number of water molecules that are in contact with the oil chains in the presence of a very large number of other water molecules.
The team combined Raman scattering and multivariate curve resolution to create an analysis method capable of managing an unprecedented signal-to-noise ratio of 10,000-to-1.
"Most people never take a spectrum with a signal-to-noise ratio greater than 100-to-1, but if we performed this experiment that way we wouldn't see anything," Ben-Amotz said. "We needed to have a higher signal-to-noise ratio because we were looking for a needle in a mountain-sized haystack."
Raman scattering involves shooting a beam of light containing photons into a sample. As the photons hit molecules within the sample, they lose or gain energy. Such measurements create a spectrum of peaks that reveal the vibrational motions of the molecules present in the sample. Shifts in the peaks' shapes can show changes in the strength of bonds between water molecules and whether the molecules are becoming more or less ordered.
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