Water is surprisingly ordered on the nanoscale

Nanometric-sized water drops are everywhere—in the air

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as droplets or aerosols, in industrially produced medications, and within rocks and oil fields. To understand the behavior of these drops, it is necessary to know how they interact with their hydrophobic environment. This interaction takes places at the curved droplet interface, a sub-nanometric region that surrounds the small pocket of water. Researchers from EPFL, in collaboration with the institute AMOLF in the Netherlands have discovered that molecules on the surface of the drops were much more ordered than expected. Their surprising results have been published in Nature Communications. They pave the way to a better understanding of atmospheric, biological and geological processes.

Unique perspective on miniscule droplets

At EPFL, Sylvie Roke has developed a unique method for examining the surface of droplets one thousandth the thickness of a human hair, with a volume of an attoliter (10−18 liters). "The method involves overlapping ultrashort laser pulses in a mixture of water droplets in liquid oil and detecting photons that are scattered only from the interface", explains Roke. "These photons have the sum frequency of the incoming photons and are thus of a different color. With this newly generated color, we can determine the structure of the interface."

Hydrogen bonding as strong as in ice

The surface of the water droplets turns out to be much more ordered than that of normal water, and is comparable to super-cooled water in which the molecules have very strong hydrogen bond interactions. In ice, these interactions lead to a stable tetrahedral configuration surrounding each water molecule. Surprisingly, this type of structure was found on the surface of the droplets even at the room temperature—50 °C above were it would normally appear.

Read more at: https://phys.org/news/2017-05-surprisingly-nanoscale.html#jCp