Water-air interface barely there

Where sea meets sky, there are lots of water molecules with an identity crisis. About a quarter of the H2O in water’s uppermost layer can’t decide whether to be liquid or gas: One hydrogen atom stays in the drink while the other pokes up, vibrating in the air.

This layer of molecular ambiguity is extremely thin and has little or no effect on the water below it, new data reported June 9 in Nature show. Right beneath the liquid’s surface, water molecules go about their business just as if the air weren’t there.

That may seem like a dull discovery, but the find is important, says Pavel Jungwirth of the Academy of Sciences of the Czech Republic in Prague, who wrote a commentary on the work in the same issue of Nature.

“In some ways this is a negative result,” Jungwirth says. “Sometimes a negative result can be very positive.”

Insights into the behavior of water molecules at this superthin surface layer may give scientists a better understanding of the bonding and behavior of pollutants or other compounds intermingling at the surface. The new data might also improve models of water’s interactions with the atmosphere and within cells, says physical chemist Dennis Hore of University of Victoria in British Columbia, who was not involved in the study.

The finding that water molecules with one hydrogen waving in the air have little effect on the arrangement of all the H2O below refutes an idea that’s been bandied about for ages — that water molecules in bulk below will fall into lockstep, organizing themselves into orderly layers, says Jungwirth.

“After the first layer, it’s all over,” he says. “A lot of people still subscribe to this long-range idea. But this shows very clearly that you don’t have long-range order in water.”

Despite covering roughly 70 percent of Earth’s surface and constituting 60 percent or more of the human body, water still puzzles scientists. For example, according to water’s structural properties it shouldn’t be liquid, but rather gas, at everyday temperatures and pressures.

“Water may be the most common substance on Earth. But from a physics standpoint, it is pretty unusual,” says study coauthor Alexander Benderskii of the University of Southern California in Los Angeles.

Much of that unusualness is attributed to hydrogen bonding, which holds water together with a strangely strong looseness. While a water molecule’s oxygen is bonded to two hydrogen atoms, the oxygen also seeks connections with the hydrogen atoms of nearby water molecules. At any given time at ordinary temperatures, an oxygen atom in water is bonded to roughly 3.6 hydrogen atoms. Yet the partners in these interactions are always changing, shifting on a picosecond time scale.

By probing water’s bonds at the interface with air, Benderskii and his colleagues determined that the hydrogen poking down into the water is bonding to oxygen atoms below with only a slightly weaker attraction than the typical hydrogen bonding of water.